High strength and ductility Mg-8Gd-3Y-0.5Zr alloy with bimodal structure and nano-precipitates
Xiaoxiao Weia, Li Jina, Fenghua Wanga, *, Jing Lia, Nan Yeb, Zhenyan Zhanga, Jie Donga
a National Engineering Research Center of Light Alloy Net Forming & State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, Chinab Powder Metallurgy Research Institute, Central South University, Changsha 410083, China
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2020 Editorial board of Journal of Materials Science & Technology Copyright reserved, Editorial board of Journal of Materials Science & Technology
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Abstract
To resolve the strength-ductility trade-off problem for high-strength Mg alloys, we prepared a high performance Mg-8Gd-3Y-0.5 Zr (wt%) alloy with yield strength of 371 MPa, ultimate tensile strength of 419 MPa and elongation of 15.8%. The processing route involves extrusion, pre-deformation and aging, which leads to a bimodal structure and nano-precipitates. Back-stress originated from the deformation-incompatibility in the bimodal-structure alloy can improve ductility. In addition, dislocation density in coarse grains increased during the pre-deformation strain of 2%, and the dislocations in coarse grains can promote the formation of chain-like nano-precipitates during aging treatment. The chain-like nano-precipitates can act as barriers for dislocations slip and the existing mobile dislocations enable good ductility.
XiaoxiaoWei, LiJin, FenghuaWang, JingLi, NanYe, ZhenyanZhang, JieDong. High strength and ductility Mg-8Gd-3Y-0.5Zr alloy with bimodal structure and nano-precipitates[J]. Journal of Materials Science & Technology, 2020, 44(0): 19-23 https://doi.org/10.1016/j.jmst.2019.10.024
1. Introduction
Advanced engineering materials have attracted the attention of materials scientists and engineers for alleviating the current challenge of the energy crisis and global warming, etc [1,2]. Magnesium (Mg) and its alloys are considered as promising engineering materials due to their light weight and high specific strength. However, low absolute strength severely restricts their wide applications. So far, alloying is one of the prevalent and effective methods to improve strength of Mg alloys [3,4]. Considerable studies present that rare earth Mg alloys, especially Mg-Gd-Y based alloys, exhibit high strength, which primarily originates from solid-solution strengthening and precipitation strengthening. Gadolinium (Gd) and yttrium (Y) elements have a large difference in atomic radius from that of Mg [5], which leads to significant strengthening effects. The solid solubility of Gd and Y in Mg matrix changes significantly with temperature [6], thus providing remarkable precipitation strengthening. Strengthening mechanisms, however, usually sacrifice ductility, which is known as strength-ductility trade-off [[7], [8], [9], [10], [11], [12]]. The primary reasons are limited slip systems and difficulties in activating non-basal slip at ambient temperature [13]. To date, strategies in dealing with the conflict mainly focus on microstructural design, especially heterogeneous structure which has positive effects on strength-ductility balance. Xu et al. [14] reported an excellent strength-ductility balance in Mg alloys, which was attributed to the bimodal structure which can relax the local strain concentration via strain transfer from the dynamically recrystallized (DRXed) regions to the deformed regions. Wu et al. [11] overcame this trade-off dilemma in titanium by architecting a heterogeneous lamella structure. They attributed the high ductility to the assistance of additional work hardening developed from storage and interaction of dislocations, which was induced by a macroscopic strain gradient and a change in stress states. Lu et al. [15,16] and Shao et al. [17] enhanced strength-ductility synergy by engineering a linear gradient in grain size in nanomaterials and Fe-Mn-C twinning-induced plasticity steel, respectively. Lu’s group considered the unique deformation mechanism of gradient nanostructures is the key to breakthrough the strength-ductility trade-off. Shao’s team considered that desirable properties originate from geometric necessary dislocations (GNDs) formed during tensile deformation, which provides additional work-hardening, especially at late stage of deformation. In addition, the superior strength-ductility combination was also discovered in a dual-constituent microstructure in Ti-Nb based gum metal [18].
Aside from microstructure design, dispersed nano-precipitates is another effective approach to overcome strength-ductility trade-off. Liu et al. [19] adopted a molecular-level liquid-liquid mixing/doping technique and obtained an optimal microstructure, where nanometric oxide particles uniformly distributed in the interior of submicrometer grains. The dispersing intragranular particles can generate, pin down and accumulate dislocations, thus resulting in the increase of strength and simultaneous improvement in ductility. Similar reports have been reported in high-entropy alloys [20] and ultrastrong steel [8]. It is noted that these highly dispersed, fully coherent or semi-coherent precipitates, which exhibits low lattice misfit with the matrix, and high anti-phase boundary energy, can significantly improve strength without trade-off in ductility.
Though the abovementioned methods have overcome the strength-ductility trade-off of Mg alloys to some extent, it is not sufficient for meeting the practical requirements in engineering services. Mg-Gd-Y based alloys are regarded as typical aging hardening alloys which can obtain high strength after aging treatment, and it is easy to obtain special microstructure through hot working process. Therefore, herein we introduce a bimodal structure and intragranular nano-precipitates in a Mg-Gd-Y-Zr alloy for solving strength-ductility trade-off. The pre-deformed GW83 K Mg alloys showed a siginificant different behavior in terms of precipitation and age-hardening behaviors when compared with as-extruded alloy, and the reasons for the good strength-ductility balance are discussed.
2. Experimental materials and procedures
2.1. Material fabrication and sample preparation
In this study, the initial material is forged Mg-8Gd-3Y-0.5 Zr (wt%) alloy, and which has been denoted as GW83 K Mg alloy. The cylindrical samples for extrusion were machined from the forged alloy. Indirection extrusion was carried out at 375 °C and then water-cooled to room temperature immediately. The extrusion ratio and ram speed are 12:1 and 0.1 mm/s, respectively. Prior to the extrusion, the forged samples were heated for 15 min at 375 °C. The as-extruded sample was tensile to different strains (2%, 5% and 8%) at ambient temperature, and then aging treatment was conducted in the oil bath at 225 °C for 16 h.
2.2. Microstructural characterization
Microstructural characterizations were conducted by using electron back scattering diffraction (EBSD), transmission electron microscope (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). EBSD observation was performed on a scanning electron microscope (SEM, MIRA3 SEM equipped with Aztec Nordlys Max3 EBSD system). The specimens for EBSD were prepared by mechanical grinding and then polished using SiC paper (1200, 2400, 4000), diamond suspension (6 μm, 3 μm, 1 μm) and OPS suspension. TEM images and HAADF-STEM images were obtained on the TALOS F200 × . The specimens for observation were cut from the tensile samples and then mechanically grinded to the thickness of 30 μm, and followed with ion milling using a Gatan 695 II precision ion polishing system.
2.3. Mechanical property testing
Dog-bone shaped specimens having the gauge length of 18 mm and cross-sectional area of 5 mm × 1.6 mm were cut from both as-extruded and peak-aged rods along the extrusion direction. Uniaxial tensile tests were conducted using a Zwick/Roell Z100 testing machine with a cross-head speed of 0.5 mm/min at ambient temperature. The tensile direction is parallel to the extrusion direction.
3. Results
Fig. 1 shows the tensile properties of the alloys in this study and literatures. As presented in Fig. 1(a), the as-extruded alloy exhibits ultimate tensile strength (UTS) of 346 MPa and tensile elongation (EL) of 19.7%. Compared with as-extruded alloy, the UTS of aged alloy increases significantly, but the EL drops to 13.7%. The sample with pre-deformation strain of 2% and aging exhibits higher UTS of 419 MPa and EL of 15.8%. In contrast, as the pre-deformation strain increases, the UTS of the alloy further increases, while the EL declines sharply. In comparison with other existing Mg-Gd-Y based alloys prepared through various processes, the alloy in this work is one of the most potential candidates for many critical engineering applications (Fig. 1(b)).
Fig. 1. (a) Engineering stress-strain curves of as-extruded and aged GW83 K Mg alloys at room temperature and (b) representative tensile properties of Mg-Gd-Y based alloys [[21], [22], [23], [24], [25], [26], [27], [28], [29], [30]].
Fig. 2 shows the optical micrograph and EBSD results of as-extruded GW83 K Mg alloy. It is noted that the as-extruded alloy presents a typically bimodal structure, which is consist of DRXed grains and elongated deformed grains. In Fig. 2(a), the optical micrograph is better to show the uniform distribution of the bimodal structure. The volume fraction and average grain size of DRXed grains are 79.8% and 1.8 μm, respectively. While, the grain size of the deformed grains ranges from 30 to 80 μm, and these grains distribute along the extruded direction. In addition, the coarse deformed grains have sharp orientation gradients within grains or near the grain boundaries, as shown in Fig. 2(c), which implies high density dislocation accumulated within them and in the vicinity of grain boundaries. However, that phenomenon is not obvious for fine grains.
Fig. 2. Optical micrograph (a) and EBSD results of as-extruded GW83 K Mg alloy: (b) inverse pole figure (IPF) map; (c) kernel average misorientation (KAM) map; (d) grain size distribution of DRXed grains. The extrusion direction is horizontal.
Fig. 3 shows the TEM images of dislocation configuration in as-extruded and pre-deformed samples. For as-extruded sample, dislocation density observed in the region adjacent to the grain boundaries is high for coarse grains (as marked by yellow arrows in Fig. 3(a)), and that in the grain center is relatively low. However, dislocation density in fine grains is very low, and even few dislocations were observed in some fine grains (Fig. 3(b)). There are consistent with the above results in Fig. 2(c). As we can see from Fig. 3(c) and (d), in the sample with pre-deformation strain of 2%, dislocation density in coarse grains significantly increases, while that in fine grains remains unchanged. Moreover, there are a few sub-grain boundaries (the white arrows in Fig. 3(d)) within the sample. As the pre-deformation strain increases to 8%, both coarse grains and fine grains were filled with high density dislocation, as shown in Fig. 3(e-g), respectively.
Fig. 3. Dislocation configuration in as-extruded and pre-deformed samples: (a, b) as-extruded; (c, d) pre-deformation strain of 2%; (e, g) pre-deformation strain of 8%, and in which (a, c, e) are in coarse grains, and (b, d, f, g) are in fine grains.
HAADF-STEM images of aged samples are shown in Fig. 4, which present the features of precipitates at different pre-deformation strains. For as-extruded alloy, precipitates within coarse grains and fine grains exhibit different distribution trend, as shown in Fig. 4(b) and (c). The precipitates uniformly distribute in fine grains, while part of that in coarse grains are chain-like distribution, and the rest of precipitates distribute between chains uniformly. The size of the chain-like precipitates is larger than that of the rest. When the pre-deformation strain of 2%, the precipitates in both coarse and fine grains present similar distribution to the coarse grains within as-extruded alloy. More importantly, the volume fraction of chain-like precipitates increases, and the space between each chain reduces. Interestingly, as the pre-deformation strain reaches to 8%, the chain-like precipitates almost disappear. The precipitate density in coarse grains increases remarkably while the precipitate size further decreases.
Fig. 4. HAADF-STEM images of precipitates in aged samples: (a-d) as-extruded; (e, f) pre-deformation strain of 2%; (g, h) pre-deformation strain of 8%, and in which (a, b, e, g) are in fine grains, and (c, d, f, h) are in coarse grains.
4. Discussion
4.1. Effect of bimodal structure on strength and ductility
By purposely deploying heterogeneous structure to enhance materials properties has been widely reported [7]. As a typical heterogeneous microstructure, bimodal structure has attracted much attention. Generally, bimodal structure can be regarded as the composite structure, which is consist of ultrafine-grained regions and coarse-grained regions [31]. According to the Hall-Patch relationship, the strength of materials varies with the grain size, and the larger the grain size, the lower the strength [32]. At the initial stage, the plastic deformation occurs preferentially in coarse grains (soft domains), while the ultrafine grains (hard domains) maintain the elastic deformation [11,31]. Therefore, the deformation-incompatibility happens, which leads to the formation of strain gradient. As the deformation continues, the strain gradient increases. To accommodate the strain gradient, plenty GNDs generates, implying back-stress existed, which control the strain hardening of the samples when their density exceeds that of the statistically-stored dislocations [7,11,33,34]. The GNDs also interact with mobile dislocations and impede their movements during plastic deformation, thus improving the strength of the alloy [35]. To generalize these ideas, we can conclude that back-stress strengthening and back-stress strain-hardening can improve the strength and ductility simultaneously.
4.2. Effect of pre-deformation on strength and ductility
In addition to the bimodal structure, the microstructure evolution during pre-deformation and aging can play a significant role to enhance the properties of the alloy. As we all know that dislocations have significant influence on the precipitation process, and considerable literatures had done profound discussion on the precipitating behavior [[36], [37], [38], [39]]. Liu et al. [36] adopted phase-field model to examine heterogeneous nucleation of β1 precipitates on various a-type dislocations (screw, edge and mixed) in an Mg-0.5 at.% Nd alloy. Considering the different in dislocation type, the morphology, aspect ratio and distribution of the β1 precipitates were significantly different. Liu et al. [37] also studied the influences of the applied stress and dislocations on the distribution of β’ precipitates in a crept Mg-Gd-Zr alloy, and pointed out that the a-type basal edge dislocations are the most optimal sites for β’ nucleation. Moreove, Qiu et al. [39] reported the variant selection by dislocations during α precipitation in α/β titanium alloys, and found that edge dislocations exhibit a much more prominent effect than screw dislocations. But few reports have been foused on which Burgers vectors is more favorable to the precipitates nucleation. In situ TEM is an effective way to solve that problem, and relevant work will be done in the future. In this paper, we pay main attention to the effect of density of dislocations and precipitates on the mechanical properties, and the detailed discussion is given as below.
Back-stress has an effect on the dislocation configuration in the coarse grain interior during pre-deformation proess, and in turn affects the precipitation process and properties of samples [11,31]. At pre-deformation strain of 2%, profuse dislocations generate within coarse grains and cannot transmit into neighboring grains, which is attributed to the back-stress [11,34]. Thus, dislocation density in fine grains is low while that in coarse grains is high. As the pre-deformation strain increases from 2% to 5%, the coarse grains sustained most of the plastic strain, and then dislocation density in coarse grains is much high and close to saturation and fine grains begin to provide stain. For triggering the deformation of fine grains, the driving force reaches sufficiently high levels as a result. At the pre-deformation strain of 8%, dislocation density increases significantly but the space for dislocation storage decreases sharply [40], which results in the decrease of ductility.
Extensive chain-like precipitates appear in both coarse and fine grains in the 2% pre-deformed sample after aging treatment. The formation of the chain-like precipitates is closely related to the dislocation configuration after pre-deformation, because high density dislocations introduced by pre-deformation can serve as heterogeneous nucleation sites. The dislocations will reduce the barrier for precipitate nucleation [41] and can also effectively accelerate mass transport by raising diffusion kinetics [42], both of which are in favor of promoting precipitation nucleation.
Compared with as-extruded alloy, dislocation density is higher in coarse grains for the 2%-deformed sample. Therefore, there are a higher volume fraction of chain-like precipitates in the 2%-deformed sample, which contributes to the good strength and ductility. The chain-like structure provides space for dislocation storage to some extent, which can provide strain in grain interior [42]. In addition, the sub-grain boundaries existed in the sample are favorable for dislocation glide through these boundaries as well [43]. However, as for the 8%-deformed sample, higher dislocation density dramatically activated more precipitates which distribute in both coarse and fine grains. High density precipitates result in high strength but poor ductility, which could ascribe to the small space between precipitates and accelerate the crack nucleation at the interface between precipitates and α-Mg matrix. Moreover, the chain-like precipitates are virtually absent in this sample, which reduces the storage capacity of the dislocations, thereby deteriorating ductility as well.
5. Conclusions
In this work, our motivation is to prepared high performance GW83 K Mg alloy by using extrusion and aging after pre-deformation. The main conclusions are as follows:
(1) The as-extruded alloy presents a typically bimodal structure, which is consist of DRXed grains and elongated deformed grains. Back-stress originated from the deformation-incompatibility can improve ductility.
(2) With the pre-deformation strain of 2%, dislocation density in coarse grains increase, while that in fine grains remain unchanged. The dislocations in coarse grains can promote the formation of chain-like nano-precipitates during aging treatment.
(3) Nano-precipitates can act as barriers for dislocations slip and the existing mobile dislocations enable good ductility.
(4) The GW83 K Mg alloy with bimodal structure and chain-like intragranular nano-precipitates exhibits YS, UTS and EL are 371 MPa, 419 MPa and 15.8%, respectively.
Acknowledgments
This work was supported financially by the National Key Research and Development Plan (No. 2016YFB0301103), the National Natural Science Foundation of China (Nos. 51771109 and 51631006) and the Shanghai Rising-Star Program (No. 16QB1402800).
24 per cubic metre) and small size (about 2.7 ± 0.2 nanometres). The minimized elastic misfit strain around the particles does not contribute much to the dislocation interaction, which is typically needed for strength increase. Instead, our strengthening mechanism exploits the chemical ordering effect that creates backstresses (the forces opposing deformation) when precipitates are cut by dislocations. We create a class of steels, strengthened by Ni(Al,Fe) precipitates, with a strength of up to 2.2 gigapascals and good ductility (about 8.2 per cent). The chemical composition of the precipitates enables a substantial reduction in cost compared to conventional maraging steels owing to the replacement of the essential but high-cost alloying elements cobalt and titanium with inexpensive and lightweight aluminium. Strengthening of this class of steel alloy is based on minimal lattice misfit to achieve maximal precipitate dispersion and high cutting stress (the stress required for dislocations to cut through coherent precipitates and thus produce plastic deformation), and we envisage that this lattice misfit design concept may be applied to many other metallic alloys.]]>
The development of materials with dual properties of high strength and high ductility has been a constant challenge since the foundation of the materials science discipline. The rapid progress of nanotechnology in recent decades has further brought this challenge to a new era. This Research News highlights a few newly developed strategies to optimize advanced nanomaterials and metallic glasses with exceptional dual mechanical properties of high strength and high ductility. A general concept of strain non-localization is presented to describe the role of multiscale (i.e., macroscale, microscale, nanoscale, and atomic scale) heterogeneities in the ductility enhancement of materials reputed to be intrinsically brittle, such as nanostructured metallic materials and bulk metallic glasses. These nanomaterials clearly form a new group of materials that display an extraordinary relationship between yield strength and the uniform elongation with the same chemical composition. Several other examples of nanomaterials such as those reinforced by nanoprecipitates will also be described.
Grain refinement can make conventional metals several times stronger, but this comes at dramatic loss of ductility. Here we report a heterogeneous lamella structure in Ti produced by asymmetric rolling and partial recrystallization that can produce an unprecedented property combination: as strong as ultrafine-grained metal and at the same time as ductile as conventional coarse-grained metal. It also has higher strain hardening than coarse-grained Ti, which was hitherto believed impossible. The heterogeneous lamella structure is characterized with soft micrograined lamellae embedded in hard ultrafine-grained lamella matrix. The unusual high strength is obtained with the assistance of high back stress developed from heterogeneous yielding, whereas the high ductility is attributed to back-stress hardening and dislocation hardening. The process discovered here is amenable to large-scale industrial production at low cost, and might be applicable to other metal systems.
Magnesium is a lightweight structural metal but it exhibits low ductility-connected with unusual, mechanistically unexplained, dislocation and plasticity phenomena-which makes it difficult to form and use in energy-saving lightweight structures. We employ long-time molecular dynamics simulations utilizing a density-functional-theory-validated interatomic potential, and reveal the fundamental origins of the previously unexplained phenomena. Here we show that the key 〈c + a〉 dislocation (where 〈c + a〉 indicates the magnitude and direction of slip) is metastable on easy-glide pyramidal II planes; we find that it undergoes a thermally activated, stress-dependent transition to one of three lower-energy, basal-dissociated immobile dislocation structures, which cannot contribute to plastic straining and that serve as strong obstacles to the motion of all other dislocations. This transition is intrinsic to magnesium, driven by reduction in dislocation energy and predicted to occur at very high frequency at room temperature, thus eliminating all major dislocation slip systems able to contribute to c-axis strain and leading to the high hardening and low ductility of magnesium. Enhanced ductility can thus be achieved by increasing the time and temperature at which the transition from the easy-glide metastable dislocation to the immobile basal-dissociated structures occurs. Our results provide the underlying insights needed to guide the design of ductile magnesium alloys.
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... Advanced engineering materials have attracted the attention of materials scientists and engineers for alleviating the current challenge of the energy crisis and global warming, etc [1,2]. Magnesium (Mg) and its alloys are considered as promising engineering materials due to their light weight and high specific strength. However, low absolute strength severely restricts their wide applications. So far, alloying is one of the prevalent and effective methods to improve strength of Mg alloys [3,4]. Considerable studies present that rare earth Mg alloys, especially Mg-Gd-Y based alloys, exhibit high strength, which primarily originates from solid-solution strengthening and precipitation strengthening. Gadolinium (Gd) and yttrium (Y) elements have a large difference in atomic radius from that of Mg [5], which leads to significant strengthening effects. The solid solubility of Gd and Y in Mg matrix changes significantly with temperature [6], thus providing remarkable precipitation strengthening. Strengthening mechanisms, however, usually sacrifice ductility, which is known as strength-ductility trade-off [[7], [8], [9], [10], [11], [12]]. The primary reasons are limited slip systems and difficulties in activating non-basal slip at ambient temperature [13]. To date, strategies in dealing with the conflict mainly focus on microstructural design, especially heterogeneous structure which has positive effects on strength-ductility balance. Xu et al. [14] reported an excellent strength-ductility balance in Mg alloys, which was attributed to the bimodal structure which can relax the local strain concentration via strain transfer from the dynamically recrystallized (DRXed) regions to the deformed regions. Wu et al. [11] overcame this trade-off dilemma in titanium by architecting a heterogeneous lamella structure. They attributed the high ductility to the assistance of additional work hardening developed from storage and interaction of dislocations, which was induced by a macroscopic strain gradient and a change in stress states. Lu et al. [15,16] and Shao et al. [17] enhanced strength-ductility synergy by engineering a linear gradient in grain size in nanomaterials and Fe-Mn-C twinning-induced plasticity steel, respectively. Lu’s group considered the unique deformation mechanism of gradient nanostructures is the key to breakthrough the strength-ductility trade-off. Shao’s team considered that desirable properties originate from geometric necessary dislocations (GNDs) formed during tensile deformation, which provides additional work-hardening, especially at late stage of deformation. In addition, the superior strength-ductility combination was also discovered in a dual-constituent microstructure in Ti-Nb based gum metal [18]. ...
1
2013
... Advanced engineering materials have attracted the attention of materials scientists and engineers for alleviating the current challenge of the energy crisis and global warming, etc [1,2]. Magnesium (Mg) and its alloys are considered as promising engineering materials due to their light weight and high specific strength. However, low absolute strength severely restricts their wide applications. So far, alloying is one of the prevalent and effective methods to improve strength of Mg alloys [3,4]. Considerable studies present that rare earth Mg alloys, especially Mg-Gd-Y based alloys, exhibit high strength, which primarily originates from solid-solution strengthening and precipitation strengthening. Gadolinium (Gd) and yttrium (Y) elements have a large difference in atomic radius from that of Mg [5], which leads to significant strengthening effects. The solid solubility of Gd and Y in Mg matrix changes significantly with temperature [6], thus providing remarkable precipitation strengthening. Strengthening mechanisms, however, usually sacrifice ductility, which is known as strength-ductility trade-off [[7], [8], [9], [10], [11], [12]]. The primary reasons are limited slip systems and difficulties in activating non-basal slip at ambient temperature [13]. To date, strategies in dealing with the conflict mainly focus on microstructural design, especially heterogeneous structure which has positive effects on strength-ductility balance. Xu et al. [14] reported an excellent strength-ductility balance in Mg alloys, which was attributed to the bimodal structure which can relax the local strain concentration via strain transfer from the dynamically recrystallized (DRXed) regions to the deformed regions. Wu et al. [11] overcame this trade-off dilemma in titanium by architecting a heterogeneous lamella structure. They attributed the high ductility to the assistance of additional work hardening developed from storage and interaction of dislocations, which was induced by a macroscopic strain gradient and a change in stress states. Lu et al. [15,16] and Shao et al. [17] enhanced strength-ductility synergy by engineering a linear gradient in grain size in nanomaterials and Fe-Mn-C twinning-induced plasticity steel, respectively. Lu’s group considered the unique deformation mechanism of gradient nanostructures is the key to breakthrough the strength-ductility trade-off. Shao’s team considered that desirable properties originate from geometric necessary dislocations (GNDs) formed during tensile deformation, which provides additional work-hardening, especially at late stage of deformation. In addition, the superior strength-ductility combination was also discovered in a dual-constituent microstructure in Ti-Nb based gum metal [18]. ...
1
2008
... Advanced engineering materials have attracted the attention of materials scientists and engineers for alleviating the current challenge of the energy crisis and global warming, etc [1,2]. Magnesium (Mg) and its alloys are considered as promising engineering materials due to their light weight and high specific strength. However, low absolute strength severely restricts their wide applications. So far, alloying is one of the prevalent and effective methods to improve strength of Mg alloys [3,4]. Considerable studies present that rare earth Mg alloys, especially Mg-Gd-Y based alloys, exhibit high strength, which primarily originates from solid-solution strengthening and precipitation strengthening. Gadolinium (Gd) and yttrium (Y) elements have a large difference in atomic radius from that of Mg [5], which leads to significant strengthening effects. The solid solubility of Gd and Y in Mg matrix changes significantly with temperature [6], thus providing remarkable precipitation strengthening. Strengthening mechanisms, however, usually sacrifice ductility, which is known as strength-ductility trade-off [[7], [8], [9], [10], [11], [12]]. The primary reasons are limited slip systems and difficulties in activating non-basal slip at ambient temperature [13]. To date, strategies in dealing with the conflict mainly focus on microstructural design, especially heterogeneous structure which has positive effects on strength-ductility balance. Xu et al. [14] reported an excellent strength-ductility balance in Mg alloys, which was attributed to the bimodal structure which can relax the local strain concentration via strain transfer from the dynamically recrystallized (DRXed) regions to the deformed regions. Wu et al. [11] overcame this trade-off dilemma in titanium by architecting a heterogeneous lamella structure. They attributed the high ductility to the assistance of additional work hardening developed from storage and interaction of dislocations, which was induced by a macroscopic strain gradient and a change in stress states. Lu et al. [15,16] and Shao et al. [17] enhanced strength-ductility synergy by engineering a linear gradient in grain size in nanomaterials and Fe-Mn-C twinning-induced plasticity steel, respectively. Lu’s group considered the unique deformation mechanism of gradient nanostructures is the key to breakthrough the strength-ductility trade-off. Shao’s team considered that desirable properties originate from geometric necessary dislocations (GNDs) formed during tensile deformation, which provides additional work-hardening, especially at late stage of deformation. In addition, the superior strength-ductility combination was also discovered in a dual-constituent microstructure in Ti-Nb based gum metal [18]. ...
1
2008
... Advanced engineering materials have attracted the attention of materials scientists and engineers for alleviating the current challenge of the energy crisis and global warming, etc [1,2]. Magnesium (Mg) and its alloys are considered as promising engineering materials due to their light weight and high specific strength. However, low absolute strength severely restricts their wide applications. So far, alloying is one of the prevalent and effective methods to improve strength of Mg alloys [3,4]. Considerable studies present that rare earth Mg alloys, especially Mg-Gd-Y based alloys, exhibit high strength, which primarily originates from solid-solution strengthening and precipitation strengthening. Gadolinium (Gd) and yttrium (Y) elements have a large difference in atomic radius from that of Mg [5], which leads to significant strengthening effects. The solid solubility of Gd and Y in Mg matrix changes significantly with temperature [6], thus providing remarkable precipitation strengthening. Strengthening mechanisms, however, usually sacrifice ductility, which is known as strength-ductility trade-off [[7], [8], [9], [10], [11], [12]]. The primary reasons are limited slip systems and difficulties in activating non-basal slip at ambient temperature [13]. To date, strategies in dealing with the conflict mainly focus on microstructural design, especially heterogeneous structure which has positive effects on strength-ductility balance. Xu et al. [14] reported an excellent strength-ductility balance in Mg alloys, which was attributed to the bimodal structure which can relax the local strain concentration via strain transfer from the dynamically recrystallized (DRXed) regions to the deformed regions. Wu et al. [11] overcame this trade-off dilemma in titanium by architecting a heterogeneous lamella structure. They attributed the high ductility to the assistance of additional work hardening developed from storage and interaction of dislocations, which was induced by a macroscopic strain gradient and a change in stress states. Lu et al. [15,16] and Shao et al. [17] enhanced strength-ductility synergy by engineering a linear gradient in grain size in nanomaterials and Fe-Mn-C twinning-induced plasticity steel, respectively. Lu’s group considered the unique deformation mechanism of gradient nanostructures is the key to breakthrough the strength-ductility trade-off. Shao’s team considered that desirable properties originate from geometric necessary dislocations (GNDs) formed during tensile deformation, which provides additional work-hardening, especially at late stage of deformation. In addition, the superior strength-ductility combination was also discovered in a dual-constituent microstructure in Ti-Nb based gum metal [18]. ...
1
2009
... Advanced engineering materials have attracted the attention of materials scientists and engineers for alleviating the current challenge of the energy crisis and global warming, etc [1,2]. Magnesium (Mg) and its alloys are considered as promising engineering materials due to their light weight and high specific strength. However, low absolute strength severely restricts their wide applications. So far, alloying is one of the prevalent and effective methods to improve strength of Mg alloys [3,4]. Considerable studies present that rare earth Mg alloys, especially Mg-Gd-Y based alloys, exhibit high strength, which primarily originates from solid-solution strengthening and precipitation strengthening. Gadolinium (Gd) and yttrium (Y) elements have a large difference in atomic radius from that of Mg [5], which leads to significant strengthening effects. The solid solubility of Gd and Y in Mg matrix changes significantly with temperature [6], thus providing remarkable precipitation strengthening. Strengthening mechanisms, however, usually sacrifice ductility, which is known as strength-ductility trade-off [[7], [8], [9], [10], [11], [12]]. The primary reasons are limited slip systems and difficulties in activating non-basal slip at ambient temperature [13]. To date, strategies in dealing with the conflict mainly focus on microstructural design, especially heterogeneous structure which has positive effects on strength-ductility balance. Xu et al. [14] reported an excellent strength-ductility balance in Mg alloys, which was attributed to the bimodal structure which can relax the local strain concentration via strain transfer from the dynamically recrystallized (DRXed) regions to the deformed regions. Wu et al. [11] overcame this trade-off dilemma in titanium by architecting a heterogeneous lamella structure. They attributed the high ductility to the assistance of additional work hardening developed from storage and interaction of dislocations, which was induced by a macroscopic strain gradient and a change in stress states. Lu et al. [15,16] and Shao et al. [17] enhanced strength-ductility synergy by engineering a linear gradient in grain size in nanomaterials and Fe-Mn-C twinning-induced plasticity steel, respectively. Lu’s group considered the unique deformation mechanism of gradient nanostructures is the key to breakthrough the strength-ductility trade-off. Shao’s team considered that desirable properties originate from geometric necessary dislocations (GNDs) formed during tensile deformation, which provides additional work-hardening, especially at late stage of deformation. In addition, the superior strength-ductility combination was also discovered in a dual-constituent microstructure in Ti-Nb based gum metal [18]. ...
1
2012
... Advanced engineering materials have attracted the attention of materials scientists and engineers for alleviating the current challenge of the energy crisis and global warming, etc [1,2]. Magnesium (Mg) and its alloys are considered as promising engineering materials due to their light weight and high specific strength. However, low absolute strength severely restricts their wide applications. So far, alloying is one of the prevalent and effective methods to improve strength of Mg alloys [3,4]. Considerable studies present that rare earth Mg alloys, especially Mg-Gd-Y based alloys, exhibit high strength, which primarily originates from solid-solution strengthening and precipitation strengthening. Gadolinium (Gd) and yttrium (Y) elements have a large difference in atomic radius from that of Mg [5], which leads to significant strengthening effects. The solid solubility of Gd and Y in Mg matrix changes significantly with temperature [6], thus providing remarkable precipitation strengthening. Strengthening mechanisms, however, usually sacrifice ductility, which is known as strength-ductility trade-off [[7], [8], [9], [10], [11], [12]]. The primary reasons are limited slip systems and difficulties in activating non-basal slip at ambient temperature [13]. To date, strategies in dealing with the conflict mainly focus on microstructural design, especially heterogeneous structure which has positive effects on strength-ductility balance. Xu et al. [14] reported an excellent strength-ductility balance in Mg alloys, which was attributed to the bimodal structure which can relax the local strain concentration via strain transfer from the dynamically recrystallized (DRXed) regions to the deformed regions. Wu et al. [11] overcame this trade-off dilemma in titanium by architecting a heterogeneous lamella structure. They attributed the high ductility to the assistance of additional work hardening developed from storage and interaction of dislocations, which was induced by a macroscopic strain gradient and a change in stress states. Lu et al. [15,16] and Shao et al. [17] enhanced strength-ductility synergy by engineering a linear gradient in grain size in nanomaterials and Fe-Mn-C twinning-induced plasticity steel, respectively. Lu’s group considered the unique deformation mechanism of gradient nanostructures is the key to breakthrough the strength-ductility trade-off. Shao’s team considered that desirable properties originate from geometric necessary dislocations (GNDs) formed during tensile deformation, which provides additional work-hardening, especially at late stage of deformation. In addition, the superior strength-ductility combination was also discovered in a dual-constituent microstructure in Ti-Nb based gum metal [18]. ...
3
2017
... Advanced engineering materials have attracted the attention of materials scientists and engineers for alleviating the current challenge of the energy crisis and global warming, etc [1,2]. Magnesium (Mg) and its alloys are considered as promising engineering materials due to their light weight and high specific strength. However, low absolute strength severely restricts their wide applications. So far, alloying is one of the prevalent and effective methods to improve strength of Mg alloys [3,4]. Considerable studies present that rare earth Mg alloys, especially Mg-Gd-Y based alloys, exhibit high strength, which primarily originates from solid-solution strengthening and precipitation strengthening. Gadolinium (Gd) and yttrium (Y) elements have a large difference in atomic radius from that of Mg [5], which leads to significant strengthening effects. The solid solubility of Gd and Y in Mg matrix changes significantly with temperature [6], thus providing remarkable precipitation strengthening. Strengthening mechanisms, however, usually sacrifice ductility, which is known as strength-ductility trade-off [[7], [8], [9], [10], [11], [12]]. The primary reasons are limited slip systems and difficulties in activating non-basal slip at ambient temperature [13]. To date, strategies in dealing with the conflict mainly focus on microstructural design, especially heterogeneous structure which has positive effects on strength-ductility balance. Xu et al. [14] reported an excellent strength-ductility balance in Mg alloys, which was attributed to the bimodal structure which can relax the local strain concentration via strain transfer from the dynamically recrystallized (DRXed) regions to the deformed regions. Wu et al. [11] overcame this trade-off dilemma in titanium by architecting a heterogeneous lamella structure. They attributed the high ductility to the assistance of additional work hardening developed from storage and interaction of dislocations, which was induced by a macroscopic strain gradient and a change in stress states. Lu et al. [15,16] and Shao et al. [17] enhanced strength-ductility synergy by engineering a linear gradient in grain size in nanomaterials and Fe-Mn-C twinning-induced plasticity steel, respectively. Lu’s group considered the unique deformation mechanism of gradient nanostructures is the key to breakthrough the strength-ductility trade-off. Shao’s team considered that desirable properties originate from geometric necessary dislocations (GNDs) formed during tensile deformation, which provides additional work-hardening, especially at late stage of deformation. In addition, the superior strength-ductility combination was also discovered in a dual-constituent microstructure in Ti-Nb based gum metal [18]. ...
... By purposely deploying heterogeneous structure to enhance materials properties has been widely reported [7]. As a typical heterogeneous microstructure, bimodal structure has attracted much attention. Generally, bimodal structure can be regarded as the composite structure, which is consist of ultrafine-grained regions and coarse-grained regions [31]. According to the Hall-Patch relationship, the strength of materials varies with the grain size, and the larger the grain size, the lower the strength [32]. At the initial stage, the plastic deformation occurs preferentially in coarse grains (soft domains), while the ultrafine grains (hard domains) maintain the elastic deformation [11,31]. Therefore, the deformation-incompatibility happens, which leads to the formation of strain gradient. As the deformation continues, the strain gradient increases. To accommodate the strain gradient, plenty GNDs generates, implying back-stress existed, which control the strain hardening of the samples when their density exceeds that of the statistically-stored dislocations [7,11,33,34]. The GNDs also interact with mobile dislocations and impede their movements during plastic deformation, thus improving the strength of the alloy [35]. To generalize these ideas, we can conclude that back-stress strengthening and back-stress strain-hardening can improve the strength and ductility simultaneously. ...
... ]. Therefore, the deformation-incompatibility happens, which leads to the formation of strain gradient. As the deformation continues, the strain gradient increases. To accommodate the strain gradient, plenty GNDs generates, implying back-stress existed, which control the strain hardening of the samples when their density exceeds that of the statistically-stored dislocations [7,11,33,34]. The GNDs also interact with mobile dislocations and impede their movements during plastic deformation, thus improving the strength of the alloy [35]. To generalize these ideas, we can conclude that back-stress strengthening and back-stress strain-hardening can improve the strength and ductility simultaneously. ...
2
2017
... Advanced engineering materials have attracted the attention of materials scientists and engineers for alleviating the current challenge of the energy crisis and global warming, etc [1,2]. Magnesium (Mg) and its alloys are considered as promising engineering materials due to their light weight and high specific strength. However, low absolute strength severely restricts their wide applications. So far, alloying is one of the prevalent and effective methods to improve strength of Mg alloys [3,4]. Considerable studies present that rare earth Mg alloys, especially Mg-Gd-Y based alloys, exhibit high strength, which primarily originates from solid-solution strengthening and precipitation strengthening. Gadolinium (Gd) and yttrium (Y) elements have a large difference in atomic radius from that of Mg [5], which leads to significant strengthening effects. The solid solubility of Gd and Y in Mg matrix changes significantly with temperature [6], thus providing remarkable precipitation strengthening. Strengthening mechanisms, however, usually sacrifice ductility, which is known as strength-ductility trade-off [[7], [8], [9], [10], [11], [12]]. The primary reasons are limited slip systems and difficulties in activating non-basal slip at ambient temperature [13]. To date, strategies in dealing with the conflict mainly focus on microstructural design, especially heterogeneous structure which has positive effects on strength-ductility balance. Xu et al. [14] reported an excellent strength-ductility balance in Mg alloys, which was attributed to the bimodal structure which can relax the local strain concentration via strain transfer from the dynamically recrystallized (DRXed) regions to the deformed regions. Wu et al. [11] overcame this trade-off dilemma in titanium by architecting a heterogeneous lamella structure. They attributed the high ductility to the assistance of additional work hardening developed from storage and interaction of dislocations, which was induced by a macroscopic strain gradient and a change in stress states. Lu et al. [15,16] and Shao et al. [17] enhanced strength-ductility synergy by engineering a linear gradient in grain size in nanomaterials and Fe-Mn-C twinning-induced plasticity steel, respectively. Lu’s group considered the unique deformation mechanism of gradient nanostructures is the key to breakthrough the strength-ductility trade-off. Shao’s team considered that desirable properties originate from geometric necessary dislocations (GNDs) formed during tensile deformation, which provides additional work-hardening, especially at late stage of deformation. In addition, the superior strength-ductility combination was also discovered in a dual-constituent microstructure in Ti-Nb based gum metal [18]. ...
... Aside from microstructure design, dispersed nano-precipitates is another effective approach to overcome strength-ductility trade-off. Liu et al. [19] adopted a molecular-level liquid-liquid mixing/doping technique and obtained an optimal microstructure, where nanometric oxide particles uniformly distributed in the interior of submicrometer grains. The dispersing intragranular particles can generate, pin down and accumulate dislocations, thus resulting in the increase of strength and simultaneous improvement in ductility. Similar reports have been reported in high-entropy alloys [20] and ultrastrong steel [8]. It is noted that these highly dispersed, fully coherent or semi-coherent precipitates, which exhibits low lattice misfit with the matrix, and high anti-phase boundary energy, can significantly improve strength without trade-off in ductility. ...
1
2016
... Advanced engineering materials have attracted the attention of materials scientists and engineers for alleviating the current challenge of the energy crisis and global warming, etc [1,2]. Magnesium (Mg) and its alloys are considered as promising engineering materials due to their light weight and high specific strength. However, low absolute strength severely restricts their wide applications. So far, alloying is one of the prevalent and effective methods to improve strength of Mg alloys [3,4]. Considerable studies present that rare earth Mg alloys, especially Mg-Gd-Y based alloys, exhibit high strength, which primarily originates from solid-solution strengthening and precipitation strengthening. Gadolinium (Gd) and yttrium (Y) elements have a large difference in atomic radius from that of Mg [5], which leads to significant strengthening effects. The solid solubility of Gd and Y in Mg matrix changes significantly with temperature [6], thus providing remarkable precipitation strengthening. Strengthening mechanisms, however, usually sacrifice ductility, which is known as strength-ductility trade-off [[7], [8], [9], [10], [11], [12]]. The primary reasons are limited slip systems and difficulties in activating non-basal slip at ambient temperature [13]. To date, strategies in dealing with the conflict mainly focus on microstructural design, especially heterogeneous structure which has positive effects on strength-ductility balance. Xu et al. [14] reported an excellent strength-ductility balance in Mg alloys, which was attributed to the bimodal structure which can relax the local strain concentration via strain transfer from the dynamically recrystallized (DRXed) regions to the deformed regions. Wu et al. [11] overcame this trade-off dilemma in titanium by architecting a heterogeneous lamella structure. They attributed the high ductility to the assistance of additional work hardening developed from storage and interaction of dislocations, which was induced by a macroscopic strain gradient and a change in stress states. Lu et al. [15,16] and Shao et al. [17] enhanced strength-ductility synergy by engineering a linear gradient in grain size in nanomaterials and Fe-Mn-C twinning-induced plasticity steel, respectively. Lu’s group considered the unique deformation mechanism of gradient nanostructures is the key to breakthrough the strength-ductility trade-off. Shao’s team considered that desirable properties originate from geometric necessary dislocations (GNDs) formed during tensile deformation, which provides additional work-hardening, especially at late stage of deformation. In addition, the superior strength-ductility combination was also discovered in a dual-constituent microstructure in Ti-Nb based gum metal [18]. ...
1
2018
... Advanced engineering materials have attracted the attention of materials scientists and engineers for alleviating the current challenge of the energy crisis and global warming, etc [1,2]. Magnesium (Mg) and its alloys are considered as promising engineering materials due to their light weight and high specific strength. However, low absolute strength severely restricts their wide applications. So far, alloying is one of the prevalent and effective methods to improve strength of Mg alloys [3,4]. Considerable studies present that rare earth Mg alloys, especially Mg-Gd-Y based alloys, exhibit high strength, which primarily originates from solid-solution strengthening and precipitation strengthening. Gadolinium (Gd) and yttrium (Y) elements have a large difference in atomic radius from that of Mg [5], which leads to significant strengthening effects. The solid solubility of Gd and Y in Mg matrix changes significantly with temperature [6], thus providing remarkable precipitation strengthening. Strengthening mechanisms, however, usually sacrifice ductility, which is known as strength-ductility trade-off [[7], [8], [9], [10], [11], [12]]. The primary reasons are limited slip systems and difficulties in activating non-basal slip at ambient temperature [13]. To date, strategies in dealing with the conflict mainly focus on microstructural design, especially heterogeneous structure which has positive effects on strength-ductility balance. Xu et al. [14] reported an excellent strength-ductility balance in Mg alloys, which was attributed to the bimodal structure which can relax the local strain concentration via strain transfer from the dynamically recrystallized (DRXed) regions to the deformed regions. Wu et al. [11] overcame this trade-off dilemma in titanium by architecting a heterogeneous lamella structure. They attributed the high ductility to the assistance of additional work hardening developed from storage and interaction of dislocations, which was induced by a macroscopic strain gradient and a change in stress states. Lu et al. [15,16] and Shao et al. [17] enhanced strength-ductility synergy by engineering a linear gradient in grain size in nanomaterials and Fe-Mn-C twinning-induced plasticity steel, respectively. Lu’s group considered the unique deformation mechanism of gradient nanostructures is the key to breakthrough the strength-ductility trade-off. Shao’s team considered that desirable properties originate from geometric necessary dislocations (GNDs) formed during tensile deformation, which provides additional work-hardening, especially at late stage of deformation. In addition, the superior strength-ductility combination was also discovered in a dual-constituent microstructure in Ti-Nb based gum metal [18]. ...
6
2015
... Advanced engineering materials have attracted the attention of materials scientists and engineers for alleviating the current challenge of the energy crisis and global warming, etc [1,2]. Magnesium (Mg) and its alloys are considered as promising engineering materials due to their light weight and high specific strength. However, low absolute strength severely restricts their wide applications. So far, alloying is one of the prevalent and effective methods to improve strength of Mg alloys [3,4]. Considerable studies present that rare earth Mg alloys, especially Mg-Gd-Y based alloys, exhibit high strength, which primarily originates from solid-solution strengthening and precipitation strengthening. Gadolinium (Gd) and yttrium (Y) elements have a large difference in atomic radius from that of Mg [5], which leads to significant strengthening effects. The solid solubility of Gd and Y in Mg matrix changes significantly with temperature [6], thus providing remarkable precipitation strengthening. Strengthening mechanisms, however, usually sacrifice ductility, which is known as strength-ductility trade-off [[7], [8], [9], [10], [11], [12]]. The primary reasons are limited slip systems and difficulties in activating non-basal slip at ambient temperature [13]. To date, strategies in dealing with the conflict mainly focus on microstructural design, especially heterogeneous structure which has positive effects on strength-ductility balance. Xu et al. [14] reported an excellent strength-ductility balance in Mg alloys, which was attributed to the bimodal structure which can relax the local strain concentration via strain transfer from the dynamically recrystallized (DRXed) regions to the deformed regions. Wu et al. [11] overcame this trade-off dilemma in titanium by architecting a heterogeneous lamella structure. They attributed the high ductility to the assistance of additional work hardening developed from storage and interaction of dislocations, which was induced by a macroscopic strain gradient and a change in stress states. Lu et al. [15,16] and Shao et al. [17] enhanced strength-ductility synergy by engineering a linear gradient in grain size in nanomaterials and Fe-Mn-C twinning-induced plasticity steel, respectively. Lu’s group considered the unique deformation mechanism of gradient nanostructures is the key to breakthrough the strength-ductility trade-off. Shao’s team considered that desirable properties originate from geometric necessary dislocations (GNDs) formed during tensile deformation, which provides additional work-hardening, especially at late stage of deformation. In addition, the superior strength-ductility combination was also discovered in a dual-constituent microstructure in Ti-Nb based gum metal [18]. ...
... ] reported an excellent strength-ductility balance in Mg alloys, which was attributed to the bimodal structure which can relax the local strain concentration via strain transfer from the dynamically recrystallized (DRXed) regions to the deformed regions. Wu et al. [11] overcame this trade-off dilemma in titanium by architecting a heterogeneous lamella structure. They attributed the high ductility to the assistance of additional work hardening developed from storage and interaction of dislocations, which was induced by a macroscopic strain gradient and a change in stress states. Lu et al. [15,16] and Shao et al. [17] enhanced strength-ductility synergy by engineering a linear gradient in grain size in nanomaterials and Fe-Mn-C twinning-induced plasticity steel, respectively. Lu’s group considered the unique deformation mechanism of gradient nanostructures is the key to breakthrough the strength-ductility trade-off. Shao’s team considered that desirable properties originate from geometric necessary dislocations (GNDs) formed during tensile deformation, which provides additional work-hardening, especially at late stage of deformation. In addition, the superior strength-ductility combination was also discovered in a dual-constituent microstructure in Ti-Nb based gum metal [18]. ...
... By purposely deploying heterogeneous structure to enhance materials properties has been widely reported [7]. As a typical heterogeneous microstructure, bimodal structure has attracted much attention. Generally, bimodal structure can be regarded as the composite structure, which is consist of ultrafine-grained regions and coarse-grained regions [31]. According to the Hall-Patch relationship, the strength of materials varies with the grain size, and the larger the grain size, the lower the strength [32]. At the initial stage, the plastic deformation occurs preferentially in coarse grains (soft domains), while the ultrafine grains (hard domains) maintain the elastic deformation [11,31]. Therefore, the deformation-incompatibility happens, which leads to the formation of strain gradient. As the deformation continues, the strain gradient increases. To accommodate the strain gradient, plenty GNDs generates, implying back-stress existed, which control the strain hardening of the samples when their density exceeds that of the statistically-stored dislocations [7,11,33,34]. The GNDs also interact with mobile dislocations and impede their movements during plastic deformation, thus improving the strength of the alloy [35]. To generalize these ideas, we can conclude that back-stress strengthening and back-stress strain-hardening can improve the strength and ductility simultaneously. ...
... ,11,33,34]. The GNDs also interact with mobile dislocations and impede their movements during plastic deformation, thus improving the strength of the alloy [35]. To generalize these ideas, we can conclude that back-stress strengthening and back-stress strain-hardening can improve the strength and ductility simultaneously. ...
... Back-stress has an effect on the dislocation configuration in the coarse grain interior during pre-deformation proess, and in turn affects the precipitation process and properties of samples [11,31]. At pre-deformation strain of 2%, profuse dislocations generate within coarse grains and cannot transmit into neighboring grains, which is attributed to the back-stress [11,34]. Thus, dislocation density in fine grains is low while that in coarse grains is high. As the pre-deformation strain increases from 2% to 5%, the coarse grains sustained most of the plastic strain, and then dislocation density in coarse grains is much high and close to saturation and fine grains begin to provide stain. For triggering the deformation of fine grains, the driving force reaches sufficiently high levels as a result. At the pre-deformation strain of 8%, dislocation density increases significantly but the space for dislocation storage decreases sharply [40], which results in the decrease of ductility. ...
... ]. At pre-deformation strain of 2%, profuse dislocations generate within coarse grains and cannot transmit into neighboring grains, which is attributed to the back-stress [11,34]. Thus, dislocation density in fine grains is low while that in coarse grains is high. As the pre-deformation strain increases from 2% to 5%, the coarse grains sustained most of the plastic strain, and then dislocation density in coarse grains is much high and close to saturation and fine grains begin to provide stain. For triggering the deformation of fine grains, the driving force reaches sufficiently high levels as a result. At the pre-deformation strain of 8%, dislocation density increases significantly but the space for dislocation storage decreases sharply [40], which results in the decrease of ductility. ...
1
2018
... Advanced engineering materials have attracted the attention of materials scientists and engineers for alleviating the current challenge of the energy crisis and global warming, etc [1,2]. Magnesium (Mg) and its alloys are considered as promising engineering materials due to their light weight and high specific strength. However, low absolute strength severely restricts their wide applications. So far, alloying is one of the prevalent and effective methods to improve strength of Mg alloys [3,4]. Considerable studies present that rare earth Mg alloys, especially Mg-Gd-Y based alloys, exhibit high strength, which primarily originates from solid-solution strengthening and precipitation strengthening. Gadolinium (Gd) and yttrium (Y) elements have a large difference in atomic radius from that of Mg [5], which leads to significant strengthening effects. The solid solubility of Gd and Y in Mg matrix changes significantly with temperature [6], thus providing remarkable precipitation strengthening. Strengthening mechanisms, however, usually sacrifice ductility, which is known as strength-ductility trade-off [[7], [8], [9], [10], [11], [12]]. The primary reasons are limited slip systems and difficulties in activating non-basal slip at ambient temperature [13]. To date, strategies in dealing with the conflict mainly focus on microstructural design, especially heterogeneous structure which has positive effects on strength-ductility balance. Xu et al. [14] reported an excellent strength-ductility balance in Mg alloys, which was attributed to the bimodal structure which can relax the local strain concentration via strain transfer from the dynamically recrystallized (DRXed) regions to the deformed regions. Wu et al. [11] overcame this trade-off dilemma in titanium by architecting a heterogeneous lamella structure. They attributed the high ductility to the assistance of additional work hardening developed from storage and interaction of dislocations, which was induced by a macroscopic strain gradient and a change in stress states. Lu et al. [15,16] and Shao et al. [17] enhanced strength-ductility synergy by engineering a linear gradient in grain size in nanomaterials and Fe-Mn-C twinning-induced plasticity steel, respectively. Lu’s group considered the unique deformation mechanism of gradient nanostructures is the key to breakthrough the strength-ductility trade-off. Shao’s team considered that desirable properties originate from geometric necessary dislocations (GNDs) formed during tensile deformation, which provides additional work-hardening, especially at late stage of deformation. In addition, the superior strength-ductility combination was also discovered in a dual-constituent microstructure in Ti-Nb based gum metal [18]. ...
1
2015
... Advanced engineering materials have attracted the attention of materials scientists and engineers for alleviating the current challenge of the energy crisis and global warming, etc [1,2]. Magnesium (Mg) and its alloys are considered as promising engineering materials due to their light weight and high specific strength. However, low absolute strength severely restricts their wide applications. So far, alloying is one of the prevalent and effective methods to improve strength of Mg alloys [3,4]. Considerable studies present that rare earth Mg alloys, especially Mg-Gd-Y based alloys, exhibit high strength, which primarily originates from solid-solution strengthening and precipitation strengthening. Gadolinium (Gd) and yttrium (Y) elements have a large difference in atomic radius from that of Mg [5], which leads to significant strengthening effects. The solid solubility of Gd and Y in Mg matrix changes significantly with temperature [6], thus providing remarkable precipitation strengthening. Strengthening mechanisms, however, usually sacrifice ductility, which is known as strength-ductility trade-off [[7], [8], [9], [10], [11], [12]]. The primary reasons are limited slip systems and difficulties in activating non-basal slip at ambient temperature [13]. To date, strategies in dealing with the conflict mainly focus on microstructural design, especially heterogeneous structure which has positive effects on strength-ductility balance. Xu et al. [14] reported an excellent strength-ductility balance in Mg alloys, which was attributed to the bimodal structure which can relax the local strain concentration via strain transfer from the dynamically recrystallized (DRXed) regions to the deformed regions. Wu et al. [11] overcame this trade-off dilemma in titanium by architecting a heterogeneous lamella structure. They attributed the high ductility to the assistance of additional work hardening developed from storage and interaction of dislocations, which was induced by a macroscopic strain gradient and a change in stress states. Lu et al. [15,16] and Shao et al. [17] enhanced strength-ductility synergy by engineering a linear gradient in grain size in nanomaterials and Fe-Mn-C twinning-induced plasticity steel, respectively. Lu’s group considered the unique deformation mechanism of gradient nanostructures is the key to breakthrough the strength-ductility trade-off. Shao’s team considered that desirable properties originate from geometric necessary dislocations (GNDs) formed during tensile deformation, which provides additional work-hardening, especially at late stage of deformation. In addition, the superior strength-ductility combination was also discovered in a dual-constituent microstructure in Ti-Nb based gum metal [18]. ...
1
2018
... Advanced engineering materials have attracted the attention of materials scientists and engineers for alleviating the current challenge of the energy crisis and global warming, etc [1,2]. Magnesium (Mg) and its alloys are considered as promising engineering materials due to their light weight and high specific strength. However, low absolute strength severely restricts their wide applications. So far, alloying is one of the prevalent and effective methods to improve strength of Mg alloys [3,4]. Considerable studies present that rare earth Mg alloys, especially Mg-Gd-Y based alloys, exhibit high strength, which primarily originates from solid-solution strengthening and precipitation strengthening. Gadolinium (Gd) and yttrium (Y) elements have a large difference in atomic radius from that of Mg [5], which leads to significant strengthening effects. The solid solubility of Gd and Y in Mg matrix changes significantly with temperature [6], thus providing remarkable precipitation strengthening. Strengthening mechanisms, however, usually sacrifice ductility, which is known as strength-ductility trade-off [[7], [8], [9], [10], [11], [12]]. The primary reasons are limited slip systems and difficulties in activating non-basal slip at ambient temperature [13]. To date, strategies in dealing with the conflict mainly focus on microstructural design, especially heterogeneous structure which has positive effects on strength-ductility balance. Xu et al. [14] reported an excellent strength-ductility balance in Mg alloys, which was attributed to the bimodal structure which can relax the local strain concentration via strain transfer from the dynamically recrystallized (DRXed) regions to the deformed regions. Wu et al. [11] overcame this trade-off dilemma in titanium by architecting a heterogeneous lamella structure. They attributed the high ductility to the assistance of additional work hardening developed from storage and interaction of dislocations, which was induced by a macroscopic strain gradient and a change in stress states. Lu et al. [15,16] and Shao et al. [17] enhanced strength-ductility synergy by engineering a linear gradient in grain size in nanomaterials and Fe-Mn-C twinning-induced plasticity steel, respectively. Lu’s group considered the unique deformation mechanism of gradient nanostructures is the key to breakthrough the strength-ductility trade-off. Shao’s team considered that desirable properties originate from geometric necessary dislocations (GNDs) formed during tensile deformation, which provides additional work-hardening, especially at late stage of deformation. In addition, the superior strength-ductility combination was also discovered in a dual-constituent microstructure in Ti-Nb based gum metal [18]. ...
1
2011
... Advanced engineering materials have attracted the attention of materials scientists and engineers for alleviating the current challenge of the energy crisis and global warming, etc [1,2]. Magnesium (Mg) and its alloys are considered as promising engineering materials due to their light weight and high specific strength. However, low absolute strength severely restricts their wide applications. So far, alloying is one of the prevalent and effective methods to improve strength of Mg alloys [3,4]. Considerable studies present that rare earth Mg alloys, especially Mg-Gd-Y based alloys, exhibit high strength, which primarily originates from solid-solution strengthening and precipitation strengthening. Gadolinium (Gd) and yttrium (Y) elements have a large difference in atomic radius from that of Mg [5], which leads to significant strengthening effects. The solid solubility of Gd and Y in Mg matrix changes significantly with temperature [6], thus providing remarkable precipitation strengthening. Strengthening mechanisms, however, usually sacrifice ductility, which is known as strength-ductility trade-off [[7], [8], [9], [10], [11], [12]]. The primary reasons are limited slip systems and difficulties in activating non-basal slip at ambient temperature [13]. To date, strategies in dealing with the conflict mainly focus on microstructural design, especially heterogeneous structure which has positive effects on strength-ductility balance. Xu et al. [14] reported an excellent strength-ductility balance in Mg alloys, which was attributed to the bimodal structure which can relax the local strain concentration via strain transfer from the dynamically recrystallized (DRXed) regions to the deformed regions. Wu et al. [11] overcame this trade-off dilemma in titanium by architecting a heterogeneous lamella structure. They attributed the high ductility to the assistance of additional work hardening developed from storage and interaction of dislocations, which was induced by a macroscopic strain gradient and a change in stress states. Lu et al. [15,16] and Shao et al. [17] enhanced strength-ductility synergy by engineering a linear gradient in grain size in nanomaterials and Fe-Mn-C twinning-induced plasticity steel, respectively. Lu’s group considered the unique deformation mechanism of gradient nanostructures is the key to breakthrough the strength-ductility trade-off. Shao’s team considered that desirable properties originate from geometric necessary dislocations (GNDs) formed during tensile deformation, which provides additional work-hardening, especially at late stage of deformation. In addition, the superior strength-ductility combination was also discovered in a dual-constituent microstructure in Ti-Nb based gum metal [18]. ...
1
2014
... Advanced engineering materials have attracted the attention of materials scientists and engineers for alleviating the current challenge of the energy crisis and global warming, etc [1,2]. Magnesium (Mg) and its alloys are considered as promising engineering materials due to their light weight and high specific strength. However, low absolute strength severely restricts their wide applications. So far, alloying is one of the prevalent and effective methods to improve strength of Mg alloys [3,4]. Considerable studies present that rare earth Mg alloys, especially Mg-Gd-Y based alloys, exhibit high strength, which primarily originates from solid-solution strengthening and precipitation strengthening. Gadolinium (Gd) and yttrium (Y) elements have a large difference in atomic radius from that of Mg [5], which leads to significant strengthening effects. The solid solubility of Gd and Y in Mg matrix changes significantly with temperature [6], thus providing remarkable precipitation strengthening. Strengthening mechanisms, however, usually sacrifice ductility, which is known as strength-ductility trade-off [[7], [8], [9], [10], [11], [12]]. The primary reasons are limited slip systems and difficulties in activating non-basal slip at ambient temperature [13]. To date, strategies in dealing with the conflict mainly focus on microstructural design, especially heterogeneous structure which has positive effects on strength-ductility balance. Xu et al. [14] reported an excellent strength-ductility balance in Mg alloys, which was attributed to the bimodal structure which can relax the local strain concentration via strain transfer from the dynamically recrystallized (DRXed) regions to the deformed regions. Wu et al. [11] overcame this trade-off dilemma in titanium by architecting a heterogeneous lamella structure. They attributed the high ductility to the assistance of additional work hardening developed from storage and interaction of dislocations, which was induced by a macroscopic strain gradient and a change in stress states. Lu et al. [15,16] and Shao et al. [17] enhanced strength-ductility synergy by engineering a linear gradient in grain size in nanomaterials and Fe-Mn-C twinning-induced plasticity steel, respectively. Lu’s group considered the unique deformation mechanism of gradient nanostructures is the key to breakthrough the strength-ductility trade-off. Shao’s team considered that desirable properties originate from geometric necessary dislocations (GNDs) formed during tensile deformation, which provides additional work-hardening, especially at late stage of deformation. In addition, the superior strength-ductility combination was also discovered in a dual-constituent microstructure in Ti-Nb based gum metal [18]. ...
1
2018
... Advanced engineering materials have attracted the attention of materials scientists and engineers for alleviating the current challenge of the energy crisis and global warming, etc [1,2]. Magnesium (Mg) and its alloys are considered as promising engineering materials due to their light weight and high specific strength. However, low absolute strength severely restricts their wide applications. So far, alloying is one of the prevalent and effective methods to improve strength of Mg alloys [3,4]. Considerable studies present that rare earth Mg alloys, especially Mg-Gd-Y based alloys, exhibit high strength, which primarily originates from solid-solution strengthening and precipitation strengthening. Gadolinium (Gd) and yttrium (Y) elements have a large difference in atomic radius from that of Mg [5], which leads to significant strengthening effects. The solid solubility of Gd and Y in Mg matrix changes significantly with temperature [6], thus providing remarkable precipitation strengthening. Strengthening mechanisms, however, usually sacrifice ductility, which is known as strength-ductility trade-off [[7], [8], [9], [10], [11], [12]]. The primary reasons are limited slip systems and difficulties in activating non-basal slip at ambient temperature [13]. To date, strategies in dealing with the conflict mainly focus on microstructural design, especially heterogeneous structure which has positive effects on strength-ductility balance. Xu et al. [14] reported an excellent strength-ductility balance in Mg alloys, which was attributed to the bimodal structure which can relax the local strain concentration via strain transfer from the dynamically recrystallized (DRXed) regions to the deformed regions. Wu et al. [11] overcame this trade-off dilemma in titanium by architecting a heterogeneous lamella structure. They attributed the high ductility to the assistance of additional work hardening developed from storage and interaction of dislocations, which was induced by a macroscopic strain gradient and a change in stress states. Lu et al. [15,16] and Shao et al. [17] enhanced strength-ductility synergy by engineering a linear gradient in grain size in nanomaterials and Fe-Mn-C twinning-induced plasticity steel, respectively. Lu’s group considered the unique deformation mechanism of gradient nanostructures is the key to breakthrough the strength-ductility trade-off. Shao’s team considered that desirable properties originate from geometric necessary dislocations (GNDs) formed during tensile deformation, which provides additional work-hardening, especially at late stage of deformation. In addition, the superior strength-ductility combination was also discovered in a dual-constituent microstructure in Ti-Nb based gum metal [18]. ...
1
2017
... Advanced engineering materials have attracted the attention of materials scientists and engineers for alleviating the current challenge of the energy crisis and global warming, etc [1,2]. Magnesium (Mg) and its alloys are considered as promising engineering materials due to their light weight and high specific strength. However, low absolute strength severely restricts their wide applications. So far, alloying is one of the prevalent and effective methods to improve strength of Mg alloys [3,4]. Considerable studies present that rare earth Mg alloys, especially Mg-Gd-Y based alloys, exhibit high strength, which primarily originates from solid-solution strengthening and precipitation strengthening. Gadolinium (Gd) and yttrium (Y) elements have a large difference in atomic radius from that of Mg [5], which leads to significant strengthening effects. The solid solubility of Gd and Y in Mg matrix changes significantly with temperature [6], thus providing remarkable precipitation strengthening. Strengthening mechanisms, however, usually sacrifice ductility, which is known as strength-ductility trade-off [[7], [8], [9], [10], [11], [12]]. The primary reasons are limited slip systems and difficulties in activating non-basal slip at ambient temperature [13]. To date, strategies in dealing with the conflict mainly focus on microstructural design, especially heterogeneous structure which has positive effects on strength-ductility balance. Xu et al. [14] reported an excellent strength-ductility balance in Mg alloys, which was attributed to the bimodal structure which can relax the local strain concentration via strain transfer from the dynamically recrystallized (DRXed) regions to the deformed regions. Wu et al. [11] overcame this trade-off dilemma in titanium by architecting a heterogeneous lamella structure. They attributed the high ductility to the assistance of additional work hardening developed from storage and interaction of dislocations, which was induced by a macroscopic strain gradient and a change in stress states. Lu et al. [15,16] and Shao et al. [17] enhanced strength-ductility synergy by engineering a linear gradient in grain size in nanomaterials and Fe-Mn-C twinning-induced plasticity steel, respectively. Lu’s group considered the unique deformation mechanism of gradient nanostructures is the key to breakthrough the strength-ductility trade-off. Shao’s team considered that desirable properties originate from geometric necessary dislocations (GNDs) formed during tensile deformation, which provides additional work-hardening, especially at late stage of deformation. In addition, the superior strength-ductility combination was also discovered in a dual-constituent microstructure in Ti-Nb based gum metal [18]. ...
1
2013
... Aside from microstructure design, dispersed nano-precipitates is another effective approach to overcome strength-ductility trade-off. Liu et al. [19] adopted a molecular-level liquid-liquid mixing/doping technique and obtained an optimal microstructure, where nanometric oxide particles uniformly distributed in the interior of submicrometer grains. The dispersing intragranular particles can generate, pin down and accumulate dislocations, thus resulting in the increase of strength and simultaneous improvement in ductility. Similar reports have been reported in high-entropy alloys [20] and ultrastrong steel [8]. It is noted that these highly dispersed, fully coherent or semi-coherent precipitates, which exhibits low lattice misfit with the matrix, and high anti-phase boundary energy, can significantly improve strength without trade-off in ductility. ...
1
2018
... Aside from microstructure design, dispersed nano-precipitates is another effective approach to overcome strength-ductility trade-off. Liu et al. [19] adopted a molecular-level liquid-liquid mixing/doping technique and obtained an optimal microstructure, where nanometric oxide particles uniformly distributed in the interior of submicrometer grains. The dispersing intragranular particles can generate, pin down and accumulate dislocations, thus resulting in the increase of strength and simultaneous improvement in ductility. Similar reports have been reported in high-entropy alloys [20] and ultrastrong steel [8]. It is noted that these highly dispersed, fully coherent or semi-coherent precipitates, which exhibits low lattice misfit with the matrix, and high anti-phase boundary energy, can significantly improve strength without trade-off in ductility. ...
1
2018
... (a) Engineering stress-strain curves of as-extruded and aged GW83 K Mg alloys at room temperature and (b) representative tensile properties of Mg-Gd-Y based alloys [[21], [22], [23], [24], [25], [26], [27], [28], [29], [30]]. ...
1
2007
... (a) Engineering stress-strain curves of as-extruded and aged GW83 K Mg alloys at room temperature and (b) representative tensile properties of Mg-Gd-Y based alloys [[21], [22], [23], [24], [25], [26], [27], [28], [29], [30]]. ...
1
2011
... (a) Engineering stress-strain curves of as-extruded and aged GW83 K Mg alloys at room temperature and (b) representative tensile properties of Mg-Gd-Y based alloys [[21], [22], [23], [24], [25], [26], [27], [28], [29], [30]]. ...
1
2010
... (a) Engineering stress-strain curves of as-extruded and aged GW83 K Mg alloys at room temperature and (b) representative tensile properties of Mg-Gd-Y based alloys [[21], [22], [23], [24], [25], [26], [27], [28], [29], [30]]. ...
1
2019
... (a) Engineering stress-strain curves of as-extruded and aged GW83 K Mg alloys at room temperature and (b) representative tensile properties of Mg-Gd-Y based alloys [[21], [22], [23], [24], [25], [26], [27], [28], [29], [30]]. ...
1
2009
... (a) Engineering stress-strain curves of as-extruded and aged GW83 K Mg alloys at room temperature and (b) representative tensile properties of Mg-Gd-Y based alloys [[21], [22], [23], [24], [25], [26], [27], [28], [29], [30]]. ...
1
2017
... (a) Engineering stress-strain curves of as-extruded and aged GW83 K Mg alloys at room temperature and (b) representative tensile properties of Mg-Gd-Y based alloys [[21], [22], [23], [24], [25], [26], [27], [28], [29], [30]]. ...
1
2017
... (a) Engineering stress-strain curves of as-extruded and aged GW83 K Mg alloys at room temperature and (b) representative tensile properties of Mg-Gd-Y based alloys [[21], [22], [23], [24], [25], [26], [27], [28], [29], [30]]. ...
1
2014
... (a) Engineering stress-strain curves of as-extruded and aged GW83 K Mg alloys at room temperature and (b) representative tensile properties of Mg-Gd-Y based alloys [[21], [22], [23], [24], [25], [26], [27], [28], [29], [30]]. ...
1
2009
... (a) Engineering stress-strain curves of as-extruded and aged GW83 K Mg alloys at room temperature and (b) representative tensile properties of Mg-Gd-Y based alloys [[21], [22], [23], [24], [25], [26], [27], [28], [29], [30]]. ...
3
2017
... By purposely deploying heterogeneous structure to enhance materials properties has been widely reported [7]. As a typical heterogeneous microstructure, bimodal structure has attracted much attention. Generally, bimodal structure can be regarded as the composite structure, which is consist of ultrafine-grained regions and coarse-grained regions [31]. According to the Hall-Patch relationship, the strength of materials varies with the grain size, and the larger the grain size, the lower the strength [32]. At the initial stage, the plastic deformation occurs preferentially in coarse grains (soft domains), while the ultrafine grains (hard domains) maintain the elastic deformation [11,31]. Therefore, the deformation-incompatibility happens, which leads to the formation of strain gradient. As the deformation continues, the strain gradient increases. To accommodate the strain gradient, plenty GNDs generates, implying back-stress existed, which control the strain hardening of the samples when their density exceeds that of the statistically-stored dislocations [7,11,33,34]. The GNDs also interact with mobile dislocations and impede their movements during plastic deformation, thus improving the strength of the alloy [35]. To generalize these ideas, we can conclude that back-stress strengthening and back-stress strain-hardening can improve the strength and ductility simultaneously. ...
... ,31]. Therefore, the deformation-incompatibility happens, which leads to the formation of strain gradient. As the deformation continues, the strain gradient increases. To accommodate the strain gradient, plenty GNDs generates, implying back-stress existed, which control the strain hardening of the samples when their density exceeds that of the statistically-stored dislocations [7,11,33,34]. The GNDs also interact with mobile dislocations and impede their movements during plastic deformation, thus improving the strength of the alloy [35]. To generalize these ideas, we can conclude that back-stress strengthening and back-stress strain-hardening can improve the strength and ductility simultaneously. ...
... Back-stress has an effect on the dislocation configuration in the coarse grain interior during pre-deformation proess, and in turn affects the precipitation process and properties of samples [11,31]. At pre-deformation strain of 2%, profuse dislocations generate within coarse grains and cannot transmit into neighboring grains, which is attributed to the back-stress [11,34]. Thus, dislocation density in fine grains is low while that in coarse grains is high. As the pre-deformation strain increases from 2% to 5%, the coarse grains sustained most of the plastic strain, and then dislocation density in coarse grains is much high and close to saturation and fine grains begin to provide stain. For triggering the deformation of fine grains, the driving force reaches sufficiently high levels as a result. At the pre-deformation strain of 8%, dislocation density increases significantly but the space for dislocation storage decreases sharply [40], which results in the decrease of ductility. ...
1
2006
... By purposely deploying heterogeneous structure to enhance materials properties has been widely reported [7]. As a typical heterogeneous microstructure, bimodal structure has attracted much attention. Generally, bimodal structure can be regarded as the composite structure, which is consist of ultrafine-grained regions and coarse-grained regions [31]. According to the Hall-Patch relationship, the strength of materials varies with the grain size, and the larger the grain size, the lower the strength [32]. At the initial stage, the plastic deformation occurs preferentially in coarse grains (soft domains), while the ultrafine grains (hard domains) maintain the elastic deformation [11,31]. Therefore, the deformation-incompatibility happens, which leads to the formation of strain gradient. As the deformation continues, the strain gradient increases. To accommodate the strain gradient, plenty GNDs generates, implying back-stress existed, which control the strain hardening of the samples when their density exceeds that of the statistically-stored dislocations [7,11,33,34]. The GNDs also interact with mobile dislocations and impede their movements during plastic deformation, thus improving the strength of the alloy [35]. To generalize these ideas, we can conclude that back-stress strengthening and back-stress strain-hardening can improve the strength and ductility simultaneously. ...
1
2016
... By purposely deploying heterogeneous structure to enhance materials properties has been widely reported [7]. As a typical heterogeneous microstructure, bimodal structure has attracted much attention. Generally, bimodal structure can be regarded as the composite structure, which is consist of ultrafine-grained regions and coarse-grained regions [31]. According to the Hall-Patch relationship, the strength of materials varies with the grain size, and the larger the grain size, the lower the strength [32]. At the initial stage, the plastic deformation occurs preferentially in coarse grains (soft domains), while the ultrafine grains (hard domains) maintain the elastic deformation [11,31]. Therefore, the deformation-incompatibility happens, which leads to the formation of strain gradient. As the deformation continues, the strain gradient increases. To accommodate the strain gradient, plenty GNDs generates, implying back-stress existed, which control the strain hardening of the samples when their density exceeds that of the statistically-stored dislocations [7,11,33,34]. The GNDs also interact with mobile dislocations and impede their movements during plastic deformation, thus improving the strength of the alloy [35]. To generalize these ideas, we can conclude that back-stress strengthening and back-stress strain-hardening can improve the strength and ductility simultaneously. ...
2
2018
... By purposely deploying heterogeneous structure to enhance materials properties has been widely reported [7]. As a typical heterogeneous microstructure, bimodal structure has attracted much attention. Generally, bimodal structure can be regarded as the composite structure, which is consist of ultrafine-grained regions and coarse-grained regions [31]. According to the Hall-Patch relationship, the strength of materials varies with the grain size, and the larger the grain size, the lower the strength [32]. At the initial stage, the plastic deformation occurs preferentially in coarse grains (soft domains), while the ultrafine grains (hard domains) maintain the elastic deformation [11,31]. Therefore, the deformation-incompatibility happens, which leads to the formation of strain gradient. As the deformation continues, the strain gradient increases. To accommodate the strain gradient, plenty GNDs generates, implying back-stress existed, which control the strain hardening of the samples when their density exceeds that of the statistically-stored dislocations [7,11,33,34]. The GNDs also interact with mobile dislocations and impede their movements during plastic deformation, thus improving the strength of the alloy [35]. To generalize these ideas, we can conclude that back-stress strengthening and back-stress strain-hardening can improve the strength and ductility simultaneously. ...
... Back-stress has an effect on the dislocation configuration in the coarse grain interior during pre-deformation proess, and in turn affects the precipitation process and properties of samples [11,31]. At pre-deformation strain of 2%, profuse dislocations generate within coarse grains and cannot transmit into neighboring grains, which is attributed to the back-stress [11,34]. Thus, dislocation density in fine grains is low while that in coarse grains is high. As the pre-deformation strain increases from 2% to 5%, the coarse grains sustained most of the plastic strain, and then dislocation density in coarse grains is much high and close to saturation and fine grains begin to provide stain. For triggering the deformation of fine grains, the driving force reaches sufficiently high levels as a result. At the pre-deformation strain of 8%, dislocation density increases significantly but the space for dislocation storage decreases sharply [40], which results in the decrease of ductility. ...
1
2003
... By purposely deploying heterogeneous structure to enhance materials properties has been widely reported [7]. As a typical heterogeneous microstructure, bimodal structure has attracted much attention. Generally, bimodal structure can be regarded as the composite structure, which is consist of ultrafine-grained regions and coarse-grained regions [31]. According to the Hall-Patch relationship, the strength of materials varies with the grain size, and the larger the grain size, the lower the strength [32]. At the initial stage, the plastic deformation occurs preferentially in coarse grains (soft domains), while the ultrafine grains (hard domains) maintain the elastic deformation [11,31]. Therefore, the deformation-incompatibility happens, which leads to the formation of strain gradient. As the deformation continues, the strain gradient increases. To accommodate the strain gradient, plenty GNDs generates, implying back-stress existed, which control the strain hardening of the samples when their density exceeds that of the statistically-stored dislocations [7,11,33,34]. The GNDs also interact with mobile dislocations and impede their movements during plastic deformation, thus improving the strength of the alloy [35]. To generalize these ideas, we can conclude that back-stress strengthening and back-stress strain-hardening can improve the strength and ductility simultaneously. ...
2
2014
... In addition to the bimodal structure, the microstructure evolution during pre-deformation and aging can play a significant role to enhance the properties of the alloy. As we all know that dislocations have significant influence on the precipitation process, and considerable literatures had done profound discussion on the precipitating behavior [[36], [37], [38], [39]]. Liu et al. [36] adopted phase-field model to examine heterogeneous nucleation of β1 precipitates on various a-type dislocations (screw, edge and mixed) in an Mg-0.5 at.% Nd alloy. Considering the different in dislocation type, the morphology, aspect ratio and distribution of the β1 precipitates were significantly different. Liu et al. [37] also studied the influences of the applied stress and dislocations on the distribution of β’ precipitates in a crept Mg-Gd-Zr alloy, and pointed out that the a-type basal edge dislocations are the most optimal sites for β’ nucleation. Moreove, Qiu et al. [39] reported the variant selection by dislocations during α precipitation in α/β titanium alloys, and found that edge dislocations exhibit a much more prominent effect than screw dislocations. But few reports have been foused on which Burgers vectors is more favorable to the precipitates nucleation. In situ TEM is an effective way to solve that problem, and relevant work will be done in the future. In this paper, we pay main attention to the effect of density of dislocations and precipitates on the mechanical properties, and the detailed discussion is given as below. ...
... ]]. Liu et al. [36] adopted phase-field model to examine heterogeneous nucleation of β1 precipitates on various a-type dislocations (screw, edge and mixed) in an Mg-0.5 at.% Nd alloy. Considering the different in dislocation type, the morphology, aspect ratio and distribution of the β1 precipitates were significantly different. Liu et al. [37] also studied the influences of the applied stress and dislocations on the distribution of β’ precipitates in a crept Mg-Gd-Zr alloy, and pointed out that the a-type basal edge dislocations are the most optimal sites for β’ nucleation. Moreove, Qiu et al. [39] reported the variant selection by dislocations during α precipitation in α/β titanium alloys, and found that edge dislocations exhibit a much more prominent effect than screw dislocations. But few reports have been foused on which Burgers vectors is more favorable to the precipitates nucleation. In situ TEM is an effective way to solve that problem, and relevant work will be done in the future. In this paper, we pay main attention to the effect of density of dislocations and precipitates on the mechanical properties, and the detailed discussion is given as below. ...
2
2017
... In addition to the bimodal structure, the microstructure evolution during pre-deformation and aging can play a significant role to enhance the properties of the alloy. As we all know that dislocations have significant influence on the precipitation process, and considerable literatures had done profound discussion on the precipitating behavior [[36], [37], [38], [39]]. Liu et al. [36] adopted phase-field model to examine heterogeneous nucleation of β1 precipitates on various a-type dislocations (screw, edge and mixed) in an Mg-0.5 at.% Nd alloy. Considering the different in dislocation type, the morphology, aspect ratio and distribution of the β1 precipitates were significantly different. Liu et al. [37] also studied the influences of the applied stress and dislocations on the distribution of β’ precipitates in a crept Mg-Gd-Zr alloy, and pointed out that the a-type basal edge dislocations are the most optimal sites for β’ nucleation. Moreove, Qiu et al. [39] reported the variant selection by dislocations during α precipitation in α/β titanium alloys, and found that edge dislocations exhibit a much more prominent effect than screw dislocations. But few reports have been foused on which Burgers vectors is more favorable to the precipitates nucleation. In situ TEM is an effective way to solve that problem, and relevant work will be done in the future. In this paper, we pay main attention to the effect of density of dislocations and precipitates on the mechanical properties, and the detailed discussion is given as below. ...
... precipitates were significantly different. Liu et al. [37] also studied the influences of the applied stress and dislocations on the distribution of β’ precipitates in a crept Mg-Gd-Zr alloy, and pointed out that the a-type basal edge dislocations are the most optimal sites for β’ nucleation. Moreove, Qiu et al. [39] reported the variant selection by dislocations during α precipitation in α/β titanium alloys, and found that edge dislocations exhibit a much more prominent effect than screw dislocations. But few reports have been foused on which Burgers vectors is more favorable to the precipitates nucleation. In situ TEM is an effective way to solve that problem, and relevant work will be done in the future. In this paper, we pay main attention to the effect of density of dislocations and precipitates on the mechanical properties, and the detailed discussion is given as below. ...
1
2019
... In addition to the bimodal structure, the microstructure evolution during pre-deformation and aging can play a significant role to enhance the properties of the alloy. As we all know that dislocations have significant influence on the precipitation process, and considerable literatures had done profound discussion on the precipitating behavior [[36], [37], [38], [39]]. Liu et al. [36] adopted phase-field model to examine heterogeneous nucleation of β1 precipitates on various a-type dislocations (screw, edge and mixed) in an Mg-0.5 at.% Nd alloy. Considering the different in dislocation type, the morphology, aspect ratio and distribution of the β1 precipitates were significantly different. Liu et al. [37] also studied the influences of the applied stress and dislocations on the distribution of β’ precipitates in a crept Mg-Gd-Zr alloy, and pointed out that the a-type basal edge dislocations are the most optimal sites for β’ nucleation. Moreove, Qiu et al. [39] reported the variant selection by dislocations during α precipitation in α/β titanium alloys, and found that edge dislocations exhibit a much more prominent effect than screw dislocations. But few reports have been foused on which Burgers vectors is more favorable to the precipitates nucleation. In situ TEM is an effective way to solve that problem, and relevant work will be done in the future. In this paper, we pay main attention to the effect of density of dislocations and precipitates on the mechanical properties, and the detailed discussion is given as below. ...
2
2015
... In addition to the bimodal structure, the microstructure evolution during pre-deformation and aging can play a significant role to enhance the properties of the alloy. As we all know that dislocations have significant influence on the precipitation process, and considerable literatures had done profound discussion on the precipitating behavior [[36], [37], [38], [39]]. Liu et al. [36] adopted phase-field model to examine heterogeneous nucleation of β1 precipitates on various a-type dislocations (screw, edge and mixed) in an Mg-0.5 at.% Nd alloy. Considering the different in dislocation type, the morphology, aspect ratio and distribution of the β1 precipitates were significantly different. Liu et al. [37] also studied the influences of the applied stress and dislocations on the distribution of β’ precipitates in a crept Mg-Gd-Zr alloy, and pointed out that the a-type basal edge dislocations are the most optimal sites for β’ nucleation. Moreove, Qiu et al. [39] reported the variant selection by dislocations during α precipitation in α/β titanium alloys, and found that edge dislocations exhibit a much more prominent effect than screw dislocations. But few reports have been foused on which Burgers vectors is more favorable to the precipitates nucleation. In situ TEM is an effective way to solve that problem, and relevant work will be done in the future. In this paper, we pay main attention to the effect of density of dislocations and precipitates on the mechanical properties, and the detailed discussion is given as below. ...
... ] also studied the influences of the applied stress and dislocations on the distribution of β’ precipitates in a crept Mg-Gd-Zr alloy, and pointed out that the a-type basal edge dislocations are the most optimal sites for β’ nucleation. Moreove, Qiu et al. [39] reported the variant selection by dislocations during α precipitation in α/β titanium alloys, and found that edge dislocations exhibit a much more prominent effect than screw dislocations. But few reports have been foused on which Burgers vectors is more favorable to the precipitates nucleation. In situ TEM is an effective way to solve that problem, and relevant work will be done in the future. In this paper, we pay main attention to the effect of density of dislocations and precipitates on the mechanical properties, and the detailed discussion is given as below. ...
1
2017
... Back-stress has an effect on the dislocation configuration in the coarse grain interior during pre-deformation proess, and in turn affects the precipitation process and properties of samples [11,31]. At pre-deformation strain of 2%, profuse dislocations generate within coarse grains and cannot transmit into neighboring grains, which is attributed to the back-stress [11,34]. Thus, dislocation density in fine grains is low while that in coarse grains is high. As the pre-deformation strain increases from 2% to 5%, the coarse grains sustained most of the plastic strain, and then dislocation density in coarse grains is much high and close to saturation and fine grains begin to provide stain. For triggering the deformation of fine grains, the driving force reaches sufficiently high levels as a result. At the pre-deformation strain of 8%, dislocation density increases significantly but the space for dislocation storage decreases sharply [40], which results in the decrease of ductility. ...
1
2019
... Extensive chain-like precipitates appear in both coarse and fine grains in the 2% pre-deformed sample after aging treatment. The formation of the chain-like precipitates is closely related to the dislocation configuration after pre-deformation, because high density dislocations introduced by pre-deformation can serve as heterogeneous nucleation sites. The dislocations will reduce the barrier for precipitate nucleation [41] and can also effectively accelerate mass transport by raising diffusion kinetics [42], both of which are in favor of promoting precipitation nucleation. ...
2
2019
... Extensive chain-like precipitates appear in both coarse and fine grains in the 2% pre-deformed sample after aging treatment. The formation of the chain-like precipitates is closely related to the dislocation configuration after pre-deformation, because high density dislocations introduced by pre-deformation can serve as heterogeneous nucleation sites. The dislocations will reduce the barrier for precipitate nucleation [41] and can also effectively accelerate mass transport by raising diffusion kinetics [42], both of which are in favor of promoting precipitation nucleation. ...
... Compared with as-extruded alloy, dislocation density is higher in coarse grains for the 2%-deformed sample. Therefore, there are a higher volume fraction of chain-like precipitates in the 2%-deformed sample, which contributes to the good strength and ductility. The chain-like structure provides space for dislocation storage to some extent, which can provide strain in grain interior [42]. In addition, the sub-grain boundaries existed in the sample are favorable for dislocation glide through these boundaries as well [43]. However, as for the 8%-deformed sample, higher dislocation density dramatically activated more precipitates which distribute in both coarse and fine grains. High density precipitates result in high strength but poor ductility, which could ascribe to the small space between precipitates and accelerate the crack nucleation at the interface between precipitates and α-Mg matrix. Moreover, the chain-like precipitates are virtually absent in this sample, which reduces the storage capacity of the dislocations, thereby deteriorating ductility as well. ...
1
2014
... Compared with as-extruded alloy, dislocation density is higher in coarse grains for the 2%-deformed sample. Therefore, there are a higher volume fraction of chain-like precipitates in the 2%-deformed sample, which contributes to the good strength and ductility. The chain-like structure provides space for dislocation storage to some extent, which can provide strain in grain interior [42]. In addition, the sub-grain boundaries existed in the sample are favorable for dislocation glide through these boundaries as well [43]. However, as for the 8%-deformed sample, higher dislocation density dramatically activated more precipitates which distribute in both coarse and fine grains. High density precipitates result in high strength but poor ductility, which could ascribe to the small space between precipitates and accelerate the crack nucleation at the interface between precipitates and α-Mg matrix. Moreover, the chain-like precipitates are virtually absent in this sample, which reduces the storage capacity of the dislocations, thereby deteriorating ductility as well. ...
, Jing Li
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