Journal of Materials Science & Technology  2020 , 38 (0): 39-46 https://doi.org/10.1016/j.jmst.2019.06.025

Research Article

Age hardening responses of as-extruded Mg-2.5Sn-1.5Ca alloys with a wide range of Al concentration

Qiuyan Huanga, Yang Liua, Aiyue Zhangbc, Haoxin Jiangbc, Hucheng Panbc*, Xiaohui Fenga, Changlin Yangd, Tianjiao Luoa, Yingju Lia, Yuansheng Yanga**

aInstitute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016, China
bKey Laboratory for Anisotropy and Texture of Materials (MOE), School of Materials Science and Engineering, Northeastern University, Shenyang, 110819, China
cResearch Center for Metallic Wires, Northeastern University, Shenyang, 110819, China
dState Key Laboratory of Solidification Processing, Northwestern Polytechnical University, Xi'an, 710072, China

Corresponding authors:   ∗Corresponding author at: Key Laboratory for Anisotropy and Texture of Materials (MOE), School of Materials Science and Engineering, Northeastern University, Shenyang, 110819, China. E-mail addresses: panhc@atm.neu.edu.cn (H. Pan)∗∗Corresponding author at: Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016, China.E-mail addresses: ysyang@imr.ac.cn (Y. Yang).

Received: 2019-03-15

Revised:  2019-06-26

Accepted:  2019-06-28

Online:  2020-02-01

Copyright:  2020 Editorial board of Journal of Materials Science & Technology Copyright reserved, Editorial board of Journal of Materials Science & Technology

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Abstract

This article aims to explore the age hardening responses of both as-extruded and as-aged Mg-2.5Sn-1.5Ca-xAl alloys (x = 2.0, 4.0 and 9.0 wt%, termed TXA322, TXA324 and TXA329, respectively) through microstructural and mechanical characterization. Results indicate that grain size of as-extruded TXA322, TXA324 and TXA329 alloys were ∼ 16 μm, ∼ 10 μm and ∼ 12 μm, respectively. A number of <a> and <c+a> dislocations were observed in all the as-extruded samples. Guinier - Preston (GP) zones were evidently identified in TXA322 alloy, while only a small number of Mg17Al12 phases existed in both TXA324 and TXA329 alloys. An aging treatment facilitated the precipitation of a high number density of GP zones within the matrix of TXA322 alloy. In contrast, no obvious nano-precipitates were in as-aged TXA324 alloy. Numerous nano-Mg17Al12 phases were formed through a following aging treatment in TXA329 alloy. In terms of mechanical properties, it is apparent that an increment in ultimate tensile strength of ∼ 46 MPa and ∼ 40 MPa was yielded in peak-aged TXA322 and TXA329 alloys, while no obvious variations in UTS were present in peak-aged TXA324 alloy, in comparison with the as-extruded counterparts.

Keywords: Magnesium alloys ; Age hardening ; Precipitations ; Dislocations ; Mechanical properties

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Qiuyan Huang, Yang Liu, Aiyue Zhang, Haoxin Jiang, Hucheng Pan, Xiaohui Feng, Changlin Yang, Tianjiao Luo, Yingju Li, Yuansheng Yang. Age hardening responses of as-extruded Mg-2.5Sn-1.5Ca alloys with a wide range of Al concentration[J]. Journal of Materials Science & Technology, 2020, 38(0): 39-46 https://doi.org/10.1016/j.jmst.2019.06.025

1. Introduction

The high-performance magnesium alloys have received considerable research interests due to their obvious advantages in weight-saving and fuel efficiency in transportation and aerospace fields [[1], [2], [3], [4]]. The proper combinations of strength and ductility are the necessary conditions that should be satisfied before the industrial usages of Mg alloys [[5], [6], [7]]. In the past decades, researchers have made use of the texture hardening, the grain refinement and the precipitation hardening to improve the strength, usually in the Rare-Earth-containing Mg alloys, and mechanical properties of Mg alloys have been obviously improved [[8], [9], [10], [11]]. For example, the highest strength of 600 MPa in Mg alloy has been achieved in the rapid solidified Mg-2.5Zn-6.8Y (in wt%) alloy [12]. By conventional aging treatment, the high strength of 403 MPa was also reported in the Mg-8.5Gd-2.3Y-1.8Ag-0.4 Zr (in wt%) alloy [13]. However, the high amount of RE element additions made the Mg-RE based alloys mentioned above expensive. Developing low-cost RE-free Mg alloys becomes critical for the practical applications [14,15].

On the other hand, the absolute strength of currently developed RE-free Mg alloy is still low [16]. For example, yield strengths (YSs) of the commercial ZK60 [17,18] and AZ31 [[19], [20], [21], [22], [23]] alloys are usually lower than 350 MPa. Recently, a new rare-earth-free Mg-Sn-Ca based wrought alloys are developed and excellent mechanical property has been achieved [24,25]. For example, tensile YS of as-extruded Mg-2Sn-1Ca alloy can reach > 300 MPa, and elongation is kept as ∼12%. The dynamic precipitation of nano-MgSnCa particles and the ultrafine DRXed grain size play the critical role in the strength enhancement [24]. By increasing Ca concentration, Pan et al. reported that the Mg-2Sn-2Ca alloy exhibits higher YS of ∼ 400 MPa. The ultra-high strength result from the numerous precipitations of nano-Mg2Ca phase, Ca segregations along the grain boundaries and the ultrafine grain size (only ∼ 0.32 μm) [25]. By increasing the Sn concentration, Chang et al. [26] reported that the Mg2Sn phases in as-extruded Mg-5Sn-0.5Ca based alloys can be refined to the submicron scale (∼ 0.2 μm) and distribute uniformly in the Mg matrix, which benefits for improving the tensile strength. Moreover, the thermal stability of CaMgSn ternary phases formed in the Mg-Ca-Sn alloy is high, and thus the creep resistance and high-temperature strength can be largely enhanced [26]. In other words, the Mg-Sn-Ca based wrought alloys have the great potentials to be developed as the next-generation cost-effective and high-performance Mg wrought alloy systems.

Recently, researches on the role of Al addition in altering microstructure and mechanical property of the Mg-Ca based alloy has also drawn particular attentions. For example, Xu et al. [27] developed a novel Mg-3.5Al-3.3Ca-0.4 Mn (wt%) wrought alloy, and the ultimate tensile strength (UTS) value can reach as high as ∼ 420 MPa The high strength can be ascribed to the nano-precipitations of both spherical Al-Mn-(Ca) phases and plate-like Al-Ca phases, the ultra-fine dynamically re-crystallized grains and the strong basal texture. Furthermore, the plate-like Al-Ca precipitates can be frequently detected in the Mg-0.5Al-0.3Ca (wt%) alloy [28] and Mg-3.6Al-3.4Ca-0.3 Mn (wt%) alloy [27], and are claimed to be the monolayer internally ordered Guinier - Preston (GP) zones. The GP zones are coherent or semi-coherent with Mg matrix and, therefore, contribute significantly to absolute strength of the Mg-Ca-Al based alloys. Recently, Cihova et al. [29] further confirmed the formation of Al-Ca-atoms enriched GP zones with atomic composition of Al2Ca by utilizing the atom probe tomography (APT), as well as the fine dispersion of Al-Mn particles in the Mg-0.6Al-0.28Ca-0.25 Mn (wt%) alloy. In this context, the Mg-Ca-Al alloy is another type of RE-free Mg system with remarkable age hardening response.

However, systematic studies on the influence of Al addition on the microstructure and mechanical properties of extruded Mg-Sn-Ca alloys have rarely been reported so far. Especially, the solid solution hardening and age hardening responses of Mg-Sn-Ca-Al alloys with a wide range of Al concentration are also unclear at present. Consequently, the different levels of Al content (0-9 wt%) was added into the benchmark Mg-2.5Sn-1.5Ca samples, to explore the age-hardening behaviors of as-extruded samples in this study.

2. Experimental

Alloy ingots with nominal compositions of Mg-2.5Sn-1.5Ca-xAl (x = 2.0, 4.0 and 9.0, termed TXA322, TXA324 and TXA 329, respectively, all in wt%) were prepared by melting pure Mg (> 99.99 wt%), pure Sn (> 99.9 wt%), pure Ca (> 99.9 wt%) and pure Al (> 99.99 wt%) by conventional casting. The molten alloy was held at 750 °C for 10 min under the protection of mixture gas of CO2 and SF6 (100:1), and then poured it into a pre-heated mold (350 °C) with a diameter of 50 mm.

The as-cast ingots were homogenized at 500 °C for 24 h, and quenched into water. Before indirect extrusion, billets with 45 mm in diameter and 100 mm in length were pre-heated at ∼ 350 °C for 0.5 h. Ingots were then extruded at temperature of ∼350 °C, the extrusion ram speed of ∼ 0.5 mm/s, and the extrusion ratio of 20:1. Aging treatment was conducted at ∼ 180 °C in a silicon oil bath for various times. The hardness was measured by using a HV-1000 type Vickers hardness tester under a load of 2 kg. The tensile bars were cut from the extruded specimens along extrusion direction in dog-bone shape with 30 mm in length and 5 mm in diameter. The tensile tests were conducted on Shimadzu AG-XPlus250KN at the initial tensile strain rate of 1 × 10-3 s-1. The mechanical properties of tensile yield strength, ultimate tensile strength and elongation to fracture are the average values of at least three individually repeated tests.

The X-ray diffraction (XRD, Philips PW3040/60 X’Pert PRO with Cu Kα radiation) was used to identify constituent phases and confirm the developed textures of samples. Microstructures over the cross sections were observed by an optical microscope (OM, Olympus GX), a scanning electron microscopy (SEM, JEOL JEM-7001 F) equipped with an energy dispersive spectrometer (EDS), and transmission electron microscopy (TEM, JEOL 2100 F). Thin foils for TEM observations were prepared by mechanical polishing (∼ 40 μm in thickness) and then ion beam thinning (GATAN, PIPS691). TEM observation was operated at an accelerating voltage of 200 kV.

3. Results and discussion

Fig. 1 shows the OM and SEM images of the as-extruded TXA322 alloy, and the corresponding EDS results are also included for determining second-phases compositions. Grains in the as-extruded alloys have been obviously refined due to the high degree of dynamic recrystallization (DRX), in comparison with the as-cast billet. The average grain size is estimated to be ∼ 16 μm according to the OM image (Fig. 1(a)). The (0002) X-ray pole figure of as-extruded TXA322 alloy can be found in inset image of Fig. 1a. A typical fiber texture with basal plane being parallel with extrusion direction (ED) is formed. Maximum texture intensity is largely weakened to be only ∼ 2.92 multiply random distributions (mrd.).

Fig. 1.   (a) Optical image, (b) SEM image and (c) the corresponding EDS results for the as-extruded TXA322 alloy, including (d) the XRD result. The (0002) X-ray pole figure of as-extruded TXA322 alloy can be found in inset image of Fig. 1(a).

The SEM image in Fig. 1(b) shows that the original eutectic microstructure in as-cast billet has disappeared after extrusion, and the micron-scale second-phases are broken into particles and distribute along the ED. Two kinds of micron phases in size of 5-10 μm can be found. They are the bright-contrast phase 1# and grey-contrast phase 3#, as shown in Fig. 1(c). Another type of dotty phase with average size lower than 1 μm also homogeneously distributes among the Mg matrix, as marked by 2#. The corresponding EDS results demonstrate that phase 1# contains 43.3 at.% Mg, 1.52 at.% Al, 24.28 at.% Ca and 30.89 at.% Sn, phase 2# contains 89.14 at.% Mg, 10.86 at.% Ca and phase 3# contains 96.44 at.% Mg, 3.26 at.% Al and 0.29 at.% Ca. Consequently, the three phases can be determined to be the MgSnCa, Mg2Ca and Al2Ca, respectively [30]. XRD results in Fig. 1(d) further confirm the existences of three phases above. In fact, the formations of MgSnCa, Mg2Ca phases have been reported in the Mg-Sn-Ca alloy [24]. The similar dotty particle was also previously detected in the Mg-Ca-Al alloy, which was identified to be the Al2Ca phase [27].

Fig. 2 shows the OM and SEM images of the as-extruded TXA324 and TXA329 alloys with high amount of Al additions. Equi-axed α-Mg grains have been formed due to high degree of DRX. The average grain sizes of as-extruded TXA324 and TXA329 alloys have be estimated to be ∼ 10 μm and ∼ 12 μm, respectively. Similarly, weak fiber textures are produced in the two samples, according to the pole figure results in the inset images of Fig. 3(a) and (b). The SEM images in Fig. 3(c) and (d) display that the micron-sized phases are broken and considerable Mg2Ca phases are gradually replaced by the Al2Ca phases with increasing the Al content from TXA324 to TXA329 alloy. The corresponding XRD patterns in Fig. 3 further confirm this change, and also demonstrate the existences of MgSnCa, Mg2Ca and Al2Ca phases.

Fig. 2.   Optical and SEM image of the as-extruded (a, c) TXA324 and (b, d) TXA329 alloys, with the corresponding pole figures inserted in the Fig. 2(a) and Fig. 2(b).

Fig. 3.   XRD patterns of the as-extruded TXA324 and TXA329 alloys.

Fig. 4 shows the bright-field TEM images of the as-extruded TXA322 alloy. The g∙b≠0 visible criterion can be used to identify the basal and/or the non-basal types of dislocations [31]. In general, dislocations containing the <c> component would be invisible under the two-beam condition of g=<10-10> , while the <a> type dislocations would be excluded under condition of g = 0001. In this context, the TEM images in Fig. 4(a) and (b) (g=<0001 > ) display that profuse <c+a>-type dislocations have been activated during the extrusion of TXA322 alloy. Besides, numerous <a>-type dislocations are also produced, according to the TEM image in Fig. 4(c) under the condition of g=<10-10> . It means that strains along both the a and c axis can be accommodated to ensure the compatible deformation. More interestingly, a number of plate-like phases can be detected to stay on the basal planes (Fig. 4(b)), which distribute either nearby or along the dislocation lines. It indicates that the dislocation-defects might provide the preferred sites for dynamical nano-precipitations during the extrusion processing, which is usually considered to be energetically favorable.

Fig. 4.   TEM images of the as-extruded TXA322 alloy under the two-beam condition of g=<0001> (a, b) and g=<10-10> (c) and SADPs and EDS results for the phase 1# existed in the Fig. 4(b) (d).

Most of these basal plate-like precipitates are less than 1 nm in thickness and about 20 nm in length. For a detail analysis of the contrast, both the selected area diffraction patterns (SADPs) and the EDS analyses are conducted. The SADPs along the <11-20> zone axis (Fig. 4(d)) reveal the appearance of extra diffraction streaks at position of 1/3 (10-10) spots, which are consistent with the GP zones observed in Mg-Ca-Al alloy [32,33]. The EDS results, as shown by the composition profile in Fig. 4(d), demonstrate that the disk-like precipitates are enriched with both Al and Ca atoms. The recent APT result in Mg-Al-Ca-Mn alloy further confirms that the nano-plate phases exhibit a composition profile with Al-to-Ca ratio of about 2 [29]. Consequently, the plate-like phases can be regarded as the GP zones with composition of Al2Ca.

Fig. 5 shows the bright-field TEM images of the as-extruded TXA324 and TXA329 alloys. For TXA324 sample, a number of <a> dislocations are presented in Mg matrix under two-beam condition of g=<10-10> (Fig. 5(a)), while few <c+a> dislocations can be found when the diffraction condition is changed to g=<0001> (Fig. 5(b)). A similar situation can be found in. In similar with that, numerous basal <a>-type dislocations can be observed in the TXA329 alloy, as shown in Fig. 5(c). However, much less <c+a> dislocations are formed in TXA329 alloy (Fig. 5(d)), in compared with TXA324 counterpart. It means that the Al addition would significantly change the deformation mode of Mg matrix during extrusion. Besides, few precipitates can be detected in the Mg matrix of as-extruded TXA324 alloy (Fig. 5(b)), while some block phases are found in the TXA329 alloy (Fig. 5(d)). According to the morphology and compositions of the block phases, they should belong to Mg17Al12 phases [10]. It suggests that the nano-precipitation behaviors have been changed in the present Mg-Sn-Ca-Al alloys with increasing the Al concentration. More importantly, these residual dislocations would be beneficial for the subsequent age-hardening responses, since the dislocation lines are usually considered as the high-energy regions and the nano-precipitations would be largely promoted.

Fig. 5.   TEM images of the as-extruded (a, b) TXA324 and (c, d) TXA329 alloys.

Fig. 6 shows the hardness variations of as-extruded TXA322, TXA324 and TXA329 alloys during heat-treated at 180 °C for up to 90 h. The hardness values of three samples increase in the early stage reach the peak levels and then decrease with prolonging the aging time. Importantly, the age-hardening responses of the three samples are totally different. A large hardness increment is achieved in the as-aged TXA322 and TXA329 alloys, which increase from the initial values of∼ 70 HV and ∼ 79 HV in as-extruded states to the peak-aged hardness values of ∼ 84 HV and ∼ 95 HV, respectively. However, a minor increase of hardness is obtained in the as-aged TXA324 alloy, which changes from the initial value of ∼ 67 HV to the peak value of ∼ 72 HV. Moreover, the time to reach the peak hardness is different, which corresponds to the ∼ 12 h, ∼ 4 h and ∼ 12 h for the TXA322, TXA324 and TXA329 alloys, respectively.

Fig. 6.   Age-hardening response of as-extruded TXA322, TXA324 and TXA329 alloys.

Besides, the peak-hardness values are in the sequence of TXA329 > TXA322 > TXA324, which is not a linear function of Al concentrations. Consequently, it is necessary to further characterize the microstructures of the as-aged alloys to clarify the mechanism for the hardness changes.

Fig. 7, Fig. 8 show the typical TEM images of the peak-aged TXA322 and TXA329 alloys. As mentioned above, the GP zones are readily precipitated in the as-extruded TXA322, and a high number density of dislocation is found to co-exist with the nano-phases. After aging treatment, the number density of GP zones is largely increased in the peak-aged TXA322 alloy, in comparison with the as-extruded counterpart, as shown in Fig. 7. However, the distributions of ageing-treatment induced precipitations are inhomogeneous within the α-Mg matrix. The interspacing of GP zones is ∼ 50 nm in the typical TEM image of Fig. 7(b), while only ∼ 20 nm in inter-distance is found in another image of Fig. 7(d). The size of GP zones is in the range of 20 ∼ 100 nm in Fig. 7b but a smaller size of 5∼ 50 nm in Fig. 7(d). Specially, it is found that the GP zones would usually precipitate along and/or nearby the dislocation lines. This can be rationalized by the fact that the dislocations can provide rapid diffusion channels for solute atoms and favor the nano-precipitations during aging treatment [25]. Because of the numerous precipitations of the metastable GP zones, the hardness of TXA322 alloy has been apparently increased to the peak value of ∼ 84 HV. In accordance with that, the tensile strength would readily increase, which would be illustrated in the following section.

Fig. 7.   TEM images of the as-aged TXA322 alloys, including the precipitations of GP zones along and nearby the dislocation lines.

Fig. 8.   TEM images of the as-aged TXA329 alloys.

TEM images of peak-aged TXA329 alloys in Fig. 8 show that some basal <a> dislocations can be observed under the two-beam condition of g=<10-10> (Fig. 8(a) and (c)), and few pyramidal dislocations can be observed when g = 0001 (Fig. 8(b) and (d)). Moreover, the number density of basal <a> dislocations is lower than that in the as-extruded counterpart, and the dynamic recovery should be one reason. At the same time, numerous second phases have been precipitated in the Mg matrix, which include the droplet-like and the plate-like nano-phases. The corresponding morphology and compositions (not shown) indicates that the nano-precipitates belong to Mg17Al12 phases, and the plate-phases mainly stay on the basal plane, which is consistent with the previous reports [10,34]. More importantly, the nano-Mg17Al12 phases distribute mainly along and/or nearby the dislocation lines, as indicated by the arrows in Fig. 8(c). It indicates that dislocations can also promote the precipitation of Mg17Al12 phases, which is similar to the case of GP zones in the peak-aged TXA322 alloy.

The tensile mechanical properties of the TXA322, TXA324 and TXA329 alloys are measured, and the corresponding strength values are listed in Table 1, which are consistent with the hardness changes mentioned above during the aging treatment. It can be seen that the as-extruded TXA322 exhibits low YS of ∼ 130 MPa, UTS of ∼ 248 MPa, with elongation (EL) of ∼ 12.5%. For the as-aged TXA322 alloy, on the other hand, the YS and UTS values have increased up to ∼ 155 MPa and ∼ 294 MPa, with a UTS increment of ∼ 46 MPa. For the TXA329 alloy, the UTS has also obviously increased from ∼ 269 MPa in as-extruded alloy to ∼ 309 MPa in the peak-aged alloy, and the UTS value is increased by ∼ 40 MPa. In contrast, no apparent increase of the UTS value is observed in the TXA324 alloy.

Table 1   Mechanical properties of the TXA322, TXA324 and TXA329 alloys.

SampleAs-extrudedAged
σs (MPa)σb (MPa)δ (%)σs (MPa)σb (MPa)δ (%)
TXA322130 ± 5248 ± 512.5 ± 2155 ± 4294 ± 79.2 ± 1
TXA324136 ± 7252 ± 311.4 ± 3140 ± 6254 ± 29.6 ± 3
TXA329157 ± 4269 ± 69.8 ± 2230 ± 6309 ± 57.5 ± 2

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With increasing of the Al concentration, the YSs of the as-extruded TXA322, TXA324 and TXA329 alloys have been gradually enhanced to be ∼ 130 MPa, ∼ 136 MPa and ∼ 157 MPa, respectively. However, it is noted that the hardness value of the TXA324 alloy (∼67 HV) is slightly lower than that of the TXA322 alloy (∼ 70 HV). It might come from the fact that the hardness can only reflect the strengthening effect from the solid-solution and nano-precipitates, while the strengthening due to grain refinement has not been involved [[8], [9], [10], [11]]. In fact, the grain size has been obviously reduced from ∼ 16 μm in TXA322 to the ∼10 μm in TXA324 alloy. Accordingly, the YS in TXA324 alloy should increase due to the extra grain refinement hardening, and becomes larger than that in the TXA322 alloy.

Moreover, despite of the similar grain size between the TXA324 and TXA329 alloys, the strength is also increased with higher amounts of Al additions. It might come from the higher concentrations of Al solute atoms that dissolved in Mg matrix of the TXA329 alloy. The numerous precipitations of Mg17Al12 phases during the following aging can confirm this assumption. With the Al content increasing up to 9 wt%, the dissolved atoms would induce large lattice-distortions and effectively drag the motions of dislocations, which is beneficial for enhancing the YS by employing the solid-solution hardening effect [[35], [36], [37], [38], [39]].

In peak-aging treatment condition, UTS increments of ∼ 46 MPa and ∼ 40 MPa have been produced in the TXA322 and TXA329 alloys, respectively. But, no obvious change of UTS is obtained in TXA324 alloy. This is primarily due to the fact that a large amount of GP zones is precipitated and in-homogeneously dispersed in the Mg matrix of peak-aged TXA322 alloy. These fine-precipitates would act as the strong obstacles and effectively pin the movement of dislocations, which are beneficial for increasing the strength values and improving hardness levels [40]. In addition, a certain amount of Mg17Al12 phases are precipitated in Mg matrix of the peak-aged TXA329 alloy. The numerous Mg17Al12 precipitates would provide extra resistances for the dislocation movements, and accordingly the strength can be largely enhanced [41]. In TXA324 alloy, on the other hand, there is no obvious nano-precipitations after aging treatment, and consequently no change of strength is observed [42].

Generally, the weak fiber texture existed in the as-extruded alloys can benefit for the elongation improvement of present Mg-Sn-Ca-Al alloys. In present as-extruded TXA322, TXA324 and TXA329 alloys, the fiber texture intensity has been decreased as low as 2-4 mrd., which means that the grain orientations have been more randomly distributed. In this sense, more slipping modes can be activated and more compatible deformation can be achieved, and consequently the high elongation can be obtained [43].. After aging treatment, the elongations of the Mg-Sn-Ca-Al alloys have been slightly decreased, which is mainly related to the growth of DRXed grains and numerous nano-precipitations.

4. Conclusions

The microstructural and mechanical properties of Mg-2.5Sn-1.5Ca alloys with various concentrations of Al addition have been investigated in both the as-extruded and as-aged conditions. Some conclusions can be drawn as follows:

(1) The grain sizes of as-extruded TXA322, TXA324, TXA329 alloys have been refined to ∼ 16 μm, ∼ 10 μm and ∼ 12 μm, respectively. A number of <a> and <c+a> dislocations can be observed in the as-extruded samples. GP zones are evidently identified in TXA322 alloy, while only a small number of Mg17Al12 phases exist in both TXA324 and TXA329 alloys.

(2) An aging treatment facilitates the precipitation of a high number density of GP zones within the Mg matrix of the TXA322 alloy. In contrast, no obvious nano-precipitates are formed in the TXA324 alloy. Numerous nano-precipitations of Mg17Al12 phases occur in TXA329 alloy during the following aging treatment.

(3) The age-hardening responses in TXA322 and TXA329 alloys are remarkable. Hardness increases from the initial values of∼ 70 HV and ∼ 79 HV in as-extruded TXA322 and TXA329 alloys to the peak-aged hardness values of ∼ 84 HV and ∼ 95 HV, respectively. However, a minor increase of hardness is obtained in the TXA324 alloy.

(4) In comparison with the as-extruded counterparts, the apparent UTS increments of ∼ 46 MPa and ∼ 40 MPa are produced in the peak-aged TXA322 and TXA329 alloys. In contrast, no obvious variation in UTS value is found in the peak-aged TXA324 alloy.

Acknowledgements

This work was supported financially by the National Key Research and Development Program of China (Nos. 2016YFB0301105 and 2016YFB0701200), the National Natural Science Foundation of China (Nos. 51701211, 51971053 and U1610253), the Fundamental Research Funds for the Central Universities (No. N170204011) and the Fund of the state Key Laboratory of Solidification Processing in NPU (No. SKLSP201920).


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