Journal of Materials Science & Technology  2019 , 35 (8): 1681-1690 https://doi.org/10.1016/j.jmst.2019.04.004

Orginal Article

Temperature-gradient induced microstructure evolution in heat-affected zone of electron beam welded Ti-6Al-4V titanium alloy

Shilin Zhangab, Yingjie Maab*, Sensen Huangac, Sabry S. Youssefab, Min Qiab, Hao Wangab, Jianke Qiuab, Jiafeng Leiab*, Rui Yangab

a Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
b School of Materials Science and Engineering, University of Science and Technology of China, Shenyang 110016, China
c School of Materials and Engineering & Key Laboratory for Anisotropy and Texture of Materials, Northeastern University, Shenyang 110819, China

Corresponding authors:   *Corresponding authors at: Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China.E-mail addresses: yjma@imr.ac.cn (Y. Ma), jflei@imr.ac.cn (J. Lei).*Corresponding authors at: Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China.E-mail addresses: yjma@imr.ac.cn (Y. Ma), jflei@imr.ac.cn (J. Lei).

Received: 2019-01-4

Revised:  2019-03-18

Accepted:  2019-03-19

Online:  2019-08-05

Copyright:  2019 Editorial board of Journal of Materials Science & Technology Copyright reserved, Editorial board of Journal of Materials Science & Technology

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Abstract

The heat-affected zone (HAZ) of electron beam welded (EBW) joint normally undergoes a unique heat-treating process consisting of rapid temperature rising and dropping stages, resulting in temperature-gradient in HAZ as a function of the distance to fusion zone (FZ). In the current work, microstructure, elements distribution and crystallographic orientation of three parts (near base material (BM) zone, mid-HAZ and near-FZ) in the HAZ of Ti-6Al-4V alloy were systematically investigated. The microstructure observation revealed that the microstructural variation from near-BM to near-FZ included the reduction of primary α (αp) grains, the increase of transformed β structure (βt) and the formation of various α structures. The rim-α, dendritic α and abnormal secondary α (αs) colonies formed in the mid-HAZ, while the “ghost” structures grew in the near-FZ respectively. The electron probe microanalyzer (EPMA) and electron back-scattered diffraction (EBSD) technologies were employed to evaluate the elements diffusion and texture evolution during the unique thermal process of welding. The formation of the various α structures in the HAZ were discussed based on the EPMA and EBSD results. Finally, the nanoindentation hardness of “ghost” structures was presented and compared with nearby βt regions.

Keywords: Ti-6Al-4V ; Electron beam welding ; HAZ ; Element distribution ; Texture

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Shilin Zhang, Yingjie Ma, Sensen Huang, Sabry S. Youssef, Min Qi, Hao Wang, Jianke Qiu, Jiafeng Lei, Rui Yang. Temperature-gradient induced microstructure evolution in heat-affected zone of electron beam welded Ti-6Al-4V titanium alloy[J]. Journal of Materials Science & Technology, 2019, 35(8): 1681-1690 https://doi.org/10.1016/j.jmst.2019.04.004

1. Introduction

Titanium alloys have many advantages such as high specific strength, wear resistance and excellent corrosion resistance. For decades, titanium alloys have been considered to be one of the most comprehensive materials that could be widely used in aerospace and marine engineering. Particularly, Ti-6Al-4V takes up more than 50% of titanium alloys market and is considered to be the most successful commercial titanium alloy [[1], [2], [3], [4], [5]]. Titanium alloys can be joined by numerous welding techniques such as gas tungsten arc welding (GTAW), laser beam welding (LBW) and electron beam welding (EBW) [[6], [7], [8], [9]]. The high-vacuum EBW technique can drastically avoid the defects induced by the chemical reaction of alloys with atmospheric gases during the welding process [[9], [10], [11]]. Moreover, the EBW process needs no filler metal and can yield deep penetration due to its high energy density and low heat input, thereby inhibiting the extension of the heat-affected zone (HAZ) and decreasing the possible deformations, welding residual stress and stress-induced internal crack. These advantages make the EBW process a reliable and efficient alloy-joining method for titanium alloys [[10], [11], [12], [13]].

The microstructural evolution and phase transformation during the EBW process along with the effect on the mechanical properties are crucial issues that influence the employment of the weldment. Many investigations [[14], [15], [16], [17]] revealed that the microstructural heterogeneity of Ti-6Al-4V EBW joint and the presence of martensite or fine α lamellas in the fusion zone (FZ) contributed to the remarkable variety of the mechanical properties between the base material (BM) and the weld joint. These factors resulted in the improved tensile properties and high-cycle-fatigue (HCF) behavior but the weakened impact toughness and low-cycle-fatigue (LCF) behavior. In order to get a proper balance between strength and plasticity, many investigations [[18], [19], [20], [21]] concentrated on the microstructure optimization in the FZ through suitable post-heat-treatment to eliminate the unstable martensite, the metastable β phase and coarsen fine α lamellas. Recent reports [22,23] found that as the connection zone between the BM and FZ, the HAZ also played a remarkable role in the mechanical behaviors due to some unique structures inside it. Compared with the FZ and BM, the HAZ of Ti-6Al-4V welded joint had the smallest magnitude of the fatigue crack growth path in a zigzag pattern and showed the highest fatigue crack growth rate [22]. Although Ti-6Al-4V weldment presented the ductile fracture pattern in the tensile test, the dimples were small and shallow in HAZ, which indicated that HAZ had the lowest toughness [23].

The HAZ of EBW joint normally undergoes a unique heat-treating process consisting of rapid temperature-rising and dropping stage, inducing the temperature gradients in HAZ as a function of the distance to FZ. Numerous studies have argued that the unique heat-treating process of HAZ leads to nonuniform microstructure and some special microstructure characterizations. For example, the “ghost” structures which usually show different α lamella sizes compared with the secondary α lamellas, were once primary α grains before welding and then lived through the β phase temperature field during the welding process, but time was not sufficient to reach chemical equilibrium [24]. However, rare literature on the microstructural evolution inside the HAZ has been systematically reported. Therefore, the aim of the present work is to offer an elaborate investigation on the microstructural evolution, alloying element distribution and orientation variation in the HAZ of Ti-6Al-4V EBW weldment. Also, the nanoindentation hardness of the “ghost” structures is compared with that of the nearby βt regions.

2. Experimental

The Ti-6Al-4V titanium plate, 30 mm in thickness, was used as the base material for the EBW weldment after rolling and annealing process. The main chemical composition of the Ti-6Al-4V plate was 6.18% Al, 4.06% V, 0.19% Fe, 0.20% O and Ti on balance (in weight percent). The beta-transformation temperature (Tβ) of the Ti-6Al-4V plate was approximately 980 ± 5°C. As shown in Fig. 1, the microstructure of the Ti-6Al-4V plate was mainly composed of primary α (αp) grains with a volume fraction of 80-85% and intergranular β platelets. The EBW process was performed at beam voltage of 150 kV, beam velocity of 5 mm/s and beam current of 60 mA. After the EBW process in a vacuum environment, specimens were cut from the weldment of the investigated surface perpendicular to the weld line (Fig. 2). The specimens were ground, polished and eroded in Kroll's reagent (1%HF, 2.5%HNO3, 96.5%H2O) before the scanning electron microscope (SEM) observation under TESCAN MAIA3. The alloying elements distribution in HAZ was analysed by the electron probe microanalyzer (EPMA) performed on an EOL JXA-8530 F system with a space resolution of 800 nm. The electron back-scattered diffraction (EBSD) analysis was performed by TESCAN MAIA3 with a field-emission source of 20 kV and a 150 ∼ 300 nm step size. The analysis of the related EBSD data was carried out with the HKL-Channel 5 software. The nanoindentation hardness of local regions in HAZ was tested by Nano Identer G200 nanoindentation device.

Fig. 1.   SEM morphology of the as-received Ti-6Al-4V alloy showing the equiaxed microstructure consisting of αp and intergranular β.

Fig. 2.   Scheme of the as-received Ti-6Al-4V plate weldment.

3. Results and discussion

Three parts of the Ti-6Al-4V EBW weldment, i.e., FZ, HAZ and BM, are shown in Fig. 3a. The microstructure of FZ is shown as the typical Widmanstatten structure that prior coarse β grains are studded by a large number of plate-like α lamellas and separated by continuous grain boundary (GB) α layers (Fig. 3b).

Fig. 3.   Macrostructure of Ti-6Al-4V alloy EBW weldment (a) and SEM image of FZ (b).

3.1. Microstructure observation in HAZ

Due to the temperature gradient in HAZ and its effect on microstructural heterogeneity, the HAZ is artificially divided into three different zones in the current work: near-BM, mid-HAZ and near-FZ (Fig. 4). The microstructural heterogeneity is mainly caused by the inhomogeneous thermal process during the welding process, i.e., the peak temperature increases from a relatively low (α + β) phase temperature to the temperature above the Tβ as getting close to the FZ position.

Fig. 4.   SEM images of HAZ of Ti-6Al-4V EBW weldment at different positions: (a) near-BM, (b) mid-HAZ, (c) local magnification of mid-HAZ, (d) near-FZ, showing various α structures (including αp, rim-α, dendritic α, αs colony, abnormal αs and “ghost” structure) indicated by the arrows.

Near-BM is relatively far from the FZ so its microstructure is almost similar to the BM. The diversification of microstructure in Near-BM (Fig. 4a) is shown as the formation of transformed β structure (βt) after the thermal welding process with fine intergranular β laths among the neighbouring αp grains. The volume fraction of αp shows a prominent decrease to 60-65% compared with the 80-85% of BM.

Mid-HAZ is located between near-BM and near-FZ, and the volume fraction of βt in this region rises obviously. Fig. 4b shows the presence of dendritic α extending from the edge of the αp grain, which is generally believed to be related to the interface disturbance mechanism [25]. Rims around the αp grains are also observed. The appearance of rim-α in titanium alloy has been reported in previous literature only under furnace cooling condition [26], while this study should be the first report of rim-α in HAZ of the weldment. It is found that there are two kinds of secondary α (αs) colonies in the mid-HAZ (Fig. 4c). Generally, the αs colonies originate inside the βt and show a clear β interface between αs colonies and αp grains. Meanwhile, minor αs colonies are likely to grow from the margin of the αp grains and there is no interface between αs colonies and αp grains, which are defined as “abnormal αs” in this study.

In near-FZ, it shows the appearance of only αs colonies studded in β matrix, while the αp grains have been totally consumed (Fig. 4d). The “ghost” structures mainly consists of αs colonies and the traces of αp grains are kept in the middle of the “ghost” structure. The disappearance of αp grain in near-FZ confirms that the peak temperature has exceeded the Tβ point during the welding process.

These findings suggest that the micromorphology characterization of the HAZ mainly includes the reduction of primary α (αp) grains, the increasing volume fraction of βt and the formation of various types of α structures (rim-α, dendritic α, αs colony, abnormal αs and “ghost” structures). The formation process of different types of α structures will be discussed in the following chapters.

3.2. Alloying elements distribution in HAZ

It is well known that the phase transformation, accompanying with alloying element partitioning during (α + β) thermal treatment, has significant influences on phase composition and mechanical behaviors of Ti-alloys [[26], [27], [28], [29]]. This study analyses the alloying elements distribution in HAZ by EPMA to reveal the element partitioning behaviors during the welding thermal process. The elements (Al and V) distributions of the three regions are shown in Fig. 5. In near-BM (Fig. 5a), it can be seen that Al (α stabilizing element) tends to concentrate in α phase while V (β stabilizing element) prefers to concentrate in the βt. As approaching to the mid-HAZ and near-FZ (Fig. 5b and c), the concentration gradient of Al (or V) in different regions decreases dramatically compared to the near-BM position. The thermal input during the welding process will increase the temperature even above the Tβ point and then promote the αp→β transformation and atoms diffusion, which will decrease the concentration gradient of Al (or V) inside the α phase or βt synergistically.

Fig. 5.   Constituent distribution of alloying elements of near-BM (a), mid-HAZ (b) and near-FZ (c) in Ti-6Al-4V EBW weldment, the left, middle and right column showing the morphology, element Al and V distribution respectively.

The line scanning maps by EPMA (Fig. 6) present a supplementary illustration of V (or Al) partitioning behavior in HAZ. In near-BM (Fig. 6b), the concentration of V shows an inhomogeneous distribution which increases exponentially with the increasing distance from the middle position of the αp grains, while the concentration variation of Al has an inverse tendency. The amplitude of V concentrating variation is obviously larger than that of Al, which could be attributed to the higher solid solubility of Al in both two regions compared with the low solubility of V in α phase [27]. As shown in Fig. 6c and d, the appearance of rim-α in mid-HAZ could be related to the V distribution (the platform pointed by the arrow in Fig. 6d). During the welding process, it is found that the peak temperature in the mid-HAZ is located in (α + β) phase region and the βt is coarsened by dissolving the adjacent αp grains. Thus, V atoms inside the interior of both αp grains and βt are subjected to diffuse towards the α/β interface, inducing the enriched concentration of V atoms at the interface. As shown in Fig. 6e and f, the αs colonies and the trace of αp grain in near-FZ show different V and Al concentration compared with the nearby βt region. This reveals that the rapid temperature rising and dropping stages during the welding process have promoted the complete αp→β→αs transformation in near-FZ, but the process time is insufficient for V or Al atoms diffusing to the chemical equilibrium.

Fig. 6.   Line scanning consequences of Al and V distributions of near-BM (a,b), mid-HAZ (c,d) and near-FZ (e,f) position in Ti-6Al-4V EBW weldment, the blue arrows showing the scanning routes.

3.3. Crystal orientation distribution

The rolling process of the Ti-6Al-4V plate in (α + β) phase region generates the deformed {10-10}<0001> texture of αp grains as shown in Fig. 7a and b. The grain boundary distribution of αp (Fig. 7d and e) shows a plenty of low-angle grain boundaries (LAGB) which are attributed to the high-temperature recovery during the annealing treatment of the plate. In contrast, a low fraction of high-angle grain boundaries (HAGB) distributes randomly among different αp grains. The high-temperature recovery redistributes the dislocations to generate the LAGB inside the αp grains.

Fig. 7.   EBSD analysis of BM in Ti-6Al-4V EBW weldment: (a) the morphology of corresponding scanning region, (b) IPF map, (c) pole figures of α phase, (d) grain boundary distribution of αp and (e) misorientation statistics of αp grains.

Fig.8 illustrates the EBSD analysis of near-BM and mid-HAZ. Compared with the BM position, the orientations of αs colonies and the neighbouring αp grains are different, which indicates that the nucleation mode of the αs colonies corresponds to the sympathetic nucleation [30,31] (Fig. 8b). The pole figures (Fig. 8c) show the exact orientation of α phase that the {0001} and <11-20> of αs are parallel to the {110} and <111> of β matrix. This is known as the Burgers orientation relationship (BOR) between the αs colonies and β phase [[32], [33], [34], [35], [36]]. The residual αp grains shown in Fig. 8a still present the orientation in accord with the typical rolling texture, illustrated by locations of ellipses in Fig. 8c. Fig. 8d shows that the grain boundary distributions of α grains (including the residual αp grain, dendritic α and αs colonies) are HAGB mainly distributing around 60° and 90°, while the high fraction of LAGB is mainly attributed to the interface of αs lamellas in the single α colony. In the undeformed titanium alloys with lamellar microstructure, the misorientations between neighbouring α colonies are dominated by 10°, 60° and 90° due to BOR during the β→α phase transformation [37,38]. Compared with the BM (Fig. 7e), the reduction of the GB misorientation from 10° to 50° in Fig. 8e indicates the decrease of αp grains, while two peak values around at 60° and 90° indicate that the volume fraction of αs lamellas is increased.

Fig. 8.   EBSD analysis of near-BM and mid-HAZ in Ti-6Al-4V EBW weldment: (a) the morphology of corresponding scanning region, (b) IPF map, (c) pole figures of α phase, (d) grain boundary distribution of α and (e) misorientation statistics of α grains.

The dendritic α phase, rim-α phase and abnormal αs colonies inside the mid-HAZ (the three red ellipses in Fig. 8a) are analysed in details. As shown in Fig. 9a, the cumulative misorientation along the green line crossing the αp grain and the rim-α is around 1°, which proves that the rim-α is a part of αp grains with different element concentration and excludes the phase transformation effect.

Fig. 9.   Cumulative misorientation in rim-α (a) and dendritic α (b) in the mid-HAZ.

As shown in Fig. 9b, the cumulative misorientation along the green line crossing the αp grain and the dendritic-α is also around 1°. Two possible processes, the interface disturbance mechanism and the LAGB effect, concerning the formation of dendritic α are considered: (1) During the continuous cooling from (α + β) phase temperature, αp grains tend to coarsen without remarkable driving force for β→αs transformation [26]. As αp grains coarsening, plenty of protuberances appear at the α/β interface due to the interface disturbance, however, the major protuberances are inhibited due to the increasing phase interfacial energy [25]. According to the Zener-Hillert function, few protuberances can break the restriction of the phase interfacial energy and finally evolve to be dendritic α [39]. (2) The LAGB offers an efficient path for atom diffusion and phase transformation during thermal cycling process of EBW. So the β laths grow preferentially along the LAGB into αp grain resulting in the appearance of dendritic α.

The abnormal αs in the mid-HAZ is individually presented and analysed in Fig.10. The α lamellas in abnormal αs colony seem to be wider than that in the normal αs colony. According to the IPF map (Fig. 10b), this region mainly consists of a small part of residual αp grain (blue), two large parts of the abnormal αs colonies (yellow) and few residual β phase (red). The pole figures in Fig. 10c confirm that blue locations are the residual αp grain and to be in accordance with the typical rolling texture. The BOR between abnormal αs colonies and β phase in the shape of the rectangle indicates that the abnormal αs colonies are transformed from the high-temperature β phase. Based on the sympathetic nucleation of α lamella [31], the formation of abnormal αs may be inferred as follow: After the rapid temperature-rising process, the interior region of αp grains is reserved, and the abnormal αs primarily nucleates in the β close to the pre-cursory α/β interface. The high (or low) concentration of Al (or V) in β region (Fig. 5), induced by the insufficient diffusion time, will facilitate the growth of α lamellas by consuming the nearby βt. The abnormal elements distribution results in the large size of α lamellas in abnormal αs during β→α phase transformation, with no β phase reserved at the interface of abnormal αs and αp grain.

Fig. 10.   EBSD analysis of abnormal αs in mid-HAZ: (a) morphology of the scanning region, (b) IPF map and (c) pole figures of the corresponding region.

Fig. 11 illustrates the EBSD image analysis in near-FZ. The pole figure (Fig. 11c) shows the exact BOR between the secondary α phase and β phase. The misorientations of the different transformed αs colonies are mainly HAGB focusing around 60° and 90°. The reduction of the misorientation statistics between 60° and 90° indicates that major αp grains have been dissolved. The “ghost” structure in near-FZ is illustrated in Fig. 12. According to the crystallographic symmetry relationship of BCC structure, β phase has six {110}β crystallographic plane and two different <111> crystallographic directions in each {110} plane. Thus, a single β grain can theoretically generate 12 possible α variants during the β→α phase transformation [32]. The cumulative misorientation along the green line (Fig. 12b) crossing different αs variants matches the theoretical result between two random α lath variants in a single β grain [40]. This result also proves that the phase transformation of αp→β→αs has been experienced in near-FZ and the “ghost” structure mainly consists of different αs colonies, but the traces of αp grains still can be observed in the middle of the “ghost” structure. During the β→αs phase transformation, the driving force of αs precipitation is relatively higher due to the high concentration of Al and low concentration of V (Fig. 6e and f). Thus, the elements distribution in “ghost” structure improves both the formation possibility and the growth rate of αs phase to form the larger α lamellas compared with the normal αs.

Fig. 11.   EBSD analysis of near-FZ in Ti-6Al-4V EBW weldment: (a) the morphology of corresponding scanning region, (b) IPF map, (c) pole figures, (d) grain boundary distribution of α and (e) misorientation statistics of α grains.

Fig. 12.   EBSD analysis of “ghost” structure in near-FZ: (a) scanning region, (b) misorientation of α phase (2°<red<10°, 10°<blue<75°, 75°<black<90°), (c) IPF map and (d) misorientations along the green arrow in (b).

An important issue emerging from these findings is that the main microstructure evolution at HAZ in different phase areas is the phase transformation, αp→β→αs. However, the unique heat-treating process consisting of rapid temperature rising and dropping stage accompanying with the temperature gradients in HAZ forms various types of α phase with unique microstructure, which are summarized in the following Table 1. All of the distinct microstructures (including αp, rim-α, dendritic α, αs colony, abnormal αs and “ghost” structures) result in obvious inhomogeneity of the HAZ and may lead to a potential influence on the mechanical properties of EBW weldment, which deserves more concern in the future investigation.

Table 1   The summary of microstructure characterization of different α phase in HAZ of Ti-6Al-4V EBW weldment.

TypePositionV distribution
(rich: +)
Orientation relationshipFormation mechanism
αp grainsNear-BM
Mid-HAZ
--{1010}<0001> textureRolling
Rim-αMid-HAZ+To be identical with αp grainElements distribution
Dendritic αMid-HAZ+Protuberances evolution
αs colonyMid-HAZ
Near-FZ
+++BOR with β phase and independent to αp grainβ→αs
Abnormal αsMid-HAZ+
“Ghost” structuresNear-FZ-

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3.4. The nanoindentation hardness of local regions in HAZ

The nanoindentation results of “ghost” structures and nearby βt regions are shown in Fig. 13. The hardness in both regions decreases with the increasing θ, which indicates that the variation of hardness is obviously dominated by the anisotropy of the hcp α phase [41]. When the load direction is parallel to the c-axis of hcp-structure, i.e., θ is equal to 0°, the < c+a > slip should be activated. As θ increases from 0° to 90°, <a > slip takes part in the plastic deformation. The critical resolved shear stress (CRSS) of < a > slip is much lower than that of < c+a > slip, which results in the reduction of hardness [42]. Besides the influence of θ, the hardness of “ghost” structures is overall higher than that of the nearby βt regions. Two possible reasons are responsible for this: (1) the volume fraction of β slices between two neighbouring αs lamellas in “ghost” structures is much lower than that in nearby βt regions. The hardness of β phase is lower compared with α phase due to the lower CRSS of β phase [43]. (2) The Al concentration in “ghost” structures is higher than that in nearby βt regions. The increase of Al concentration contributes to enhancement of nanoindentation hardness [44].

Fig. 13.   Variations of nanoindentation hardness with the declination angle of the loading direction relative to the c-axis of HCP crystal in the Near-FZ zone.

4. Conclusions

The microstructure observation, elements distribution, crystallographic orientation and nanoindentation hardness of HAZ in Ti-6Al-4V EBW weldment were investigated. The findings of this study have a number of important implications as follows:

(1) During the rapid temperature rising and dropping process of welding, the main phase transformation in the HAZ is αp→β→αs process with the decreasing of αp and increasing of βt from near-BM to near-FZ. Various α structures, including the rim-α, dendritic α and abnormal αs colony in the mid-HAZ and the “ghost” structures in the near-FZ, have been generated in HAZ during the phase transformation process.

(2) Accompanying with the phase transformation, elements redistribution occurs during the thermal process in the three HAZ regions. The atoms diffusion will reduce the concentration gradients at the α/β interface especially in mid-HAZ and near-FZ. However, due to the rapid thermal process, the atoms diffusion is far from equilibrium state, which will result in the high Al concentration and low V concentration in the αs colonies (such as the abnormal αs colony and “ghost” structures).

(3) The rim-α in αp grains is attributed to the nonuniform distribution of V atoms inside the αp grains induced by the atoms diffusion at the α/β interface. The dendritic α may be evolved from the protuberance which survives in the coarsening process of αp grains during temperature dropping process of welding. Both rim-α and the dendritic α exhibit the identical crystallographic orientation with the interior αp, which indicates that the two α structures should be related to the epitaxial growth of αp grains and exclude the phase transformation effect.

(4) The abnormal αs colony and the “ghost” structures are both originated from β→αs transformation. The high (or low) concentration of Al (or V) in the two α structures facilitates the growth of αs, resulting in the large size of αs lamella. The newly generated αs lamellas in the abnormal αs colony and the “ghost” structures show the Burgers orientation relationship with β phase, and the misorientations of the αs lamellas are concentrated to 60° and 90° around.

(5) The nanoindentation hardness of “ghost” structures and nearby βt regions is obviously dominated by the anisotropy of the hcp α phase. The hardness of “ghost” structures is higher than that of the nearby βt regions possibly due to the low volume fraction of β phase and high Al concentration.

Acknowledgements

This work was supported by Strategic Priority Research Program of the Chinese Academy of Sciences (XDB06050100), Natural Key Research and Development Program of China (2016YFC0304201, 2016YFC0304206) and Natural Science Foundation of China (No. 51871225). The authors also would like to acknowledge Prof. Dongsheng Xu and Prof. Qingmiao Hu for the very useful discussions.

The authors have declared that no competing interests exist.


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