J. Cent. South Univ. (2021) 28: 1932-1945

DOI: https://doi.org/10.1007/s11771-021-4665-2

Effects of tensile temperatures on phase transformations in zirconium by molecular dynamics simulations

AN Ke-ying(安克滢), OU Xiao-qin(欧小琴), AN Xing-long(安星龙),ZHANG Hao(张灏), NI Song(倪颂), SONG Min(宋旼)

State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2021

Abstract: The effects of tensile temperatures ranging from 100 K to 900 K on the phase transition of hexagonal close-packed (HCP) zirconium were investigated by molecular dynamics simulations, which were combined with experimental observation under high resolution transmission electron microscopy. The results show that externally applied loading first induced the HCP to body-centered cubic (BCC) phase transition in the Pitsch-Schrader (PS) orientation relationship (OR). Then, the face-centered cubic (FCC) structure transformed from the BCC phase in the Bain path. However, the HCP-to-BCC transition was incomplete at 100 K and 300 K, resulting in a prismatic-type OR between the FCC and original HCP phase. Additionally, at the temperature ranging from 100 K to 600 K, the inverse BCC-to-HCP transition occurred locally following other variants of the PS OR, resulting in a basal-type relation between the newly generated HCP and FCC phases. A higher tensile temperature promoted the amount of FCC phase transforming into the BCC phase when the strain exceeded 45%. Besides, the crystal stretched at lower temperatures exhibits relatively higher strength but by the compromise of plasticity. This study reveals the deformation mechanisms in HCP-Zr at different temperatures, which may provide a better understanding of the deformation mechanism of zirconium alloys under different application environments.

Key words: zirconium; phase transformation; molecular dynamics simulation; deformation mechanism; tensile temperature

Cite this article as: AN Ke-ying, OU Xiao-qin, AN Xing-long, ZHANG Hao, NI Song, SONG Min. Effects of tensile temperatures on phase transformations in zirconium by molecular dynamics simulations [J]. Journal of Central South University, 2021, 28(7): 1932-1945. DOI: https://doi.org/10.1007/s11771-021-4665-2.

1 Introduction

Zirconium, as a Group IV transition metal, has been widely used in nuclear reactor core materials, aerospace materials and military special steels due to its extremely low thermal neutron absorption rate, preeminent heat resistance, amazing resistance to corrosion and radiation damage, and excellent mechanical properties [1, 2]. In the equilibrium phase diagram, zirconium is in a hexagonal close-packed (HCP) structure at room temperature and in a body-centered cubic (BCC) structure at a temperature above 1135 K [3, 4]. Specifically, HCP phase transforms into a unique hexagonal structure at room temperature at a pressure of 2.2 GPa [5, 6]. Since mechanical properties of metals are closely related to their phase composition, extensive attention has been paid to the mechanisms of solid-solid phase transitions [7-9]. For example, the strain-induced HCP-FCC phase transformation has been widely observed in metals and alloys, including hafnium [10, 11], zirconium [12, 13], titanium [14-17] and their alloys [18-20]. There are two different orientation relationships (ORs) forming between the HCP and FCC phases [17, 21-24]:1) the basal-type (B-type) OR represented by and 2) the prismatic-type (P-type) OR represented by and It is widely accepted that the B-type phase transition proceeds by the glide of Shockley partial dislocations on every second {0001} basal plane of the HCP structure [12, 13]. However, there are discrepancies regarding the mechanism of the P-type transformation in previous studies. HONG et al [15] reported that the P-type phase transition involves the glide of Shockley partial dislocations on everyprismatic plane of the HCP structure. ZHAO et al [12] put forward that the P-type transformation in zirconium is accomplished through the pure-shuffle and shear-shuffle mechanisms. First-principle calculation and experimental studies by YANG et al [17] indicated that the P-type phase transformation involves the slip of dislocations, adjustment and expansion of crystal lattices. In addition, externally applied strain may also lead to HCP twinning to coordinate deformation effectively [25]. For HCP zirconium, there are two types of twins [26, 27]: the tension twins represented by and twins, the compression twins represented byand twins.

In recent years, molecular dynamics (MD) simulation is proved to be a useful method to explore the microstructural evolution in a single crystal and the corresponding deformation mechanism at the atomic scale [28-35]. For example, previous investigation [36] found a P-type transition of the HCP-Ti nanocrystalline column by MD simulations. ISLAM et al [37] studied the effect of tensile strain directions on the phase transformation of zirconium film by MD method. In this study, MD simulation was used to study the effect of temperature on the microstructure and mechanical performance of a single zirconium crystal during uniaxial tension along the direction. The pathways of phase transformations in pure Zr were discussed on the basis of the MD simulations, which were further compared with experimental observation by high resolution transmission electron microscopy (HRTEM) from the atomic scale. It is expected that the present study will contribute to a better understanding of the deformation mechanisms of zirconium and its alloys in practice.

2 Modelling and experimental methods

In the present work, the interaction between Zr-Zr atoms was described using the embedded atomic method (EAM) potential (#2) developed by MENDELEV et al [38]. This EAM potential used in the present study can properly describe the lattice parameters, cohesive energy, melting temperature and other physical characteristics of the BCC, FCC, HCP phases in zirconium. Besides, the EAM potential has been used to study the phase transformation in Zr by MD simulations in previous studies [39, 40]. The system consists of a single HCP crystal including 864000 atoms. The box size equals 19.46 nm×33.49 nm×31.27 nm in the x-axis,y-axis and z-axis, which are parallel to the directions, respectively. The initial configuration was first relaxed in a vacuum environment at a constant temperature (100, 300, 600 and 900 K) in the NPT ensemble (or barostat and thermostat of the Nosé-Hoover type) for 10000 steps (equaling 5 ps). After relaxation, the HCP crystal was stretched along the x direction under vacuum condition. The strain rate equaled 0.01 ps^-1. The temperature was fixed at 100, 300, 600 and 900 K, respectively. Each simulation lasted for 50 ps. Periodic boundary conditions in x, y and z directions were applied in all MD simulations in this study. The adaptive common neighbor analysis (a-CNA) was applied to understand structural changes during tensile deformation [41]. The OVITO software was used to visualize MD configurations [42]. All MD simulations were performed using the open-source LAMMPS code [43].

For the experimental observation under HRTEM, the material used in this work was high purity Zr (>99.9%), which has an average grain size of around 50 μm. A small bar with dimensions of 50 mm×20 mm×5 mm was cut from the raw material and then annealed at 923 K for 2 h. Then, the bar thickness was reduced by 60% through cold rolling at 298 K. The samples for HRTEM test were cut from the cold-rolled specimen and mechanically grinded until a thickness of 40 μm, followed by electro-polishing. The HRTEM observations were performed using a Titan G2 60-300 Cs-corrected TEM operated at 300 kV.

3 Results and discussion

3.1 Microstructural evolution and mechanical performance

The microstructural evolution of the HCP-Zr single crystal deformed along the x direction and at four different temperatures is shown in Figure 1. The simulation results show that phase transformations took place inside the parent HCP phase under a uniaxial tensile loading at varied temperatures. Figures 1(a) and (b) show that intersecting BCC bandings appeared in the HCP crystal at the temperatures of 100 and 300 K, respectively, when the strain ε equaled 10%. With the strain increasing to 20%, both the BCC-to-FCC and the inverse BCC-to-HCP phase transitions occurred simultaneously. The FCC structure grew gradually until obtaining a single FCC crystal with a few stacking faults as the strain reached 50%. No significant differences existed between the two systems, except for more unidentified atoms in the HCP crystal at 100 K than that at 300 K in the initial stage of deformation. The two stepwise HCP→BCC→FCC and HCP→BCC→HCP phase transformations were also observed in the HCP-Ti crystalline during tension along the direction by MD simulations [44].

When the tensile temperature increased to 600 K, the trend of microstructural evolution in the system was in overall similar to that at 100 and 300 K during the tensile deformation, as seen from Figure 1(c). However, there were more BCC atoms transformed from the matrix HCP phase at 600 K at a strain of 20%. When the temperature was as high as 900 K, it can be seen from Figure 1(d) that almost all matrix HCP structure transformed into the BCC phase at a strain of 20%. When the strain increased to 30%, however, the BCC structure transformed to the FCC phase with minor stacking faults forming locally in the system at 900 K. Part of the FCC structure transformed back to the BCC structure with significant texture feature, which can coordinate the plastic deformation in the late deformation stage.

Figure 1 Phase change in HCP crystal at tensile temperature:(The x, y and z axes of the simulation cells are paralleling to the and [001]_HCP directions of the HCP crystalline, respectively. Atom colors correspond to different phases: red-HCP, green-CC, blue-BCC, white-unrecognized)

By comparing the microstructure evolution of HCP-Zr at different temperatures, it can be found that the number of BCC atoms increases with the increase of temperature at the initial deformation stage. Previous studies [45, 46] reported that the metal zirconium exists in an HCP structure at room temperature and in a BCC structure when the temperature rises above 1135 K but below the melting point. It indicates that the BCC phase is stable at high temperature, while the HCP phase is stable at low temperature. Therefore, it is reasonable that more BCC atoms were obtained at higher temperatures in the early deformation stage in the current MD simulations. In addition, the high-temperature stability of the BCC phase also determines that it is difficult for its transformation back to the HCP phase at higher temperatures.

3.2 Deformation paths

From the above analysis, it is seen that the microstructural evolution of the HCP crystal at low tensile temperatures of 100 and 300 K was different from that deformed at 600 and 900 K. The following section discussed in detail the corresponding deformation mechanisms of the HCP crystal at varied temperatures, specifically, at low temperature (100, 300 K), intermediate temperature (600 K) and high temperature (900 K).

At low temperatures (100 K, 300 K), externally applied loading first induced the phase transition from the HCP to BCC structure in a intersecting banding shape in the Pitsch-Schrader (PS) relation [47]: (0001)_HCP||(110)_BCC and (see Figure 2(a)). Then, FCC structure was generated within the BCC banding junctions in the Bain path [48]: (001)_BCC||(100)_FCC, (see Figure 2(b)). As tension continued, the BCC bands were absolutely occupied by the FCC phase. This led to the formation of the HCP/FCC interface at the contacting sites between the FCC phase and HCP matrix (defined as “HCP-M”). It is interesting to note the OR of the HCP-M/FCC interface, which is indicated by and [0001]_HCP|| [001]_FCC in Figure 2(c). Figure 2(d) is an HRTEM image showing the interface between the HCP matrix and FCC lamellae in the cold-rolled Zr. The distances of and (110)_FCC crystal planes measured from the HRTEM image are ~2.80 A and ~1.67 A, respectively, which agree with their corresponding lattice parameters reported in Refs. [13, 49]. Based on the HRTEM result, the lamellae are in the FCC phase and the ORs between the HCP matrix and FCC lamellae obey the P-type OR: and which conforms to the MD simulation result observed in Figure 2(c). Besides, this P-type relation was also reported in previous experimental studies [10, 12, 15-17]. According to the MD simulation results, the P-type interface formed between the FCC phase and HCP matrix. Nonetheless, the FCC phase is not directly converted from the HCP but from the parent BCC phase. Therefore, the product FCC lamellae inherited the banding morphology of the parent BCC phase.

Figure 3 presents a schematic pathway of the serial HCP-BCC-FCC transitions. The HCP-BCC transition includes two parts: 1) the HCP lattice (a_HCP=3.220 A) is extended by 10.6% along the x-direction, compressed along the y-and z-[0001]_HCP directions by 9.7% and 3.4%, respectively; 2) atoms on every other (0001)_HCP plane move along the direction for a displacement d=1/3 With the externally applied tensile loading rising, the BCC structure (a_BCC=3.562 A) continues to transform into an FCC structure (a_FCC=4.545 A), which involves an expansion in the x direction by 27.6 % and a shrink by 9.8% both in the y and z directions [48]. Correspondingly, the angle between theand directions changes from θ_HCP=60° to θ_BCC=70.5° and eventually to θ_FCC=90°.

In previous studies [27, 50, 51], it is proposed that the formation mechanisms for deformation twinning involve the glide of partial dislocations or pure-shuffle mechanism. However, different twinning mechanism was found based on the MD simulation results at the atomic level. In addition to the BCC-FCC transition, the inverse BCC-HCP transition occurred but followed varied PS variants, because the BCC lattice is highly symmetrical. Thus, it resulted in the formation of diversely oriented HCP grains, i.e., the newly generated HCP-N1 and HCP-N2 grains at 300 K, as indicated in Figures 4(a) and (b). The ORs between HCP structures with varied crystallographic orientations and the parent BCC phase are presented by: 1) (0001)_HCP-M||(110)_BCC, 2) 3) The angle between either two of the (110)_BCC, and (011)_BCC planes is 120°. Therefore, the angles between their corresponding paralleling (0001)_HCP-M, (0001)_HCP-N1 and (0001)_HCP-N2 planes are also 120°, resulting in a twinning relationship between either two of the HCP-M, HCP-N1, HCP-N2 grains, as shown in Figure 4(c). This formation mechanism of the twinning was also found in the deformed titanium crystalline by MD simulations in previous studies [44]. The HRTEM image of the twinning structure in the cold-rolled pure Zr is illustrated in Figure 4(d).

Figure 2 (a) Snapshot showing PS OR at ε=9.55% with view direction perpendicular to plane at 300 K;(b) Snapshot showing Bain OR at ε=9.85% with view direction perpendicular to (0001)_HCP plane at 300 K; (c) Snapshot showing P-type relation between HCP matrix and FCC phase at ε=14% with view direction perpendicular to (0001)_HCP plane at 300 K; (d) HRTEM image of P-type OR between HCP and FCC structures in cold-rolled zirconium at 298 K

Figure 3 Schematic showing HCP-BCC-FCC phase transition path

From Figures 5(a) and (b), it is seen that both the newly generated HCP (defined as “HCP-N”) and FCC phases grew by consuming the BCC phase until they eventually contacted to form HCP-N/FCC interface in the B-type relationship: (0001)_HCP-N||(111)_FCC, Then, the FCC structure developed into the newly generated HCP phase via the slipping of Shockley partial dislocations, as reported in previous experimental studies [11, 14]. The transition mechanism was presented in Figures 5(a)-(c): the slip of Shockley partial dislocations on every other {0001} basal planes changes the original … ABABAB …stacking sequence for the HCP structure into the … ACBACB …stacking sequence for the FCC structure. The HRTEM image in Figure 5(d) gives an example for the commonly observed B-type FCC/HCP interface in the cold-rolled Zr by experiment.

The microstructural evolution and the corresponding mechanisms for the system stretched at 600 K were similar to that deformed at low temperatures (100 and 300 K). However, it is worth noting that the FCC structure transformed into BCC structure following the Nishiyama-Wassermann (NW) OR [52] ((011)_BCC||(111)_FCC when the strain exceeded about 45%, as shown in Figure 6(a).

Figure 4 (a, b) Snapshots showing PS ORs between new-forming HCP (HCP-N1, HCP-N2) and parent BCC structure at ε=13.15% and 300 K, respectively; (c) Snapshot showing twins at ε=13.15% and 300 K; (d) HRTEM image of twins in cold-rolled zirconium at 298 K

Figure 5 (a-c) Snapshots showing mechanism of phase transition from HCP-N to FCC structure following B-type OR at a strain of 22.5%, 34.0% and 41.7 %, respectively, at 300 K; (d) HRTEM image of B-type OR between HCP and FCC structures in cold-rolled zirconium at 298 K

At the temperature of 900 K, the deformation path of the HCP matrix was different from that deformed at lower temperatures (100, 300 and 600 K). In the early deforming stage, the parent HCP phase transformed completely into the BCC structure following the PS OR. Thus, a single BCC crystal was obtained, which then transformed into a single FCC crystal following the Bain path. When the strain exceeded 45%, the BCC phase nucleated in the FCC structure but in the NW OR ((011)_BCC||(111)_FCC,and the Kurdjumov-Sachs (KS) OR [53] ((011)_BCC||(111)_FCC, to further accommodate deformation, as shown in Figure 6(b). The details related to the FCC-BCC transformation were discussed in another paper [54].

To sum up, phase transformations that took place in the HCP-Zr crystal at temperatures from 100 to 900 K were summarized in Figure 7. At low temperatures (T=100, 300 K), the HCP matrix (HCP-M) first transformed into the BCC structure obeying the PS relation. Then, the BCC-FCC transition occurred following the Bain path. Simultaneously, the inverse BCC-HCP transition occurred following varied variants of the PS OR (i.e., HCP-N1, HCP-N2), as shown in Figure 6(a). As the uniaxial tension continued, the FCC/HCP-M interface formed in a P-type relation while the FCC/HCP-N interfaces obeyed a B-type OR. Additionally, the new-forming HCP structures (HCP-N1, HCP N2) and the original HCP crystal were in a twinning relation with each other. Deformation path of the HCP crystal at a medium temperature (T=600 K) was similar to that at lower temperatures (T=100, 300 K), as shown in Figure 7(b). The difference is that the FCC structure transformed into the BCC structure following the NW OR in the later stage. At a high temperature (T=900 K), the HCP matrix first transformed into a single BCC crystal in the PS OR and then into a single FCC crystal in the Bain OR. As the externally applied loading increased, the FCC phase transformed back into the BCC phase but in the NW and KS ORs, as shown in Figure 7(c).

Figure 6 Snapshot showing:

Figure 7 Deformation mechanisms at tensile temperature of:

3.3 Tensile property

Figures 8(a)-(d) indicate the evolution of the phase compositions of the system deformed at 100, 300, 600 and 900 K, respectively. It can be seen that phase transition started at different strains. A higher tensile temperature induced the start of phase transition at a smaller strain. Figure 8(e) shows the stress-strain curves of four systems, which first experienced an elastic deformation stage and then a plastic deformation stage. During the elastic deformation stage, the stress increases linearly with the strain. The HCP-BCC phase transition led to the softening of the HCP crystal. The softening mechanism by phase transformation was also reported in other metals, such as iron and titanium, in previous studies [44, 55-58]. With the increase in tensile temperature, the yield stress deceases. The strength yields at the highest strain of 7.7% at 100 K and at the smallest strain of 5.15% at 900 K. The HCP-BCC phase transformation occurs when the internal stress induced by severe lattice distortion combined with high local strain effect exceeds the critical stress required for the nucleation of BCC phase in the matrix [59]. It was reported in previous MD simulations that tensile temperatures affect the phase transformation path in pure iron as well [55]. In order to transform to a product phase, the parent phase needs to surpass an activation energy that decreases with temperature [55, 59]. Thus, it can be referred that the onset of the HCP-BCC phase transformation in Zr in the present study requires a lower driving force at a higher temperature due to the lower energy barrier.

Figure 8 (a-d) Evolution of phase percentages for systems at tensile temperatures of 100, 300, 600 and 900 K, respectively; (e) Stress-strain curves; (f) Dislocation length in systems at temperatures of 100, 300, 600 and 900 K

When the externally applied strain exceeds the yield strain, the stress of the four curves suddenly drops due to the onset of the HCP-BCC phase transition. It is noted that the stress at different temperatures all experiences a temporary platform when the strain increases up to about 10%. Besides, it is found in Figures 8(a)-(c) that the a-CNA curves also undergo a platform at the corresponding strain point. The reason for the emergence of those platforms is different from that in previous investigations [30, 60], where the twinning fibers form due to a substantial work hardening and a twinning-induced plasticity effect. In the present paper, such platform periods may correspond to an incubation time that is necessary for the nucleation of a new structure (HCP or FCC) in the BCC bandings [55, 59]. In the plastic deformation stage, the stress at different temperatures fluctuates due to the joint effect of dislocation generation and phase transitions between the HCP, FCC and BCC structures in the crystal. Specifically, for the tensile temperature of 900 K, the BCC-to-FCC phase transition started and completed at a strain of 12% and 35% (marked by “A” and “B” in Figure 8(d)), respectively; then the inverse FCC-to-BCC phase transition occurred at strain ε ≈ 47% (marked by “C” in Figure 8(d)). Those three points correspond to the three “turning points” (marked accordingly by “A”, “B” and “C” in Figure 8(e)) on the stress-strain curve of the system stretched at 900 K. For the other three temperatures, the simulation cell experienced two stepwise HCP→BCC→HCP and HCP→BCC→ FCC phase transitions, which lead to a continuous decrease of the stress before ε≈20%. After that, it was mainly dominated by the B-type HCP-FCC phase transition via the slip of Shockley partial dislocations.

As the tensile strain increased, a large quantity of dislocations were activated in systems, especially at low temperatures ranging from 100 to 600 K, as shown in Figure 8(f). Dislocations began to appear at strain ε≈10%, 10% and 19% (marked by “a”, “b” and “c” in Figure 8(f)) at 100, 300 and 600 K, respectively. These three strain points correspond simultaneously to the start of BCC-to-HCP/FCC phase transitions, as shown in Figures 8(a)-(c). The reason is that FCC phase formed a B-type and P-type interface with the HCP-N and HCP-M structure, respectively, which induced the accumulation and motion of various dislocations, i.e., Shockley partial dislocations, inside the crystalline. With the strain increasing up to around 25%, almost all BCC atoms transformed into the FCC/HCP structures and dislocation density reached the maximum in three systems. The reason is that dislocations diminished with the propagation of B-type HCP-N/FCC interfaces. Comparatively, dislocations began to appear at a larger strain ε≈25% (marked by “d” in Figure 8(f)) at 900 K. Stacking faults were generated in FCC phase at strains ranging between 20% and 30% (see Figure 1(d)), resulting in an increase in partial dislocations at the B-type HCP/FCC interfaces. When the strain reached 37%, the stacking faults in the HCP structure began to be consumed by the FCC phase, leading to the reduction of dislocation density in the system at 900 K.

Simulation results in the present study indicate that different deformation temperatures affect phase transitions and their transformation paths in HCP zirconium, which lead to different microstructures and thus mechanical performance including the strength and plasticity of the bulk material. On the one hand, the increase of tensile temperature brings down the yield strength of the Zr crystal due to phase transitions; on the other hand, phase transitions initiating at a smaller deformation strain can release the stress concentration inside the crystal and thus improve the plasticity at a higher temperature. Besides, lower tensile temperatures also produced higher dislocation density. The accumulation of abundant of dislocations tended to enhance the strength of the Zr bulk. Additionally, twinning occurred at lower deformation temperatures. The phase transitions, twinning and dislocation activation depend strongly on the tensile temperatures and are closely correlated to the final mechanical performance of Zr and Zr-based alloys [27, 61]. The present simulation results may lay a foundation for further research on zirconium and its alloys.

4 Conclusions

In the present paper, uniaxial tensile tests were performed on an HCP zirconium crystal at different temperatures ranging from 100 to 900 K by MD simulations. The serial HCP→BCC→FCC phase transitions occurred during uniaxial tension at all temperatures. However, the HCP→BCC transition following the Bain path was incomplete at 100 and 300 K, resulting in the formation of P-type FCC/HCP-matrix interfaces. In addition, the inverse BCC→HCP phase transition occurred but followed varied PS variants below 600 K, which resulted in the generation of B-type interfaces between the FCC and new HCP structure. Additionally, the original and new HCP structures were twinned relative to the twinning plane. The simulation cell at 900 K was composed of an FCC single crystal when the strain exceeded 45%, and then BCC phase formed in the FCC structure in the NW and KS ORs to coordinate deformation. The crystal stretched at lower temperatures exhibits higher strength but by the compromise of plasticity due to a combinational functions of phase transformations, dislocations and twinning. It is expected that the present research will provide a better understanding of the deformation mechanisms of zirconium and its alloys, which may contribute to the development of zirconium alloys with both high strength and excellent ductility.

Contributors

AN Ke-ying performed simulations, analyzed data, and wrote original draft. OU Xiao-qin analyzed data, supervised the writing, review and editing of the paper, administrated the project and funding. AN Xing-long provided the TEM images. ZHANG Hao analyzed simulation data. NI Song administrated the project and funding. SONG Min administrated the project and funding, revised the paper.

Conflict of interest

AN Ke-ying, OU Xiao-qin, AN Xing-long, ZHANG Hao, NI Song, and SONG Min declare that they have no conflict of interest.

Data availability

The raw data required to reproduce these findings are available on request.

Acknowledgements

We would like to thank the financial support from Natural Science Foundation of China (51901248, 51828102), Natural Science Foundation of Hunan Province (2018JJ3649) and Project of Innovation-driven Plan in Central South University (2019CX026). The Advanced Research Center of Central South University is sincerely appreciated for TEM technical support.

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(Edited by YANG Hua)

中文导读

分子动力学模拟拉伸温度对锆相变的影响

摘要：本文采用分子动力学模拟方法研究了不同拉伸温度(100~900 K)对密排六方结构(HCP)的锆单晶相变的影响，并将模拟结果与实验观察中高分辨率透射电子显微图片相结合。结果表明：外加载荷会诱发沿Pitsch-Schrader(PS)的位向关系(OR)的HCP到体心立方结构(BCC)的相变，随后，BCC相沿贝茵路径(Bain path)转化为FCC相。在100 K和300 K温度下，HCP-BCC的相变不完全，导致FCC和原始HCP相之间呈柱面型关系；当温度在100~600 K时，会发生局部的BCC-HCP逆相变，其位向关系符合PS关系的其他变体形式，从而导致新生成的HCP和FCC相呈基面型关系。当应变高于45%时，较高的拉伸温度能够促进FCC相向BCC相的转变。另外，在较低的拉伸温度下，晶体表现出较高的强度但塑性有所降低。本研究揭示了HCP-Zr在不同温度下的变形机理，为进一步了解锆合金在不同应用环境下的变形规律提供了依据。

关键词：锆；相变；分子动力学模拟；变形机制；力学性能

Foundation item: Projects(51901248, 51828102) supported by the National Natural Science Foundation of China; Project(2018JJ3649) supported by the Natural Science Foundation of Hunan Province, China; Project(2019CX026) supported by the Innovation-driven Plan in Central South University, China

Received date: 2020-07-19; Accepted date: 2021-01-07

Corresponding author: OU Xiao-qin, PhD, Lecturer; Tel: +86-13667339046; E-mail: Xiaoqin.Ouyang@csu.edu.cn; ORCID: https://orcid.org/0000-0001-8543-7532