Abstract
To study the deformation twins and plastic anisotropy of AZ31 magnesium alloy, based on the plastic constitutive theory of rate-correlated crystals, a magnesium alloy model with different initial textures, including slip and twin deformation mechanisms, was established by the finite element method. Besides, the volume fraction of twin crystals was introduced into the model. The relationship among texture evolution, twins, and mechanical properties during compression was studied. Results show that the plastic behavior of crystal depends largely on the initial texture, and the difference in the initial texture leads to the obvious anisotropy of compression behavior: high axial yield and tensile strength and low radial yield and tensile strength. During the compressive plastic deformation process, with increasing the deformation, the volume fraction of activated twin crystal is increased. In addition, the higher the volume fraction of radial compressive activated twin crystal, the lower the volume fraction of axial compressive activated twin crystal. The points of obvious twin crystal appearance in the simulation coincide with those of stress mutation. When the volume fraction of twin crystal reaches a certain value, the stress suddenly changes, the crystal orientation changes significantly, and the new slip system is activated, reflecting the influence of the coupling of slip and twin crystal mechanisms on the mechanical properties of AZ31 magnesium alloy.
Magnesium alloy is widely used as structural material in aerospace, communication electronics, automobile, and other fields due to its lightweight, high specific strength and stiffness, good cutting and casting performance, abundant sources, and easy recover
Taylor et a
Thus, in order to study the effects of initial textures on the twins, texture evolution, and mechanical properties of AZ31 magnesium alloy during the compression process, the magnesium alloy models were established based on the crystal plasticity finite elements. The deformation textures of magnesium alloys during the compressive process could be achieved via the user-defined material interface (UMAT) in finite element software. In order to verify the crystal plasticity finite element model, cubic specimens along the axial and radial directions were cut from the extruded AZ31 magnesium alloy bar, and the uniaxial compression experiment was conducted. The microstructure and deformed texture were characterized by EBSD and then compared with the simulation results. This research provided a theoretical basis for processing and development of magnesium alloy.
Based on the crystal plasticity constitutive model, the corotational stress rate () of Cauchy stress with the material coordinate axis was established with the lattice elastic deformation rate tensor
(1) |
where σ is the stress; I is the second-order unit tensor; L is the elastic tensor; Ω is the antisymmetric spin tensor; means decomposable tensor into lattice parts.
The crystal slip obeys Schmidt criterion. Thus, the slip ratio of arbitrary slip system α () depends on the Schmidt stress with current stress σ:
(2) |
where is Schmidt stress with current stress σ; ρ is the density of material after deformation; ρ0 is the density of material before deformation of 1.78 g/c
(3) |
where represents the corotational stress rate on the lattice coordinate axis. The shear strain rate can be achieved through the hardening equation based on the rate-correlated model in this research.
The rate-correlated plastic limit is rate-independent. According to the Schmidt criterion, the slip rate of α-slip system depends on the corresponding shear stres
(4) |
where represents the reference strain rate; represents the current strength of the slip system; n represents the rate sensitive index. With n→∞, the strain rate is independent.
The strain hardening can be expressed by
(5) |
where is the hardening modulus of slip with α and β slip systems; β represents a different slip system. represents the self-hardening coefficient of α slip system, and represents the latent hardening coefficient between α and β slip systems, as follows:
(6) |
(7) |
where is the critical value of the initial partial shear stress; is the partial shear stress; is the hardening coefficient when the material yields; q is a constant; γ is the Taylor cumulative shear strain on all slip systems; h(γ) is the intermediate variable. Thus, γ can be expressed by
(8) |
where t represents the time.
According to Ref.[
(9) |
where τ and ξ are time variables; is the twin shear strain of AZ31 magnesium allo
When the cumulative sum of volume fractions of deformed twins in all twin systems is greater than the turning threshold, the twin crystals completely replace the parent grains. The grains are turned along the direction of the largest twin system in the twin crystals, and the turned twin crystal orientation etw is as follows:
(10) |
where emt is the crystal orientation matrix of the untwinned parent grains.
Extruded AZ31 magnesium alloy bar with the diameter of 40 mm was used in this research. The compression specimen of 8 mm×8 mm×12 mm was cut along the radial and axial directions, as shown in

Fig.1 Schematic diagram of compression specimen

Fig.2 EBSD microstructure of AZ31 magnesium alloy
hot extrusion molding process, and the initial grains gradually change to the isoaxial crystal
In this research, the crystallographic orientation was obtained from EBSD analysis for the rate-correlated crystal plasticity finite element model. Three groups of Euler angles were obtained. According to
(11) |
where φ1, φ2, and Φ are the Euler angles; u, r, h, v, s, k, w, t, and l represent the grain orientation.

Fig.3 Schematic diagrams of microstructure model construction: (a) 3D EBSD microstructure; (b) 3D finite element model; (c) 3D simulated grain morphology
represented a grain and different colors reflected different orientations. Therefore, the simulation model was established and the accurate prediction of the deformation texture of the magnesium alloy during compressive process was realized by
UMAT in the finite element software.

Fig.4 Cloud maps of stress distributions of constitutive models along radial direction (a) and axial direction (b) during compressive process

Fig.5 True stress-true strain curves of AZ31 magnesium alloys during compressive deformation of 4% (a) and 8% (b)

Fig.6 Simulated (a) and experimental (b) texture evolution of AZ31 magnesium alloys obtained along axial direction
According to

Fig.7 Simulated (a) and experimental (b) texture evolution of AZ31 magnesium alloys obtained along radial direction

Fig.8 Volume fractions of activated twin crystals in AZ31 magnesium alloys obtained along axial direction (a) and radial direction (b) after compression

Fig.9 Volume fraction curves of activated twin crystals of AZ31 magnesium alloys obtained along axial direction (a) and radial direction (b) after compression

Fig.10 Grain boundary distributions of AZ31 magnesium alloys obtained along axial direction (a) and radial direction (b) after compressive deformation of 8%
1) Based on the rate-correlated crystal plasticity theory, a magnesium alloy model with different textures is constructed, including the slip and twinning mechanisms, and the volume fraction of twin crystals is introduced.
2) The plastic behavior of crystals largely depends on the initial texture of alloys. The difference in initial textures leads to the anisotropy of compressive plastic deformation behavior. The radial compressive yield strength and tensile strength are low, and the axial compressive yield strength and tensile strength are high.
3) During the compressive plastic deformation process, with increasing the deformation, the volume fraction of activated twin crystals is increased. The higher the volume fraction of radial activated twin crystals, the lower the volume fraction of axial activated twin crystals.
4) The points of obvious twin crystal appearance in the simulation coincide with those of stress mutation. When the volume fraction of twin fraction reaches about 0.02, the stress changes abruptly. The stress growth rate is reduced sharply with increasing the strain. At this time, the crystal orientation changes, which reflects the influence of the coupling effect of slip and twinning mechanisms on the mechanical properties of alloys.
References
Li Yuyu, Yang Bowen, Han Tingzhuang et al. Materials Science and Engineering A[J], 2022, 845: 143 234 [Baidu Scholar]
Trang T T T, Zhang J H, Kim J H et al. Nature Communications[J], 2018, 9(1): 2522 [Baidu Scholar]
Hu Hongjun. Rare Metal Materials and Engineering[J], 2021, 50(2): 416 [Baidu Scholar]
Xue Chun, Chu Zhibing, Su Hui et al. Rare Metal Materials and Engineering[J], 2020, 49(12): 4041 [Baidu Scholar]
Liu Shuaishuai, Liu Han, Chen Xiang et al. Journal of Materials Science and Technology[J], 2022, 113: 271 [Baidu Scholar]
Chu Zhibing, Su Hui, Li Wei et al. Rare Metal Materials and Engineering[J], 2020, 49(9): 3265 [Baidu Scholar]
Huang Xinde, Xin Yunchang, Cao Yu et al. Journal of Materials Science and Technology[J], 2022, 109: 30 [Baidu Scholar]
Su Hui, Chu Zhibing, Xue Chun et al. Materials Research Express[J], 2020, 7(8): 86 503 [Baidu Scholar]
Xiong Hanqing, Wu Yiping, Jia Yuzhen et al. Rare Metal Materials and Engineering[J], 2020, 49(2): 429 [Baidu Scholar]
Hou Dewen, Zhu Yuzhi, Wen Haiming et al. Materials Science and Engineering A[J], 2021, 823: 141 748 [Baidu Scholar]
Taylor G I, Elam C F. Proceedings of the Royal Society of London[J], 1923, 102(719): 643 [Baidu Scholar]
Asaro R J, Rice J R. Journal of the Mechanics and Physics of Solids[J], 1977, 25(5): 309 [Baidu Scholar]
Asaro R J. Journal of Applied Mechanics[J], 1983, 50(4): 921 [Baidu Scholar]
Peirce D, Asaro R J, Needleman A. Acta Metallurgica[J], 1982, 30(6): 1087 [Baidu Scholar]
Hill R, Rice J R. Journal of the Mechanics and Physics of Solids[J], 1972, 20(6): 401 [Baidu Scholar]
Mareau C, Daymond M R. Acta Materialia[J], 2010, 58(9): 3313 [Baidu Scholar]
Weber G G, Lush A, Zavaliangos A et al. International Journal of Plasticity[J], 1990, 6(6): 701 [Baidu Scholar]
Nielsen K L Niordson C F. Journal of the Mechanics and Physics of Solids[J], 2014, 63: 113 [Baidu Scholar]
Li Heng, Zhang Zhao, Yang Heng et al. Forging Technology[J], 2021, 46(4): 11 (in Chinese) [Baidu Scholar]
Shi Dongfeng, Liu Tianmo, Wang Tianyu et al. Journal of Alloys and Compounds[J], 2017, 690: 699 [Baidu Scholar]
Wang M, Xu X Y, Wang H Y et al. Acta Materialia[J], 2020, 201: 102 [Baidu Scholar]
Lan Y T, Zhong X C, Quan G F et al. Transactions of Nonferrous Metals Society of China[J], 2015, 25(1): 249 [Baidu Scholar]
Su Hui, Chu Zhibing, Xue Chun et al. Rare Metal Materials and Engineering[J], 2021, 50(10): 3446 [Baidu Scholar]
Ganapathysubramanian S, Zabaras N. Computer Methods in Applied Mechanics and Engineering[J], 2004, 193(45‒47): 5017 [Baidu Scholar]
Ganapathysubramanian S, Zabaras N. International Journal of Plasticity[J], 2005, 21(1): 119 [Baidu Scholar]
Wang Wenbiao. Research on the Mechanism of Microstructure and Texture Evolution of Polycrystalline Copper During Cold Rolling[D]. Chongqing: Chongqing University of Technology, 2019 (in Chinese) [Baidu Scholar]
Rice J R. Journal of the Mechanics and Physics of Solids[J], 1971, 19(6): 433 [Baidu Scholar]
Sun Chaoyang, Guo Xiangru, Huang Jie et al. Acta Metallurgica Sinica[J], 2015, 51(3): 357 (in Chinese) [Baidu Scholar]
Chino Y, Kimura K, Hakamada M et al. Materials Science and Engineering A[J], 2008, 485(1‒2): 311 [Baidu Scholar]
Song Guangsheng, Zhang Jianqiang, Zhang Shihong. Rare Metal Materials and Engineering[J], 2020, 49(1): 288 (in Chinese) [Baidu Scholar]
Xia Q X, Yuan S, Xiao G F et al. Materials Today Communications[J], 2021, 27: 102 469 [Baidu Scholar]
Song Guangsheng, Zhang Jianqiang, Niu Jiawei et al. Rare Metal Materials and Engineering[J], 2019, 48(11): 3563 (in Chinese) [Baidu Scholar]
Wang Zhongtang, Yan Cao, Song Guangsheng et al. Rare Metal Materials and Engineering[J], 2012, 41(2): 221 (in Chinese) [Baidu Scholar]