Formation Mechanism and Deformation Behavior of AZ31 Magnesium Alloy Bimodal Structure
ZHOU Wenhui, XIONG Jintao, HUANG Sicheng, WANG Penghao, LIU Yong()
Jiangxi Key Laboratory of Light Alloy, School of Advanced Manufacturing, Nanchang University, Nanchang 330031, China
Cite this article:
ZHOU Wenhui, XIONG Jintao, HUANG Sicheng, WANG Penghao, LIU Yong. Formation Mechanism and Deformation Behavior of AZ31 Magnesium Alloy Bimodal Structure. Acta Metall Sin, 2025, 61(3): 488-498.
The magnesium alloy exhibits a notable plasticity limitation due to its hcp structure. In recent years, the development of a bimodal structure, consisting of deformed coarse grains and recrystallized fine grains, has emerged as an effective strategy to balance the strength and plasticity of magnesium alloys, offering a new avenue for property. This optimization study investigates the formation mechanism and deformation behavior of the bimodal structure in AZ31 magnesium alloy by controlling the extrusion process. The formation of the bimodal structure is attributed to the incomplete dynamic recrystallization during plastic deformation and the particle-stimulated nucleation effect of the secondary phase. During deformation, fine grains endure higher stresses, while coarse grains accommodate more strain. The fine grains significantly contribute to the improved strength of the AZ31 magnesium alloy, while the coordinated deformation of the coarse grains ensures excellent plasticity. Leveraging the superior deformation capability of the bimodal structure, fine-grained AZ31 magnesium alloy was successfully fabricated through further extrusion, achieving outstanding mechanical properties: a tensile strength of 265 MPa, a yield strength of 112 MPa, and an elongation of 19%. This demonstrates the synergistic enhancement of strength and plasticity.
Fund: National Key Research and Development Program of China(2022YFC2905204);National Natural Science Foundation of China(52061028);Major Research and Development Projects of Jiangxi Province(20223-BBE51021);Maturation and Engineering of Major Scientific and Technological Achievements in Jiangxi Province(20243BDD40002);Program of One Thousand Talented People of Jiangxi(S2021GDKX-0864)
Corresponding Authors:
LIU Yong, professor, Tel: 13576087535, E-mail: liuyong@ncu.edu.cn
Fig.1 Schematics of the extrusion process and sampling positions of AZ31 magnesium alloy (ED—extrusion direction) (a, b) extrusion dies with extrusion ratios of 2.5 (a) and 5 (b) (c) schematic of extrusion direction (d, e) sampling diagrams of ϕ330 mm (d) and ϕ140-220 mm (e) (R—radius) (f) dimensions of tensile specimens (unit: mm)
Fig.2 XRD spectra of alloys (a); statistical analysis of coarse grain size (b), OM image (c), and SEM image and EDS result (d) of coarse-grained AZ31 magnesium alloy
Fig.3 Typical OM images of bimodal structure AZ31 magnesium alloy (Blue circle is fine equiaxed grains (FEGs), yellow circle is row stacked grains (RSGs), and red circle is long elongated grains (LEGs); DRX—dynamic recrystallization) (a) //ED (b) ⊥ED
Fig.4 Distributions of the secondary phase of the bimodal structure AZ31 magnesium alloy (The black arrows indicate Mg17Al12; inset in Fig.4a shows the locally magnified image; inset in Fig.4b is the compositions of the points) (a) //ED (b) ⊥ED
Fig.5 OM images of microstructure of fine-grained AZ31 magnesium alloy (a) //ED (b) ⊥ED
Fig.6 Statistics of DRXed grain size and volume fraction in bimodal structure (a) and fine-grained (b) AZ31 magnesium alloys
Fig.7 Cross-sectional microstructures of tensile samples of bimodal structure AZ31 magnesium alloy after ~2% (a), ~10% (b), and ~18% (c) deformation (Arrows in Figs.7a-c indicate the microcracks); typical bimodal deformable structure (d, e); and sampling positions (f)
Fig.8 Mechanical properties of bimodal structure (a, b) and fine-grained (c, d) AZ31 magnesium alloys (UTS—ultimate tensile strength, YS—yield strength, EL—elongation) (a, c) engineering stress-strain curves (b, d) strength and elongation
Fig.9 Engineering stress-strain curves (a), work hardening curves (Inset in Fig.9b is the locally magnified curves) (b); and tensile fracture morphologies of coarse-grained (c), bimodal structure (d), and fine-grained (e) AZ31 magnesium alloys (The red dashed line in the image outlines the fractured second phase)
Reinforcement method
YS / MPa
UTS / MPa
EL / %
Ref.
Coarse-grain
43 ± 4
170 ± 9
13 ± 0.9
This work
Bimodal structure
77 ± 9
217 ± 12
18 ± 0.7
Fine-grain
112 ± 12
265 ± 14
19 ± 1.0
Grain refinement
-
200 ± 6
16 ± 1.0
[7]
Suppressing intergranular deformation
380 ± 7
430 ± 11
13 ± 0.8
[22]
Grain refinement and texture change
220 ± 4
253 ± 7
13 ± 0.9
[23]
Grain refinement
250 ± 7
310 ± 10
14 ± 1.0
[24]
Grain refinement
203 ± 6
200 ± 10
12 ± 1.0
[25]
Grain refinement
-
290 ± 5
13 ± 0.6
[26]
Table 1 Comparisons of mechanical properties of the AZ31 magnesium alloy conventionally and/or severely deformed by means of various metal forming techniques[7,22-26]
Fig.10 Mechanism of bimodal structure formation in AZ31 magnesium alloy (a) coarse-grained AZ31 (b) bimodal structure AZ31 (c) fine-grained AZ31 (d-f) nucleation mechanisms in dynamically recrystallized grains
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