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Acta Metall Sin  2016, Vol. 52 Issue (9): 1123-1132    DOI: 10.11900/0412.1961.2016.00051
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STUDY ON THE DYNAMIC RECRYSTALLIZATION SOFTENING BEHAVIOR OF AZ80 MAGNESIUM ALLOY
Yun CAI1,Chaoyang SUN1(),Li WAN2,Daijun YANG2,Qingjun ZHOU2,Zexing SU1
1 School of Mechanical and Engineering, University of Science and Technology Beijing, Beijing 100083, China
2 Capital Aerospace Machinery Company, Beijing 100076, China
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Abstract  

Magnesium alloys are considered as one of the lightest structural metallic materials with excellent properties such as high specific strength, superior damping characteristics and electromagnetic shielding performance. In order to improve the mechanical properties of magnesium alloys, the hot rolling, hot extrusion and other hot forming processes are often introduced to produce the high performance parts. Both of the two softening mechanisms, dynamic recovery and dynamic recrystallization (DRX), occur during the hot deformation. As an important softening mechanism in hot processing, DRX is beneficial to obtaining fine grains structure, eliminating defects and improving mechanical properties for magnesium alloys. In this work, isothermal compression tests of AZ80 magnesium alloy were conducted on Gleeble thermo-mechanical simulator in the temperature range of 200 to 400 ℃ and strain rate range of 0.001 to 1 s-1. In view of the dynamic hardening and softening mechanisms, the softening behavior of AZ80 magnesium alloy, dominated by dynamic recrystallization, was depicted. Dynamic recrystallization volume fraction was introduced to reveal the power dissipation during the microstructural evolution which was indicated by the strain rate sensitivity value based on the dynamic material model. To quantify the dynamic recrystallization softening behavior by the strain rate sensitivity (SRS) value, the SRS value distribution maps were constructed depending on various temperatures and strain rates. Therefore, the critical conditions and evolution process were studied in terms of temperatures and strain rates, while features of the SRS value distribution maps at different strains were deeply investigated. It can be concluded that the value of dynamic recrystallization critical condition decreases and dynamic recrystallization volume fraction increases when the temperature increases and strain rate decreases during the deformation. The strain rate sensitivity was positive correlated with the dynamic recrystallization volume fraction. It has been verified effectively by the analysis of microstructure that the region in which the strain rate sensitivity value is above 0.21 corresponds to the higher dynamic recrystallization volume fraction and lower strain rate.

Key words:  magnesium alloy      dynamic recrystallization      deformation behavior      strain rate sensitivity     
Received:  01 February 2016     
Fund: Supported by Joint Fund of National Natural Science Foundation of China and Chinese Academy of Engineering Physics (No.U1330121), National Natural Science Foundation of China (No.51575039) and Open Research Fund of Key Laboratory of High Performance Complex Manufacturing, Central South University (No.Kfkt2015-01)

Cite this article: 

Yun CAI,Chaoyang SUN,Li WAN,Daijun YANG,Qingjun ZHOU,Zexing SU. STUDY ON THE DYNAMIC RECRYSTALLIZATION SOFTENING BEHAVIOR OF AZ80 MAGNESIUM ALLOY. Acta Metall Sin, 2016, 52(9): 1123-1132.

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https://www.ams.org.cn/EN/10.11900/0412.1961.2016.00051     OR     https://www.ams.org.cn/EN/Y2016/V52/I9/1123

Fig.1  Typical stress-strain and θ-ε curves of dynamic recovery of AZ80 magnesium alloy at strain rate 0.1 s-1 and temperature 250 ℃ (θ—work-hardening rate, σ—true stress, ε—true strain, σsat—saturation stress, σp—peak stress, σc—critical stress, σss—steady-state stress, σ0—initial stress, εc—critical strain, εp—peak strain, σDRV—dynamic recovery (DRV) curve, σDRX—dynamic recrystallization (DRX) curve, Δσ—softening stress, KM model—Kocks-Mecking model)
Fig.2  Flow stress-strain curves of AZ80 magnesium alloy under strain rates of 0.001 s-1 (a), 0.01 s-1 (b), 0.1 s-1 (c) and 1 s-1 (d)
Fig.3  Curves of θ-σ (a) and -(?θ/?σ)-σ (b) of AZ80 magnesium alloy at strain rate 0.1 s-1 and temperature 250 ℃
Fig.4  Curves of θ-σ (a, c) and -?θ/?σ)-σ (b, d) of AZ80 magnesium alloy at 0.01 s-1 and different temperatures (a, b), and different strain rates and 350 ℃ (c, d)
Fig.5  Relationships among σss, σc, σsat and σp
Fig.6  Determination of k and n of AZ80 magnesium alloy in dynamic recrystallization kinetics model (XDRX—volume fraction of DRX, k—material constant, n—Avrami constant)
Fig.7  XDRX evolution of AZ80 magnesium alloy at various conditions ((2)~(6)—verify the dynamic recrystallization kinetic model, (2)—Fig.9b, (3)—Fig.9c, (4)—Fig.9d, (5)—Fig.9e, (6)—Fig.9f)
(a) strain rate 0.001 s-1 (b) temperature 400 ℃
Fig.8  SRS values m at the strains of 0.6 (a) and 0.9 (b) for AZ80 magnesium alloy ((2)~(6)—verify the SRS values distribution map, (2)—Fig.9b, (3)—Fig.9c, (4)—Fig.9d, (5)—Fig.9e, (6)—Fig.9f)
Fig.9  Microstructures of AZ80 magnesium alloy at different conditions
(a) initial (b) 250 ℃, 0.001 s-1, m=0.27 (Fig.7 (2)
and Fig.8 (2))(c) 300 ℃, 0.001 s-1, m=0.30 (Fig.7 (3) and Fig.8 (3)) (d) 400 ℃, 0.001 s-1, m=0.26 (Fig.7 (4) and Fig.8 (4))
(e) 400 ℃, 0.1 s-1, m=0.12 (Fig.7 (5) and Fig.8 (5)) (f) 400 ℃, 1 s-1, m=0.04 (Fig.7 (6) and Fig.8 (6))
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