采用等温压缩实验获得了变形温度为200~400 ℃, 应变速率为0.001~1 s-1的AZ80镁合金的流变应力曲线, 考虑动态硬化及软化特性描述了AZ80镁合金热变形过程动态再结晶主导的软化行为. 提出基于动态材料模型的应变速率敏感性指数表征动态再结晶引起的能量耗散, 该指数通过引入动态再结晶体积分数描述微观组织演化的耗散功. 考虑变形温度和应变速率构建了不同应变的应变速率敏感性指数图, 实现应变速率敏感性指数对动态再结晶软化行为的量化表征. 在此基础上, 研究了变形温度、应变速率对动态再结晶临界条件及演化过程的影响, 重点分析了不同应变的应变速率敏感性指数图特征. 结果表明: 随着变形温度的升高和应变速率的降低, 动态再结晶软化临界应变减小, 动态再结晶体积分数增加; 应变速率敏感性指数与动态再结晶体积分数正相关, 指数大于0.21的区域对应着高动态再结晶体积分数, 且均位于低应变速率下, 并通过动态再结晶软化的微观组织进行了验证.
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.
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