Deformation behavior and microstructure evolution of an AM80 magnesium alloy at large strain rate range
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摘要: 采用INSTRON准静态压缩试验机和分离式霍普金森压杆装置，研究固溶态AM80镁合金在室温准静态和冲击载荷下的变形行为及组织演变。准静态载荷下，流变应力随应变率（3×10-5~4×10-1 s-1）的升高逐渐降低，表现为负应变率敏感性；冲击载荷下，流变应力随应变率（7.00×102~5.20×103 s-1）的升高而升高，呈现出明显的正应变率敏感性。冲击载荷下AM80镁合金的变形机制以基面滑移和孪生为主，大量细小致密的形变孪生以及适量非基面滑移的启动是AM80镁合金在冲击载荷下流变应力明显高于准静态载荷的重要原因。此外，随应变率的升高，AM80镁合金变形的均匀性明显增强，当应变速率升至3.65×103 s-1时，冲击变形所引起的局部绝热温升软化大于应变硬化与应变速率硬化的总和，部分晶粒产生了明显的动态回复，使得孪晶密度和变形均匀性反而降低。
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关键词:
- AM80镁合金 /
- 高速冲击 /
- 应变率敏感性 /
- Johnson-Cook本构方程 /
- 形变孪生
Abstract: In order to understand the deformation behavior and microstructure evolution of a solution treated AM80 magnesium alloy under quasi-static and impact loadings, quasi-static and high-speed impact compression tests at room temperature were performed by an Instron universal compression machine and a slip Hopkinson pressure bar apparatus, respectively. Under quasi-static loadings, the flow stress of the AM80 magnesium alloy decreases gradually with the increase of the strain rate (3×10-5 s-1 ≤ $\dot \varepsilon $ ≤ 4×10-1 s-1), showing a negative strain rate sensitivity. While, it increases with the increase of the strain rate (7.00×102 s-1 ≤ $\dot \varepsilon $ ≤ 5.20×103 s-1) under impact loadings, demonstrating a significant positive strain rate sensitivity. Basal slip, mechanical twining as well as proper non-basal slip are the deformation mechanisms for the AM80 magnesium alloy under impact loadings. A larger number of dense tiny mechanical twins under impact loadings are the fundamental reasons for the significantly higher flow stress as compared with that under the quasi-static loadings. In addition, the deformation uniformity of the AM80 magnesium alloy increases significantly as the strain rate increases. When the strain rate increases to 3.65×103 s-1, dynamic recovery is detected in the same grains at the location of c, because the softening caused by the adiabatic temperature rise due to localized deformation is greater than the sum of strain hardening and strain rate hardening, which leads to a significant reduction in the density of deformation twins. As a result, the deformation uniformity declines finally. -
图 3 AM80镁合金在准静态和高速冲击载荷下的真应力-真应变曲线
Figure 3. True stress-true strain curves of AM80 magnesium alloy at quasi-static and high-speed impact loads
图 4 准静态和高速冲击载荷下AM80镁合金的应变硬化率曲线
Figure 4. Strain hardening rate curves of AM80 magnesium alloy at quasi-static and high-speed impact loads
图 5 特定应变下真应力与应变率的关系曲线
Figure 5. True stress as a function of strain rate at specified stains
图 6 高速冲击实验与J-C本构拟合真应力-真应变曲线
Figure 6. True stress-true strain curves of high-speed impact and J-C constitutive fitting results
图 8 显微观测点在压缩试样中的相对位置
Figure 8. Relative positions for microstructure observation in compression sample
图 9 应变率为3×10-5 s-1时压裂后的金相图
Figure 9. OM images of the fractured sample at 3×10-5 s-1 strain rate
图 10 应变率为4×10-1 s-1时试样压裂后的金相图
Figure 10. OM images of the fractured sample at 4×10-1 s-1 strain rate
图 11 试样在2.15×103 s-1的应变率下压缩至0.28应变后的金相图
Figure 11. OM images of the fractured sample with compressive strain of 0.28 at 2.15×103 s-1 strain rate
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