低温等径角挤压制备1050铝合金在磁退火后的均匀延伸率和屈服下降现象

Uniform Elongation and Yield-Drop Phenomenon in Magnetically Annealed 1050 Aluminum Alloy Prepared by CryoECAP

摘要:

采用低温等径角挤压（cryoECAP）制备了超细晶（UFG）1050铝合金。采用拉伸试验、透射电子显微镜和电子背散射衍射等方法，研究了UFG 1050铝合金在90~210 ℃、无磁场和12 T强磁场下退火4 h后的拉伸行为和显微组织。1050铝合金经cryoECAP退火后，晶粒尺寸为0.70~1.28 μm，极限抗拉伸强度与屈服强度之比小于1.24，均匀延伸率小于2.3%。随着退火温度从90 ℃上升到210 ℃，屈服下降现象变得明显，这是因为在拉伸变形过程中，为了维持所施加的应变速率，可动位错有所减少。均匀延伸率从1.55%下降到0.55%，位错密度从5.6×10¹⁴ m^-2下降到4.2×10¹³ m^-2，大角度晶界含量从63.8%增加到70.8%，使得位错湮灭速率提升，从而导致了应变硬化能力的降低。在90~210 ℃的强磁场退火条件下，低含量的大角度晶界（61.7%~66.2%）可以提供一个较慢的位错湮灭速率，从而导致较高的均匀延伸率（0.64%~1.60%）和更慢的屈服点后的流变应力下降。

Abstract:

The ultrafine grained (UFG) 1050 aluminum alloy was prepared by equal channel angular pressing at cryogenic temperature, namely cryoECAP process. The tensile behavior and microstructures of UFG 1050 aluminum alloy after annealing at 90–210 °C for 4 h without and with high magnetic field of 12 T were investigated by tensile tests, transmission electron microscope, and electron backscattered diffraction analyses. After cryoECAP and annealing treatments, the 1050 aluminum alloy has ultrafine grains with 0.7–1.28 μm in size, the ratio of ultimate tensile strength to yield strength is less than 1.24, and the uniform elongation is less than 2.3%. With increasing the annealing temperature from 90 °C to 210 °C, the yield-drop phenomenon becomes more obvious due to the decrease in mobile dislocations to maintain the applied strain rate during tensile deformation. The uniform elongation decreases from 1.55% to 0.55%, the dislocation density reduces from 5.6×10¹⁴ m^-2 to 4.2×10¹³ m^-2, and the fraction of high-angle grain boundaries (HABs) increases from 63.8% to 70.8%. These phenomena cause the higher annihilation rate of dislocations, thereby leading to the degradation of strain hardening effect. During annealing under high magnetic field at 90–210 °C, the low fraction of HABs (61.7%–66.2%) can provide a slower annihilation rate of dislocations, therefore resulting in the higher uniform elongation (0.64%–1.60%) and slower decrease in the flow stress after the yield peak.