Ti-6Al-4V合金超塑性变形中的组织演变及变形机制

Microstructural evolution during superplastic deformation of Ti-6Al-4V alloy

920℃、应变速率为1×10-3和2×10-4 s-1时,对不同初始晶粒尺寸(2.6、6.5和16.2μm)的Ti-6Al-4V合金进行超塑性拉伸变形。采用光学显微镜、透射电镜观察变形后的显微组织。结果表明,初始晶粒尺寸的不同对超塑性变形中的组织演变及变形机制有着显著的影响。拉伸变形中晶粒明显粗化,变形诱发晶粒长大是超塑性变形组织的重要特征之一;随着变形程度的增大,应变诱发的晶粒长大显著增大,并且远大于静态长大的增幅。对于细晶粒材料(2.6和6.5μm),位错运动协调的界面滑动是其变形的主要机制。而对于晶粒较粗的材料(16.2μm),超塑变形机制是晶界滑动与晶内位错运动的共同作用。随着晶粒尺寸的增大,以晶界滑动为主的变形方式逐渐转向以晶内位错运动为主。

Superplastic tensile tests were carried out for Ti-6Al-4V alloy using different initial grain sizes （2.6, 6.5 and 16.2 μm） at 920 ℃ with an initial strain rate of 1 × 10^-3 and 2 × 10^-4 s^-1. To get an insight into the effect of grain size on microstructural evolution and superplastic deformation mechanisms, the microstructures of deformed alloy were investigated by using an optical microscope and transmission electron microscope. The results indicate that there is dramatic difference in the superplastic deformation mode of free and coarse grained Ti-6Al-4V alloys. Grain growth is clearly observed, and strain inducing grain growth is one of the important microstructural characteristics during superplastic deformation. Moreover, grain growth induced by strain becomes dramatic with the increase of deformation, which is obviously larger than the static growth. For free grained material, boundary sliding accommodated by dislocation motion is the main deformation mechanisms. For coarse grained material, superplastic deformation is accommodated by the mixture of grain boundary sliding and intragranular dislocation motions. It indicates a transition from boundary sliding to matrix deformation with the coarsening of grains.