Ti-6Al-4V钛合金超塑成形连续动态再结晶机理及亚晶粒转动

Mechanism of continuous dynamic recrystallization of Ti−6Al−4V alloy during superplastic forming with sub-grain rotation

研究Ti-6Al-4V合金在超塑性成形过程中的连续动态再结晶机理,并重点研究取向差演变机制。在温度为890、920和950℃,应变速率为10^(-4)~10^(-2)s^(-1)的条件下进行拉伸试验,采用电子背散射衍射与本构模型相结合的方法描述显微组织演变。几何必需位错在小角晶界处形成过程中起作用。此外,小角晶界转化为大角晶界的机制包括:(1)大角晶界向小角晶界的迁移;(2)由动态回复不充分的残余位错在小角晶界处聚集;(3)亚晶粒的旋转也会促进晶粒形核。基于物理本构模型的计算结果,验证在小角晶界转变为大角晶界前亚晶粒会发生旋转。特别地,在晶界角取向差的演化过程中,位错运动在取向差达到饱和值前起主导作用,否则亚晶粒旋转将起主导作用。

The continuous dynamic recrystallization(CDRX)mechanism of Ti−6Al−4V alloys during the superplastic forming(SPF)was investigated,aiming to clarify the dominant mechanism for various misorientation evolution processes.The tensile tests were designed at 890,920,and 950℃ with 10^(-4)~10^(-2)s^(-1),the electron back-scattered diffraction(EBSD)observations and constitutive model were applied to describing the microstructure evolution.The geometrically necessary dislocations(GNDs)are responsible for the low angle grain boundaries(LAGBs)evolution.Furthermore,the mechanism of the transformation from LAGBs to high angle grain boundaries(HAGBs)includes:(1)the migration from HAGBs to LAGBs;(2)the remaining dislocations after dynamic recovery will move to the LAGBs;(3)the sub-grain rotation also promotes the grain nucleation.Based on the physical constitutive model,the sub-grain rotation is verified just before the transformation from LAGBs to HAGBs.Particularly,the dislocation motion is dominant before the misorientation reaches the saturated value during the grain boundary misorientation evolution,otherwise,the sub-grain rotation will be activated.