Dynamic-coarsening behavior of an α/β titanium alloy

Abstract

The dynamic-coarsening behavior of Ti-6Al-4V with an equiaxed α microstructure was established via isothermal hot-compression testing of cylindrical samples cut from an ultra-fine-grain-size (UFG) billet. Compression experiments were conducted at 900 and 955 °C, strain rates between 10−4 and 1 s−1, and imposed true strains between 0 and 1.4. Following deformation, quantitative metallography revealed marked coarsening of the primary α particles at low strain rates (10−4 and 10−3 s−1). The dynamic-coarsening rate followed rn vs time kinetics, in which n was between 2 and 3, or behavior between those of bulk-diffusion and interface-reaction controlled. An examination of the temperature and strain-rate dependence of theoretical coarsening rates, however, strongly suggested that bulk diffusion (with n=3) was more important. The dynamic-coarsening behavior was also interpreted in the context of the observed plastic-flow behavior. At low strain rates, high values of the strain-rate sensitivity (m>0.5) and the overall shape of log stress-log strain rate plots indicated that the majority of the imposed strain was accommodated by grain-boundary sliding (gbs) and only a small amount via dislocation glide/climb processes. In addition, an analysis of the flow hardening that accompanied dynamic coarsening indicated that the flow stress varied approximately linearly with the α particle size, thus providing support for models based on gbs accommodation by dislocation activity in grain-mantle regions.