Dynamic-coarsening behavior of an alpha/beta titanium alloy

Abstract

The dynamic-coarsening behavior of Ti-6Al-4V with an equiaxed alpha microstructure was established via isothermal hot-cornpression testing of cylindrical samples cut from an Ultra-fine-grain-size (UFG) billet. Compression experiments were conducted at 900 and 955 degrees 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 alpha particles at low strain rates (10(-4) and 10(-3) s(-1)). The dynamic-coarsening rate followed r(n) vs time kinetics, in which it 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 to,, 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 alpha particle size, thus providing support for models based on gbs accommodation by dislocation activity in grain-mantle regions.