Effect of initial microstructure on plastic flow and dynamic globularization during hot working of Ti-6Al-4V

Plastic flow behavior and globularization kinetics during subtransus hot working were determined for Ti-6Al-4V with three different transformed beta microstructures. These conditions consisted of fine lamellar colonies, a mixture of coarse colonies and acicular alpha, and acicular alpha. Isothermal hot compression tests were performed on cylindrical samples at subtransus temperatures and strain rates typical of ingot breakdown (i.e., T similar to 815 degrees C to 955 degrees C, epsilon(radical anion) similar to 0.1 s(-1)). For all three material conditions, true stress-true strain curves exhibited a peak stress followed by noticeable flow softening; the values of peak stress and flow softening rate showed little dependence on starting microstructure. On the other hand, the kinetics of dynamic globularization varied noticeably with microstructure. By and large, the globularization rate under a given set of deformation conditions was most rapid for the fine acicular microstructure and least rapid for the mixed coarse-colony/acicular structure. At temperatures close to the beta transus, however, the difference in globularization rates for the three microstructures was less, an effect attributed to the rapid (continuous) coarsening of the laths in the acicular microstructure during preheating prior to hot working. The absence of a correlation between the globularization kinetics and the observed flow softening at low strains suggested platelet/lath bending and kinking as the primary deformation mechanism that controls the shape of the flow curves.