Effect of Cooling Rate on Microstructure Evolution and Plastic Flow of Ti-6Al-4V

The effect of cooling rate following subtransus solution treatment on microstructure evolution and plastic flow of Ti-6Al-4V was established. For this purpose, three types of tests were performed using samples of Ti-6Al-4V with an initial structure of equiaxed α in a matrix of β and a cooling rate of 11, 42, or 180 K/min: (i) Static cooling following solution treatment without or with a prestrain, (ii) constant-strain-rate hot compression during concurrent cooling, and (iii) static cooling to a specified temperature followed by constant-strain-rate isothermal hot compression. The volume fraction of equiaxed α developed during cooling was strongly dependent on cooling rate, but pre- or concurrent deformation resulted in relatively-small changes in this quantity. In addition, the cooling rate through its effect on the growth kinetics of equiaxed α had a noticeable effect on plastic-flow behavior under both isothermal and non-isothermal conditions. In both instances, the retention of the high-temperature microstructure (characterized by a low fraction of equiaxed α) during rapid cooling gave rise to lower flow stresses than samples with equilibrium equiaxed phase fractions. By contrast, when secondary α was formed during cooling, higher flow stresses were generated due to a Hall-Petch-like effect. The results were interpreted using models for the diffusional growth of equiaxed α, the onset of nucleation of secondary α, and predictions of the plastic-flow response of equiaxed two-phase microstructures based on a self-consistent approach. Unlike previous findings which indicated a large increase in the rate of dissolution of equiaxed α due to concurrent deformation/pipe diffusion during heating transients, the present work did not reveal a corresponding enhancement of growth during cooling.