Coarsening kinetics of primary alpha in a near alpha titanium alloy

Abstract

In this study, the coarsening behavior of primary alpha after hot deformation of a near alpha titanium alloy (Ti-1100) was investigated in the context of heat treatment parameters. Ti-1100 slab with initial martensitic microstructure was rolled to a moderate level of strain (ε ˜ 0.8) at 850 °C revealing the small number of the spheroidized alpha particles. After rolling, only 10 min of heat treatment at 1000 °C was needed to reach a uniform bimodal microstructure. Decreasing the heat treatment temperature to 930 °C resulted in complete spheroidization of the alpha laths after elapsing 5 h. Evaluation of the microstructure heat treated at 930 °C, 980 °C, and 1000 °C up to 50 h showed that the primary alpha particles were coarsened with heat treatment time. The linear relation of the cube of average particles radius with the heat treatment time and the scaling behavior of the particle size distribution proved that the coarsening was a volume diffusion controlled process. The Lifshitz-Slyozov-Wagner (LSW) model could describe the coarsening process in this alloy from low (∼20%) to high (∼85%) volume fraction of the primary alpha without considering any further modification for the changes in the volume fractions. The rate limiting level for the primary alpha coarsening was found to be the outward diffusion of Mo.