Primary Alpha Grain Coarsening Behavior of Ti-6Al-2Zr-1Mo-1V Alloy in the Alpha plus Beta Two-Phase Field

Grain coarsening of titanium alloys takes place easily at high temperatures, which significantly affects the mechanical properties of the material. In this study, the coarsening mechanisms and kinetics of the Ti-6Al-2Zr-1Mo-1V (TA15) alloy in the (alpha + beta) two-phase field were investigated by heat treatment experiments. The experimental results showed that the microstructural morphology evolved from the cuboidal, rod-like, or plate-like at the initial stages to the equiaxed or spherical. The coarsening mechanism was different at different temperature. At lower temperature (900 A degrees C), the coarsening exponent (n) was close to 4.1. This indicated that the coarsening process was controlled by diffusion along grain boundaries. At higher temperatures (940 and 970 A degrees C), the values of n were equal to 3.2 and 3.3, respectively. It could be concluded that the coarsening processes were mainly controlled by diffusion through the matrix. Furthermore, the activation energy of diffusion (Q (act)) and the coarsening rate constant (K) were calculated and compared with the theoretic values. That confirmed the coarsening mechanism mentioned above.