Mechanisms and kinetics of static spheroidization of hot-worked Ti-6Al-2Sn-4Zr-2Mo-0.1Si with a lamellar microstructure

The effect of imposed strain ε, annealing temperature T _A, and annealing time τ on the static spheroidization behavior of Ti-6Al-2Sn-4Zr-2Mo-0.1Si having an initial lamellar microstructure was investigated. For this purpose, the samples were compressed isothermally at 1173 K (900°C) to ε = 1.0 and subsequently annealed at 1228 K (955°C) ≤ T _A ≤ 1253 K (980°C) for 10 minutes ≤ τ ≤ 24 hours. For each test condition, metallography was performed to evaluate the change in aspect ratio (AR) and thus quantify the structural evolution from a lamellar to an equiaxed morphology. The average AR decreased rapidly during short annealing times as a result of sub-boundary-induced boundary splitting, but it decreased at a considerably slower rate during subsequent long-time, diffusion-controlled termination migration. The overall time to complete the static globularization was thus governed largely by termination migration. To model the observations, a kinetic equation describing the static spheroidization of two-phase titanium alloys was developed. A comparison of experimental results and predictions showed that the model can provide a reasonable prediction of the time required to complete diffusion-controlled migration of the edges of thin lamellar fragments that are circular or elliptical in shape.