Microstructure evolution and static recrystallization during hot rolling and annealing of an equiaxed-structure TC21 titanium alloy

Abstract

An equiaxed-structure TC21 titanium alloy, consisting of predominant equiaxed α grains and remained β phase, was hot rolled at 750 °C, and subsequently annealed at different temperatures. Based on electron back-scattered diffraction (EBSD) observation, the microstructure evolution and static recrystallization (SRX) during rolling and annealing were investigated. An apparent evolution of grain morphology occurred during rolling and annealing. The equiaxed α grain was elongated along the transverse direction (TD) during hot rolling, and then got an equiaxed morphology due to the SRX and boundary splitting during annealing. In contrast, subsequent annealing had a limited effect on the textures. The Kernel Average Misorientation (KAM) calculation showed that the β phase endured a larger deformation amount than α phase, which induced a more preferential SRX in β phase. Meanwhile, the different SRX mechanism between α and β phases caused a different evolution of SRX and grain morphology with increasing annealing temperature. The continuous SRX (cSRX) in α phase made the SRX fraction increase and grain size decrease with increasing annealing temperature, with an exception of a grain growth at 940 °C. However, SRX occurred in β phase by subgrain coalescence, which induced the increase of grain size and decrease of SRX fraction with increasing annealing temperature. Conclusively, 880 °C was an appropriate annealing temperature to get a uniform and refined microstructure for the hot-rolled TC21 titanium alloy.