Thermal stability and microstructure evolution in nanocrystalline Ti-5Al-2Sn-2Zr-4Mo-4Cr – A HRTEM study

Thermal stability and microstructure evolution in nanocrystalline Ti-5Al-2Sn-2Zr-4Mo-4Cr were investigated. X-ray diffraction (XRD), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were applied to characterize grain size change and microstructure evolution after annealing at the temperatures ranging from 300 °C to 650 °C. The results showed that nanograins slowly grew from ∼11.0 nm to ∼32.8 nm as the annealing temperature increased to 500 °C at which the grains were generally homogeneous in size. Re-ordering of grain boundaries (GBs) was shown to be the responsible for the change in grain size annealed at 500 °C and lower temperatures, in which sharp GBs occurred due to strain relaxation and dislocations annihilation. However, an abnormal grain growth was observed at annealing temperatures of 550 °C and 600 °C in which grains grew to ∼85.5 nm and ∼168 nm, respectively. It was further confirmed that the abnormal grain growth was due to synergistic effect of GBs migration, grain rotation, phase boundaries (PBs) migration, and intrinsic difference in α/β PBs.