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 degrees C to 650 degrees C. The results showed that nanograins slowly grew from similar to 11.0 nm to similar to 32.8 nm as the annealing temperature increased to 500 degrees 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 degrees 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 degrees C and 600 degrees C in which grains grew to similar to 85.5 nm and similar to 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 alpha/beta PBs. (C) 2017 Elsevier B.V. All rights reserved.