Thermal Stability of Nanocrystalline BCC-Ti Formed by Phase Transformation During Surface Mechanical Attrition Treatment

This paper reports the transformation of HCP-Ti into BCC-Ti in the Ti–6Al–4V alloy induced by surface mechanical attrition treatment (SMAT). The processes of surface nanocrystallization (SNC) and phase transformation were investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM). The results show that the average grain size in the surface layer gradually decreased with increasing SMAT duration, but plateaued at 10nm after 90min of SMAT, while the proportion of BCC-Ti in the surface layer gradually increased. The refined grains displayed equiaxed grain morphology with a random crystallographic orientation. The thermal stability of nanocrystalline BCC-Ti was investigated by subjecting it to isothermal annealing treatment in the temperature range of 450–800∘C. BCC-Ti nanocrystallites were shown to exhibit excellent thermal stability up to 650∘C, whereas those in HCP-Ti started to recrystallize at approximately 550∘C.