Microstructural investigation of stress-assisted α″-α' phase transformation in cold-rolled Ti-7.5Mo alloy

The present study investigated the microstructure and stress-assisted α″-α' phase transformation of a solution-treated Ti-7.5Mo alloy being cold-rolled with a series of different thickness reductions using transmission electron microscopic (TEM) and electron backscatter diffraction (EBSD) techniques. The EBSD/IPF results indicated a substantially random texture in ST sample. When the alloy was mildly cold-rolled, a texture toward 〈1 0 1-0〉 preferred orientation was established, which gradually shifted toward 〈2 1-1-0〉 with increased reduction in thickness. The TEM results indicated that substantially all fine platelets in the solution-treated (ST) sample had an orthorhombic α″ phase, and that a stress-assisted α″-to-α' phase transformation took place when the alloy was cold-rolled, confirming the XRD results. In the mildly rolled (CR20) sample, the existence of α' phase could be easily identified, indicating that the stresses involved in the mild rolling process were sufficient to initiate the α″-α' phase transition. On the other hand, in the heavily rolled (CR80) sample, the presence of residual α″ phase was still observed throughout the sample, indicating that the α″-to-α' phase transition was never complete even after a severe cold rolling process. The SAD analysis indicated an orientation relationship, α'(1 2-1 0)//α″(2 0 2-) and α' [0. 0. 0. 1]//α″ [0. 2. 0], existing between the two phases.