Severe plastic deformation induced precipitation of the ordered α₂-Ti₃Al phase in Ti–5Al–2Sn–2Zr–4Mo–4Cr

The typical two-phase titanium alloy of Ti–5Al–2Sn–2Zr–4Mo–4Cr was subjected to severe plastic deformation (SPD) by using high energy shot peening (HESP) at room temperature. Here, we experimentally investigated the SPD-induced atomic diffusion and phase transformation in the HESP processed Ti–5Al–2Sn–2Zr–4Mo–4Cr. The experimental results revealed that the ellipsoid-shaped precipitates with about 30–50 nm in thickness occurred in the surface layer of the HESP processed Ti–5Al–2Sn–2Zr–4Mo–4Cr, which was characterized to be the ordered α₂-Ti₃Al phase with the D0₁₉ crystal structure. The underlying formation mechanism of these ordered α₂-Ti₃Al precipitates was attributed to the SPD-induced atomic diffusion behaviors at room temperature, i.e., SPD-induced diffusive phase transformation, significantly different from the conventional ordering transformation driven by thermodynamics at high temperature. During HESP, the extremely high stress and strain level provided the potential conditions for precipitation of α₂-Ti₃Al phase; meanwhile, the generation of high-density dislocations accelerated the atomic diffusion, thereby promoting the nucleation of α₂-Ti₃Al phase. Furthermore, the significant reduction of grain size to nanometer contributed to the stability of α₂-Ti₃Al phase at room temperature.