Introducing transformation twins in titanium alloys: an evolution of α-variants during additive manufacturing

Titanium alloys can experience a cooling-induced phase transformation from a body-centred cubic phase into a hexagonal close-packed phase which occurs in 12 crystallographically equivalent variants. Among them, variant selection II, 60°/12¯10, is very close to the orientation of {101¯1}12¯10 twins (57.42°/12¯10). We propose that the cyclic thermal loading during additive manufacturing introduces large thermal stresses at high temperature, enabling grain reorientation that transforms the 60°/12¯10 variant boundaries into the more energetically stable 57.42°/12¯10 twin boundaries. This transformation twinning phenomenon follows a strain accommodation mechanism and the resulting boundary structure benefits the mechanical properties and thermal stability of titanium alloys.