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

Abstract

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°/ (Formula presented.), is very close to the orientation of (Formula presented.) twins (57.42°/ (Formula presented.)). We propose that the cyclic thermal loading during additive manufacturing introduces large thermal stresses at high temperature, enabling grain reorientation that transforms the 60°/ (Formula presented.) variant boundaries into the more energetically stable 57.42°/ (Formula presented.) 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.