Introducing transformation twins in titanium alloys: an evolution of α-variants during additive manufacturing
Haotian Wang, Q. Chao, Lin Yang, Matthew J. Cabral, Zizheng Song, B. Y. Wang, Sophie Primig, Wei Xu, Zibin Chen, Simon P. Ringer, Xiaozhou Liao
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°/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.