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°/ $ \left\langle {1\bar{2}10} \right\rangle $ , is very close to the orientation of $ \{{10\bar{1}1} \}\left\langle {1\bar{2}10} \right\rangle $ twins (57.42°/ $ \left\langle {1\bar{2}10} \right\rangle $ ). We propose that the cyclic thermal loading during additive manufacturing introduces large thermal stresses at high temperature, enabling grain reorientation that transforms the 60°/ $ \left\langle {1\bar{2}10} \right\rangle $ variant boundaries into the more energetically stable 57.42°/ $ \left\langle {1\bar{2}10} \right\rangle $ 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.

Keywords

• titanium alloy
• additive manufacturing
• twinning mechanism
• variant selections