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 degrees/< 1 (2) over bar 10 >, is very close to the orientation of {10 (1) over bar1} < 1 (2) over bar 10 > twins (57.42 degrees/< 1 (2) over bar 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 degrees/< 1 (2) over bar 10 > variant boundaries into the more energetically stable 57.42 degrees/< 1 (2) over bar 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. [GRAPHICS] . IMPACT STATEMENT A new twinning mechanism, transformation twinning, was discovered in a Ti- 6Al-4V alloy fabricated by selective laser melting. The resulting high density of transformation twins impact the global mechanical properties significantly.