Phase-Field Simulations of α Colony Formation Associated with Pre-existing α Phase in Titanium Alloys

Fine α colonies within heat-affected zone bands are commonly observed in titanium alloy components fabricated by directed energy deposition (DED) additive manufacturing. Unlike grain boundary colonies, these colonies are formed within the interior of β grains and are triggered by the residual (pre-existing) α phase. Since the pre-existing α often includes multiple α variants with various orientations and transformation strains, it is still unclear which specific combinations of pre-existing α variants can induce the formation of α colonies. In this study, the effect of various combinations of pre-existing α variants on subsequent α precipitation was systematically investigated through phase-field simulations. It was revealed that the pre-existing variant pairs with [112¯0]_α/60deg-type, [107¯173]_α/60.83deg-type and [717¯100]_α/90deg-type α/α orientation relationships (referred to as the promoted α/α-ORs) can induce the formation of α colonies. In the cases of three pre-existing variants, it is interesting that the colonies can be induced not only by the combinations containing at least one promoted α/α-OR, but also by some special combinations containing no promoted α/α-ORs. Thus, the frequency of α colony formation triggered by the three-variant combinations is larger than that by the two-variant combinations. It was illustrated that the existence of the third variant can alter the local stress conditions generated by the other two pre-existing variants. This modification in the stress field facilitates the formation of a large region, where the elastic interaction energies promote the nucleation and growth of a single type of new α variant, and thus results in the formation of α colony.