Variant selection during BCC to HCP phase transformation in selective laser melted Ti-6Al-4V

Abstract

The transformed α grains of selective laser melted Ti-6Al-4V display a strong (0001) fiber texture along the building direction, whereas during SLM printing, the parent high temperature β grains have preferentially grown in the <001> parallel to the building direction. In the current work, we have studied the phenomenon of variant selection occuring during the cubic to hexagonal phase transformation by investigating in detail the distribution of misorientation pairs and intervariant boundaries observed in orientation contrast maps acquired by EBSD. It was found that local variant selection within a single parent grain governed the martensitic transformation as it was observed that the experimentally assessed distribution of intervariant axis/angle pairs significantly differed from the one that can be expected if the Burgers orientation relationship applies with equal probability of the 12 possible variants. The variant selection could be explained by considering the degree of self-accommodation, which is corroborated by the characteristic morphologies formed by three- variant clusters as observed in the orientation contrast maps. Irrespective of the misorientation, the grain boundary characterization distribution revealed a strong anisotropic distribution of the internal interfaces with a tendency of alignment of these interfaces with the {hki0} prismatic planes. The dominant [10¯553¯]/ 63.26° intervariant boundaries displayed primarily twist and 180-tilt (3¯210) boundary planes.