An Orientation Relationship Between Parent Grains and Its Application to Variant Selection of Transformed α in Titanium Alloys

Abstract

The variant selection (VS) of the child phase at the grain boundary (GB) during solid phase transformation has been extensively reported in polycrystalline alloys and has proved to be a key factor in determining the final microstructure, texture, and mechanical properties of alloys. This study developed a new definition of the crystallographic orientation relationship (OR) between two parent grains, called phase transformation (PT) misorientation, which could be used to evaluate the VS tendency occurring at GBs. Specifically, PT misorientation (θp) is the minimum angle of a grain rotating around a particular crystallographic orientation to where the grain can produce the same child variant (crystallographic orientation) with another grain. We examined the β→α phase transformation in titanium alloy to confirm the effect of θp on the VS tendency of GBα. The study demonstrated that the probability of VS occurrence is largely affected by the magnitude of θp; in particular, the θp dominates the VS when the angle is less than 5 deg. Compared with the empirical rule for VS using the deviation angle of the common {110} pole of two adjacent β grains, a small θp was found to be more reliable for evaluating the VS tendency. Moreover, we also analyzed the correlation between θp and misorientation (θ), as well as their combined effects on VS. The effectiveness of θp on VS prediction at various cooling rates was also discussed. As a new parameter for describing the OR between two adjacent grains, we believe that θp is expected to tailor GB engineering in extended alloys during thermal-mechanical processes to achieve a satisfactory microstructure and texture.