An Orientation Relationship between Parent Grains and its Application to Variant Selection of Transformed α in Titanium Alloy

The variant selection 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 variant selection 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 variant selection tendency of GBα. The results demonstrated the probability of variant selection occurrence is largely affected by the magnitude of θp; in particular, the θp dominates the variant selection when the angle is less than 5˚. Compared with the empirical rule for variant selection 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 variant selection tendency. Moreover, we also analyzed the correlation between θp and misorientation (θ), as well as their combined effects on variant selection. As a new parameter for describing the OR between two grains, θp is expected to tailor the GBs in various alloys during thermal-mechanical processes to achieve a satisfactory microstructure and texture.