Variant selection of grain boundary α by special prior β grain boundaries in titanium alloys

During beta-processing of alpha/beta and beta titanium alloys, variant selection (VS) of grain boundary alpha (GB alpha) is one of the key factors in determining the final transformation texture and mechanical properties. It has been observed frequently that GB alpha prefers its < 0 0 0 1 >(alpha) pole to be parallel to a common < 1 1 0 >(beta) pole of the two adjacent beta grains and results in a micro-textured region across the grain boundary (GB) and, as a consequence, slip transmission may take place more easily across that GB. In order to investigate how such a special prior beta GB contributes to VS of GB alpha, we develop a crystallographic model based on the Burgers orientation relationship (BOR) between GB alpha and one of the two beta grains. The model predicts all possible special beta grain boundaries at which GB alpha is able to maintain BOR with both beta grains. A new measure for VS of GB alpha, (beta Delta J beta(BOR)) i.e., a measure of the deviation of the actual OR between the GB alpha and the non-Burgers grain from the BOR, is proposed. For the particular alloy chosen for experimental observations, Ti-5553, it is found that when the misorientation angle theta(m) of ( beta Delta J beta(BOR)) (instead of the closeness between two closet {1 1 0}(beta) poles between two beta grains widely used in literature) is less than 15 degrees, misorientation between the two beta grains dominates the VS of GB alpha and, in particular, the variant with minimum theta(m) is always selected for GB alpha. A possible effect due to grain boundary plane inclination on VS is also discussed. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.