Effect of low-angle grain boundaries on morphology and variant selection of grain boundary allotriomorphs and Widmanstatten side-plates

Morphology and variant selection (VS) of grain boundary (GB) allotriomorphs and Widmanstatten side plates of alpha phase in an alpha/beta titanium alloy, Ti-6Al-4V (wt%), are investigated using a three-dimensional phase field model. The structures of low-angle GBs (misorientation theta(m) <= 10) are modeled as discrete dislocation networks using Frank-Bilby theory. It is shown that cc allotriomorphs and side-plates compete with each other during precipitation and the final morphology and selected a variants exhibit a strong correlation with the GB dislocation structures. While the side-plate morphology is more preferred by a symmetrical tilt GB with theta(m) similar to 10, it can also be induced by a pure twist GB with theta(m) <= 5 degrees. Quantitative analysis indicates that precipitate morphology and VS are determined by the interplay among (i) elastic interaction between a nucleating cc precipitate and the GB dislocation networks, (ii) growth anisotropy determined by the relative inclination of the habit plane with respect to the GB dislocations, (iii) density of nucleation sites for the same variant and coalescence during growth, and (iv) spatial confinement from simultaneously nucleated neighboring a variants of dissimilar types. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.