The precipitation mechanism of secondary α-phase and formation of a bimodal microstructure in Ti6242S alloy cooled from the α plus β phase field

In this study, the precipitation mechanism of secondary alpha-phase and formation of a bimodal microstructure in Ti6242S alloy cooled from the alpha + beta phase field were investigated. It was found that grain boundary alpha (alpha(GB)) precipitated preferentially not only at the triple junction of the beta/beta boundary, but also at the equiaxed alpha(p)/beta interface and then grew along the beta/beta boundary. The nucleation and growth of the intragranular alpha (alpha(1)) phase were influenced by beta grain boundary (GB), the pre-existing equiaxed alpha (alpha(p)), and adjacent alpha(1) grain. alpha(1) grain preferentially precipitated at equiaxed alpha(p)/beta interface regardless of the presence or absence of a Burgers orientation relationship (BOR) between alpha(p) and beta phases; when there was no BOR, alpha(1) preferentially precipitated due to self-accommodation (i.e. a special orientation relationship) between the alpha(p) and adjacent alpha(1) grains, to minimize the total transformation-strain energy. Meanwhile, two types of nucleation and growth modes (interface instability and sympathetic nucleation) coexisted in their precipitations. Moreover, several microtextured areas (large areas of alpha phase with similar crystallographic orientations) were observed in the obtained bimodal microstructure. This can be attributed to the fact that the special a variant always occupied a large area by preferential nucleation and growth, which further confirmed the role of the BOR and self-accommodation. (C) 2020 Elsevier B.V. All rights reserved.

Automated summary

This study investigated the precipitation mechanism of secondary alpha-phase and the formation of a bimodal microstructure in Ti6242S alloy cooled from the alpha + beta phase field. It was found that both grain boundary alpha and intragranular alpha were influenced by various factors, and two types of nucleation and growth modes coexisted. The formation of several microtextured areas in the bimodal microstructure was attributed to the preferential nucleation and growth of a special alpha variant, confirming the role of Burgers orientation relationship and self-accommodation.