Uncovering the hierarchical clusters in the heat-affected zone of an electron beam welded α/β titanium alloy joint

Using high-resolution transmission Kikuchi diffraction (TKD) and transmission electron microscopy (TEM), we examined the hierarchical clusters that form in situ in the heat-affected zone (HAZ), which are commonly referred to as "ghost" structures, of bimodal titanium alloy Ti-5Al-2Sn-2Zr-4Mo-4Cr (wt%, TC17). The ghost structures are enriched with Al elements but poor in Mo and Cr compared to the surrounding & beta; matrix. TKD results show that the ghost structure in middle-HAZ mainly consists of & alpha;L laths with a high-angle grain boundary, which exhibits the classic Burgers orientation relationship (BOR) with the host matrix, while it encircles the & alpha;P grains in far-HAZ. And the ghost structure is evidenced to form via in-complete martensitic transformation. TEM results further confirm that the ghost structure is composed of & alpha;L and tiny & beta;L laths with BOR, with the former being enriched with Al and poor with Cr and Mo, while the latter is the opposite. Interestingly, two & alpha;L variant clusters with a check-mark morphology are frequently observed viewed along [0 0 01]& alpha;L//[110]& beta;L directions, which are dominated by the crystallographic and geometrical relationships between & alpha; and & beta; phases. Based on the microstructural characterization, it is hypothesized that the ghost structure is transformed from the initial & alpha;P phase, due to the coupling effect of high thermal stress (which induces the formation of a large number of dislocations) and element diffusion caused by sudden temperature increase and plunge cooling in the HAZ during the welding process.& COPY; 2023 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.