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

Abstract

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 β matrix. TKD results show that the ghost structure in middle-HAZ mainly consists of αL laths with a high-angle grain boundary, which exhibits the classic Burgers orientation relationship (BOR) with the host matrix, while it encircles the αP grains in far-HAZ. And the ghost structure is evidenced to form via incomplete martensitic transformation. TEM results further confirm that the ghost structure is composed of αL and tiny βL laths with BOR, with the former being enriched with Al and poor with Cr and Mo, while the latter is the opposite. Interestingly, two αL variant clusters with a check-mark morphology are frequently observed viewed along [0001]αL//[110]βL directions, which are dominated by the crystallographic and geometrical relationships between α and β phases. Based on the microstructural characterization, it is hypothesized that the ghost structure is transformed from the initial αP 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.