The decomposing crystallography of sandwich-like microstructure from deformed martensite in Ti-10V-2Fe-3Al alloy

Abstract

Crystal defects are critical in the microstructure control of titanium alloys, as they determine the precipitation characteristics of α phase. In this study, the decomposition of stress-induced martensite in Ti-10V-2Fe-3Al is investigated to understand the effect of martensite on α precipitation. The crystallography between α and β phases is analyzed using Electron Backscatter Diffraction (EBSD) and Transmission Electron Microscopy (TEM). During decomposition, the martensite is first transformed into α, and then to β_T, with a misorientation of 50°/<110> with matrix β_M. During further aging, α begins to precipitate at the β_M-β_T boundaries, and α phase exhibits two types of orientation relationships with β phase, i.e. Type I: one involving a double Burgers orientation relationship (BOR) and Type II: the other involving a single-sided BOR with β_T while the other side follows $\left(\overline{1}10\right)\beta M\sim \left(\overline{1}2\overline{1}3\right)\alpha \left[\overline{1}\overline{1}1\right]\beta M\sim \left[\overline{1}010\right]\alpha$ with β_M. As for Type I, the selection of single α variant is due to the constraint of the β grains, $<1\overline{1}1{>}_{\beta T}$ and $<1\overline{1}\overline{1}{>}_{\beta M}$ are parallel to ${\left[\overline{1}2\overline{1}0\right]}_{\alpha }$ and ${[\overline{11}20]}_{\alpha }$ respectively and its habit plane {334} is close to β_M-β_T boundary. In Type II, the selected α variant, which has a {334} habit plane, also lies close to the β_M-β_T boundary. This finding indicates that the variant selection rule of the habit plane governs α precipitation during martensite decomposition.