Grain-Resolved Temperature-Dependent Anisotropy in Hexagonal Ti-7Al Revealed by Synchrotron X-Ray Diffraction

TL;DR

This study uses high energy x-ray diffraction microscopy to measure grain-resolved, temperature-dependent anisotropic thermal expansion coefficients in hexagonal Ti-7Al, revealing internal stress evolution and microstructural changes during heating.

Contribution

It provides the first grain-resolved, in situ measurements of anisotropic CTEs in Ti-7Al, clarifying discrepancies in literature and linking microstructural evolution to anisotropic expansion.

Findings

CTEs change ratio between 'a' and 'c' directions with temperature

Dissolution of $eta_2$ precipitates affects CTEs

Grain-scale stresses reconfigure due to anisotropic expansion

Abstract

Hexagonal metals have anisotropic coefficients of thermal expansion causing grain-level internal stresses during heating. High energy x-ray diffraction microscopy, a non-destructive, in situ, micromechanical and microstructural characterization technique, has been used to determine the anisotropic coefficients of thermal expansion (CTEs) for Ti-7Al. Two samples of polycrystalline $\alpha$-phase Ti-7Al were continuously heated from room temperature to 850 $^\circ$C while far-field HEDM scans were collected. The results showed a change in the ratio of the CTEs in the 'a' and 'c' directions which explains discrepancies found in the literature. The CTE additionally appears to be affected by the dissolution of $\alpha_2$ precipitates. Analysis of the grain-resolved micromechanical data also shows reconfiguration of the grain scale stresses likely due to anisotropic expansion driving crystallographic slip.