Effect of coating layer structures and surface treatments on the oxidation behavior of a Ti–50at.%Al alloy

A Ti–50at.%Al alloy, coated by a two-step Cr (at 1573 K for 7.2 and 18 ks) and Al-diffusion (at 1573 K for 36 ks) process, was oxidized in air under thermal cycling between 1173 K and room temperature for up to 3600 ks. The coated layer consisted of a three layer structure: an outermost TiAl2 (τ phase), outer TiAl (Al-rich γ-phase), and intermediate γ, β, and Laves phase layers. The coated TiAl with an intermediate layer of a fine, three phase structure oxidized slowly due to a protective α-Al2O3 layer, while a coarse structured coating was oxidized catastrophically due to the formation of cracks. Vickers hardnesses of a β and Laves mixture as well as a γ and Laves one are around 700–800 Hv, whereas the τ phase and a γ and τ mixture showed 300–400 Hv. The Al content and its profile in the outermost and outer layers were almost unchanged before and after the 3600 ks oxidation, and it seems to arise from a slow diffusion of Al, Ti, and Cr through the three-phase intermediate layer. It was suggested that the fine, three-phase structured coating layer was very effective to suppress degradation due to both mutual diffusion and cracking.