Two-step Cr and Al diffusion coating on TiAl at high temperatures

The formation of an oxidation resistive coating layer on a TiAl alloy was investigated with a two-step Cr (at 1573 K for up to 72 ks) and then Al (at temperatures between 1273 and 1573 K for 36 ks) pack diffusion process at high temperatures. The coated TiAl was oxidized in air at 1173 K for up to 1252.8 ks under a thermal cycling condition. The Cr diffusion coating layer consisted of gamma, beta, and Laves phases, which were transformed during cooling from the beta-phase formed at 1573 K. The coating layer formed by the Al diffusion at 1473 and 1573 K consisted of an outermost TiAl2, outer Al-rich gamma-TiAl, intermediate gamma, beta, and Laves phases, and a diffusion zone; there is little Al-diffusion at 1273 and 1373 K. The TiAl coated by the two-step Cr and Al diffusion process at 1573 K showed very good oxidation resistance in air at 1173 K due to the formation of a protective alpha-Al2O3 scale. After oxidation for up to 1,252.8 ks the coating layer maintained a structure with three phases gamma, Laves, and beta, which acts as a diffusion barrier to Al and Ti. (C) 2003 Elsevier Science Ltd. All rights reserved.