Preparation and oxidation resistance of a crack-free Al diffusion coating on Ti22Al26Nb

A crack-free Al diffusion coating has been developed to improve the oxidation resistance of Ti22Al26Nb. It was produced by a two-step method; an Al film was deposited on the substrate alloy by arc ion plating followed by a diffusion process conducted at 873 K in pure Ar to form the Al diffusion coating. The two-step method lowers the temperature required to form the diffusion coating, which dramatically decreases the thermal stress developed in the coating and results in it being crack-free. The oxidation resistance of the non-coated Ti22Al26Nb alloy in isothermal and cyclic tests in air at 1073 K was poor, but the coated specimens possessed excellent oxidation resistance because a protective α-Al2O3 scale formed. The life of the Al diffusion coating greatly depends upon the rapid initial formation of a protective Al2O3 scale and interdiffusion between coating and substrate. Once the stable Al2O3 scale has formed and the composition changes from (Ti, Nb)Al3 into (Ti, Nb)Al2, the coating has a long life.