Effect of Cr addition on the properties of aluminide coating layers formed on TiAl alloys

TiAl alloys were coated by two different processes with a simple aluminide coating and a Cr+Al-type composite coating for the improvement of oxidation resistance of the alloy. Simple aluminizing was applied to TiAl–XCr (X=1, 4, 6 at.%) alloys. A Cr+Al-type composite coating was performed by Cr evaporation on the TiAl substrate followed by pack aluminizing. A simple aluminide coating applied to the Cr-added TiAl alloys showed improvement of ductility and oxidation resistance. This was mainly due to grain refinement of the Cr-added coating layer. The coating layer formed by the composite coating process consisted of the Al4Cr outer layer and TiAl3 inner layer. However, these coating layers were transformed to a Ti(Al,Cr)3 layer with a L12 structure during oxidation. This layer showed much better ductility compared to other coating layers and superior oxidation resistance in cyclic oxidation tests.