Al+Y codeposition using EB-PVD method for improvement of high-temperature oxidation resistance of TiAl

Abstract

An "Al + Y codeposition" on TiAl by the single EB-PVD method has been developed to improve the oxidation resistance of TiAl. To determine the optimum codeposition condition, the Al + Y codepositions with various ratios of Al and Y are evaluated through the isothermal and cyclic oxidation tests. The oxidation resistance of TiAl can be improved extensively by the AIS Y codeposition due to the formation of a gradient coating of Al and Y. The Al + Y codeposition with the ratio of Al:2Y for evaporation source material is proved to be the best. With a proper ratio of Al:Y, the Al + Y codeposition forms two distinctive layers of the oxides during high-temperature oxidation; Al2O3 in the inner layer and (Y, Al)O type oxide in the outer layer. In addition to the inner Al2O3 layer, the formation of the outer (Y, Al)O type oxide layer further improves the stability of the coatings. The stability of the Al + Y codeposition greatly depends upon the alloying element of TiAl substrate or oxidation resistance of the TiAl substrate alloy. The non-alloyed TiAl shows a poor coating stability, whereas TiAl-2.8Nb and Alloy K5 show a good coating stability under severe thermal stresses during cyclic oxidation since a stable Al2O3 can form on the surface of these alloys. (C) 1999 Elsevier Science S.A. All rights reserved.