Effect of a yttrium coating on the oxidation behavior of Ni3Al (II)

Abstract

A Y-coated Ni3Al with a postheat treatment shows much better oxidation resistance than a Y-coated Ni3Al without a postheat treatment. In order to explain the effect of postheat treatment at low oxygen pressure, postheat treatment after Y-ion plating was performed in flowing hydrogen as a function of time. During these treatments, the Y-coated layer was modified into a (Y, Al)O-type oxide by reaction between Y2O3 and Al2O3. The thickness of the modified Y-layer was related to oxygen pressure and time. The Y-modified layer formed by postheat treatment acts as a barrier to the transport of oxygen. The fine ( Y, Al)O-type oxide can easily relieve growth stress by permitting easy plastic deformation, and the ( Y, Al) O-type oxide layer can absorb the thermal stress developed in the Al2O3 layer. The tensile stress generated by the difference in thermal-expansion coefficients of the ( Y, Al) O-type oxide and the Al2O3 layer compensates the large compressive stress generated by the difference in thermal-expansion coefficients of the Al2O3 layer and the Ni3Al alloy.