EFFECT OF YTTRIUM COATING ON THE OXIDATION BEHAVIOR OF NI3AL

Yttrium-coated Ni3Al With post heat treatment has shown good high-temp erature oxidation resistance. To understand the effect of the Y-coatin g and post heat treatment on the oxidation resistance of Ni3Al, the sp ecimens were coated with Y by an ion-plating method, and heat treatmen t was performed at low oxygen level before or after the Y-coating was applied. Performance of the Y-coated Ni3Al was evaluated by isothermal and cyclic oxidation tests. A simple deposition of Y on Ni3Al did not change the oxidation kinetics, but the post heat treatment after Y-io n plating significantly decreased the oxidation rate of Ni3Al. The sca le formed on Y-coated Ni3Al with post heat treatment after Y-ion plati ng showed a fine and dense structure which was grain refined by the pr esence of a (Y, Al)O-type oxide in the scale. The coated Y layer becom es a Y-Al compound during heat treatment. The presence of the (Y, Al)O -type oxide in grain boundaries or the lattice of Al2O3 modify the dif fusion rate of Al and oxygen, and the oxide microstructure during oxid ation. Improvement of cyclic-oxidation resistance of Ni3Al by the Y-co ating occurs because the presence of (Y, Al) O-type oxide develops fin e-grain oxides which can easily relieve the growth stress.