| The yttrium (Y) coated Ni₃Al with post heat treatment has shown a good high temperature oxidation resistance. To understand the effect of Y-coating and post heat treatment on the oxidation resistance of Ni₃Al, the specimens were coated with Y by ion-plating method, and heat treatment was performed before or after Y-coating at low oxygen level. 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-ion plating decreased significantly the oxidation rate of Ni₃Al. The scale formed on the Y-coated Ni₃Al with post heat treatment after Y-ion plating showed a fine and dense structure which was grain refined by the presence of the (Y,Al)O-type oxide in the scale. The coated Y layer becomes a Y-Al compound after post heat treatment. The presence of (Y,Al)O-type oxide in grain boundaries of Al₂O₃ modifies the diffusion rates of Al and oxygen, and the oxide microstructure during oxidation. Improvement of cyclic oxidation resistance of Ni₃Al by the Y-coating is attributed to the fact that the finegrained oxides developed by the presence of (Y,Al)O-type oxide can easily relieve the growth stress. |
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