| The Y-coated Ni₃Al with post heat treatment shows much better oxidation resistance than the Y-coated Ni₃Al without post heat treatment. In order to explain the effect of post heat treatment at low oxygen pressure, post heat treatment after Y-ion plating is performed in flowing hydrogen as a function of time. During post heat treatment, the Y-coated layer is modified into (Y, Al)O-type oxide by reaction between Y₂O₃, and Al₂O₃. Thickness of the modified Y-layer is related to oxygen pressure and time. The Y-modified layer formed during post heat treatment acts as a barrier to the transport of oxygen. The fine (Y,Al)O-type oxide can relieve easily growth stress by permitting easy plastic deformation, and can absorb the thermal stress developed in the Al₂O₃ layer. The tensile stress generated by the difference in thermal expansion coefficient of the (Y,Al)O-type oxide and Al₂O₃, layer compensates the large compressive stress generated by the difference in thermal expansion coefficient of the Al₂O₃layer and Ni3Al alloy. |
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