| The effect of heat treatment on the surface precipitation of trace impurities and on the subsequent oxidation of an Invar alloy was investigated. An Fe-36% Ni Invar alloy with Mn, Cr, Si, S and C as impurities was heat-treated at 1100˚C in vacuum as well as in 60% N₂＋ 40% H₂gas. Surface analysis with AES, SEM and EPMA has revealed that BN compound and nonmetallic elements such as S, C and O precipitated on the surface of the Invar alloy after a heat treatment in vacuum, while metallic impurities like Mn, Al and Si precipitated as oxide particles especially along grain boundaries after a heat treatment in 60% N₂＋ 40% H₂gas. And it was found that the oxidation occured preferentially near the precipitated nonmetallic and metallic oxide particles. The oxide layers formed on vacuum heat-treated alloys showed the best adherence, while the oxide layers formed without any prior heat treatment spalled on the outer-inner oxide interface. |
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