Phase Relationship in the Ni-Mo-B-O System and Oxidation Property of a Ni-NiMo2B2 Two-Phase Alloy

The oxidation property of a Ni-NiMo2B2 alloy which is expected to be a new heat resistant-material was investigated. The phase relationship of the Ni-Mo-B-O system at 973 K was determined to clarify the oxidation path of the alloy from the point of view of thermodynamics. The phase relationship suggested that Ni3B2O6, NiMoO4, MoO3, MoO2, NiO and B2O3 are formed in the atmosphere on the oxidation path of the alloy. Oxidation tests of the alloy were carried out at 773, 973 and 1173 K. The oxide film formed on the surface of the alloy was composed of Ni3B2O6, NiMoO4, MoO3, MoO2, NiO and B2O3 as expected from the phase relationship. The oxide film was very dense and appeared to be protective for the alloy, and as a result, the oxidation resistance of the alloy was very excellent.