Microstructural and oxidation study of chromia forming molybdenum-tungsten based alloys

The microstructure and oxidation behaviour of tungsten and molybdenum based alloys containing chromium, and group VIII metals as sintering agents, have been investigated. Good properties are observed with use of palladium which enhances the diffusion of chromium towards the metal-oxide interface. Contrary to palladium, nickel leads to a catastrophic oxidation of the sample: it segregates at the oxide/alloy interface and induces the formation of a non-protective oxide layer constituted with Cr2O3 and NiWO4. Catastrophic oxidation occurs also when the refractory metals are transformed into volatile oxides: this is the case for the alloys with a high molybdenum concentration. Contrary to molybdenum, a high tungsten level leads to a high oxidation resistance, even at high temperature (1300 degreesC.)