Effects of Mo on the evolution of microstructures and mechanical properties in Co-Al-W base superalloys

Abstract The solidification paths, microstructures and mechanical properties of two Co-Al-W base superalloys with different additions of tungsten and molybdenum were investigated. Mo addition does not influence the volume fraction of the γ′ phase; however, the morphology of the γ′ phase changes from cuboidal to near cuboidal. The μ phase and β phase were identified in both alloy systems. Although the melting point of the μ phase is decreased by Mo addition, the solidification paths remain the same: L→ L 1 + γ → L 2 + γ + μ → γ + μ + β → γ + μ + β + γ′. The precipitation of a γ′ phase envelop around the interdendritic regions in Co-Al-W alloy systems has never been reported. The formation of a surrounding γ′ phase was caused by the diffusion of elements between secondary phases and the substrate during the heat treatments. The microhardness of the μ phase, which can be easily influenced by Mo addition, is the highest in both alloys, followed by the microhardnesses of the β phase and the substrate. In the fracturing process, microcracks were generated in the μ phase, the number of which increased with Mo addition.