Effect of Mo on microstructural stability of a 4th generation Ni-based single crystal superalloy

The microstructural stability during thermal exposure at 900 °C of Ni-based single crystal superalloys with various Mo content was investigated. The coarsening of γ′ precipitates and formation of TCP (topologically close-packed) phases were discussed in detail. The LSW theory and TIDC theory were applied to study the coarsening behavior of γ′ particles, and both models fitted well with the experimental results. Increasing Mo content enhanced the partitioning behavior of alloying elements, thus increasing the lattice misfit and interfacial stress of γ′/γ phases. Additionally, Mo addition also decreased γ′ coarsening rate by retarding the diffusion process of elements. However, Mo addition significantly promoted the precipitation of TCP phases, consuming massive solution strengthener elements, such as Re, W and Mo. TCP precipitation may significantly limit the creep life at 900 °C.