The effect of Fe addition on γ′ precipitates during thermal exposure in Ni-Co-based single crystal superalloys

The effect of Fe addition on γ′ precipitates in Ni-20Co-xFe-12Cr-7.5Al-7.5Ti-1.5 W (where x = 0 or 4 at.%) single crystal superalloys during thermal exposure process was systematically studied. The results showed that the alloys presented a good microstructure stability and no any harmful TCP phases were formed in the long-term aged microstructures (such as, 1000 °C for 1000 h). And, the Fe addition had an obvious influence on the volume fraction of γ′ precipitates, γ/γ′ lattice misfit, and the morphology of γ′ precipitates. The higher Fe content enabled the γmatrix to contain more Al and Ti elements, resulting in a lower volume fraction of γ′ precipitates. Simultaneously, the higher content of Al and Ti in γmatrix enlarged the lattice constant of γmatrix and decreased the γ/γ′ lattice misfit from 0.30% to 0.11%, which further induced the morphology of γ′ precipitates changing from the cuboidal shape to the spheroidal shape. Furthermore, the coarsening behavior of γ′ precipitates was analyzed and a corresponding mechanism was proposed that the alloy with Fe addition had a higher coarsening rate of γ′ precipitates during thermal exposure process compared to the alloy without Fe addition, which might result from a lower diffusion activation energy and a larger γ/γ′ interfacial energy.