Alloy design of a Mo-rich single crystal superalloy based on d-electron concept and coarsening of γ′ phase during long-term aging

This study designed a low-density Mo-rich second generation Ni based single crystal superalloy (Alloy Mo) based on d-electron concept, and then investigated microstructural stability and coarsening behavior of γ′ phase during long-term aging. Through reasonable composition design, Alloy Mo (with Md_t value of 0.970) exhibited favorable microstructural stability, i.e. topologically close-packed (TCP) phase was not observed after long-term aging at both 980 °C and 1100 °C (aging for 50 h to 500 h). Furthermore, based on the literature data compiled in this study, formation tendency of TCP phase and Md_t values of both W-rich and Mo-rich single crystal superalloys revealed that critical Md_t value (without forming TCP phase) of Mo-rich single crystal superalloys was 0.970, which was lower than that of W-rich single crystal superalloys (0.990). This difference arose because Mo element exhibited a stronger tendency than W element to partition to γ phase. Moreover, Mo/Re and Mo/Cr interactions were stronger than W/Re and W/Cr interactions. Consequently, as compared with W addition, Mo addition more significantly increased enrichment tendencies of Re and Cr elements in γ phase, which further increased supersaturation degree of γ phase and ultimately increased formation tendency of TCP phase during long-term aging. Furthermore, coarsening behavior of γ′ phase in Alloy Mo during long-term aging followed Lifshitz-Slyozov-Wagner (LSW) coarsening model. As compared with typical W-rich second generation single crystal superalloys, both coarsening coefficient of γ′ phase and effective diffusion coefficient of Alloy Mo were comparable to those of typical W-rich second generation single crystal superalloys.