New insights on the interaction of solutes with the defects during creep deformation of CMSX4 Ni-based single crystal superalloy

Solutes segregation effects to the defect structures produced during the creep of superalloys are believed to influence their high-temperature performance. This study reveals the selective segregation of solutes to the defects in a multi-component single crystal γ/γ` Ni-base superalloy (CMSX4). The defect structures formed in γ` are superlattice-extrinsic-stacking faults (SESFs), micro-twins, and anti-phase boundaries (APBs). The former two were formed during creep at 800 °C (applied stress 800 MPa), while APBs were formed at 1000 °C (applied stress 200 MPa). Atomic-scale compositional and structural analysis of these defects reveals a clear dependence of segregation behavior on their local internal fault structure. More specifically, during micro-twinning, we propose that the type of observed solutes diffusing toward the fault depends on minimizing the fault energy at each step of its structural evolution. The results indicate variation in Al composition along the twin boundaries and higher composition of W/Ta confined at the Al-depleted portion of the boundaries. Based on the segregation behavior, we introduce an additional step of reordering in Kolbe`s mechanism of micro-twinning that can be one of the rate-limiting steps for creep deformation. At 1000 °C, we show enrichment of Mo/W along with the Re at the core of dislocations pinned at γ/γ` interfaces. We also observe these solutes at the APB`s fault plane, indicating the occurrence of solute drag during the shearing of γ`. This emphasizes and reveals the importance of the solutes Mo/W in contributing towards the “Re effect” during high-temperature creep. Interestingly, Hf was also enriched to the APB fault plane, which might be due to its relatively large negative solution enthalpy in the local disordered structure of APB. These new observations suggest, other than Re, the possible creep rate limiting solutes whose content can be optimized for future alloys with higher temperature capabilities for more energy-efficient and cost-effective turbine engines with lower CO ₂ emissions.