Challenging the Limit of Strengthening by γ/γ′ Lattice Misfit on the High Temperature Creep Properties of Ni-Base Single Crystal Superalloy

Abstract TMS-238MoIr previously achieved a record creep rupture life of 4044 hours at 1100 °C/137 MPa—more than twice that of the sixth-generation Ni-base single crystal superalloy TMS-238—corresponding to a temperature capability of 1136 °C (Larson–Millar conversion by 1000 hours rupture life under 137 MPa). This alloy was designed to enhance the high-temperature low-stress creep performance of TMS-238 by adding 1.5 at. pct Mo and 2 at. pct Ir. This study investigates the mechanisms behind this exceptional high-temperature creep performance and its limitations at other conditions by comparing it to a counterpart alloy, TMS-238MoRu (TMS-238 + 1.5 at. pct Mo and 2 at. pct Ru). At 1100 °C/137 MPa, TMS-238MoIr's superior life was attributed to the combined effects of a refined γ / γ ′ interfacial dislocation network achieved through increased magnitude of lattice misfit by Mo addition, and the suppression of topologically close-packed (TCP) phases by Ir. In contrast, TMS-238MoRu failed in only 870 hours due to extensive TCP precipitation. The high γ / γ ′ coherency stress and decreased stacking fault energy in both modified alloys induced stacking faults in the γ matrix during the initial heat treatment, before the creep test. These pre-existing faults served as potent nucleation sites for TCP phases at temperatures below 900 °C. Consequently, TCP precipitate-assisted deformation significantly reduced the creep lives of both concept alloys at 900 °C/392 MPa. These findings demonstrate that lattice misfit tuning must be carefully balanced with stacking fault energy and γ / γ ′ phase stability to optimize creep performance across a wide range of temperatures in advanced Ni-base superalloys.