Abstract: Single crystal (SX) superalloys are required for full-operating-temperature mechanical properties. How-ever, the quest for intermediate temperature (IT) resistance often encounters a perplexing phenomenon: anomalous yielding behavior coupled with an unexpected loss of ductility. This study delved into the tensile behavior of a [111]-oriented SX superalloy from room temperature (RT) to 1150 °C, uncover-ing temperature-dependent tensile mechanisms where the interplay among phases and deferent de-fects governs plastic deformation. Desirable high strength-ductility properties were observed at IT, show-casing comparable strength with increased ductility. Microstructural evidences show that the primary strengthening effects stem from coupled interface boundary strengthening and anti-phase boundary (APB) strengthening, while the plasticity arises from planer defects transitioning from the stacking fault (SF) within γphase at small strains, to superlattice SFs, ultimately to the erasure of superlattice SFs, leaving cutting dislocation pairs in γʹ phase. Energy analysis of APB and SF, along with adherence to Schmid laws, reinforce the plausibility of such intricate defect interactions. The strength-ductility balance can be ascribed to the collective effect of preferentially generated dislocations and prompt formation of SF. This strategy of sequential defects' competition provides a new route for solving the strength-ductility trade-offof alloys.

Key words: SX superalloy, Anomalous yielding, Anti-phase boundary, Stacking fault, Deformation mechanism