On the origin of yield strength anomaly in a γ/γ′ strengthened high entropy alloy

Yield strength anomaly (YSA), characterized by increased strength at elevated temperatures, is crucial for the performance of superalloys in high-temperature applications. However, the observation of YSA is seldomly reported in high entropy alloys (HEAs). This work unveils the origin of YSA in a γ′-strengthened HEA, Co₃₇.₆Ni₃₅.₄Al₉.₉Cr₅.₉Mo₄.₉Ti₃.₅Ta₂.₈, containing >75% γ′ phase. The alloy exhibits a peak yield strength of 772 ± 11 MPa at 770 °C. Scanning and transmission electron microscopy (S/TEM) reveal the formation of superlattice intrinsic stacking faults (SISFs) on non-coplanar {111} planes, whose interactions result in a high density of Lomer-Cottrell (L-C) locks, thereby contributing to the strength anomaly. The deformation substructure evolution below and above the anomaly peak temperature (670 °C and 870 °C) revealed γ′ precipitates are sheared by SISF- and anti-phase boundary-coupled super-partial dislocations. The persistence of L-C locks at 870 °C leads to higher strength than 670 °C, underscoring their role in strengthening in the low stacking fault energy system at elevated temperatures.