Achieving the excellent intermediate-temperature strength-ductility synergy in a fine-grained FeCrNi-based medium entropy alloy with heterogeneous precipitation

Abstract Fe-Cr-Ni austenitic alloys are extensively utilized in the hot-end components of nuclear light water reactors, turbine disks, and gas compressors. However, their low strength at elevated temperatures constrains their engineering applications. In this study, we developed a novel precipitation-strengthened alloy system by incorporating Al and Si elements into a FeCrNi equiatomic alloy. The results indicate that the FeCrNiAlxSix (at%, x = 0.1, 0.2) alloys possesses heterogeneous precipitation structure characterized by a micron-scale σ phase at the grain boundaries and a nano-scale BCC (B2) phase within the grains. An exceptional strength-ductility synergy across a wide temperature range is achieved in FeCrNiAl0.1Si0.1 alloy, attributed to the grain refinement and precipitation strengthening. Notably, at 873 K, a yield strength of 694 MPa, an ultimate tensile strength of 818 MPa, and a uniform elongation of 18 % are attained. The dislocation shearing mechanism for B2 phases and the Orowan bypass mechanism for σ phase, coupled with a high density of nano-twins and stacking faults in the matrix, contribute to the excellent mechanical properties at cryogenic and ambient temperatures. Moreover, the emergence of serrated σ phase and micro-twins in the matrix plays a crucial role in the strengthening and toughening mechanisms at intermediate temperatures. This study offers a novel perspective and strategy for the development of precipitation-hardened Fe-Cr-Ni austenitic alloys with exceptional strength-ductility synergy over a broad temperature range.