Mitigating embrittlement of sigma phase in dual-phase high-entropy alloys through heterostructure design

The design of dual-phase high-entropy alloys (HEAs) often involves extensive alloying, which can lead to the formation of topologically close-packed (TCP) phases, significantly reducing tensile ductility. Balancing the high hardness of TCP phases while minimizing their embrittling effects is crucial for developing high-performance HEAs. This study, which focuses on the brittle sigma phase, proposes an innovative heterogeneous structural coupling design strategy that simultaneously enhances the strengthening effect of the sigma phase while minimizing its embrittlement role. A (FeCoCrNi)₉₀Al₁₀ HEA with sigma phase is employed as the model material, where a bimodal grain heterogeneous structure is achieved through a short-term high-temperature annealing process at 850 °C for 5 min. A small amount of sigma phase precipitates (∼0.8 vol.%) in the recrystallization (RX) region, modulating the hardness difference between the RX and non-recrystallized (NRX) regions. This induces significant heterogeneous deformation-induced (HDI) stress, while promoting coordinated deformation between regions, thereby triggering continuous work hardening and plastic deformation. As a result, the HEA exhibits an exceptional combination of high strength (1412 MPa) and ductility (14.9 %). The underlying deformation mechanism involves strain hardening driven by HDI stress, which strengthens the RX region and minimizes local strain mismatch between the sigma phase and the FCC matrix, suppressing the nucleation and propagation of interfacial cracks. The present approach presents a promising pathway for co-designing strength and ductility in metallic materials susceptible to TCP phase formation.