Tuning needle-like precipitation for enhanced strength-ductility synergy in a non-equiatomic FeNiCoCuTi high-entropy alloy

Precipitation strengthening is an effective strengthening strategy widely utilized in high-entropy alloys (HEAs) with a single-phased face-centered cubic (fcc) structure. In recent research works, reinforcing phase adopted are mostly focused on equiaxed or nearly equiaxed structures (e.g., spherical, cubic, and rod-like), while relatively rare studies on the strengthening effects of needle-like precipitates with large aspect ratios. The η-D024 phase, like the L12 strengthening phase most commonly used in fcc-structured HEAs, features an ordered Ni3Ti-type structure and also exhibits a comparable strengthening effect. However, since the η phase often co-precipitates with other precipitates in alloy system, the strengthening effect of the sole η-D024 phase in fcc-structured alloys remains to be further explored. In this study, microstructural evolution, phase transformation, and mechanical behaviors of a non-equiatomic Fe27Ni27Co26Cu10Ti10 HEA were systematically investigated. Results show that following high-temperature heat treatment, the microstructure of the studied HEA transforms from a combination of the fcc, L12, Cu-rich, and η phases in the as-cast state to a fcc+η structure in the heat-treated state. Meanwhile, the mechanical properties of the heat-treated HEA are significantly improved, with a total elongation increasing from approximately 0.9% to 7.5%. The enhanced ductility of the heat-treated alloy can be attributed to the strong hindering effect of numerous needle-like η phase at the grain boundaries, which restricts crack propagation and dislocation movement. This study develops a novel η-strengthened FeNiCoCuTi HEA, expanding the selection of available reinforcing phases in fcc-structured alloys and providing valuable insights into the phase transformation and strengthening effect of the η-D024 phase.