Precipitation mechanism and age hardening behavior in a FeCoNiCr-based multi-principal element alloy

Multi-phase synergistic strengthening can substantially improve the strength of alloys by utilizing a variety of phases. However, the application of this strengthening approach in the multi-principal element alloys (MPEAs) is still limited. An ultra-high strength Fe₅₆Co₂₈Ni₁₀Cr₅(AlMo)₁ (at.%) alloy with multiple strengthening phases was prepared by blending CoFeNi alloy and 18Ni300 alloy powders via selective laser melting. The alloy demonstrated an ultra-high peak hardness of 692.5±4.8 HV after aging for 128 h at 400°C, which was caused by the synergetic strengthening of multiple phases. The size, number density, composition, and spatial distribution of various phases as a function of aging time were systematically characterized by atom probe tomography and transmission electron microscopy. The results show that the matrix exhibits a body-centered cubic (BCC) structure. The secondary phases were identified to be the Al₂O solidification phase with face-centered cubic (FCC) structure, Cr-rich α' phase and NiAl-rich phase with BCC structure, and NiMo-rich phase with hexagonal close-packed (HCP) structure. The precipitation mechanisms of these phases were discussed based on experimental results. This study reveals the key role of synergistic strengthening from multiphase precipitation in MPEA, supplying the theoretical foundation for designing innovative MPEA.