Microstructure and mechanical property of novel L1₂ nanoparticles-strengthened CoFeNi-based medium entropy alloys

A novel L1₂ nanoparticles-strengthened (CoFeNi)₈₄Ti₈V₈ medium entropy alloy (MEA) is fabricated. It has typically equiaxed grains (mean grain size: ∼65 μm) in which the near-spherical L1₂ particles (mean size: ∼23 nm) are uniformly distributed with a volume fraction of ∼45%. This L1₂ nanoparticle is identified as the novel (Ni,Co,Fe)₃(Ti,V) phase. (CoFeNi)₈₄Ti₈V₈ MEA has an excellent combination of high yield strength (∼950 MPa) and ultimate tensile strength (∼1394 MPa) and good ductility (∼25%). Compared to single-phase (fcc) CoFeNi base alloy, the yield strength and ultimate tensile strength of (CoFeNi)₈₄Ti₈V₈ MEA increase by ∼428% and ∼215%, respectively. This is mainly attributed to the precipitation strengthening of L1₂ nanoparticles. The dimple features in the fracture surface imply a good ductility. Moreover, the main deformation mode is dominated by dislocation slips and microbands.