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

A novel L1(2) nanoparticles-strengthened (CoFeNi)84Ti8V8 medium entropy alloy (MEA) is fabricated. It has typically equiaxed grains (mean grain size: ~65 mu m) in which the near-spherical L1(2) particles (mean size:-23 nm) are uniformly distributed with a volume fraction of ~45%. This L1(2) 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)(84)Ti8V8 MEA increase by ~428% and ~215%, respectively. This is mainly attributed to the precipitation strengthening of L1(2) nanoparticles. The dimple features in the fracture surface imply a good ductility. Moreover, the main deformation mode is dominated by dislocation slips and microbands.