Precipitation phase and twins strengthening behaviors of as-cast non-equiatomic CoCrFeNiMo high entropy alloys

To regulate multiple strengthening behaviors of as-cast high entropy alloys (HEAs) with a single phase face centered cubic (FCC) structure, the Mo element has been alloyed to the non-equiatomic CoCrFeNi HEA. Co₃₀Cr₃₀(FeNi)_40-xMo_x (x = 0, 2, 4, 6, 8, and 10 at%, hereinafter, simplified as HMo0, HMo2, HMo4, HMo6, HMo8, and HMo10, respectively) HEAs were prepared. The microstructure, tensile properties, and deformation behavior of as-cast alloys were investigated. The results show that the phase composition of HMo0-HMo4 is a single phase solid solution. The microstructure transforms from single phase FCC to FCC and σ dual phases when Mo content is more than 4 at%. The morphology of σ phase changes from a dispersed rod-like structure to a network structure. Md value is utilized to predict the formation of σ phase. Mo elements with large atomic size cause severe lattice distortion and reduce stacking fault energy (SFE) in the FCC matrix phase. The yield strength increases gradually from 227 MPa to 688 MPa with the increase of Mo content. HMo4 has high strain hardening ability to realize the synergy of tensile strength and plasticity of 689 MPa and 102.43%. HMo6 has high yield strength (486 MPa) and tensile strength (770 MPa), but without causing serious embrittlement (43.91%). The improvement of yield strength is attributed to solid solution strengthening and precipitated strengthening. The twins and SFs are activated due to the low SFE in HMo4. The dislocation movement is further hindered and the dislocation mean-free path is reduced due to the increase of twin boundary, which contributes to high strain hardening ability. For dual-phase alloy, the dispersed hard σ phase effectively impedes dislocation movement, causing stress concentration and crack initiation within the σ phase. The crack propagation is passivated in the soft FCC matrix to maintain a certain plasticity. Excessive network σ phases damage plasticity.