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-x)Mo-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 & USigma; dual phases when Mo content is more than 4 at%. The morphology of sigma phase changes from a dispersed rod-like structure to a network structure. Md value is utilized to predict the formation of sigma 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 sigma phase effectively impedes dislocation movement, causing stress concentration and crack initiation within the sigma phase. The crack propagation is passivated in the soft FCC matrix to maintain a certain plasticity. Excessive network sigma phases damage plasticity. (C) 2022 Published by Elsevier B.V.

Automated summary

To regulate the strengthening behaviors of as-cast HEAs, molybdenum (Mo) was added to the CoCrFeNi HEA. With increasing Mo content, the phase composition of the as-cast alloy transformed from single phase solid solution to a dual-phase structure. The addition of Mo caused lattice distortion and reduced stacking fault energy, resulting in increased yield strength. The HMo4 alloy exhibited good plasticity and high yield strength and tensile strength.