Calculation-driven design of off-equiatomic high-entropy alloys with enhanced solid-solution strengthening

Addition of Mo to the face-centered cubic (fcc) NiCoCrFe base alloy is an attractive method for improving the solid-solution strengthening of high-entropy alloys (HEAs). However, the low solubility of Mo in the equiatomic base alloy limits implementation. In this study, we used thermodynamic and ab initio calculations to develop an off-equiatomic NiCoCrFe-based HEA with an Mo content of up to 11.11 at%. Thermodynamic phase diagrams were constructed for the derived quinary subsystems with various Mo contents, and potential precipitate-free HEA compositions were determined. The degrees of lattice distortion (DLD) in seven selected HEAs were evaluated by statistically analyzing bond lengths determined using ab initio calculations. This approach could accurately predict the relative magnitudes of DLDs for multiple off-equiatomic HEAs in the studied system. Consequently, an off-equiatomic Ni1.8Co0.95Cr0.8Fe0.25Mo0.475 HEA was designed with enhanced lattice distortion, solid-solution strengthening, and yield strength. Microscopic analysis confirmed that the designed HEA exhibited a single fcc lattice, while excess Mo was detected at the grain boundary (GB) in a coarse-grained sample. It was deduced that slight GB segregation had a negligible influence on the lattice concentrations and solid-solution strengthening.