The effect of lattice misfit on the mechanical properties of L12-strengthened medium-entropy alloys with low stacking fault energy

L12 precipitates reinforced face-centered cubic (FCC) high entropy alloys have attracted broad attention because of their good strengthening and ductility behaviors. Unlike conventional design strategy that the lattice misfit between L12 and FCC matrix is relatively small, herein we designed a L12-FCC (nominal composition of Co20Cr10Fe5Ni50Al10Ta5) HEA with large lattice misfit by adding Ta into CoCrFeNiAl alloy. Experimental results indicate that the degree of lattice misfit is inversely proportional to the aging time, and the reduction of lattice misfit is closely related to the trend of changes in the matrix lattice constant. In addition, this study also qualitatively analyzes the effect of large lattice misfit on the strength and plasticity of alloys. Increasing lattice misfit helps to improve alloy strength, while also reducing plasticity. The reason for this is that the large amount of high stress field generated by large lattice misfit at the phase boundary helps to hinder dislocation movement, but it can also cause stress to be too concentrated during early deformation, leading to fracture. Under the premise of low stacking fault energy, partial dislocations can shear L12 precipitation through other coherent interfaces and form stacking faults. These findings not only provide a relationship between lattice misfit and aging time, but also provide new ideas for designing FCC-L12 dual phase high entropy alloys with appropriate lattice misfit.