Tensile behaviors and hetero-deformation-induced hardening mechanisms in medium entropy alloys with dual gradients of grain size and coherent nanoprecipitate

Abstract

Heterogeneous structures with dual gradients of grain size and coherent L12 nanoprecipitation have been designed and fabricated in a Co34.46Cr32.12Ni27.42Al3Ti3 medium entropy alloy. These heterogeneous structures with dual gradients show improved tensile properties over both the single grain-size gradient structure and the coarse grained structures with homogeneously distributed L12 nanoprecipitates. Hetero-deformation-induced hardening rate was observed to be higher and the strain hardening capacities at each depth were also found to be higher in the structures with dual gradients, as compared to the single grain-size gradient structure. Deformation twins and stacking faults were observed at both the topmost and center layers, while deformation bands at two directions were only found at the topmost layer after tensile deformation in the single grain-size gradient structure. Strong interactions between defects and coherent L12 nanoprecipitates can be observed at all depths after tensile deformation in the structures with dual gradients, resulting in strong precipitation hardening. While, stronger precipitation hardening was achieved at the topmost layer due to the higher volume fraction and smaller spacing of coherent L12 nanoprecipitates, which could delay the early necking trend at the topmost layer for better tensile ductility in the structures with dual gradients.