Abstract: Structures with single gradient and dual gradients have been designed and fabricated in an Al_0.5Cr_0.9FeNi_2.5V_0.2 medium entropy alloy. Structures with dual gradients (with increasing grain size and a decreasing volume fraction of nanoprecipitates from the surface to the center) were observed to show much better dynamic shear properties compared to both structures with single grain-size gradient and coarse-grained structures with homogeneously distributed nanoprecipitates. Thus, the dual gradients have a synergetic strengthening/toughening effect as compared to the sole effect of a single gradient and the sole precipitation effect. Initiation of the adiabatic shear band (ASB) is delayed and propagation of ASB is slowed down in structures with dual gradients compared to structures with single gradients, resulting in better dynamic shear properties. A higher magnitude of strain gradient and higher density of geometrically necessary dislocations are induced in the structures with dual gradients, resulting in extra strain hardening. Higher density dislocations, stacking faults, and Lomer-Cottrell locks can be accumulated by the interactions between these defects and B2/L1₂ precipitates, due to the higher volume fraction of nanoprecipitates in the surface layer of the structures with dual gradients, which could retard the early strain localization in the surface layer for better dynamic shear properties.

Key words: Medium-entropy alloys, Gradient structures, Precipitates, Strain hardening, Dynamic properties, Adiabatic shear band