Unusual phase transformation and novel hardening mechanisms upon impact loading in a medium entropy alloy with dual heterogeneous structure

Dynamic shear properties and corresponding deformation mechanisms of a CrCoNi-based medium entropy alloy (MEA) with both coherent nanoprecipitate and heterogeneous grain structure were investigated utilizing hat-shaped specimens in Hopkinson-bar and microstructure characterization. The present MEA with dual heterogeneous structure is found to have extraordinary dynamic shear properties, as compared to the other advanced metals and alloys, including the CrCoNi MEA with similar heterogeneous grain structure while with single FCC phase. A formula for considering effects of strain hardening, strain rate and thermal softening was utilized to calculate the critical shear strain for onset of adiabatic shear band for the present MEA, and the experimental results were found to be well predicted by the theoretical estimations. The formations of microscopic shear bands with low-angle grain boundary type, nano-grains, deformation twins, stacking fault network, and Lomer-Cottrell locks were found to be responsible for the multiple strain hardening mechanisms during dynamic shear deformation for both unaged and aged samples. An unusual phase transformation (γ→σ) was observed for only aged samples under dynamic shear loading, which was not found under quasi-static conditions before, should have significant contribution on strain hardening due to the precipitation by-passing hardening by the hard σ nano-particles.