Dynamic shear behavior of forged CrMnFeCoNi high entropy alloy using hat-shaped specimens over a wide range of temperatures and strain rates

Abstract

Dynamic shear deformation generally occurs in machining, punching and other manufacturing processes, so it is of key significance to study the shear properties of materials under high strain rate for guiding machining and realizing the application of materials. In this work, the dynamic shear behavior of forged CrMnFeCoNi high entropy alloys at different temperatures and strain rates was studied by using hat-shaped specimens. Dynamic loading at a wide temperature range (-100–650 °C) and ultra-high strain rate (5 × 105 s−1) was achieved. To investigate the influence of specimen geometry on shear properties, the shear widths (the horizontal width between the upper hat portion and the lower brim portion of the specimen) were set as 50 μm,100 μm, 200 μm and 400 μm, respectively. The results showed that the adiabatic shear was a precursor to failure, and the resistance to adiabatic shear localization of this alloy was related to the strain rate and geometric effect. The shear strength decreased gradually with the increase of temperature. The microstructure of the shear zone was characterized. The central ASB was composed of nanograins with the size of 200–300 nm, containing nano-twins and dislocations. The hardness of the central ASB was 70 % higher than that of the matrix due to the strengthening effect of fine grains. The grains in the transition region were deformed along the shear direction to form slender sub grains containing a large number of dislocations and nano-twins, resulting in strain hardening. The cracks propagated along the ASB, and the ductile fracture characterized by dimples was the main fracture mode.