Influence of high strain rate deformation on the microstructure and mechanical behavior of NiCoCrFe high entropy alloy

The paper reported a high-entropy alloy, NiCoCrFe, which was produced in a Vacuum Induction Melting (VIM) furnace. In order to homogenize the microstructure, the as-cast ingot was rolled, annealed and machined to appropriate dimensions. Then the Split Hopkinson Pressure Bar (SHPB) was used to evaluate the specimens under various strain rates. True stress-strain curves of this test showed similar patterns in all strain rates, with two peaks that can be assigned to the competition between strain hardening and thermal softening. Modified Johnson-Cook model applied to characterize flow behavior of the alloy. X-ray diffraction results indicated that the alloy was single-phase with FCC crystal structure at the casting condition, after thermomechanical operation and after high-strain rate tests. The specimens were further examined by EBSD. The results indicated the impressive presence of twins in the samples deformed at strain rate of 1500s-1, confirms the significant contribution of twinning in the strengthening process; Regarding the low SFE of the mentioned alloy, it can be said that Twin-Induced Plasticity (TWIP) has occurred. By strain rate enhancement to 3000s-1, finer and elongated grains were observed while twinning decreased because of SFE increase as a result of temperature elevation due to adiabatic heating during deformation. According to KAM map, there was strain gradient in mentioned sample, which results in production of GNDs and GNDs pile-up leading to strain hardening by creating obstacles to further slip. Moreover, in the hat-shaped samples, which used for shear localization resistance evaluations, bundles of nano-twins were observed whose formation largely prevented strain localization and make the above alloy suitable for applications exposed to high-speed impacts.