Slip to Twinning Transition in the Microstructure of CoCrFeMnNi High Entropy Alloy under High Strain Rate Compression

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High entropy alloys (HEAs) is a new concept of alloying design system which using multi-major components in the alloy. Little research has been done on the mechanical properties of CoCrFeMnNi high entropy alloy. This thesis focuses on the mechanical behavior and the microstructure transformation of CoCrFeMnNi HEA, with equimolar concentration of each component, under high strain rate (HSR) compression. The HSR compression experiment was carried out using Kolsky pressure bar at strain rate of 5000 s-1 and 8000 s-1. The HSR experiments results is compared with the deformation under quasi-static compression at strain rate of 10-3 s-1. The alloy exhibits higher yield strength, Vickers hardness, and strain hardening rate under high strain rate compression compared to conventional alloy system, such as stainless-steels, in the same strain rate conditions. The pre-strain and post-strain microstructures were studied using scanning electron microscopy and transmission electron microscopy. At HSR condition, twinning is the preferable deformation mechanism with certain grain orientations while slip band formation dominates the deformation under quasi-static strain rate. Furthermore, not only there are nanotwins formed at the HSR conditions, but there is zero nano-twinning formation in the quasi-static condition in spite of the high yield strength and high flow stress during the process. A deformation mechanism alteration of slip to twinning transition is observed when increasing strain rate from 10-3 s-1 to 8000 s-1. The great mechanical properties that results from high strain hardening, nano-twinning formation, and HEA core effects make the CoCrFeMnNi alloy an ideal potential material for military armor applications and structural applications.%%%%M.S., Materials Science and Engineering – Drexel University, 2017