Microstructural evaluation of thermal-sprayed CoCrFeMnNi0.8V high-entropy alloy coatings

The aim of this work is to improve the understanding of the effect of the cooling rate on the microstructure of high-entropy alloys, with a focus on high-entropy alloy coatings, by using a combined computational and experimental validation approach. CoCrFeMnNi0.8V coatings were deposited on a steel substrate with high velocity oxy-air-fuel spray with the employment of three different deposition temperatures. The microstructures of the coatings were studied and compared with the microstructure of the equivalent bulk high-entropy alloy fabricated by suction casting and powder fabricated by gas atomization. According to the results, the powder and the coatings deposited by low and medium temperatures consisted of a BCC microstructure. On the other hand, the microstructure of the coating deposited by high temperature was more complex, consisting of different phases, including BCC, FCC and oxides. The phase constitution of the bulk high-entropy alloy included an FCC phase and sigma. This variation in the microstructural outcome was assessed in terms of solidification rate, and the results were compared with Thermo-Calc modelling. The microstructure can be tuned by the employment of rapid solidification techniques such as gas atomization, as well as subsequent processing such as high velocity oxy-air-fuel spray with the use of different spray parameters, leading to a variety of microstructural outcomes. This approach is of high interest for the field of high-entropy alloy coatings

Investigation of the strain rate sensitivity of CoCrFeMnNiTix (x = 0, 0.3) high-entropy alloys using the shear punch test

High entropy alloys (HEAs) are a novel class of metallic materials that exhibit a unique blend of properties due to their chemical composition and atomic arrangement. This research aims to investigate the strain rate sensitivity (SRS) of two HEA CoCrFeMnNiTix (x = 0, 0.3) alloy compositions through the use of shear punch testing. This method has been proven to provide reliable results for both HEA materials, including the CoCrFeMnNiTi0.3 HEA composition which was found to be inherently brittle and contained both σ-phase and Laves phase compounds with a hardness close to 14 GPa and a soft FCC phase. Among all the testing temperatures (room temperature to 400 °C) and deflection rates (0.2, 2 and 10 mm.min−1) used, only the CoCrFeMnNi HEA alloy was found to exhibit SRS at room temperature (m = 0.0333), while for the other HEA alloy variant and testing conditions, the SRS was found to be zero. From empirical correlations and finite element analysis (FEA), the calculated value for m ranged from 0.0333 to 0.0359, thus evidencing that the FEA simulations provide an accurate and suitable means of capturing the deformation behaviour of such alloys when subjected to shearing