As-Cast Microstructures of High Entropy Alloys Designed to Be TaC-Strengthened

In this work two new alloys were obtained by extrapolation from a well known high entropy alloy, the equimolar CoNiFeMnCr one. This was done by the addition of carbon and of tantalum, Ta being one of the strongest MC-former elements. They were produced by conventional casting under inert atmosphere. The obtained microstructures were characterized by X-ray diffraction, metallography, electron microscopy, and energy dispersion spectrometry. Their hardness was also measured by hardness indentation. In parallel, the original CoNiFeMnCr alloy was also synthesized and characterized for comparison. The reference HEA alloy is single-phased with an austenitic structure, while the two {Ta, C}-added alloys are double-phased, with an austenitic matrix and interdendritic script-like TaC carbides. The matrixes of these HEA/TaC alloy are equivalent toan equimolar CoNiFeMnCr alloy to which 2 wt.% Ta is present in solid solution. The presence of the TaC carbides caused a significant increase in hardness which suggests that the HEA/TaC alloys may be mechanically stronger than the HEA reference alloy at high temperature