Institut Jean Lamour (University of Lorraine),
Keywords: high entropy alloys, HfC carbides, TaC carbides, microstructures, high temperature properties
Summary:In this work, the replacement by more common elements (Ni, Fe, Mn), in Co-based superalloys, of the main part of the critical cobalt element was undertaken. Taking two superalloys with interesting high temperature chemical and mechanical properties as references (Co–25Cr–0.25C–3.7Hf and Co–10Ni–30Cr–0.2C–4Ta), three alloys were elaborated by casting: a well–known equimolar CoNiFeMnCr HEA alloy and two versions with C and Hf or Ta to promote the formation of HfC or TaC carbides in quantities similar to the two previously cited Co–based superalloys. The CoNiFeMnCr equimolar alloy is single–phased while its versions containing either Hf and C or Ta and C are both double–phased: a dendritic matrix composed of the equimolar Hf–free CoNiFeMnCr solution with interdendritic script–like eutectic HfC carbides for the alloy containing additionally Hf and C, and a dendritic matrix composed of Co, Ni, Fe, Mn and Cr with contents all close to 20 wt.% and containing about 2 wt.%Ta, with interdendritic script–like eutectic TaC carbides for the alloy containing additionally Ta and C. Since Hf is – as previously encountered in chromium–rich cobalt, nickel and iron based alloys – totally absent in the CoNiFeMnCr matrix, the equimolar alloy added with Hf and C is really the association of the HEA alloy and the HfC carbides (both phases are clearly distinct). This is not true for the equimolar alloy added with Ta and C since a part of Ta and of C as entered the matrix. In addition to the microstructural characterization of these HEA+MC composites, results of hardness, oxidation or creep deformation at high temperature will be presented.