Behavior at elevated temperature of cobalt-based superalloys strengthened by ZrC carbides
Résumé
Zirconium is an element which is commonly used for trapping the impurities (e.g. sulfur) during the casting of metallic alloys to prevent any detrimental effect on their mechanical properties. Zr is also a possible base element for some alloys used in the nuclear industry (e.g. Zircaloy-4). In addition, zirconium is also a MC carbide-former element which may bring superalloys potential high strength at high temperature. As many MC carbides, eutectic ZrC may precipitate at solidification with high imbrication with the matrix. They are also very stable at elevated temperatures, in term of morphology and volume fraction. ZrC are thus possibly very efficient for the strengthening of alloys needing high resistance to creep deformation at high temperature. Despite these advantages, ZrC are curiously rarely used for this purpose and this study aims to test this alternative strengthening solution in refractory cobalt-based alloys.
Two Co-based alloys, Co-25Cr-0.25C-1.9Zr and Co-25Cr-0.50C-3.8Zr, were elaborated by casting to investigate their high temperature microstructures and properties. SEM/BSE examinations were carried out to verify if the expected microstructures (matrix with dendritic development, single MC nature and script-like morphology of carbides) were well obtained. Differential Thermal Analysis, flexural 3-points creep and thermogravimetric oxidation were performed to explore the potentials of high temperature properties and to identify the eventual problems to solve before benefiting of such new high performance refractory alloys.
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