Experimental and thermodynamic study of the influence of the base elements on the carbides natures in {Ni,Co}-based {25Cr, 0.4C, 6Ta}-containing alloys

Primary carbides may be important for the high temperature strength of polycrystalline Cr-rich cast alloys. TaC are among the best carbides for this role. Their presence depends on the base elements of the alloys. This dependence is here studied in the case of a series of Cr-rich alloys based on Ni and/or Co and containing Ta and C in equal molar fractions. Real alloys were cast and exposed at 1400 K and 1510 K, and their as-cast and aged microstructures were characterized. In parallel thermodynamic calculations using Thermo-Calc and a home-made database were carried out. It appears that TaC is the single carbide present in the alloy stabilized at high temperature only if the Co content is higher than the Ni one. Discrepancies appeared between calculations and the experimental results, showing that the used database must be improved. The experimental part of this work provides microstructures data which can be used to test databases and to enrich them if necessary.