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Microstructure stability of three carbides-strengthened cobalt-based alloys in the first times of thermal cycling

Patrice Berthod

The alloys for applications at high temperaturemaymicrostructurally evolve in service. This is notably the case of the volume fractions and of the morphologies of the carbides reinforcing the alloys issued from certain families, as the cobalt-based superalloys. If these phenomena are obvious after a long time spent at high temperature, they may start during the first hours of thermal cycling, with more or less consequences, this notably depending on the amount and the natures of the strengthening carbides. In this work, two types of carbides among the most used ones, were considered in a Co-30Cr alloy base: tantalum carbides and chromium carbides, either separately or present together. Four maximal temperatures were studied, between 1050 and 1250°C, for the 2 hours isothermal stage of the thermal cycle.After three cycles themicrostructures of the alloyswere analysed by SEMobservations, DRX runs and image analysismeasurements to characterize the microstructure evolution from the initial as-cast state. This evidenced a rapid evolution of both morphology and volume fraction of carbides which should continue during the following cycles, as shown by the great differences still remaining with the thermodynamic calculations performed for the stage temperatures.