Improvement of the high temperature oxidation of TaC-strengthened Co(Cr)-based cast superalloys by the addition of nickel

In this work, nickel is added in substitution to cobalt in the composition of a cast cobalt-based chromium-rich superalloy designed to obtain a double-phased {fcc matrix + TaC} microstructure. Two nickel amounts were tested, 13 and 26 wt-%Ni, for 25 wt-%Cr, 0.4 wt-%C and 6 wt-%Ta, the rest being cobalt. The obtained as-cast microstructures are not different from the starting alloy's one, contrarily to the hardness which became lower. The oxidation tests at 1200 degrees C for 170 h demonstrated a significant improvement of the alloy's behaviour, especially for an addition of 26 wt-%Ni. This seems to be attributed to an easier Cr diffusion towards the oxidation front as suggested by the observed Cr contents in extreme surface as well as by the lower Cr concentration gradients. In addition, the TaC seemed to be morphologically more stable in the presence of nickel in the alloys than in the absence of Ni.