A study of the dependence on the Co and Ni proportions of the oxidation at elevated temperature of TaC-strengthened {Ni and Co}-based cast superalloys

Abstract

Cobalt and nickel may play antagonist roles on the microstructures and high temperature properties of cast chromium-rich superalloys containing the same tantalum and carbon molar fractions. Since such alloys may be considered for uses at temperature levels as high as 1200 °C, the whole range of base compositions was explored with six model quaternary alloys with Ni and Co contents varying from 0 to 100% of the remaining elements, the 25 wt%Cr, 0.4 wt%C and 6 wt%Ta being deducted. The interdendritic carbide network of the as-cast microstructures is made of chromium carbides and tantalum carbides for the Ni-richest alloys and of TaC only for the Co-richest ones. {170 h, 1200 °C}-exposure carried out in laboratory air led to important morphology deterioration for the two Ni-richest alloys but limited fragmentation of the TaC in the four Co-richest alloys. At the same time, the most of the alloys more or less well behaved in oxidation, except the two Co-richest alloys for which the too hindered volume diffusion of chromium did not sustain the chromia-forming behavior. Notably the Ni-free cobalt-based alloys suffered from starts of catastrophic oxidation in several surface locations. The quaternary alloy bases allowing taking benefit of the best compromise between hot oxidation resistance and possibly good mechanical behavior at elevated temperature was the model alloys with 14–28 wt%Ni.