Oxidation Behavior and Structure Stability at 1250 °C of Chromium-Rich TaC-Containing Cast Alloys Based on Nickel and Cobalt

In this work, six refractory nickel and cobalt-based cast alloys, rich in chromium and designed to be strengthened by tantalum carbides, were cast in controlled atmosphere. Samples were especially prepared to investigate their surface behavior in synthetic air at 1250 °C for three days. Their oxidized states were first characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) on surface, prior to following metallography processing. Second, oxides and subsurfaces were examined by SEM and energy dispersion spectrometry (EDS) on cross sections. Results show that the surface and subsurface deterioration by oxidation clearly depend on the relative quantities in Ni and Co in the chemical composition of the alloys. All alloys well-resisted oxidation due to their chromium contents, but beginnings of catastrophic oxidation were noticed for the alloys containing much more cobalt than nickel. Tantalum, present with particularly high contents, played an important role in the oxidation phenomena and for the microstructure stability at elevated temperature. It notably influenced the adherence of the external scale and led to highly stable heat-resistant TaC carbides, the fraction of which is governed by the base element.