Titanium carbides feature among the MC carbides which tend to crystallize with a script-like shape recognized to be favorable to mechanical strength at high temperature for equi-axed cast refractory alloys. Recent results demonstrated that script-like TiC can be successfully obtained in Co-based alloys while they are less stable than chromium carbides in nickel-based alloys. The present work aims to study whether the addition of cobalt to nickel-based alloys may stabilize TiC. This double investigation, by thermodynamic calculations and by the elaboration of a series of nickel-based alloys with an increasing part of cobalt, demonstrates that the progressive enrichment in cobalt tends favoring the presence of TiC instead chromium carbides. To obtain a majority of TiC, script-like shaped and in significant quantity, cobalt must be more present than nickel in the alloys. But this enrichment in cobalt deteriorates the good resistance against hot oxidation.

Six alloys, based on Ni and/or Co with Co/(Ni+Co) ratio varying from 0 to 1, and containing 25wt.%Cr, 0.4wt.%C and 1.6wt.%Ti, were synthesized by conventional foundry way. Their as-cast microstructures were controlled by electron microscopy. Parts of these alloys, properly prepared on surface, were then exposed to atmospheric air during 20 hours at 1400K. The obtained surface states and sub-surfaces were characterized by optical observations and cross-sectional examinations by electron microscopy. The nickel-richest alloys contained more chromium carbides than titanium carbides while this was the opposite for the cobalt-richest alloys. This confirm the results of a recent study. It was furthermore found here that the alloys with many chromium carbides and few TiC were much better in oxidation at high temperature than the ones with many TiC and few chromium carbides. By considering the apparent involvement of titanium in the oxidation process it seems that the major influent parameter is directly the base element, independently on the nature of the obtained primary carbides.

Many of the cast superalloys chosen for specific applications at high temperatures, such as tools devoted for shaping molten glass, are based on nickel or cobalt. They contain chromium and carbon to achieve good resistances against both mechanical stresses and hot oxidation/corrosion, by favoring the formation of reinforcing carbides at solidification and the development of a protective oxide scale of chromia during service. In presence of tantalum or titanium in the chemical composition of the alloys, high performance MC carbides may be obtained but this depends on the base element. One recently observed how the respective proportions of nickel, cobalt and chromium may promote the formation of TiC or TaC at the expense of chromium carbides. The ratings chosen for No, Co and Cr may have high influence on the oxidation of the alloys at high temperature this is what was studied in this work.