Effect of the Ni/Co Ratio on the High-Temperature Oxidation Behavior of {Ni&Co}-Based {25Cr–0.4C–1.6Ti, wt%}-Containing Cast Alloys

Abstract

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.