THE EFFECTS OF REACTIVE ELEMENT ADDITIONS AND SULFUR REMOVAL ON THE ADHERENCE OF ALUMINA TO NI-BASE AND FE-BASE ALLOYS

The effects of reactive element additions to alumina-forming alloys (s ingle-crystal Ni-base and ferritic Fe-Cr-Al alloys) and the effect of hydrogen annealing to remove sulfur on the oxide adherence to these al loys have been studied. The results have shown that desulfurization by hydrogen annealing can result in improvements in cyclic oxidation com parable to that achieved by doping with reactive elements. The results have also shown that then is less stress generation during the cyclic oxidation of Y-doped FeCrAl compared to Ti-doped or desulfurized FeCr Al. This indicates that the growth mechanism, as well as the strength of the oxide/alloy interface, influences the ultimate oxidation morpho logy and stress state which will certainly affect the length of time t he alumina remains protective. It has been shown to be possible to est imate the amount of sulfur available to segregate to the alloy/oxide i nterface and how this is influenced by reactive element additions or h ydrogen annealing. If these calculations can be made more quantitative it should be possible to engineer alumina-forming alloys for optimum resistance to cyclic oxidation e.g. by combining an appropriate desulf urization treatment and choice of reactive element addition.