The reactive element effect (R.E.E.) : a tentative classification

Abstract
The oxidation resistance of many high temperature materials, especially gas turbine materials, can be enhanced by applying protective coatings. In this study, aluminide and chromaluminide coatings were applied on a number of superalloys, viz., IN738, René 80, B1900, IN100 and IN713, using both high and low activity pack cementation process, and subjected to cyclic oxidation. Oxidation behaviour of the coated material was found to be dependent on the type of pack used, i.e. low or high activity pack and also on the composition of the base alloy. It is well established that small additions of so-called "reactive elements" increase the oxidation resistance of alumina former superalloys. The beneficial effects of these "reactive elements" are currently regarded as being of two kinds : 1) an improvement in oxide to metal adhesion, and 2) a reduction in oxidation rate for some systems. The results on the effect of certain alloying elements, such as Mo, Ta, Ti and Hf on the oxidation behaviour of these aluminide coatings are presented in this paper and a classification of the "reactive elements" is proposed.