Role of Active Elements and Oxide Dispersions in the Development of Oxidation-Resistant Alloys and Coatings

摘要

The effect in more detail and establishing the critical parameters to aid the design of alloys having improved oxidation resistance were studied. Alloys in the CoCrAl system which form Al sub 2 O sub 3 scales were studied, Co-10Cr-11Al being the basis composition. The alloys also contained between 0 and 1 wt % Y or Hf. Oxidation kinetics in dry air were measured over the temperatures range 1000 to 1200 exp 0 C for times up to 1000 h; thermal cycling tests were also carried out. The morphological development of the scales was studied in detail. Preferential oxidation of Hf and to a lesser extent Y, results in the formation of numerous oxide protrusions penetrating into the substrate alloy and these are largely responsible for the improvement in scale adherence. The pegs consist predominantly of Al sub 2 O sub 3 which has grown inwards encapsulating the Hf- or Y-rich internal oxide particles. The size and distribution of the oxide protrusions play a decisive role in maintaining scale adherence over long exposure periods. Hf additions are better in this respect than Y additions, because other than at very low Y contents, most of the Y is present as the intermetallic yttride, which gives rise to relatively large, more widely spaced pegs. Hf is completely soluble in the alloy up to at least 1.5 wt %, and thus the consequent, more uniform distribution of Hf in the alloy matrix results in a better size and distribution of the oxide pegs at the alloy-scale interface. The distribution can be improved even further by a pre-internal oxidation treatment which converts the Hf to a fine dispersionof oxide particles in the alloy. These act as sites for peg development and a finer, more uniform distribution is achieved. (ERA citation 05:001446)