The Influence of Ion Implantation of Reactive Elements on the Growth of Cr2O3 Scales on Ni-20% Cr at High Temperature
The additions of reactive elements, such as yttrium or cerium, to nickel-chromium alloys have considerable beneficial effects on the development, growth and mechanical stability of Cr2O3 scales in oxygen at high temperature. Several mechanisms have been proposed to account for these effects, involving processes within the alloy, at the alloy/scale interface or within the bulk scale, but none is universally accepted. In order to investigate the phenomenon further, a study of the influence of ion-implanted reactive elements on the subsequent oxidation behaviour of Ni-20%Cr has been carried out. Implanted cerium or yttrium are particularly effective in increasing the rate of establishment of the healing protective Cr2O3 scale and in decreasing its rate of growth. The latter effect can be associated with a change in diffusion processes in the oxide. The scale on the unimplanted surfaces grows outwards following diffusion of Cr3+ ions from the alloy/scale to the scale/gas interface. Here, the contribution of oxygen diffusion is relatively small. However, the scale on the implanted surfaces grows inwards following diffusion of oxygen from the scale/gas to the scale/alloy interface. On exposure of the implanted surfaces to the environment, the implanted reactive element species are oxidized to form small (5 to 50 nm) particles at or near the metal surface. As the Cr2O3 scale develops, these are incorporated into it as discrete particles, with only a very small solubility in the oxide. However, there is a general suppression of outward diffusion of Cr3+ ions through the scale, enabling inward transport of oxygen to predominate. Possible reasons for these effects are discussed and related to the distribution of the reactive elements in the scale.
KeywordsReactive Element Internal Oxidation Oxidation Metal Outward Diffusion Cr203 Layer
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