Ruzica Petkovic-Luton

The influence of yttrium on oxide scale growth and adherence

Alloys and coatings for high-temperature service are designed to form selectively chromia scales, alumina scales, or, to a limited extent, silica scales upon exposure to the environment. For such oxide scales to be protective, they should be both slow growing and adherent. It turns out that the addition of yttrium to such alloys can often impart both characteristics to the oxide scale. However, the actual operating mechanisms continue to be a matter of controversy among researchers in the area of oxidation. In the present study, the growth and adherence of alumina and chromia scales on alloys containing yttrium, either as an oxide dispersion or as an intermetallic phase, have been investigated in conjunction with detailed oxide scale characterization using the techniques of scanning electron microscopy (SEM), transmission electron microscopy (TEM), and secondary ion mass spectrometry (SIMS). The results of the study are used for critical assessment of the proposed mechanisms, especially the more recent ones, and to suggest some new mechanisms for adherence.

Mixed-oxidant attack of high-temperature alloys in carbon- and oxygen-containing environments

The corrosive degradation of high-temperature alloys in environments containing more than one oxidant cannot, in general, be predicted from a knowledge of the response of the materials to the individual oxidants. In the present study, the phenomenological changes associated with the degradation of iron-nickel-chromium base alloys in carbon-oxygen environments have been investigated by examining the microstructural changes in samples exposed to such environments for extended periods of time. The results of these studies have led to the formulation of a model which proposes that the material exposed to the reaction environment experiences five stages of microstructural changes close to the surface before severe degradation sets in. The end of Stage V is the start of severe degradation, which contributes to a complete modification of the microstructure. This, in turn, leads to a rapid deterioration of the mechanical properties of the material.

Crystal structure of intermetallic phase in Fe--20Cr--4Al--0. 5Y alloy by convergent beam electron diffraction

The crystal structure and chemical composition of the intermetallic phase in a Fe--20%Cr--4%Al--0.5%Y (wt. %) alloy were investigated by electron microscopy. Convergent beam diffraction studies revealed that the intermetallic phase forms in three different crystal structures that could coexist in a single grain of the phase. The dominant crystal structure was shown to be hexagonal (a = 0.85, c = 0.84 nm) with a space group most likely to be P6/sub 3//mmc. Within the hexagonal phase, regions of a rhombohedral crystal structure (a = 0.85, c = 1.26 nm) were observed that had grown in without an apparent phase boundary separating the two crystal structures. The third crystal structure was determined to be monoclinic (a = 0.97, b = 0.85, c = 1.07 nm, and beta = 97.3/sup 0/) and formed by twinning on the )101-bar1) planes of the hexagonal phase. The chemical compositions of regions with different crystal structures were comparable and the stoichiometry of the intermetallic phase corresponds to (Fe,Cr)/sub 17/(Al,Y)/sub 2/. The relationship of the observed crystal structures to those previously reported is discussed.