ANALYSIS AND MODELING OF TRANSPORT PROCESSES IN ALUMINA SCALES ON HIGH-TEMPERATURE ALLOYS

The oxidation behaviour of a number of FeCrAl-based oxide dispersion strengthened (ODS) alloys of the type MA 956 was investigated at temperatures between 1100-degrees-C and 1200-degrees-C. The main emphasis was placed on the effect of alloy yttria content on scale composition and structure. Studies of three model alloys with various alloy yttria contents in the range 0.02-0.7% showed an increase in the oxide growth rate with increasing yttria content. Alumina scales on alloys with 0.17% and 0.7% yttria grow almost exclusively by oxygen diffusion, whereas the scale on the alloy with 0.02% yttria exhibits a significant contribution from cation diffusion. By using SIMS depth-profiling and SIMS-imaging it was found that the oxygen diffuses through the alumina scale via oxide grain boundaries. TEM-studies revealed that the yttria is incorporated in the scale grain boundaries as precipitations and as a segregation layer. This behaviour of the yttrium is believed to be responsible for its influence on scale growth mechanisms. A mathematical model has been developed which describes the alumina scale growth by oxygen diffusion through grain boundaries. The model accounts for the observed increase in aluminium oxide grain size in scale growth direction.