Nano-scale Si segregation and precipitation in Cr2Al(Si)C MAX phase coatings impeding grain growth during oxidation

摘要

We recently reported that the columnar grain width of Cr_(2)AlC MAX phase coatings increases during oxidation (4?h at 1120°C) by 80%, while for 0.7 at.% Si additions to Cr_(2)AlC, coarsening of only 12% was observed. Here, we use nm scale compositional and microstructural investigations to identify significant differences between Cr_(2)AlC and Cr_(2)Al(Si)C. In particular, needle-shaped precipitates coarsen into globular Cr_(3)Si precipitates upon oxidation in the Si-containing MAX phase. We infer that the presence of these precipitates, which are located predominantly along grain boundaries in the MAX phase, retards coarsening during oxidation by Zener pinning.