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

ABSTRACT We recently reported that the columnar grain width of Cr2AlC MAX phase coatings increases during oxidation (4？h at 1120°C) by 80%, while for 0.7 at.% Si additions to Cr2AlC, coarsening of only 12% was observed. Here, we use nm scale compositional and microstructural investigations to identify significant differences between Cr2AlC and Cr2Al(Si)C. In particular, needle-shaped precipitates coarsen into globular Cr3Si 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. GRAPHICAL ABSTRACT IMPACT STATEMENT This is the first report of Zener pinning in MAX phases. We show that Si additions to Cr2AlC cause the formation of precipitates, which retard grain coarsening during oxidation