Garnet stability in the Al–Ca–Mg–Si–Y–O system with implications for reactions between TBCs, EBCs, and silicate deposits

This work expands the understanding of the garnet solid solution field in the yttria-calcia-magnesia-alumina-silica system. The crystal chemistry was used to define the theoretical extent of the garnet homogeneity range based on site-specific substitution rules and charge neutrality constraints. Compositions lying within this range were equilibrated at 1400°C. The resulting phase assemblages were analyzed to determine the identity and compositions of the equilibrium phases. Quantitative chemical analysis was used to infer that Mg²⁺ preferentially partitions into the intermediate-sized octahedral site compared to the larger dodecahedral site, that Ca²⁺ has a higher solubility in the dodecahedral site than Mg²⁺, and that the presence of Mg²⁺increases the total Si⁴⁺ solubility in the garnet structure. The results were also used to map the extent of the garnet solid solution phase field at 1400°C. The findings are discussed in the context of efforts to understand the interactions between Y-containing thermal and environmental barrier coatings and silicate deposits.