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 degrees 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 Mg(2+)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 degrees 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.