Quantitative analysis of grain boundary segregation and fracture behavior in Ca-doped magnesium aluminate spinel.

Grain boundary structure and chemistry play crucial roles in determining atomic diffusion, grain growth, and mechanical behavior in magnesium aluminate spinel (MgAl2O4, spinel). The grain boundary structure and chemistry can be modified by doping with cations, such as Ca2+, to facilitate grain boundary segregation and tailor such properties. However, the nanoscale details of the segregation behavior of calcium and its mechanism, and hence the dopant behavior influence on the grain boundary mechanical properties, have not yet been clearly understood. In this study, the segregation behavior of Ca2+ in spinel was quantitatively analyzed by X-ray energy dispersive spectrometry (XEDS) and electron energy-loss spectrometry (EELS), and compared to undoped spinel. Composition profiles obtained from EELS spectrum-imaging datasets across grain boundaries showed calcium segregation at the boundary core correlated with magnesium depletion. Additionally, energy-loss near-edge structure (ELNES) analysis of the Mg–K edge indicated that changes in the bonding status of the Mg2+ sublattice occurred after Ca-doping. It is implied that Ca2+ cations replace Mg2+ at tetrahedral sites in spinel. The calcium enrichment at grain boundaries obtained through quantitative XEDS analysis was in the range of 0.1–0.9 atoms/nm2 (0.03—0.26 monolayers). Calcium segregation had no detrimental effect on the indentation fracture toughness estimated through microindentation. The fracture behavior remains unaltered compared to undoped spinel, presenting primarily transgranular cracking. These results suggest that magnesium replacement by calcium atoms at the grain boundary core occurred with no compromise on the fracture toughness or fracture behavior, allowing it to maintain similar mechanical integrity as undoped spinel. [ABSTRACT FROM AUTHOR]