Correlation between grain-boundary segregation behaviors of calcium and yttrium and enhanced fracture toughness in magnesium aluminate spinel

Polycrystalline magnesium aluminate spinel, or simply spinel, with near theoretical density is a transparent ceramic material with multiple applications in extreme environmental conditions, which require an enhanced fracture toughness. For this reason, in this study, the effect of 500 ppm doping level of calcium (Ca) and yttrium (Y) on the segregation behavior and mechanical properties of spinel was quantitatively investigated. Calcium and yttrium doping reduced the grain-boundary plane anisotropy for grain boundaries with rotations about the [111] axis. More tilt boundaries and fewer twist boundaries were found in the doped samples compared to the undoped condition. Direct observations by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging revealed that yttrium atoms preferentially occupy aluminum sites at grain boundaries in spinel. Quantitative electron energy-loss spectrometry (EELS) analysis in the vicinity of grain boundaries indicated that calcium atoms preferentially occupy magnesium sites in the Ca-doped spinel samples and confirmed the substitution of aluminum by yttrium atoms in the Y-doped sample. Quantitative X-ray energy-dispersive spectrometry (XEDS) analysis employing the zeta-factor method indicated that the maximum segregation levels at grain boundaries were 0.8 +/- 0.1 calcium atoms/nm2 (0.23 +/- 0.03 monolayers) and 2.4 +/- 0.06 yttrium atoms/nm2 (0.45 +/- 0.11 monolayers). Enhanced indentation fracture toughness was found in samples with calcium and yttrium doping compared to undoped spinel. The enhanced fracture toughness in Ca- and Y-doped spinel samples, in comparison with undoped spinel, was primarily attributed to more pronounced crack deflections and a more tortuous crack path.