| Abstract Scope |
Nanocrystalline magnesium aluminate spinel (MgAl2O4) exhibits exceptional hardness in excess of 20 GPa for grain sizes below 50 nm. Using spinel samples with grain sizes ranging over 4-100 nm formed by EC-PAS (environmentally controlled pressure-assisted sintering) at the US Naval Research Laboratory, we studied the thermal stability and mechanical properties from room temperature to 400C for samples using x-ray diffraction and nanoindentation, respectively. Annealing of spinel samples at 400C, a significant fraction of the sintering temperature (640-850C), for 24 hours resulted in no grain size evolution. Elevated temperature nanoindentation testing resulted in a minor, but transient reduction in hardness that was recovered upon return to room temperature. Variations in the degree of flow serration were also observed. Additionally, we measured statistically significant transient and permanent changes to the elastic moduli of the samples, potentially indicating an internal relaxation of the structure unrelated to grain size. |