Enhanced fracture toughness in nonstoichiometric magnesium aluminate spinel through controlled dissolution of second phase alumina

Dissolution of Al2O3 particles into a MgAl2O4 (spinel) matrix is accompanied by a volumetric expansion that is predicted to lead to a compressive stress field upon cooling, resulting in a promising microstructure for enhanced toughening of transparent spinel. This study explores the conditions to form such a microstructure by hot-pressing powders of Al2O3 and spinel, at temperatures that promote dissolution of the Al2O3. Tough, particulate-reinforced composites were formed under lower temperatures and shorter times, but single phase, cubic spinel was formed at 1700 degrees C for 10 hours. The single phase spinel made in this way exhibited a toughness of 2.26 +/- 0.17 MPam, significantly higher than the equivalent nonstoichiometric spinel made by traditional methods, 1.72 +/- 0.06 MPam. X-ray diffraction measurements revealed lattice parameter changes consistent with the dissolution of Al2O3 into spinel. Mechanics modeling reveals that toughening arises due to the volume expansion as Al2O3 dissolves into the spinel matrix.