Strength of Flame - Fusion - Grown Magnesium - Aluminum Spinel

Alumina-rich single crystals of Mg-Al spinel with molar ratios of 1.7 to 3.5 were grown by flame fusion. The fracture load of these crystals was measured. It was found (1) that in unannealed crystals the strength increased with increasing alumina content and (2) that strength was influenced by the temperature of heat treatment. The strength increased with increasing annealing temperature above 650°C up to a maximum value; the temperature at which this value was reached depended on the molar ratio of the spinel and the duration of annealing. This behavior can be explained as a precipitation strengthening, as is typical of alloys. Pre-precipitation is thought to be the cause of the observed strengthening.