Microhardness investigations on manganese aluminate spinels

Vickers microhardness measurements were carried out on single-crystalline manganese aluminates having compositions ranging from Mn_1.83Al_1.17O₄ to Mn_0.91Al_2.06□_0.03O₄. Hardness increases with increasing alumina content, but the hyperstoichiometric composition shows a decline of hardness probably due to structural vacancies. In Mn_1.83Al_1.17O₄ the crack pattern round indentations is independent of relative indenter orientation and can be accounted for by the presence of a {110} glide system. Apparent hardness anisotropy is related to the position of these glide planes in the surface. In all other harder specimens of this series, other glide systems also become operative during indentation. The occurrence of a pre-precipitation phase after the annealing of specimens is revealed by an increase in hardness, followed by a decline which is due to over-ageing.