Effect of Crystal Orientation on Plastic Deformation of Magnesium Oxide

Stress‐strain data for single crystals of MgO tested in compression with 〈110〉 and 〈111〉 loading axes are presented for temperatures ranging from 26〉 to 1250°C. Stress‐strain data for polycrystalline MgO are also presented over the same temperature range. Single crystals with a 〈111〉 loading axis were found to deform plastically on the {100}〈110〉 slip systems at temperatures above 350°C. The total strain at fracture for polycrystalline MgO at room temperature was about 0.6%; above 600°C it was about 2%. The general inability of the {110}〈110〉 slip systems of this structure to satisfy the Taylor requirement, i.e., the necessity of five independent slip systems, ease of cleavage, and slip nonuniformity, limits polycrystalline ductility at low temperatures. At higher temperatures, slip can occur on {100}〈110〉 slip systems, thus providing the additional slip, systems necessary to satisfy Taylor's criterion; also, stress‐induced climb and high dislocation mobility inhibit cleavage fracture.