Thermal‐Mechanical History and the Strength of Magnesium Oxide Single Crystals: I, Mechanical Tests

Abstract

Various dislocation impurity interactions in magnesium oxide and their effect on mechanical properties between room temperature and 1000°C are discussed. (1) Impurity precipitate particles in single crystals can be redistributed by heat treatment. Their size and density affect the lattice resistance to the motion of fresh dislocations and thus affect the yield strength at room temperature. Above 1200°C impurities go into solid solution and the strength decreases to 0.7 times the as‐received fully‐precipitated value. (2) Fresh dislocations can be locked by heating above 600°C to produce yield phenomena at room temperature. The upper yield strength increases with time and temperature particularly when the aging temperature exceeds 1000°C. Locking is attributed to the diffusion of impurities and point defects to dislocations. (3) The basic temperature dependence of yield strength contains two regions: (a) Up to approximately 500°C the strength decreases rapidly, suggesting a dislocation cutting mechanism during flow. Material preheated above 1200°C shows a stronger dependence than as‐received material. (b) Above 500° C strength decreases linearly at a rate equal to the decline in elastic modulus. Superimposed on this basic temperature dependence curve there is, for material preannealed above 1200° C, a hump due to reprecipitation between 500° and 1000°c.