TEM study of the recovery process of cyclically deformed MgO single crystals

Single crystals of MgO were subjected to plastic strain-controlled push-pull cyclic deformation at elevated temperatures. Below 400° C the crystals were very brittle and failed with a few fatigue cycles. At 470° C a large number of cycles could be obtained before failure, and the cyclic stress-strain response showed a period of rapid hardening followed by a period of decreasing hardening rate. TEM investigations of the lower temperature samples show structures of isolated dislocation dipoles, multipoles and debris. At 470° C dense bundles of dislocations were observed aligned perpendicular to the Burgers vector direction. The regions between the bundles were relatively dislocation free, but they contained a high density of debris. Bowed out screw dislocations are observed between the edge dislocation bundles, suggesting that screw dislocations were largely mobile. Comparisons are made with the cyclic deformation and structure of fcc metals and other NaCl structure single crystals.