The occurrence of yield drops during the high temperature bending of magnesium oxide single crystals

Abstract Yield drops are often observed during bending deformation of magnesium oxide single crystals at high temperature. It is suggested that these are the result of strong dislocation pinning produced by the heat treatments preceding the deformation. Because fresh dislocations are introduced into the compression face of the crystal by contact with the loading knife-edge, the initial deformation of the crystal occurs plastically on the compression face, but elastically on the tension face. The yield drop is caused by the onset of plastic flow in the tension region, which converts the crystal into a normal symmetrically deforming beam. The unusual stress configuration before the yield allows large stresses to be attained on the tension face although the applied load need not be large. Estimates of the strength of the pinning from the magnitude of the yield drop alone, assuming the beam to have behaved completely elastically up to the yield point, can thus be widely in error. The yield drop can never exceed 50%, whereas surface stresses >15 times the normal flow stress for fresh dislocations have been observed in the tension region without plastic flow occurring.