Cyclic deformation of molybdenum single crystals II. Dislocation substructures and cyclic hardening in crystals deformed at 400 K

Abstract

The study of the cyclic deformation behaviour in tension-compression of molybdenum single crystals described in Part I has been completed with the observation by transmission electron microscopy (TEM) of the dislocation substructures developed at 400 K. The dislocation microstructures' morphologies were determined, the dislocation densities measured and the dislocation Burgers vectors evaluated. The different regions of the c.s.s. curve were correlated with characteristic dislocation substructures.

At low plastic-strain amplitudes the dislocation substructure consists of poorly developed tangles and, as the plastic-strain amplitude increases, dislocation bundles, walls and cells are formed and developed. Probably the most important feature is the presence of secondary dislocations, mainly with conjugate Burgers vectors, which increase very strongly with increasing plastic-strain amplitude. It is the activity of secondary sources which governs the cyclic hardening behaviour of the crystals. This activity of secondary sources is related to the plastic-strain amplitude imposed and the peak stress achieved during the first few cycles of deformation, and is responsible for the cyclic hardening behaviour in each region of the c.s.s. curve. Hence the characteristic shape of the c.s.s. curve obtained at the transition temperature is fully explained.