Wu Jiansheng

Dislocation behaviour during cyclic deformation of niobium single crystals

Abstract Dislocation structures formed during cyclic deformation of niobium single crystals have been studied. Single crystals of both [321] and [110] orientations have been cyclically deformed in tension-compression under strain control. Moreover, the mechanical property data are supplemented by the direct observation of dislocation arrangements by transmission electron microscopy. In the case of [321] orientation single crystals the hardening curve can be divided into three stages, i.e. first, rapid-hardening and saturation stages associated with various dislocation structures. In the first stage the dominan features of the dislocation configurations are high density networks and debris loops. In the rapid-hardening stage the main feature is the formation of dislocation bundles. In the saturation stage the characteristic features are the dislocation bundles and the screw dislocations between bundles. The imposed strain is accomodated mainly by the motion of screw dislocation travelling to and fro between the bundles. For the [110] orientation single crystals the hardening curves are similar to those of the [321] orientation except that dislocation bundles of more than one direction appear during the first few cycles. Three-dimensional cell, two-dimensional cell and bundle structures are summarized as the saturated dislocation structures of b.c.c. metals.