Exhaustion mechanisms in the Preyield domain of niobium single crystals at low temperatures

Pure niobium single crystals were tested by tension and by stress-relaxation at stresses below the macroscopic yield stress and at temperatures between 4.2 and 70 K. The slope of the stress-strain curve and the amount of stress relaxation were investigated as a function of the nature and density of dislocations initially present in the sample. In the preyield domain the dislocation mobility was found to be governed by a thermally activated phenomenon associated to an exhaustion mechanism. This behaviour was quantitatively discussed in a model, in which edge dislocations undergo fragmentation, when the stress increases, as a result of their interaction with interstitial impurities.