Optimized tensile conditions for thein situX-ray topographic observation of glide dislocations in single crystals

In order to observe in situ, by X-ray topography, the dislocation mechanisms responsible for the microplastic deformation of crystals, several tensile methods have been used in the past. In a review of the different types of tensile stages it appears that, for most of them, the crystal was subjected to bending stresses, 80 that a quantitative interpretation of the observed phenomena could hardly be given. The magnitude of bending effects and their consequences on dislocation configurations are analysed in this paper. It is shown, by experiments on a new type of tensile stage and a finite-element computation, that a set-up with the two ends of the crystal cemented firmly onto guided holders is preferable to an imperfect freedom at the grips. The proposed method, applied to the microdeformation of cadmium and zinc crystals, produces dislocation configurations which are characteristic of a uniform tensile stress. The theoretical computation shows that the technique cannot get rid of a local enhancement of the stress near the crystal ends, but keeps the bending effect smaller than a few per cent, even when the crystal has been attached in a slightly oblique position.