Eine neue Methode zur Herstellung verformungsfreier Einkristall‐Zugproben hochschmelzender Metalle und einige Ergebnisse über de plastische Deformation von Niob‐Einkristallen

A new method is described for the direct production of strain‐free (tensile) specimens of high melting‐point metals by electronbeam zone‐melting. Measurements of the purity of such crystals show that purification occurs mainly by the evaporation of impurities, accompanied by a relatively small zone‐melting effect. Tensile tests on niobium single crystals, prepared in this way, show that the plastic properties depend strongly on orientation. Thus, for the three corner‐orientations, the <100>‐orientation has the highest U.T.S. and the highest uniform and total elongation, and also shows a two‐step work hardening rate. The <110>‐orientation has only a one‐step, but steep, work hardening rate. For the 〈111〉‐orientation no uniform type of curve could be found: this orientation shows the effect of “repeated work softening”. For orientations inside the stereographic triangle the development of an “easy glide” range is observed. This range is, however, somewhat different according to whether the crystal axes are shifted from the <100>‐ or <110>‐orientation. The “easy glide” range is followed by appreciable work‐hardening, and in a later stage by a recovery range. The centre of the “easy glide” range seems to lie near the 〈123〉‐orientation. All, the niobium single crystals investigated show pronounced yield points, the yield‐point drop depending on the orientation and perfection of the crystal. The 〈111〉‐orientation has the smallest, and the 〈110〉‐orientation the largest yield‐point drop. In addition a tendency is observed for large yield‐point drops to occur along the line 〈110〉–〈112〉. These results are discussed with regard to the possible active slip planes; the observed yield‐point effects are explained by the behaviour of dislocations between the upper and lower yield points.