Asymmetry of the yield stress of Nb-6W single crystals

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Abstract
The effect of orientation and the sense of the applied stress on the yield stress was studied in single crystals of pure Nb and Nb-6 a/o W. Two orientations were examined, one with maximum resolved shear stress on the [111](101) system and the other with maximum resolved shear stress on the [111](211) system. Room temperature results on Nb—6W indicate that the yield stress depends upon the sense of the applied stress in such a way that the yield stress is a maximum for (211) slip in tension and is a minimum for (211) slip in compression. Asymmetry of the yield stress also exists in Nb—6W at low temperatures. Within limits of experimental error, there appears to be almost no asymmetry of the yield stress of zone-refined Nb for the two orientations tested. The results indicate that thermally activated substitutional solid solution strengthening must take into account the anisotropic nature of the dislocation core in body-centred cubic alloys.