Cyclic deformation of Nb single crystals II. Influence of orientation on cyclic hardening, shape changes and stress asymmetry

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Abstract
Niobium single crystals with seven different orientations of the stress axis have been deformed cyclically in tension-compression at room temperature and at a constant strain rate of 6 × 10−4 s−1 over a range of total strain amplitudes between 0.3 × 10−3 and 6 × 10−3. The cyclic hardening behaviour has been found to be strongly dependent upon the crystal orientation. Those crystals whose c.s.s. curves exhibit four well-defined regions do not obey a cyclic equation of state, but for certain crystal orientations, the c.s.s. curves are flat and independent of the cyclic strain history. The magnitude of the crystal shape changes during cyclic deformation, varies with the crystal orientation, and for some orientations no changes in shape take place. These results are explained in terms of both the asymmetric slip and the observed active glide systems in tension and compression. The stress asymmetry was also found to vary in magnitude and sign with the orientation of the crystal axis in agreement with the asymmetric glide properties of the screw dislocations. Good evidence is obtained for the influence of the normal components of the stress tensor on the c.r.s.s. for glide on {110} and/or {112} planes.