Cyclic deformation of Nb single crystals I. Influence of temperature and strain rate on cyclic hardening, shape changes and stress asymmetry

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Abstract
Niobium single crystals with single glide orientation have been cyclically deformed in tension-compression, at total axial strain amplitudes between 0.3 × 10−3 and 6.0 × 10−3. The cyclic hardening, the changes in shape of the crystals and the asymmetry of the hysteresis loop, have been studied at different temperatures (between 250 K and 354 K) and different strain rates (between 3.0 × 10−5 ' s−1 and 1.2 × 10−2 s−1). The cyclic stress-strain curves exhibit four different hardening regions, the main characteristic being the existence of a “plateau” region at low strain amplitudes where a limited amount of hardening or softening is observed. The length of this plateau increases as the temperature is decreased or the strain rate increased, and an interpretation is given in terms of the relative absence of dislocation-dislocation interactions. The shape of the cyclic stress-strain curves is found to depend on the cyclic strain history of the crystals, especially at the small strain amplitudes. The changes in shape which take place during cyclic deformation are explained in terms of the observed active glide systems in tension and compression, and good agreement is found between the shape changes measured and those predicted from geometrical considerations. The stress asymmetry in the saturation stresses increases as the temperature is lowered or the strain rate is increased. This stress asymmetry is consistent with the asymmetric glide properties of screw dislocations, and evidence is obtained for the influence of the normal stress components on the c.r.s.s. for slip on {110) planes.