Discontinuous flow in Cu-2Be at various strain rates

Homogenized Cu-2 wt.% Be-0.3 wt.% Co wires of different diameters, D, show in a tensile test in a He bath at a strain rate of 5 × 10−3 s−1 regular stress serrations. By increasing the strain rate above a characteristic value (varying nearly as D−0.5) the regular serrations change gradually to extended patterns. We show by considering a local thermal equilibrium state at the site of a stress drop that this dependence of characteristic strain on sample diameter is in accordance with the assumption of a critical local sample temperature (independent of the diameter) occuring during the stress drop. The variation of stress causing the patterns at high strain rates suggests the existence of propagating slip bands there. For strain rate changes from 1.4 × 10−6 to 1.4 × 10−5 s−1 in the continuous part of the deformation curve we find a small but positive stress rise which does not change appreciably with deformation thus ruling out a gradual change from positive to negative strain rate sensitivity as a cause for the onset of the serrations.