Anomalous Plasticity in the Cyclic Torsion of Micron Scale Metallic Wires

The plasticity of micron scale Cu and Au wires under cyclic torsion is investigated for the first time by using a torsion balance technique. In addition to a size effect, a distinct Bauschinger effect and an anomalous plastic recovery, wherein reverse plasticity even occurs upon unloading, are unambiguously revealed. The Bauschinger effect and plastic recovery have been observed in molecular dynamics and discrete dislocation dynamics simulations of ideal single-crystal wires; the results here are an excellent confirmation that these effects also occur in experiment in nonideal polycrystalline wires. A physical model consistent with the simulations is described in which the geometrically necessary dislocations induced by the nonuniform deformation in torsion play the key role in these anomalous plastic behaviors.

Figure 1

(a) Schematic of the experimental apparatus. Typical time dependence of torque in two modes is shown in the inset. (b) Typical cyclic torsion data for a Cu wire with $2a=20\mu \mathrm{m}$ under twist control. The broken straight lines show the elastic response. The plastic strain recovery upon unloading ${\epsilon }_{\mathrm{rp}}$ is marked on the plot.

Figure 2

Multiple cyclic responses of Cu and Au wires with increasing strain amplitudes in twist control: (a) Cu, $2a=18\mu \mathrm{m}$, (b) Cu, $2a=35\mu \mathrm{m}$, (c) Cu, $2a=42\mu \mathrm{m}$, and (d) Au, $2a=20\mu \mathrm{m}$. The elastic response is indicated by the dashed line, and the yield points in the forward and reverse directions are marked by circles. Oscillations in the curves are attributed to slight vertical misalignment.

Figure 3

(a) Responses of the Cu and Au wires to repeated forward loading. All load-unload cycles of both metals show a pronounced hysteresis. (b) SEM image of a Cu wire with $2a=42\mu \mathrm{m}$ after the test. The arrows point out distinct slip bands created by multiple slip systems.

Figure 4

Cyclic torsion data of the Cu wires with $2a=18$, 35, and $42\mu \mathrm{m}$ for the initial loading cycles. The yield points in the forward and reverse directions are marked by circles. The wires show significant Bauschinger and size effects.