The cyclic hardening and saturation behaviour of copper single crystals

Abstract

An experimental study has been performed on the cyclic deformation of copper single crystals at constant plastic resolved shear-strain amplitudes, γ pl, over a range 1.55 × 10 −5 < γ pl < 2 × 10 −2. The increase of the (initial) cyclic hardening rates with increasing γ pl is attributed to a change in the work-hardening mechanisms responsible. Similarities to unidirectional stage I and stage II work-hardening at low and at high γ pl, respectively, are pointed out. The dependence of the saturation resolved shear stresses τ s, on γ pl (cyclic stress-strain curve) reveals three regimes of cyclic saturation. The dominant intermediate “plateau” regime extends over about two decades of γ pl(6 × 10 −5 < γ pl < 7.5 × 10 −3) and is related to inhomogeneous deformation localized in persistent slip bands (PSB's). In this range, τ s is almost independent of γ pl and represents the stress required for the deformation in the PSB's. The fact that PSB's do not form below the “threshold” value of γ pl ∼ 6 × 10 −5 implies that a fatigue limit exists in copper. Details of the nucleation and propagation of the PSB's are inferred from softening observed prior to saturation and from characteristic changes of the shapes of the hysteresis loops. The observations indicate that, with increasing γ pl, the formation of the PSB's in the plateau regime is facilitated increasingly by long-range internal stresses. At very low γ pl (< 6 × 10 −5) and at very high γ pl (>7.5 × 10 −3) the deformation is more homogeneous. Primary slip dominates at low γ pl and secondary slip becomes more pronounced only at higher γ pl.