The strain-rate dependence of the flow stress of copper single crystals

Abstract

An experimental study is described of the strain-rate sensitivity of the flow stress of Cu single crystals in all stages of deformation and over a wide temperature range. The curves relating the increase Δτ in flow stress on increasing the strain-rate by a fixed amount to the flow stress τ are generally divisible into three regions a, b and c. Region a is linear and characteristic of easy glide. At the end of this region the rate of increase of Δτ with strain increases, and a transition region, b, occurs in which Δτ varies with τ much more slowly than in easy glide. Another linear region, c, of Δτ versus τ, of intermediate slope, begins during stage II and is characteristic of the latter part of stage II and of stage III of the work-hardening curves. The lower the test temperature, the greater is the range in stage II in which the linear region c applies. There is no change in the curves of Δτ versus τ at the onset of stage III. The slope of region c is similar for crystals deforming by single or double slip, and is of the same order of magnitude as that for polycrystals. The results agree with those of previous workers in that the temperature dependence of the flow stress is greater during easy glide than in stages II and III. The two linear regions, a and c, are thought to correspond to two different arrangements of dislocations, the transition from one type of arrangement to the other taking place in region b. The occurrence of linear regions can be explained on the long range stress, forest, or jog theories of work-hardening. However, while the similarity in the slopes of the curves of Δτ versus τ for single crystals deforming in single or double slip, and for polycrystals, follows naturally from the forest and jog theories, it does not seem to be so easily explicable in terms of the current long-range stress theory.