Cyclic deformation response of planar-slip materials and a new criterion for the wavy-to-planar-slip transition

Abstract

This paper will start with the review of mechanical response and dislocation structure evolution of single crystals of planar-slip alloys during cyclic deformation. Experimental results with typical planar-slip materials have demonstrated that, unlike typical wavy-slip crystals, planar-slip materials do not exhibit ‘real’ cyclic saturation behaviour, nor is there any evidence for the formation of persistent slip bands and dislocation ladder structures. Comparisons of the following three aspects of cyclic deformation, namely the mechanical response, the surface morphology and the dislocation structures, between wavy-slip and planar-slip materials will then be presented. Although it is a recognized fact that, on many occasions, the value of stacking-fault energy (SFE) can be employed as the criterion for distinguishing planar slip from wavy slip, detailed comparisons have shown that one cannot always obtain a satisfactory result if such a criterion for the transition is employed alone. Other considerations, such as the criterion based on the reciprocal width of the stacking fault and the approach based on a short-range-ordered structure, are then discussed. Further study has indicated that a more indicative factor for the transition of wavy-slip mode to planar-slip mode, at least in many Cu-based alloys, can be deduced in terms of the free-electron-to-atom ratio e/a of the alloy. Still further analysis and comparison show that the transition actually occurs when the SFE no longer decreases rapidly with the increase in the e/a ratio, that is in the solute concentration. In fact, a critical value or a critical range of e/a ratios for such a transition in these alloy systems could indeed be determined. To recognize the range of application of the e/a ratio approach, other alloy systems have also been examined and will be discussed in this paper.