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.

Acknowledgements

This paper was prepared for the International Symposium in memory of the late Professor Z. S. Basinski for his great contribution to the scientific fields of plasticity and cyclic deformation.

Thanks are due to Dr Bo Gong for his excellent experimental work while he was in my research group. Specifically, the author would like to express his sincere appreciations to Professor H. Mughrabi for his encouragement and suggestion for carrying out this study. Gratitude is also due to Professor C. Laird, Professor D. Embury, Professor L. M. Brown and Professor P. Lukáš for their helpful discussions during preparation of this report. Finally, the author would like to express his gratitude to Ms Cindy Y. Ren and Ms Farnoursh Heidarzadeh for their help in preparing the figures and tables in this report.

Notes

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† Frequently the SFE for the same alloy may have very different values provided by different references. Hence, whenever information is available, more accurate values measured by weak-beam TEM studies are used. Otherwise, values quoted in Murr’s book are employed.

‡ In this and the following sections, the sign of the slope is ignored and only the magnitude of the slope is considered.

† For the e/a versus G relation, see Neighbours and Smith (1954) and Fuchs (1936); for the e/a versus σ_y relationship, see Hibbard (1958); for the the e/a versus SFE relationship, see Howie (1961).