Pergamon Scripta Metallurgica et Matcrialia,Vol. 31, No. 11, pp. 1587-1592, 1994 Copyright ©1994 ElsevierScience Ltd Printed in the USA. All rights reserved 0956-716X/94 $6.00 + 00 A MODEL OF THE FORMATION OF STRAIN BURSTS DURING CYCLIC DEFORMATION MICHAEL ZAISER and PETER HAHNER* Max-Planck-Institut f(ir Metallforschung, Institut f~r Physik P. O. Box 800 665, D-70506 Stuttgart, Germany * Present Address: European Commission, Institute for Advanced Materials, Joint Research Centre, 1-21020 Ispra (Va), Italy. (Received May 20, 1994) (Revised July 14, 1994) 1. Introduction This paper presents a quantitative model of the formation of slip avalanches ("strain bursts") which are observed during cyclic deformation of single-glide orientated crystals. By means of a dislocation-dynamical approach, the scaling relations found by experiment are related to the physical parameters characterizing the microstructural evolution. The strain-burst phenomenon, discovered and subjected to detailed experimental studies by Neumann [1- 4], is observed during stress-amplitude controlled cyclic deformation of single-glide-orientatedface-centred cubic or hexagonal monocrystals. If the applied stress amplitude is slowly increased in the course of such a test, the crystal may respond by large oscillations of the cyclic strain amplitude. This manifests itself as a regular sequence of sharp peaks ("strain bursts") during each of which the strain amplitude rises by a factor of 2 to 100, depending on the experimental conditions. Two prerequisites for the appearance of this phenomenon have been identified: (i) Deformation has to occur in single glide, i.e., strain bursts are neither observed in polycrystals nor in monocrystals where several glide systems are active. They can also be suppressed in single-glide-orientated crystals by prestraining into stage II of the work hardening curve. (ii) Tile phenomenon is restricted to sufficiently low deformation temperatures and/or high cycle frequencies. This gives evidence that thermal activation of dislocation motion must be suppressed to some extent, i.e., a nonvanishing thermal flow stress contribution is required [5]. Within a sequence, the n-th burst may be characterized completely by a small set of parameters. One may use the external shear stress at the onset of the burst, f~2~, the maximum strain amplitude reached during the burst, "~("), and the burst width expressed by the stress range Ar,~t, or the number An of cycles within a burst and the cumulative strain 7~, i.e. the sum of the strain amplitudes during these cycles. These parameters are interrelated by a set of characteristic scaling relations that are compiled in Table 1. It is important to note that these scaling relations are highly reproducible over a wide range of different experimental conditions and perhaps form the most remarkable peculiarity of the strain-burst phenomenon. TABLE 1: Scaling relations between the characteristic parameters of the strain bursts (6rcycl= shear-stress increment per cycle). Shear-stress difference between successive bursts Maximum shear-strain amplitude within a burst Cumulative shear-strain during a burst .(n=bl) __ .~(n) ~(~l) text -e.t ~ 'e*t (I) ~,(,0 ~ ,ext-~('q (II) (") -~(~) (III) ~cum ~ %xt Burst width (shear-stress interval) Arext ~ t~Tcycl Burst width (number of cycles) An = const(f~(~),(~Tcycl) (IV) 1587