Acta Materialia 50 (2002) 653–662 www.elsevier.com/locate/actamat Evolution of dislocation glide kinetics during cyclic deformation of copper G.C. Kaschner a , J.C. Gibeling b,* a MST-8, Structure/Properties Relationships, Los Alamos National Laboratory, Los Alamos, NM 87545, USA b Division of Materials Science and Engineering, Department of Chemical Engineering and Materials Science, University of California, One Shields Avenue, Davis, CA 95616, USA Received 15 November 2000; recieved in revised form 28 August 2001; accepted 28 August 2001 Abstract Strain rate change tests were performed during low cycle fatigue of polycrystalline copper using plastic strain as the control variable. The evolution of dislocation interactions was observed by evaluating the activation area and true stress as a function of cumulative plastic strain. Activation areas at each of three plastic strain amplitudes, e p /2 = 0.2, 0.4, and 0.6%, have initial values of approximately 2000b 2 which decrease to 600b 2 during cyclic loading to saturation. This observation suggests a transition from forest dislocation cutting to increasing contributions of cross-slip as the predominant rate-controlling mechanisms of dislocation motion. Haasen plots of normalized inverse operational acti- vation area (b 2 /a) for specimens cycled to saturation exhibit a deviation from linearity similar to that observed for monotonic deformation. This nonlinearity corresponds to a failure of the Cottrell–Stokes law that correlates with the development of characteristic dislocation structures during cyclic deformation. Tests performed at various stresses at saturation reveal a linear dependence of b 2 /a on true stress. The athermal stress, s b =86.5 MPa, measured at saturation by extrapolating the activation area data compares favorably with the value determined from a Bauschinger analysis, s b =80 MPa, at a plastic strain amplitude of 0.6%. In addition, athermal stress values vary with plastic strain amplitude as expected, resulting in a constant value of approximately s b /s=0.5.  2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Low cycle fatigue; Mechanical properties (thermally activated processes); Strain rate change; Dislocations; Copper 1. Introduction The plastic deformation of metals is commonly explained in terms of dislocation interactions: dis- locations intersecting other dislocations, encoun- * Corresponding author. Tel.: +1-5307520400; fax: +1- 5307521031. E-mail address: jcgibeling@ucdavis.edu (J.C. Gibeling). 1359-6454/02/$22.00  2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII:S1359-6454(01)00362-7 tering particles, interacting with solute atmos- pheres, etc. Whether the deformation occurs in response to a single, monotonic application of forces or is due to alternating forces as in cyclic deformation, many of the same types of dislocation interactions are observed. A basic knowledge of these interactions is essential to our ability to design new structural materials. For that reason, fundamental studies performed on single crystals have been compared with studies of polycrystalline