A DISLOCATION-BASED MODEL FOR ALL HARDENING STAGES IN LARGE-STRAIN DEFORMATION

A new model is presented to describe the hardening behaviour of cell-f orming crystalline materials at large strains. Following previous appr oaches, the model considers a cellular dislocation structure consistin g of two phases: the cell walls and the cell interiors. The dislocatio n density evolution in the two phases is considered in conjunction wit h a mechanical analysis for the cell structure in torsional deformatio n in which the cell walls are lying at 45 degrees with respect to the macroscopic shear plane and are strongly elongated in the direction pe rpendicular to the applied shear direction. Guided by recent results o n the volume fraction of cell walls [Muller, Zehetbauer, Borbely and U ngar, Z. Metallk. 1995, 86, 827], the cell-wall volume fraction is con sidered to decrease as a function of strain. Within a single formulati on, all stages of large strain behaviour are correctly reproduced in a n application for copper torsion. Moreover, strain rate and temperatur e effects are accounted for correctly and the predicted dislocation de nsities are in accord with experimental measurements. It is suggested that the factor responsible for the occurrence of hardening Stages IV and V is a continuous decrease of the volume fraction of the cell wall s at large strains. A significant effect of the deformation texture va riation on strain hardening is also discussed. (C) 1998 Acta Metallurg ica Inc. Published by Elsevier Science Ltd. All rights reserved.