Strain field measurements and simulation of dislocation propagation for nanoscale hardening precipitation Patricia Donnadieu 1 —Guy F. Dirras 2 — Joël Douin 3 1) Laboratoire de Thermodynamique et Physico-Chimie Métallurgique, ENSEEG, CNRS / INPG Domaine Universitaire, BP 75, F38402 St Martin d’Hères Tél.33(0)4 76 82 66 86 / Fax. : 33(0) 4 76 82 66 44/ E-mail : donnadie@ltpcm.inpg.fr 2) Laboratoire de LPMTM-CNRS, Institut Galilée, / Université Paris XIII Av. J.B. Clément -F93193 Villetaneuse Tél.33(0)1 49 40 34 88 / Fax. : 33(0)1 49 40 39 38 / E-mail : dirras@galilee.univ-paris13.fr 3) Laboratoire d’Etudes des Microstructures, CNRS/ONERA, 29 av. Division Leclerc, F92322 Châtillon Tél.33(0)1 46 73 44 42 / Fax. : 33(0)1 46 73 41 55 / E-mail : douin@onera.fr ABSTRACT. Hardening in Al alloys frequently results from the modification of dislocation propagation by a large density of nanometric precipitates. To study the precipitate/dislocation interaction in Al-Mg-Si alloys, we have carried out a TEM study of deformed samples complemented by a new approach combining image analysis and simulation of dislocation motion. The analysis of HRTEM image allows to measure the strain field which is further introduced in the simulation of the dislocation propagation. In the Al-Mg-Si alloy, the simulation indicates that dislocation motion in the matrix occurs by circumventing the rod shaped precipitates through activation of the Orowan process while the lath shaped precipitates can be sheared. This is in agreement with the observation of dislocation loops and traces of shear of the lath precipitates in deformed samples. KEYWORDS precipitation hardening, Al-Mg-Si alloys, TEM, dislocation, precipitate strain field 1. Introduction Nanoscale precipitation is frequently responsible for strengthening in materials such as Aluminum alloys. The microstructure typical of hardening is a large dispersion of nanometric precipitates. Actually the hardening effects (both hardening rate and yield strength increase) result from the interaction of dislocations with the nanoscale precipitates which act as obstacles to the dislocation motion. Basically the dislocations are pinned by the precipitates; under the increasing applied stress they will finally shear or by-pass the obstacles. As dislocations are moving in the matrix, their propagation, and hence the interaction mechanisms, are influenced by the strain field due to the precipitates. This strain field depends on precipitate characteristics: morphology, size, structure, composition and orientation relationships with the matrix. Most of these parameters are unknown since the hardening precipitates usually do not belong to the phase diagram. Direct study of dislocation/precipitate interaction can be extremely difficult for microstructures presenting a dense dispersion of nanoscale precipitates. Therefore we propose an alternative approach: image analysis combined to the simulation of dislocation propagation. On the one hand, the phase image method, proposed by Hytch et al. (1997), allows extracting quantitative information on displacements from high resolution transmission electron microscopy (HRTEM) images. On the other hand the simulation of the dislocation propagation allows to predict the interaction mechanisms in the precipitate strain field. The combination of image analysis and simulation provide then a tool to examine the dislocation behaviour when approaching the precipitates. Such tool is particularly appropriate to our case since no hypothesis on the structure composition nor the elastic properties of precipitates has to be used, the strain field created by the precipitates being the experimental one given by direct measurement. Besides, observation of deformed samples carried out in order to test the simulation results will be also reported. The system chosen as model case here is a Al-Mg-Si alloy (6XXX series) at peak hardening (T6 state). In this state, a large density of precipitates is