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Slip trace characterisation of Ni3Al by atomic force microscopy. Christophe Coupeau, Tomas Kruml1 and Joël Bonneville Université de Poitiers, LMP, UMR-CNRS 6630, SP2MI, F-86962 Futuroscope Cedex, FRANCE. 1 Ecole Polytechnique Fédérale de Lausanne (EPFL), DP-IGA, CH-1015 Lausanne, SWITZERLAND. ABSTRACT We examined by atomic force microscope the slip traces produced on Ni3Al single crystals pre-deformed up to nearly 1% plastic strain at three temperatures in the anomaly domain: 293K, 500K and 720K. It is observed that, whatever the deformation temperature, the slip traces essentially belong to the primary octahedral slip system. The lengths of the slip lines become shorter and shorter with increasing temperature, while the number of dislocations that constitutes the lines is approximately constant. These results are interpreted in terms of a decreasing mean free path of the mobile dislocations when the temperature is raised. The implications of these results in the understanding of the flow stress anomaly are underscored. INTRODUCTION Positive temperature dependence (PTD) of the flow stress of the Ni3Al intermetallic compound has not yet received a satisfactory explanation. While several theoretical works succeed in explaining the increase of the flow stress with temperature, they generally fail to predict other characteristic parameters of the plastic deformation, such as the work-hardening rate (which also exhibits a PTD) and the strain-rate sensitivity of the flow stress (for a review see [1]). The proposed models usually consider that a thermally activated cross-slip process plays a key role in understanding the PTD of the flow stress, but, depending on the dislocation dynamics considered, cross-slip process leads with increasing temperature either to a decrease in the dislocation velocity or to a decrease in the mobile dislocation density. Indeed, on the one hand, direct measurements of the stress and temperature dependences of the dislocation velocity have been carried out in the anomaly domain of Ni3Al by the double etching technique [2,3]. These two studies have shown that, at a given stress, the screw dislocation velocity decreases when the temperature is raised. In addition, small activation areas, i.e. A < 100 b2 (b being the Burgers vector of a superpartial dislocation), have been reported in both investigations. These latter results contrast with the very low strain-rate sensitivity of the flow stress obtained by deformation experiments performed either at different strain-rates (see for instance [4]) or by strain-rate changes [5,6]. On the other hand, variation in the density of mobile dislocations has been estimated by indirect techniques [7,8], based on transient tests performed during constant strain-rate experiments. These studies have demonstrated that, in the PTD range of the flow stress, high exhaustion of the mobile dislocations takes place, which is associated with large activation areas [9]. This study is aimed at quantifying the respective contributions of the mobile dislocation density and di