On the Interaction Between Mg Solute Atoms and Dislocations in Al-Mg Binary Alloys
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On the Interaction Between Mg Solute Atoms and Dislocations in Al-Mg Binary Alloys D. Zhang and R.C. Picu Department of Mechanical, Aerospace and Nuclear Engineering Rensselaer Polytechnic Institute, Troy, NY 12180 ABSTRACT The interaction between solute atoms and dislocations is known to lead to negative strain rate sensitivity and poor formability. The negative rate sensitivity causes inhomogeneous flow and the Portevin-LeChatelier (PLC) effect. These observations motivate the present study of the solute-dislocation interaction in binary Al-Mg alloys. We report here on three issues. First, we determine the size and shape of stable Mg clusters at stationary dislocations of edge, 60o and pure screw type. We then evaluate the accuracy with which clustering is predicted by the classical pressure field-solute interaction formula, i.e. within the assumption that solute do not interact and the pressure field of the dislocation is unperturbed by the solute. Second, we investigate to what extent the presence of the cluster perturbs the far field of the dislocation. Finally, the effect of the solute on the stacking fault energy is investigated. INTRODUCTION Magnesium is added to aluminum to improve strength. However, it is found to have a detrimental effect on formability at room temperature. In the temperature range ~ –100 to +100oC, the ductility decreases and the strength is constant or increases slightly with increasing temperature. The strain rate sensitivity is negative in this range, which leads to strain localization. The deformation is localized in bands that leave undesirable traces on the surface of the formed part. This is the domain of temperature in which dynamic strain aging reflects in the macroscopic behavior of the material. Dynamic strain aging consists in the interaction of fast diffusing solute atoms, such as magnesium, with dislocations. Solute is attracted to dislocation cores due to the primarily elastic interaction with the strain field of the line defect. If the dislocation is stationary for sufficiently long time, a cluster forms at the core. This binds the defect and an additional energy/stress is necessary to unbind it from the cluster. This static aging mechanism was transferred to the dynamic case and it was proposed, beginning with Cottrell [1], that solute clusters are either dragged by dislocations, or form during the arrest time at obstacles. A later model that acknowledged that solute diffusion in absence of a large concentration of vacancies is too slow to account for the phenomenon, considered that dislocation pinning actually occurs through pipe diffusion [2,3]. According to this model, pipe diffusion would take place from decorated stationary dislocations to the undecorated mobile ones. A large number of theoretical and experimental studies have been dedicated to this issue [e.g. 4,5]. Most of the experimental work was performed on Al and Cu-based dilute alloys in which the PLC effect is measurable at room temperature. The theoretical and numerical studies were mostly non-spec
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