On the Shearing Mechanisms of Β" Precipitates by the Dislocations Gliding on the {100}al Planes in Al–7Si–Mg Alloys

In this study, some deformation and shear behaviors of the β" phase in Al–7Si–Mg alloy were found to be caused by the dislocations on {100}Al planes instead of {111}Al planes. Corresponding shearing mechanisms were systematically investigated by combing high-resolution transmission electron microscopy (HRTEM) and first-principles calculations. All the possible shearing process on the {100}Al planes, such as the shearing along the cross-section, the width, and the length of the needle has been considered and studied. The optimal atomic slip layers and the energetically most favorable paths were obtained by the generalized-stacking-fault energy (GSFE) curves. It was found that, for the shearing along the (100)β'' and (001)β'' planes, the cross planes at the middle of the Si2 pillars in the β" phase are the easiest slip planes. This can be attributed to the lowest energy barriers and the destruction and recombination of Mg–Si and Si–Si covalent bonds near the slip planes during shearing. Based on these conclusions, the formation mechanisms of the black trailing shadows and the step-like morphology in the deformed β" phase can be well explained. These findings provide a deeper understanding of the deformation and shear behavior of β" precipitates.