| Summary: | There are a number of experiments showing that the ductility of aluminum alloys decreases during age-hardening heat treatment. Observing the grains of age-hardened aluminum alloys at the micron scale, one can notice that there are precipitate-free zones (PFZs) along the grain boundaries. PFZ has yield stress three times lower than the grain interior (bulk) due to absence of alloying elements. As a result, PFZ is suspected to be the reason for ductility reduction of alloys. On the other hand, a number of experiments performed on specimens with micron-scale dimensions have shown that the plastic deformation of crystalline materials is size-dependent. These micron-scale dimensions which can influence the mechanical behavior, such as yield stress or hardening, are not taken into account in the conventional plasticity theory, therefore another theory has been developed. That theory is Strain Gradient Plasticity (SGP). The specific SGP theory used here is a so called ‘higherorder theory’ in the sense that higher order stresses as well as additional boundary conditions are included in the theory. SGP theory also includes length scale parameters in order to be dimensionally consistent. On a recent study conducted by Fourmeau et al. (Fourmeau, 2015), transmission electron microscopy (TEM) is used to display the geometrical properties and the chemical composition of PFZ in the AA7075-T651 aluminum alloy. It is observed that the width of PFZ is about 20 to 40 nm. In the present thesis, the properties for PFZ and bulk material provided by that study are used for a micromechanical finite element model of a microstructure including the bulk, PFZ and the grain boundary (GB). A uniaxial loading condition is applied to the representative volume element (RVE) and SGP theory is hired in order to capture the plastic strain fields as well as the stress triaxiality in PFZ and bulk region. Moreover a damage criterion is employed and studied for models with PFZ and without PFZ to understand the role of PFZ in reduction of the ductility of aluminum alloys. It is found that the damage parameter is much higher in the presence of PFZ. Finally, the void growth is studied by adding voids at critical locations to the model.
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