UBC Theses and Dissertations
The effect of microstructure on strain localization in the precipitate free zone of Al-Mg-Si alloys Mansouri Arani, Mojtaba
An important parameter to consider for selecting Al-Mg-Si alloys for the automotive industry is their ductility as extruded parts may experience severe plastic deformation, e.g. in a crash. Several studies have investigated the effect of microstructural features on mechanical properties and, in particular, ductility as characterized by the true fracture strain in a tensile test. However, most of the data in the literature is based on qualitative experimental observations, or more recently, polycrystal plasticity simulations. The lack of quantitative measurements limits the predictive capability of such models. Therefore, the current study aims to make a quantitative approach to the measurement of microstructural parameters and the plastic strain distribution within grains and the PFZ. Having this perspective, the effect of cooling rate after solution treatment and the effect of aging on the size and number density of grain boundary precipitates and PFZ width was quantified. In T6 condition it was found that slow cooling (~ 6 C/s) results in the formation of ~ 300 nm wide PFZ near the grain boundaries whereas the PFZ width was ~ 55 nm in fast quenched (~ 1500 C/s) sample. It was observed that the magnitude of strain localization is affected by the magnitude of yield stress mismatch between the PFZ and the grain interior. The effect of significant strain localization within the PFZ in slow cooled samples is reflected in the observed fraction of inter-granular fracture and fracture strains. For example, in the T6 condition, the air cooled sample exhibited a fully intergranular fracture with strain to fracture of ~ 0.35 whereas the water quenched sample showed a mixture of intergranular and transgranular fracture with strain fracture of ~0.75. Finally, it is found that non-shearable precipitates and dispersoids make the distribution of slip within the grains more uniform. It should be noted that although the addition of dispersoids increased transgranular fracture mode, but it did not affect the fracture strain in this sample. This may be due to the competition between void nucleation on grain boundary precipitates and dispersoids inside grains.
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