Effect of ceramic particles on precipitation in an Al-Mg-Si alloy with silicon excess during ageing

Providing a good balance between lightweight and high mechanical properties, the aluminum-based metal matrix composites (MMC) became an interesting alternative for specific industrial applications. However, considering an Al-Mg-Si alloy with a high silicon excess, the ceramic particles added as reinforcement can act on : i) the precipitation kinetics of the coherent and semi-coherent phases, ii) the precipitation sequence of the alloy and iii) the loss of mechanical strength from a peak-aged microstructure obtained by a T6 condition. In order to understand the influence of reinforcement on these aspects, the composite was characterized during isothermal ageing between 100°C and 350°C, allowing us to propose an experimental Isothermal Transformation Curve. Compared to the unreinforced alloy, heterogeneous precipitation of disordered semi-coherent phases occurs on dislocations and the precipitation kinetics were found to be accelerated in the composite leading to an acceleration of the loss of strength from the T6 state, due to the precipitation of the Type-C phase. A study performed on a deformed alloy, demonstrated that the most of the differences observed between the unreinforced alloy and the composite can be explained by the high dislocation density generated in the matrix of the composite due to the presence of ceramic particles. Finally, the JMAK approach turned out to be a powerful tool to model the decrease in mechanical strength occurring during isothermal treatments from T6 state.