Plasticity and dislocation mechanisms in aluminium alloys containing precipitate free zones

Age hardening of commercial Al-Mg-Si aluminium alloys results in microstructures characterized by needle-shaped Mg- and Si-rich precipitates. At peak strength, the most prominent of these is the non-equilibrium β`` phase. This precipitate are coherent with the fcc Al lattice, and can cause considerable hardening by restricting dislocation movement. Precipitate free (denuded) zones, with no or a very limited amount of strengthening precipitates, are formed around grain boundaries after the solution heat treatment. The width of these zones is around 100-500 nm depending on cooling rate and alloy composition. They strongly influence the global mechanical properties of the alloy due to strain localization and following ductile fracture initiated within or close to these zones. To understand the influence of precipitate free zones on the global material behaviour, a detailed understanding of the local plasticity and dislocation mechanisms is necessary. Aluminium alloys containing precipitate free zones have therefore been studied by modelling and simulations, ranging from crystal plasticity to molecular dynamics, and experiments using nanohardness measurement and transmission electron microscopy.