The Role of Grain Boundary Precipitates during Intergranular Fracture in 6xxx Series Aluminium Alloys

During ageing, 6xxx aluminium alloys will develop a microstructure characterised by needle-shaped Mg/Si-rich precipitates in the bulk, precipitate-free zones along the grain boundaries and larger Mg/Si-rich precipitates on the grain boundary. Depending on, among other things, the size of the precipitate-free zone, these alloys are prone to intergranular fracture. The role of the grain boundary precipitates during the initiation and propagation of the intergranular fracture is still not fully understood. Transmission Electron Microscopy has been used to characterise the grain boundaries and grain boundary precipitates. The precipitates were found to be of the beta ' type surrounded by a layer of U2 structure. The atomic details of relevant interfaces of Al-beta ' were characterised for further investigation. Density Functional Theory simulations were performed on the bulk precipitate structures and on the interfaces obtained experimentally. The decohesion energy of these interfaces was calculated and compared to bulk values. In addition, simulated tensile tests were performed in order to find values for the tensile strength sigma(t). The dependence of the interfacial energy and tensile strength of beta ' grain boundary precipitates were found to depend on the orientation and type of interface in addition to the amount of defects on the interface.