Modelling of grain boundary segregation, precipitation and precipitate-free zones of high strength aluminium alloys
Aluminium alloys of the 7000 series with their high strength to weight ratio characteristics have been considered as a excellent choice in airftwne structure manufacturing. The commercial application of the alloys, however, may be restricted by their poor characteristics such as resistance to stress corrosion cracidng which, it is reported, strongly depends on the gain boundary precipitation and the widths of precipitate free-zones. Both of these would be affected by segregation. It is the main aim of the work described in this thesis to construct a combined model in which the kinetics of the segregation of the solute atoms, the nucleation of the grain boundary precipitates, the precipitate growth and coarsening are taken into account and the model gives the prediction of the state of the grain boundary and its environment as a function of heat treatment. A numerical approach is used to perform the combined analysis. The collector plate area before the growth of the precipitates begins depends on the density of the precipitate nuclei at the grain boundaries. The mean collector plate area is very sensitive to Tmp, which is the temperature at which most segregation is considered completed, the concentration of Zn and Mg of the alloy, the temperature at which the nucleation takes place and the gain boundary structures. Within the temperature range of interest here, the higher the temperature and/or, the lower the concentrations of Zn and Mg and/or the smaller the grain boundary energy to die precipitate/matrix interface energy ratio, the fewer are the equilibrium precipitate nuclei and the bigger the mean collector plate area. Ile changing of the mean collector plate area with time during the growth is caused by the coalescence of the precipitates at the grain boundaries. The widths of the precipitate-free zones is very sensitive to the quenching rate, the diffusion coefficient of the impurity-vacancy complexes and the ageing temperature. Within the temperature range of interest here, The widths of the precipitate-free zones increases with the increase in ageing temperature and decreases with increases in the ageing time. The situation where both segregation and precipitation take place, the SCC susceptibility is mainly controlled by the inter-particle spacing of the grain boundary precipitates. The decrease in the inter-particle spacing of grain boundary precipitates promotes the propagation speed of SCC along the GBs, and this induces the decrease in the resistance to stress corrosion cracking. Apart fi-orn this main work, successful strategies for effectively using the image analysis system in analysing TEM micrographs have been developed.