A model for two-step ageing

In commercial practice, two-step ageing is commonly used in Al-Zn-Mg alloys to produce a fine dispersion of eta' precipitates to accentuate the mechan ical properties and resistance to stress corrosion cracking, While this is true in Al-Zn-Mg alloys, two-step ageing leads to inferior properties in Al -Mg-Si alloys. This controversial behaviour in different alloys can be expl ained by Pashley's Kinetic model. Pashley's model addresses the stability o f clusters after two-step ageing. In the development of the model, the surf ace energy term between cluster and matrix is taken into account white the coherency strains between the cluster and matrix are not considered. In the present work, a model is developed which takes into account the coherency strains between cluster and matrix and defines a new stability criterion, i nclusive of strain energy term. Experiments were done on AA 7010 aluminium alloy by carrying out a two-step ageing treatment and the results fit the n ew stability criterion, Thus it is found that the new model for two-step ag eing is verified in the case of Al-Zn-Mg alloy.