A model for two-step ageing

[摘要] In commercial practice, two-step ageing is commonly used in Alâ€�?�Znâ€�?�Mg alloys to produce a fine dispersion of 𝜂′ precipitates to accentuate the mechanical properties and resistance to stress corrosion cracking. While this is true in Alâ€�?�Znâ€�?�Mg alloys, two-step ageing leads to inferior properties in Alâ€�?�Mgâ€�?�Si alloys. This controversial behaviour in different alloys can be explained by Pashley’s Kinetic model. Pashley’s model addresses the stability of clusters after two-step ageing. In the development of the model, the surface energy term between cluster and matrix is taken into account while 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, inclusive 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 new stability criterion. Thus it is found that the new model for two-step ageing is verified in the case of Alâ€�?�Znâ€�?�Mg alloy.

[学科分类] 材料工程

[关键词] Two-step ageing;delayed ageing;clusters;coherency strains.