On the form of the age-hardening response in high strength aluminium alloys

The form of the age-hardening response in high strength aluminium alloys has been modelled using appropriate versions of the Orowan equation and precipitation-hardening equations developed for rationally-oriented precipitate plates. When the shape and orientation of precipitates are taken into account, the change in critical resolved shear stress (CRSS) produced following isothermal ageing of the Al–Cu–Li and Al–Cu alloys can be modelled exclusively using equations assuming interfacial strengthening and Orowan hardening, respectively. Combined with direct observations of the interaction between dislocations and the T1 and θ′ precipitates, respectively, in fractured tensile samples, these results suggest that it is not necessary to invoke a transition from predominantly dislocation shearing to Orowan looping to account for the form of the ageing curves of these alloys.