Precipitate coarsening during overaging of Al–Zn–Mg–Cu alloy

Abstract

T he overaging kinetics of Al-Zn-Mg-Cu alloy 7050 at 430-455 K (157-182°C) was investigated by electrical resistivity and tensile property measurements, which were supplemented by T EM and diVerential scanning calorimetry (DSC). Overaging in this alloy system was shown to follow classical coarsening behaviour as characterised by a time−1/3 particle size dependence. Changes in solute supersaturation, as measured by electrical resistivity, could be directly related through the Gibbs-T hompsonFreundlich equation to particle size variations, which were inferred from the DSC results. Strength changes, when interpreted in terms of the dislocation looping mechanism, also fit the classical coarsening theory. T he activation energy for the coarsening reaction was estimated to be about 160 kJ mol−1, which is 30-40 kJ mol−1 higher than that for solute (Mg,Zn) diVusion. T his diVerence is ascribed to the solution enthalpy of the coarsening precipitate. Precipitate dissolution under non-isothermal conditions (DSC) had an activation energy (~135 kJ mol−1) closer to that for diVusion.MST /4278T he author is in

A model for the electrical conductivity of peak-aged and overaged Al-Zn-Mg-Cu alloys

“…Consequently, the study of electrical conductivity of 7xxx alloys has been of key interest to industrial manufacturers as well as material scientists, with an aim to predict and improve the stress corrosion performance. Several models or elements of models for the dependence of conductivity of Al-based alloys on composition and heat treatment have been reported [9,10,11,12,13]. For instance, a general description of thermodynamics of the Al alloy system and the kinetics of phase precipitation (based on JMAK kinetics [14]) was given by Bratland et al [9].…”

“…This model considered conductivity increase on overageing due to the reduction in electron scattering resulting from the coarsening of precipitates at constant volume fraction, and the model was applied to one 7xxx alloy. In contrast, Dorward [11] proposed a model that considered conductivity increase on overageing due to the reduction in the amount of dissolved atoms resulting from the coarsening of precipitates, and the model was applied to one 7xxx alloy (a 7050 alloy) for three ageing temperatures. Femminella and co-workers [12,13] used adaptive numerical (AN) approaches to model the electrical conductivity of nine 7xxx alloys.…”

“…According to the regular solution model, the solvus of η′ for η′ particles that are large (i.e. in the absence of any capillary effects [11]) can then be approximated as: (23) This type of simplified approach was explored in an earlier, simpler version of a conductivity model, which led to an accurate description of the conductivity data [46]. In the present, more detailed model, we will take account of coarsening of precipitates and the effect on metastable solubility through the capillary effect (see e.g.…”

“…In the present, more detailed model, we will take account of coarsening of precipitates and the effect on metastable solubility through the capillary effect (see e.g. [11]). To describe particle growth during coarsening we will approximate the precipitates as being spherical and adopt the Lifshitz-Slyozov-Wagner (LSW) theory (see e.g.…”

“…where r is the average particle radius, r o is the average particle radius at the start of the coarsening stage, t o , and K is a temperature dependent factor (see [11]). The capillary effect that is related to this coarsening will affect the metastable solvus of η′ particles, such that the metastable solvus for small particles that are coarsening is given by:…”