A model for the yield strength of overaged Al–Zn–Mg–Cu alloys

Abstract

A model for the yield strength of multi-component alloys is presented and applied to overaged Al-Zn-Mg-Cu alloys (7xxx series). The model is based on an approximation of the strengthening due to precipitate by-passing during precipitate coarsening and takes account of ternary and higher order systems. It takes account of the influence of supersaturation on precipitation rates and of volume fraction on coarsening rates, as well crystallographic texture and recrystallisation. The model has been successfully used to fit and predict the yield strength data of 21 Al-Zn-Mg-Cu alloys, with compositions spread over the whole range of commercial alloying compositions, and which were aged for a range of times and temperatures to produce yield strengths ranging from 400 to 600 MPa. All but one of the microstructural and reaction rate parameters in the model are determined on the basis of microstructural data, with one parameter fitted to yield strength data.The resulting accuracy in predicting unseen proof strength data is 14 MPa. In support of the model, microstructures and phase transformations of 7xxx alloys were studied by a range of techniques, including differential scanning calorimetry (DSC), electron backscatter diffraction (EBSD) in an SEM with a field emission gun (FEG-SEM). Published as: Acta Mater., 2003, Vol. 51, pp. 5131-5150 IntroductionSince the 1940s significant progress has been made in quantitatively predicting the individual strengthening effects in simple metallic alloy systems, such as solution strengthening, strengthening due to coherent particles, dispersion strengthening, grain boundary strengthening, etc. In complex heat treatable commercial alloys, a superposition of a multitude of strengthening effects occurs, and the prediction of the yield strength as a function of composition, thermo-mechanical processing, and heat treatment attracts great interests, both from commercial and academic perspective. Recent published literature on modelling of the yield strength of Al based alloys [1,2,3,4,5,6,7] Published as : Acta Mater., 2003, Vol. 51, pp. 5131-5150 Most commercial 7xxx alloys can be categorized in two groups depending on the type of grain structure controlling element that is added. Zr containing alloys will contain β′ phase (Al 3 Zr, L1 2 structure) particles, whilst Cr containing alloys will generally contain Al 7 Cr particles. In aluminium metallurgy, these grain structure controlling particles are generally called dispersoids.The type of dispersoids will influence the recrystallisation and quench sensitivity of the alloy.Through varying recrystallisation, the dispersoids also influence the texture and the related anisotropy.In the present paper we will present a model that is designed for modelling and predicting strength in overaged multi-component alloys, in which the composition of the precipitates is variable, and closely related to the alloy composition. The model is mostly based on physical principles, but to accommodate the complexities introduced by the multi-com...

Microstructural evolution in a spray-cast aluminum alloy during equal-channel angular pressing

“…2(a) shows the DSC thermograms recorded over the entire range of heating temperatures and Fig. 2(b) shows a magnified view for temperatures above 640 K. These various curves are generally similar to those reported for other 7xxx-series alloys when treated to peak-aged and over-aged tempers [11,12]. For ease in interpretation, the primary thermal effects are labeled from I to VI in Fig.…”

A model for precipitation kinetics and strengthening in Al–Cu–Mg alloys

“…where i c is the solute concentration at the precipitate/matrix interface, m V is the molar volume of the precipitates and e c is the equilibrium solute concentration that is calculated using the regular solution model [7,8] (with formation enthalpy S ΔH taken as 75kJ/mol [8]). …”

“…There has been considerable interest in the modelling of precipitation kinetics and strengthening in age hardening aluminium alloys [1,2,3,4,5,6,7,8,9,10,11,12,13]. Most of the published work on modelling of the precipitation kinetics in aluminium alloys is based on two main approaches [1][2][3][4][5][6][7][8][9][10].…”

“…Most of the published work on modelling of the precipitation kinetics in aluminium alloys is based on two main approaches [1][2][3][4][5][6][7][8][9][10]. In one of the approaches, the modelling of the nucleation, growth and impingement of precipitates are based on the concepts of the Johnson-Mehl-AvramiKolmogorov (JMAK) model [14,15,16] and the modelling of the coarsening kinetics is based on the Lifshitz-Slyozov-Wagner (LSW) theory [17,18].…”

VPPA welds of Al-2024 alloys: Analysis and modelling of local microstructure and strength

“…( ) is the average size during the coarsening stage [43], which is taken to be in line with the classical coarsening theory, and 0 l is the average size at the start of coarsening. …”