Materials Science Forum Vols. 519-521

Abstract: A multi-mechanistic model for microstructure development and strengthening during nonisothermal treatment of precipitation strengthened Al-Cu-Mg based alloys is derived. The formation kinetics of the precipitates is modelled using the Kampmann and Wagner numerical model that accounts for complete transformation from the nucleation to the coarsening stages. The increase in critical resolved shear strength of the grains due to the precipitates is based on two mechanisms i.e. the modulus strengthening mechanism for the shearable Cu:Mg co-clusters and the Orowan strengthening mechanism for the non-shearable S phase precipitates. The contributions due to solute and dislocation strengthening are also included in the strength calculations. The model is verified by comparing the predicted results with differential scanning calorimetery and hardness data on 2024 aluminium alloys. The microstructural development and strength/hardness predictions of the model are in reasonable agreement with the experimental data and the differences are discussed in terms of requirements for further model development.

Abstract: This paper reviews some of the practical outcomes of exploiting secondary precipitation during the heat treatment of aluminium alloys and discusses current understanding of this phenomenon. Recent studies have utilised the techniques of positron annihilation spectroscopy (PAS), 3D atom probe (3DAP) as well as the more traditional transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) to investigate early precipitation events that occur during secondary ageing at low temperatures (~20-65°C). This work has confirmed that clustering and GP zone formation can modify the nature and distribution of precipitates that form when ageing is subsequently resumed at more elevated temperatures. Prospects for achieving further improvements in heat treatment schedules and alloy compositions to take greater advantage of secondary precipitation are also considered.

Abstract: The precipitation kinetics path in multi-component alloys may involve a competition between atomic mobilities and precipitates thermodynamic stability. Cluster dynamics modelling (CDM) is a simulation method that allows to describe this competition without introducing any heuristic assumptions as, for example, in the classical theory of nucleation. CDM consists in solving numerically, for each time increment, the master equations expressing the balance of solute exchanges (absorption and emission) between clusters/precipitates. A key issue is the energetics of the nano-clusters in the nucleation range. The computation of the precipitate size distribution function allows the complete description of the precipitates kinetic evolution, in chemical composition and in size. The method is applied to the precipitation of the Al3(Zr,Sc) L12 phase in Al solid solutions. The model predicts fairly well in the precipitation path some observed coupling effects between the two solutes, particularly during the nucleation stage.

Abstract: A simplified numerical model for the solid state phase transformation from Al6(Mn,Fe) to α-Al(Mn,Fe)Si phase in 3xxx alloys has been constructed. In this model, the phase transformation is assumed to be initiated by the heterogeneous nucleation of α-Al(Mn,Fe)Si dispersoids at the interface between Al6(Mn,Fe) particle and matrix and the growth of the α- Al(Mn,Fe)Si phase into the Al6(Mn,Fe) particle is controlled by the diffusion of Si from the matrix. The model has been implemented into a numerical homogenization model. The simulation results show that the implementation of the phase transformation model improves much the prediction results of the homogenization model on the evolution of solid solution level of alloying elements and the volume fraction evolution of dispersoids in 3xxx alloys during homogenization.

Abstract: Alloys designed to optimise strength and extrudability have a lower alloy Mg to Si ratio than has commonly been used in AA6060 and AA6063 alloys. Intermetallic phases have an impact on alloy design since they tie up some of the Mg and Si alloy content. The effect of Mg and Si alloy content on the type of intermetallic phases present has been investigated using TEM, SEM and Thermocalc analysis. Results for Al-Mg-Si-Fe alloys with 0.46 - 0.70 wt%Mg, 0.27 - 1.24 wt%Si and 0.09 - 0.22 wt%Fe are presented. The occurrence of a-AlFeSi (various stoichiometries), b- Al5FeSi, p-Al8FeSi6Mg3, Mg2Si and Si has been found to depend on alloy composition within the ranges examined.

Abstract: Positron annihilation spectroscopy in two versions (lifetime and coincidence Doppler broadening) has been applied to investigate solute/vacancy interactions when minor amounts (<1wt.%) of Ag or Cu are added to the alloy Al-4Zn-3Mg (wt.%) during ageing at 150°C. The results show early clustering of vacancies with Zn (and with Cu, if present). Ag displays a strong interaction with vacancies in competition with Mg and forms clusters that may help further aggregation of the other alloying elements during artificial ageing. High Mg concentration is observed at the misfit interfaces of semi-coherent or incoherent precipitates.

Abstract: The commercial 7xxx series Al alloys are based on medium strength Al-Zn-Mg and high strength Al-Zn-Mg-Cu systems. The medium strength alloys are weldable, whilst the high strength alloys are nonweldable. On the other hand, the Cu-free, weldable alloys suffer from poor SCC resistance. It is the purpose of this article to provide quantitative data and microstructural analysis to demonstrate that small additions of either Ag or Sc to Al-Zn-Mg and Al-Zn-Mg-Cu alloys bring about very significant improvement in SCC resistance and weldability, respectively. The improvement in SCC resistance of the Cu-bearing alloys due to over aging and retrogression and reaging (RRA) is further discussed in light of a similar improvement in the SCC resistance of these alloys, when peak aged, due to Ag and Sc additions.

Abstract: A kinetic model has been developed to simulate the precipitate size distribution and the resulting yield strength during ageing of 7xxx alloys. The η phase is the only one considered. The kinetic model is mean field: precipitates of different sizes see each other through the average solid solution. Precipitates are assumed to be homogeneous in concentration and are allowed to change chemistry. Local equilibrium is assumed at the matrix-precipitate interface; the equilibrium concentrations are corrected by the curvature effect. Values of the equilibrium concentrations at the matrix-precipitate interface are solved by an iterative method: the resulting flux for each element must be compatible with equilibrium conditions and with the changing stoechiometry of the considered precipitate while maximizing the energy gained. The yield strength is derived from the precipitate size distribution through a mixture law combining the effect of each individual precipitate. The model can take into account non-isothermal treatments and can therefore simulate complicated multi-stage ageing treatment as well as a FSW weld. Results of the model are discussed and compared measurements.

Abstract: Intergranular corrosion is a significant concern for Al-Mg alloys when subjected to a corrosive salt-water environment. To address this issue, the standard composition of a 5XXX series aluminum alloy (AA5083) was modified in an attempt to improve the alloy’s overall corrosion resistance through alloying and thermal processing. The concept being that through alloying and heat treatments, desirable precipitate phases such as τ- and/or τ-copper rich phase(s) that are known to offer corrosion resistance would potentially form that could effectively improve intergranular corrosion behavior. Therefore, the chemical composition of standard AA5083 was modified by adding various amounts of copper and zinc. Sensitization heat treatments were then performed to determine the specific conditions under which these phases would form. LOM, SEM, STEM imaging and conventional TEM were used to analyze microstructural features. Corrosion was attributed to a network of detrimental Mg-rich grain boundary precipitates in the standard alloy. Alloying with Cu and Zn can offer improved intergranular corrosion behavior. The mechanism seems to be either by delaying or eliminating precipitation at the grain boundaries.

Abstract: In industrial process, like creep-ageforming, materials are aged under load. To investigate the influence of an applied stress on the ageing behaviour of Cu bearing Al-7xxx series alloys, a 7475 alloy was aged under a constant tensile stress and analysed by means of Small Angle X-ray Scattering (SAXS) and TEM. Mechanical testing was also employed, to determine if there was any effect on the materials strength. The results show that during the early stages of ageing significant interactions takes place, which preferentially aligns one type of GP zone, as well as affecting their size and volume fraction. During the second stage ageing treatment, the applied stress was observed to cause more rapid over-ageing, by promoting the formation of the η phase.