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
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.