Cluster hardening in Al-Cu-Mg alloys: analysis of Cu-Mg atomic clustering

The usual inference of the term age hardening is that alloy strengthening occurs through precipitation
hardening. However, the proposal that certain Al-Cu-Mg and Al-Zn-Mg-Cu alloys
may be strengthened through the formation of a very fine dispersion of pre-precipitate solute
clusters has received significant attention and interest in recent years, including significant
discussion at the last several International Conferences on Aluminum Alloys [1-6]. These
studies follow the original observations of rapid hardening in these systems [7]. The proposition
of cluster hardening is attractive because of the fine length scales over which the
strengthening mechanisms might operate, so providing flexible utilization of alloy solute and
considerable scope for design of this nanoscale microstructure or nanostructure. Factors such
as the cluster chemistry and the dispersive properties of clusters could be tailored for specific
property targets. These clusters may also be used to effect particular transformation pathways,
leading to particular precipitate nucleation reactions that could not otherwise occur [4,8].
Because these solute clusters are very small in size, perhaps involving only a few solute atoms,
and because the clustered atoms occupy substitutional positions on the parent Al matrix,
the challenges for detailed quantitative characterization of solute atom clustering are formidable.
Yet such experiments are crucial to advance our understanding of the phenomena occurring
during the early stages of ageing in Al alloys. In fact, such path-finding experiments are
essential in order to establish a phenomenological framework for subsequent modeling and
computational analyses that can ultimately enable the phenomena to be used in alloy design.
Atom probe tomography (APT) is a powerful technique with sufficiently high spatial resolution
in three dimensions to be able to resolve the position and chemical identity of individual
atoms [9]. Even with this resolving power, the systematic identification and analysis of atomic
clusters must consider microscope aberrations, detector shortcomings and, given the large
size of the data files now routinely acquired, must rely on rigorous data mining computations.
This paper summarizes recent experimental studies on the nature of atomic clustering in Al1.1Cu-xMg
(at. %) alloys that lie in the α+S phase field and which exhibit a remarkable rapid
hardening effect during the early stages of ageing and prior to precipitation when x ≥ 0.5 at. %
[10]. Our transmission electron microscopy (TEM) and APT studies reveal the decomposition
processes within the solid solution and provide insights into the mechanisms of this novel and
most significant strengthening effect.