**High Strength Al-Alloys: Microstructure, Corrosion and Principles of Protection**

Anthony E. Hughes1, Nick Birbilis2, Johannes M.C. Mol3a, Santiago J. Garcia3b, Xiaorong Zhou4 and George E. Thompson4 *1CSIRO Materials Science and Technology, Melbourne 2Department of Materials Engineering, Monash University, Clayton 3TU Delft, Department of Materials Science and Engineeringa and Novel Aerospace Materials, Aerospace Engineeringb, Delft 4School of Materials, The University of Manchester, Manchester 1,2Australia 3Netherlands 4United Kingdom* 

## **1. Introduction**

Aluminium alloys have highly heterogeneous microstructures compared to many other metal alloys. This heterogeneity originates from alloy additions and impurities which combine to produce both the desired microstructure as well as undesired, large particles, called constituent particles (and residual impurity particles) which have a range of compositions. In corrosion science these latter particles are commonly referred to as intermetallic (IM) particles. The heterogeneous nature of aluminium alloys is most evident in members of the high strength alloys of the 2xxx, 6xxx, 7xxx and 8xxx and most particularly the 2xxx series alloys where alloy additions are required to obtain the high strength to weight ratio properties of these materials.

For many years now, the study of corrosion in these alloys was, and in many instances continues to be, a phenomenological exercise. So the literature on this subject largely involves studies of a small number of intermetallic (IM) particles under a variety of conditions which are difficult to relate to each other in order to form a more general model of corrosion in highly heterogeneous aluminium alloys. This is particularly true for the 2xxx series of alloys which lacks a system to unambiguously categorise these IM particles compositional variation makes it difficult to relate these particles with well know composition, crystallography and electrochemistry. The difficulty in devising such a system should not be underestimated since the intermetallic particles form at various stages during manufacture, individual particles have compositionally different phase domains and their distribution including the spatial relationship to one another is often dictated by the processing route. Nevertheless, in recent years there have been significant advances in the understanding of both the microstructure of some high strength alloys as well as its influence on corrosion. These advances have their foundations in the wider accessibility to a range of newer electrochemical and physicochemical characterisation techniques. The use of advanced electrochemical techniques has led to a greater understanding of the properties of the intermetallic particles themselves and

High Strength Al-Alloys: Microstructure, Corrosion and Principles of Protection 225

thermomechanical processing. Only a small number of metals have sufficient solubility to serve as major alloying elements (Das 2006) and alloys derived from these few form the basis of the present classes of commodity Al-alloys. Magnesium, zinc, copper and silicon have significant solubility, whilst additional elements (of <1% solubility) are also used to confer improvements to alloy properties, namely grain refinement, and such elements include manganese, chromium, zirconium, titanium and less commonly (due to cost) scandium (Hatch 1984; Polmear 2004). Alloying of Manganese with Fe-containing intermetallic particles reduces the electrochemical activity of these Fe-containing particles

The low strength of pure aluminium (~10 MPa) mandates alloying. The simplest strengthening technique is solution hardening, whereby alloying additions have appreciable solid solubility over a wide range of temperatures and remain in solution after many

The most significant increase in strength for aluminium alloys is derived from age hardening (often called precipiation hardening) which can result in strengths as high as 800 MPa. The principal of age hardening requires that the solid solubility of alloying elements decreases with temperature. The age hardening process can be summarised by the following

i. solution treatment at a temperature within a single phase region to dissolve the alloying

iii. decomposition of the supersaturated solid solution at ambient or moderately elevated

The fundamental aspects of decomposition of a supersaturated solid solution are complex (Raviprasad, Hutchinson et al. 2003; Kovarik, Miller et al. 2006; Winkelman, Raviprasad et al. 2007). Typically however, Guinier-Preston (GP) zones and intermediate phases are formed as precursors to the equilibrium precipitate phase (Hatch 1984) (Figure 1 reveals a

Fig. 1. Dark field scanning transmission electron micrograph of coarse Al2CuMg precipitate

particles in an AA2xxx (Al-Cu-Mg) alloy - imaged down <100> zone axis

ii. quenching of the alloy to obtain what is termed a supersaturated solid solution

thus improving the corrosion resistance of the alloy (Polmear 1995)

temperature to form finely dispersed precipitates.

thermal cycles.

element(s)

stages:

their roles in corrosion of alloys. The physico-chemical studies have led to a better characterisation of the composition of intermetallic particles, and, importantly of their spatial distributions. The convergence of the electrochemical and physicochemical approaches, in combination with modelling, is leading to a statistical basis for understanding the influence of the intermetallic particles on corrosion of aluminium alloys. This chapter therefore aims to summarise our current understanding of the microstructure

of high strength aluminium alloys, particularly the more microstructurally complex alloys such as the 2xxx, and 7xxx series alloys. It then looks at how different components of the microstructure contribute to corrosion processes and finishes by examining the principles of protection of aluminium alloys using traditional and newer techniques used to assess the degree of protection.
