**AA2xxx**

230 Recent Trends in Processing and Degradation of Aluminium Alloys

required to remove these electrochemically active layers (Leth-Olsen, Nordlien et al. 1997; Mol, Hughes et al. 2004; Hughes, Mol et al. 2005). However, the presence of fine grains alone in the deformed layer, with grain boundaries free of oxide particles, is insufficient to hinder

Importantly, the NSDL has significant influence on properties such as the electrochemical and corrosion behaviour as well mechanical properties, material joining and optical properties. The high population of grain boundaries and severe deformation in the deformed layer promote precipitation of intermetallic particles during subsequent heat treatment (Liu, Zhou et al. 2007). For example, a near-surface deformed layer on AA6111 automotive closure sheet alloy can be generated by mechanical grinding during rectification, as shown in Figure 3 (left). Subsequent paint baking, i.e. thermal exposure at 180C for 30 minutes, promotes the precipitation of Q phase (with various compositions: Al5Cu2Mg8Si6 (Pan, Morral et al. 2010), Al4CuMgSi4 (Hahn and Rosenfield 1975)) particles, ~20 nm diameter, at preferred grain boundaries within the deformed layer (Figure 3 centre), but with no precipitates being formed in the underlying bulk alloy. The presence of Q phase precipitation in the near-surface deformed layer increases dramatically the susceptibility of the alloy to cosmetic corrosion that propagates intergranularly, with micro-galvanic coupling between the Q phase precipitates and the adjacent aluminium matrix providing

grain coarsening during typical annealing treatments (Zhou 2011).

(a) (b) (c)

**2.7 High strength aluminium alloys AA2xxx and AA7xxx** 

Fig. 3. Transmission electron micrographs of ultramicrotomed section of AA6111 aluminium

Microstructural variation in the high strength Al-alloys exists over a range of scales as reported in Table 2. At the atomic and nanoscopic scale the microstructure is related to the mechanical properties of the alloy. This microstructure involves defect structures, hardening precipitates and dispersoid particles. The high strength of the 2xxx and 7xxx series alloys is due to the hardening precipitates with dispersoids playing a secondary role. Dispersoids can pin grain growth limiting grain size thus making a small contribution to increased

alloy after SHT, mechanical grinding and 30 minutes at 180C: (a) bright field image, revealing a near-surface deformed layer and (b) dark field image at increased magnification, revealing grain boundary precipitates. (c) transmission electron micrograph showing

the driving force (Figure 3(right)).

intergranular corrosion

The AA2xxx series of alloys are among the most complicated to analyse. While there have been several reports of the compositions of different phases within this group, most have focused on the legacy alloy AA2024-T3, which, unfortunately, is one of the most complex of the 2xxx series of alloys. Perhaps one of the better known of these works was published in 1950 by Phragmen (Phragmen 1950). He examined all the binary, ternary, quaternary quinternary and senary compositions in order to under stand the IM particles in AA2024. Unfortunately the classification of these particles relied heavily on metallographic techniques (etches) and optical microscopy meaning that assignment of IM particles was, in many cases made on appearance and not on composition. These types of studies, however,

High Strength Al-Alloys: Microstructure, Corrosion and Principles of Protection 233

study IM particles in AA2024-T3 and AA7075-T6 and found that compositions containing Al, Cu, Fe, Mn and Si, had a rhomohedral structure which did not match the hexagonal structure previously reported for particles of this type for example Al8Fe2Si or Al10Mn3Si. Classifying IM particles by shape is used both in metallurgy and corrosion. It is commonly used to distinguish between phases that contain Al-Cu-Fe-Mn-Si which tend to be angular and S (Al2CuMg) and θ (Al2Cu) phase which tend to be rounded. This distinction is alluded to in standard texts describing the microstructure of AA2024-T3 (Hatch 1984). This is the least reliable method of identification since it has recently been demonstrated that Al7Cu2Fe also has a rounded structure and is a similar size to S and θ-phase constituent particles

Fig. 4. Analyses in wt% for Cu, Mg, Mn, Si and Fe in ten different batches of AA2024-T3(51)

 Fe Si Mn Mg Cu

10 μm

suggesting that assignement by shape could easily lead to mis-identification.

Batch 10 Batch 9 Batch 0 Batch 7 Batch 6 Batch 1 Batch 2 Batch 3 Batch 4 Batch 5

purchased over the period 1995 to late 2000

**(a) (b)**

Fig. 5. (a) backscattered and (b) secondary electron images of an IM particle with compositional domains. Corroded in 0.1 M NaCl for 30 minutes at room temperature

To understand the complexity of the AA2024-T3 microstructure we begin with the compositional variation between several batches of sheet product (Figure 4). The major alloying components in the 2xxx series are Cu and Mg and they generally have similar mole

Wt %

form the basis for modern assignment of IM particles. From the perspective of obtaining the desired mechanical properties at the nanometer scale, characterisation has focused on the evolution of the alloy microstructure. Corrosion *initiation*, however, is much more closely related to the large constituent particles whose compositions are based on major alloying elements. Corrosion propagation involves all scales of the alloy microstructure.

Copper and magnesium are the two major alloying additions in AA2xxx wrought alloys and because of the Cu, this series is less resistant to corrosion than alloys of other series. Much of the thin sheet made of these alloys is produced as an Al-clad composite, with a relatively pure aluminium alloy as the outer layer, but thicker sheet and other products in many applications have no protective cladding. Electrochemical effects on corrosion can be stronger in these alloys than in alloys of many other types because of two factors: larger variations in electrochemical activity with variations in amount of copper in solid solution and, under some conditions, the presence of non-uniformities in solid solution concentration. The decrease in resistance to corrosion with increasing copper content is exacerbated by the corrosion process itself by the formation of minute copper particles or films deposited on the alloy surface as a result of corrosion.

To begin to understand the microstructure and its influence on corrosion it is important to know compositions of second phase intermetallic particles. However, the characterisation of these IM particles in the corrosion literature is poor with virtually no studies that relate composition with crystal structure. Hence, there is a need for a detailed identification system which links compositional variation within IM particles, with crystallography and electrochemical character. The objective is to move from a purely phenomenological description of corrosion to a level of understanding where the corrosion process can be predicatively modelled. This level of understanding is primarily aimed at developing structural health management algorithms for maintenance management and several approaches to this are already outlined in the literature (Hughes, Hinton et al. 2007; Cavanaugh, Buchheit et al. 2010; Ralston, Birbilis et al. 2010), (Trueman 2005). The determination of appropriate metrics for the compositional and electrochemical characteristics for IM particles is not so straight forward and is addressed below.

IM particles in AA2024-T3 are currently identified and categorised by one or a mixture of the following:


Composition and electrochemistry are the most useful categories for corrosion studies and the convergence of these two systems is desirable and has already been achieved for alloys with simpler microstructure. This means that in many alloys a particular composition can be associated with a specific electrochemical behaviour. This is not the case, for example, in AA2024-T3 where there is large compositional variation as described below. The crystallography of IM phases is not so useful in corrosion studies since it is likely that specific crystal structures do not have a one to one relationship with either composition or the electrochemistry in AA2024-T3. What generally happens in the corrosion literature is that once the composition of an IM particle is determined then the particle is assigned a standard stoichiometry which is derived from crystallographic studies. So there is an assumption that a particular composition has a specific crystallographic structure which may not be valid. For example, Wei and coworkers (Gao, Feng et al. 1998; Wei, Liao et al. 1998) used TEM to

form the basis for modern assignment of IM particles. From the perspective of obtaining the desired mechanical properties at the nanometer scale, characterisation has focused on the evolution of the alloy microstructure. Corrosion *initiation*, however, is much more closely related to the large constituent particles whose compositions are based on major alloying

Copper and magnesium are the two major alloying additions in AA2xxx wrought alloys and because of the Cu, this series is less resistant to corrosion than alloys of other series. Much of the thin sheet made of these alloys is produced as an Al-clad composite, with a relatively pure aluminium alloy as the outer layer, but thicker sheet and other products in many applications have no protective cladding. Electrochemical effects on corrosion can be stronger in these alloys than in alloys of many other types because of two factors: larger variations in electrochemical activity with variations in amount of copper in solid solution and, under some conditions, the presence of non-uniformities in solid solution concentration. The decrease in resistance to corrosion with increasing copper content is exacerbated by the corrosion process itself by the formation of minute copper particles or

To begin to understand the microstructure and its influence on corrosion it is important to know compositions of second phase intermetallic particles. However, the characterisation of these IM particles in the corrosion literature is poor with virtually no studies that relate composition with crystal structure. Hence, there is a need for a detailed identification system which links compositional variation within IM particles, with crystallography and electrochemical character. The objective is to move from a purely phenomenological description of corrosion to a level of understanding where the corrosion process can be predicatively modelled. This level of understanding is primarily aimed at developing structural health management algorithms for maintenance management and several approaches to this are already outlined in the literature (Hughes, Hinton et al. 2007; Cavanaugh, Buchheit et al. 2010; Ralston, Birbilis et al. 2010), (Trueman 2005). The determination of appropriate metrics for the compositional and electrochemical

characteristics for IM particles is not so straight forward and is addressed below.

IM particles in AA2024-T3 are currently identified and categorised by one or a mixture of

Composition and electrochemistry are the most useful categories for corrosion studies and the convergence of these two systems is desirable and has already been achieved for alloys with simpler microstructure. This means that in many alloys a particular composition can be associated with a specific electrochemical behaviour. This is not the case, for example, in AA2024-T3 where there is large compositional variation as described below. The crystallography of IM phases is not so useful in corrosion studies since it is likely that specific crystal structures do not have a one to one relationship with either composition or the electrochemistry in AA2024-T3. What generally happens in the corrosion literature is that once the composition of an IM particle is determined then the particle is assigned a standard stoichiometry which is derived from crystallographic studies. So there is an assumption that a particular composition has a specific crystallographic structure which may not be valid. For example, Wei and coworkers (Gao, Feng et al. 1998; Wei, Liao et al. 1998) used TEM to

elements. Corrosion propagation involves all scales of the alloy microstructure.

films deposited on the alloy surface as a result of corrosion.

the following: i. composition ii. electrochemistry ii. crystallography

iv. shape

study IM particles in AA2024-T3 and AA7075-T6 and found that compositions containing Al, Cu, Fe, Mn and Si, had a rhomohedral structure which did not match the hexagonal structure previously reported for particles of this type for example Al8Fe2Si or Al10Mn3Si. Classifying IM particles by shape is used both in metallurgy and corrosion. It is commonly used to distinguish between phases that contain Al-Cu-Fe-Mn-Si which tend to be angular and S (Al2CuMg) and θ (Al2Cu) phase which tend to be rounded. This distinction is alluded to in standard texts describing the microstructure of AA2024-T3 (Hatch 1984). This is the least reliable method of identification since it has recently been demonstrated that Al7Cu2Fe also has a rounded structure and is a similar size to S and θ-phase constituent particles suggesting that assignement by shape could easily lead to mis-identification.

Fig. 4. Analyses in wt% for Cu, Mg, Mn, Si and Fe in ten different batches of AA2024-T3(51) purchased over the period 1995 to late 2000

Fig. 5. (a) backscattered and (b) secondary electron images of an IM particle with compositional domains. Corroded in 0.1 M NaCl for 30 minutes at room temperature

To understand the complexity of the AA2024-T3 microstructure we begin with the compositional variation between several batches of sheet product (Figure 4). The major alloying components in the 2xxx series are Cu and Mg and they generally have similar mole

High Strength Al-Alloys: Microstructure, Corrosion and Principles of Protection 235

Al2CuMg Al51.62Cu24Mg23.52 Fe(0.08), Mn(0.05), Cr(0.09) Zn(0.75)

Al2Cu Al62.98Cu33.83Mg2.82 Fe(0.11), Mn(0.04), Cr(0.17) Ni(0.06)

Table 4. Composition of IM particles in AA2024-T3 determined by Gao et al. (Gao, Feng et

Wei and co-workers(Gao, Feng et al. 1998) identified S-phase, θ-phase and the remainder were categorised as (Fe,Mn)xSi(Al,Cu)y indicating no Al7Cu2Fe. Electron diffraction of these phases found an unidentified rhombohedral structure and particles with the general composition of (Fe,Mn)xSi(Al,Cu)y were reported as variants of Al8Fe2Si or Al10Mn3Si. Buchheit et al.(Buchheit, Grant et al. 1997) performed an electron microprobe analysis of 652 particles and identified S-phase and a range of phases containing Al, Cu, Fe and Mn as listed in Table 5. In the most recent study, Boag et al. (Boag, Hughes et al. 2009), examined around 82,000 compositional domains in 18,000 IM particles and identified the compositions in Table 6. An example of these compositional domains within IM particles is shown in Figure 6. (Al,Cu)21(Mn,Fe)4Si are widespread in the alloy and were assigned to the (Fe,Mn)xSi(Al,Cu)y phase identified by Wei and co-workers. The S-phase and θ-phase predominantly occur as domains within individual, but composite particles. Al7Cu3Fe, in most instances, are a third group of particle and have domains, generally within their centre of Al10(Cu, Mg). A periphery phase was observed around S/θ phase composite particles, which appeared to coincide with a precipitate free zone (Boag 2009; Boag, Hughes et al.

Fig. 6. Microprobe image IM particles in AA2024-T3 showing compositional domains. = (Al,Cu)21(Mn,Fe)4Si, = Al2CuMg, = Al2Cu, = Al7Cu3Fe, = Al10(Cu, Mg), =

**Particle Type Measured Stoichiometry Other elementse** 

2009). There was also a periphery phase surrounding many particles.

Al2CuMg (EPMA) Al52.8Cu24.5Mg22.7 -

al. 1998)

Al3(Cu,Fe,Mn)

(Al,Cu)ySi (Fe,Mn)x Al67.6 Cu3.36Fe14.89 Mn6.87Si6.27 Mg (0.43)

fractions. Cu, Mn and Mg have specified compositional bands and the bands for Cu are superimposed on Figure 4. Clearly, most batches fall within specification, although two batches breach the upper specification limit by 0.3 to 0.4 wt%. The IM particles are formed from these elements and the variation in composition from batch to batch manifests itself in compositional variation of the IM particles. The origin of this variation is described above and is related to the source material.

The reason why compositional variation from batch to batch represents a difficulty is that the transition elements can be substitutional in many intermetallics. For example Ayer et al. (Ayer, Koo et al. 1985) found Zn and Ni in Al7Cu2Fe, and Gao et al. (Gao, Feng et al. 1998) found considerable compositional variation within a phase nominated as (Fe,Mn)xSi(Al,Cu)y with some particles containing mainly Fe and Cu with small amounts of Mn and others had significant Mn and Si; similar results were reported by Boag et al.(Boag, Taylor et al. 2010). Gao et al. (Gao, Feng et al. 1998) even found small amounts of Cr and Zn in Al2CuMg. These variations can make it difficult to classify the particles on the basis of composition alone. In addition to composition variation, many individual IM particles often contain compositional domains within the particle. Figure 5 shows backscattered and secondary electron images of an IM particle with different composition domains. The bright regions in the backscatter images are Cu and Fe rich as described above whereas the darker parts of the IM particle contain more Mn and Si. It is not know whether these domains have different crystal structures. The dark band around the bottom of the IM particle represents the beginning of a form of corrosion called trenching which is often observed around cathodic IM particles. It is clear that this corrosion has initiated in the matrix adjacent to the Cu and Fe rich part of the IM particle indicating greater electrochemical activity of this part of the particle.

From the literature, standard texts such as Hatch (Hatch 1984) lists the IM particles in AA2024-T3 and AA7075-T6 as presented in Table 3. These compositions are derived from metallurgical studies. At the top of the Table are the 'as-cast" compositions and in the bottom the wrought compositions. According to these standard texts, heat treatment dissolves much of the Al2Cu and Al2CuMg whereas all Fe-containing IM particles convert to Al7Cu2Fe. Thus the wrought composition contains the IM particles listed in Table 3. Compositional analyses in several other studies have some overlap with the composition of phases (based on stoichiometry) listed in Table 3. Compositional analyses from the three most comprehensive studies in the literature are presented Tables 4 to 6.



fractions. Cu, Mn and Mg have specified compositional bands and the bands for Cu are superimposed on Figure 4. Clearly, most batches fall within specification, although two batches breach the upper specification limit by 0.3 to 0.4 wt%. The IM particles are formed from these elements and the variation in composition from batch to batch manifests itself in compositional variation of the IM particles. The origin of this variation is described above

The reason why compositional variation from batch to batch represents a difficulty is that the transition elements can be substitutional in many intermetallics. For example Ayer et al. (Ayer, Koo et al. 1985) found Zn and Ni in Al7Cu2Fe, and Gao et al. (Gao, Feng et al. 1998) found considerable compositional variation within a phase nominated as (Fe,Mn)xSi(Al,Cu)y with some particles containing mainly Fe and Cu with small amounts of Mn and others had significant Mn and Si; similar results were reported by Boag et al.(Boag, Taylor et al. 2010). Gao et al. (Gao, Feng et al. 1998) even found small amounts of Cr and Zn in Al2CuMg. These variations can make it difficult to classify the particles on the basis of composition alone. In addition to composition variation, many individual IM particles often contain compositional domains within the particle. Figure 5 shows backscattered and secondary electron images of an IM particle with different composition domains. The bright regions in the backscatter images are Cu and Fe rich as described above whereas the darker parts of the IM particle contain more Mn and Si. It is not know whether these domains have different crystal structures. The dark band around the bottom of the IM particle represents the beginning of a form of corrosion called trenching which is often observed around cathodic IM particles. It is clear that this corrosion has initiated in the matrix adjacent to the Cu and Fe rich part of

the IM particle indicating greater electrochemical activity of this part of the particle.

most comprehensive studies in the literature are presented Tables 4 to 6.

AA2024-T3 AA7075-T6

Unreacted (Mn,Fe)3SiAl12

Al20Mn3Cu2 (Dispersoid)

Table 3. Typical breakdown of constituent particles in AA2024-T3 and AA7075-T6

(Mn,Fe)3SiAl12

Al2Cu (θ-phase) Al2CuMg (s-phase)

Al3(Fe,Mn) Al6(Fe,Mn)

Al7Cu2Fe

Mg2Si

From the literature, standard texts such as Hatch (Hatch 1984) lists the IM particles in AA2024-T3 and AA7075-T6 as presented in Table 3. These compositions are derived from metallurgical studies. At the top of the Table are the 'as-cast" compositions and in the bottom the wrought compositions. According to these standard texts, heat treatment dissolves much of the Al2Cu and Al2CuMg whereas all Fe-containing IM particles convert to Al7Cu2Fe. Thus the wrought composition contains the IM particles listed in Table 3. Compositional analyses in several other studies have some overlap with the composition of phases (based on stoichiometry) listed in Table 3. Compositional analyses from the three

> (Fe,Cr)3SiAl12 Mg2Si

> (Fe,Cr)3SiAl12 Al7Cu2Fe

Zn2Mg ((Zn,Cu,Al)2Mg)

Al18Mg3Cr2 (Dispersoid)

and is related to the source material.

**Treatment Phases** 

Wrought Al2CuMg

Cast

Ingot formation


Table 4. Composition of IM particles in AA2024-T3 determined by Gao et al. (Gao, Feng et al. 1998)

Wei and co-workers(Gao, Feng et al. 1998) identified S-phase, θ-phase and the remainder were categorised as (Fe,Mn)xSi(Al,Cu)y indicating no Al7Cu2Fe. Electron diffraction of these phases found an unidentified rhombohedral structure and particles with the general composition of (Fe,Mn)xSi(Al,Cu)y were reported as variants of Al8Fe2Si or Al10Mn3Si. Buchheit et al.(Buchheit, Grant et al. 1997) performed an electron microprobe analysis of 652 particles and identified S-phase and a range of phases containing Al, Cu, Fe and Mn as listed in Table 5. In the most recent study, Boag et al. (Boag, Hughes et al. 2009), examined around 82,000 compositional domains in 18,000 IM particles and identified the compositions in Table 6. An example of these compositional domains within IM particles is shown in Figure 6. (Al,Cu)21(Mn,Fe)4Si are widespread in the alloy and were assigned to the (Fe,Mn)xSi(Al,Cu)y phase identified by Wei and co-workers. The S-phase and θ-phase predominantly occur as domains within individual, but composite particles. Al7Cu3Fe, in most instances, are a third group of particle and have domains, generally within their centre of Al10(Cu, Mg). A periphery phase was observed around S/θ phase composite particles, which appeared to coincide with a precipitate free zone (Boag 2009; Boag, Hughes et al. 2009). There was also a periphery phase surrounding many particles.

Fig. 6. Microprobe image IM particles in AA2024-T3 showing compositional domains. = (Al,Cu)21(Mn,Fe)4Si, = Al2CuMg, = Al2Cu, = Al7Cu3Fe, = Al10(Cu, Mg), = Al3(Cu,Fe,Mn)

High Strength Al-Alloys: Microstructure, Corrosion and Principles of Protection 237

(Meng and Frankel 2004). All AA7xxx alloys are more resistant to general corrosion than AA2xxx alloys, but less resistant than wrought alloys of other groups. The AA7xxx series alloys are among the aluminium alloys most susceptible to SCC and Cu is beneficial from

While the total weight of alloying components in AA7075-T6 is higher than AA2024-T3 by around 1 to 2% the microstructure tends to be simpler, in terms of the number and identification of IM particle types. Hardening precipitates are generally of the family ηphase (Zn2Mg) and dispersoids are of the composition Al20Cu2Mn3 and Al18Mg3Cr2. Like AA2024-T3 reports of constituent particle compositions vary. Gao et al.(Gao, Feng et al. 1998) report two phases: Al23Fe4Cu and SiO2. However, constituent particles compositions reported by others authors suggest Al7Cu2Fe, Al2Zn, Al3Zr and Mg2Si (Birbilis and Buchheit

Clustering of IM particles is an emerging area of importance in understanding pit initiation and stabilisation (Chen, Gao et al. 1996; Park, Paik et al. 1996; Park, Paik et al. 1999), (Ilevbare, Schneider et al. 2004), (Liao, Olive et al. 1998; Schneider, Ilevbare et al. 2004; Harlow, Wang et al. 2006), (Cawley and Harlow 1996; Hughes, Boag et al. 2006; Mao, Gokhale et al. 2006; Hughes, Wilson et al. 2009; Hughes, MacRae et al. 2010). Clustering may be important at several different length scales and perhaps even times scales (for corrosion processes). Clustering at length scales similar to the IM particle size can be attributed to IM particle fracture during mechanical processing and, in some instances to non-equilibrium microstructures. An explanation of IM particle clustering reported for larger scale of a few hundred microns is not clear. A study by Mao et al., (Mao, Gokhale et al. 2006) revealed both short range (size similar to the particle dimensions) and long range (few hundred times the particle size) clustering in AA7075 alloy plate material. Clearly the clustered structures are elongated in the rolling direction and they have a range of different sizes. Hughes and co-workers (Hughes, Boag et al. 2006; Hughes, Wilson et al. 2009; Hughes, Muster et al. 2010) reported significant clustering in AA2024-T3 alloy sheet between phase domains within IM particles, as well as between IM particles themselves. In their study they identified strong clustering behaviour between S-phase and θ-phase, S-phase and the Al7Cu2Fe phase and to a lesser extent between S-phase and IM particles with an average stoichoimetry of (Al,Cu)21(Fe,Mn)4Si . In that particular study the microstructure consisted of individual particles which had compositional domains of S and θ, which represented a

Clustering behaviour has also been reported for a number of other aluminium alloys including AA6061-T6, AA7075-T6 and AA5005 (Cawley and Harlow 1996; Hughes, Boag et al. 2006). Coupling between IM particles types of different electrochemical activity has been observed at stable pit sites and attributed to their initiation (Liao, Olive et al. 1998; Boag, Taylor et al. 2010). On the other hand Wei and co-workers (Chen, Gao et al. 1996; Liao, Olive et al. 1998) and Ilevbare et al. (Ilevbare, Schneider et al. 2004) have concluded that clustering in AA2024-T3 and AA7075-T6 alloys leads to large stable pits, primarily through excessive lateral trenching. Cawley and Harlow (Cawley and Harlow 1996) found that IM particles in AA2024-T3 alloys tended to be clustered whereas the pits tended to be randomly distributed because the spatial relationships between IM particles is lost during

the standpoint of resistance to SCC.

2005; Wloka and Virtanen 2008).

the highest degree of clustering.

excessive corrosion.

**2.8 Clustering** 

What is clear from these studies is that there is no definitive composition for particles that contain Al, Cu, Mn, Fe, and Si (with small additions of other elements) and it is clear from observations such as those in Figure 5 that these regions of compositional variation have different electrochemical activity. In addition to the compositional variation there is evidence of considerable microstructural variations within the compositional field defined for AA2024-T3. For example θ-phase has been reported recently in studies of AA2024-T3, whereas no θ-phase was detected by Buchheit et al. (Buchheit, Grant et al. 1997) and Hughes and co-workers have examined different batches of AA2024-T3 sheet product and detected some batches with only S-phase and some with S-phase/ θ-phase composite particles(Boag, Taylor et al. 2010).

### **AA7xxx**

In AA7xxx wrought alloy the major alloying element is zinc along with magnesium or magnesium plus copper in combinations that develop various levels of strength. Those containing copper have the highest strengths and have been used as structural materials, primarily in aircraft applications.


Table 5. Composition of IM particles in AA2024-T3 determined by Buchheit et al. (Buchheit, Grant et al. 1997)


Table 6. Composition of IM particles in AA2024-T3 determined by Boag et al. (Boag, Hughes et al. 2009)

The AA7xxx wrought alloys are anodic to AA1xxx wrought aluminium and to other aluminium alloys. Resistance to general corrosion of the copper-free wrought AA7xxx alloys is good, approaching that of the wrought AA3xxx, AA5xxx and AA6xxx alloys. The coppercontaining alloys of the AA7xxx series, such as 7049, 7050, 7075, and 7178 have lower resistance to general corrosion than those of the same series that do not contain copper (Meng and Frankel 2004). All AA7xxx alloys are more resistant to general corrosion than AA2xxx alloys, but less resistant than wrought alloys of other groups. The AA7xxx series alloys are among the aluminium alloys most susceptible to SCC and Cu is beneficial from the standpoint of resistance to SCC.

While the total weight of alloying components in AA7075-T6 is higher than AA2024-T3 by around 1 to 2% the microstructure tends to be simpler, in terms of the number and identification of IM particle types. Hardening precipitates are generally of the family ηphase (Zn2Mg) and dispersoids are of the composition Al20Cu2Mn3 and Al18Mg3Cr2. Like AA2024-T3 reports of constituent particle compositions vary. Gao et al.(Gao, Feng et al. 1998) report two phases: Al23Fe4Cu and SiO2. However, constituent particles compositions reported by others authors suggest Al7Cu2Fe, Al2Zn, Al3Zr and Mg2Si (Birbilis and Buchheit 2005; Wloka and Virtanen 2008).
