Aluminium Alloy Characterisation

At the nanoscale there are hardening precipitates which provide the high strength of these alloys. At larger scales there are dispersoid particles and at the 10s of microns scales there are intermetallic particles. The characterization techniques available to our group include electron backscatter diffraction, electron microprobe analysis and standard SEM with EDS. In recent years Li detection using soft X-ray emission spectroscopy has been added to our range of techniques. This is extremely important for investigation of some of the Al-Cu-Mg-Li (2xxx series) and Al-Li alloys (8xxx series). These techniques allow characterisation from the submicron range up to the macroscopic scale.

With such large datasets. it is important to have the right tools to analyse the data quickly and efficiently. The group has developed a software package called Chimage which provides a range of different types of techniques for analysing large scale datasets the technique of choice used for analysing this data is the scatter plot. The scatter plot plots the K-ratio of one element with respect to a second element this provides a “map” of the compositional distribution of these elements. Figure 2 shows such a scatter plot for copper and iron for AA2099. 

In the case for aluminium alloy 2099 the five compositions in figure 2 are presented in Table 1. It can be seen that there are only four structural types assigned to the five compositions. In this case the structure Al₁₃(Fe,Mn)₄ has two compositional variants, one with slightly higher Cu than the other. This compositional “resolution” could not have been attained using standard EDS showing how the combination of the microprobe data and software analysis provides a much more subtle tool for discriminating compositions.

The importance of determining the compositions of these intermetallic particles is useful for understanding corrosion initiation sites on the surfaces of these high strength aluminium alloys. The IM particles form galvanic couples with the aluminium matrix and also between particles  themselves. This type of galvanic couple usually leads to corrosion of the surrounding aluminium matrix the strength of the couple is determined by the composition of the intermetallic particle. 

Another strength of the group also has access to soft X Ray emission spectroscopy (SXES) principally for lithium detection. This is installed on two machines. On our Jeol 8530 SXES runs in parallel with a number of other techniques including wavelength dispersive X Ray spectroscopy (WDS x4), Energy dispersive spectroscopy and photoluminescence spectroscopy. Thus data from all these techniques can combined into it is single datasets thus giving lithium detection at the same time as other spectroscopies.

¹Note that this phase was determined using EBSD and the composition presented in the first column was determined from a sampling volume that was generally much larger than the particles themselves. Also the beam energy at 15 keV is too high for B detection
²The Al:TM ratio was determined from the atomic percent figures