Soft x-ray measurement of light elements
Li distribution in aircraft alloys
Aluminium alloys, both cast and wrought have complex microstructures. The complex microstructures includes variation in grain size and distribution, internal stress distribution, and intermetallic (IM) particle composition, size, shape and distribution. IM particles form through alloying of deliberately added elements with each other or with impurities. Alloying elements are added for a range of reasons which are targeted towards optimising the mechanical properties as well as reducing corrosion susceptibility. Our group has experience in characterization high strength aluminium alloys from the 2000 series. These are the precipitate hardened alloys and the microstructure extends from the nanoscale up to tens of microns. 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.
In the first example we show an electron microprobe map of aluminium 2024 T3. The image shows a range of different composition particles with up to five different types of compositions. This data was collected using an electron microprobe (Jeol 8500). The CSIRO Mineral Resources’ electron microprobe can map large areas up to the macroscopic scale at microscopic resolution. This means that this technique can provide statistical information about the composition of the IM particles.
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.
Examples of two EBSD patterns
Cu-Fe Scatter plot. Compositions for each particle type can be determined by analysing the groups of data at the endpoints of the different arms such as those circled in the blue ellipses. Their structure has been determined using EBSD.
It can be seen that there are several “groups” of common Cu and Fe K-ratios. These groups represent different compositional types of IM particles in AA2099. Quantitative analyses can be made of the different groups and their positions can be assigned resulting in a phase map of the surface. From this sort of approach it is possible to map all the different intermetallic composition types within an alloy. Their crystallography can be determined using EBSD this giving a complete picture of the IM particles types.
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.
| Microprobe Composition (at. %) | EBSD Assignment | Al:TM ratio |
| Al83.9Fe7.1Cu3.4Mn4,4Zn1.1 | Al6(Fe,Mn) | 5.2 |
| Al75.2Cu9.3Mn7.0Fe6.8Zn1.7 | Al13(Fe,Mn)4 | 3.1 |
| Al75.6Cu11.5Mn3.9Fe6.9Zn2.0 | Al13(Fe,Mn)4 | 3.1 |
| Al76.2Cu14.6Mn1.1Fe5.5Zn2.1 | Al37Fe12Cu2 | 3.2 |
| Al87.8Cu7.9Mn1.1Fe0.2Zn1.9Mg0.4 | Al7Cu2Fe | 7.9 |
| Al70.3Ti17.8Cu1.7Mn0.2Fe0.7Zn0.8Mg0.41 | TiB21 | Al:Ti=4 |
| Al81.6Cu1.2Mn0.2Fe3.1Zn0.9Si10.9Mg0.6 | N.D. | Al:Si=7.5 |
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
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.
In Figure 4 shows addresses lithium distribution in one IM particle. One the left are two SXES spectra aggregated from one map showing Al L edge and the Li Kα edge in individual IM particle types. Both particle types appears to have some Li in them. ON the right are tow images which show backscatter (BSE) and total SXES emission contrast. Spectra from these different contrast regions can be extracted (c and d). These spectra (e and f) show that the contrast variation is likely due to differences in Li concentration between different regions within the one particle. Some preliminary work has shown that the regions containing more Li may be more susceptible to corrosion.
Left: (a) and (b) SXES spectra from two different IM compositions identified in Table 1. The Li Kα edge is indicated by the blue dashed line. Right: two maps showing both backscatter contrast within an Al7Cu2Fe particle as well as corresponding SXES contrast. Regions of different contrast are shown in c and d. e shows the position of the particles in the Cu-Fe scatter plot and f shows the difference in the soft X-ray spectra.