Introduction
Lithium (Li)-containing compounds and alloys are critical to many key technologies of the twenty-first century, from Li-ion batteries used to power mobile electronic devices and cars to lightweight structural alloys. Progress in these fields has been remarkable given the lack of a method to determine lithium content at the microscale. Commonly, Energy Dispersive Spectroscopy (EDS) in the Scanning Electron Microscope (SEM) is employed for microanalysis. However, this has not been possible for elements with atomic number (Z) <4 as the characteristic X-rays emitted (e.g., Li K at 55 eV) are easily attenuated by the sample or presence of an oxide layer or contamination and require the use of highly specialized detectors. Even so, a limit of detection of ~20 wt. % Li and the inability to perform quantitative measurements due to the dependence on the Li bonding state present significant issues [1]. However, quantification of Li in the SEM was demonstrated recently using a composition by difference method based on EDS and quantitative Backscattered Electron Imaging (qBEI) [2]. EDS analysis was used to quantify elements Z = 4 – 94, while qBEI was used to determine the mean atomic mass (the qBEI signal being a function of atomic number
for Z = 1 – 94). The fraction of light elements (Z = 1 – 3) was calculated and, given the MgLi alloy analyzed, assumed to be Li. Using this method, detection of <5 wt. % Li was demonstrated with
acceptable accuracy (~1 wt. %).
Results and Discussion
We extend the composition by difference method to generate quantitative, spatially resolved elemental maps of a MgAlLi alloy. The sample was cast with a nominal composition of Mg52.6Li18.3Al29.1 wt. %. The sample was prepared by broad beam argon milling using a Gatan Ilion® polisher. To minimize reaction with the atmosphere, the sample was transferred to the SEM immersed in isopropanol. A field emission SEM was used to collect EDS and qBEI maps at 3 and 5 kV, respectively, selected to reveal the sample microstructure while ensuring comparable sampling depths of the signals. EDS spectra were captured using an EDAX Octane Elite EDS System, and quantified elemental maps were calculated in the APEX software. qBEI was performed using a Gatan OnPoint™ backscattered electron detector and image analysis was performed using the DigitalMicrograph® software.
In good agreement with thermodynamic simulations using Thermo-Calc software, secondary and backscattered electron images (Figure 1) revealed a eutectic microstructure with 61:39 area % while EDS maps revealed a Mg-rich matrix and Al-rich MgAl secondary phase (Figure 1c). Despite careful sample handling, high carbon and oxygen concentrations plus surface pitting in some regions provide evidence of reaction with the atmosphere. Sample topography is known to affect the backscattered electron yield and observed topographic features correlated with anomalous ‘dark’ features in the qBEI data (arrowed). To avoid incorrect
data interpretation, these regions were excluded from the elemental maps that were calculated.
Figure 1. a) Secondary and b) backscattered electron images of MgLiAl alloy; c) elemental map revealing Mg matrix and Al-rich secondary intermetallic phase.
Magnesium, aluminum, and, for the first time, lithium elemental maps were calculated using the composition by difference method [2] (Figure 2). The matrix was determined to be Mg90.6Li9.4 wt. % with little spatial variation; however, the second phase exhibited wide compositional variation from Mg26Li11Al63 to Mg10Li43Al47 (mean Li content of 35.5 wt. %). These results agree well with thermodynamic calculations predicting a Mg-rich matrix with BCC Li configuration and an FCC AlLi secondary intermetallic phase capable of accommodating broad ranges of Mg-content.
Figure 2. Secondary electron image and elemental metal fraction maps (by wt. %) of the same region of the MgLiAl alloy; white pixels are regions excluded from the analysis due to influence of topography (identified by arrows in the secondary electron image).
Conclusion
The results demonstrate that single-digit mass percentages of Li can be mapped quantitatively in the SEM using the composition by difference method. Limitations of the method are known to include surface topography, as well as the presence of unknown quantities of H or He (or voids). Nevertheless, the methodology offers distinct advantages compared to specialized “Li” EDS detectors.
References
[1] P. Hovington et al., Scanning 38 (2016) p571–578
[2] JA. Österreicher et al., Scripta Materialia 194 (2021) 113664