An Examination of the Composition and Microstructure of Coarse Intermetallic Particles in AA2099-T8, Including Li Detection

Abstract

Electron and proton microprobes, along with electron backscatter diffraction (EBSD) analysis were used to study the microstructure of the contemporary Al-Cu-Li alloy AA2099-T8. In electron probe microanalysis, wavelength and energy dispersive X-ray spectrometry were used in parallel with soft X-ray emission spectroscopy (SXES) to characterize the microstructure of AA2099-T8. The electron microprobe was able to identify five unique compositions for constituent intermetallic (IM) particles containing combinations of Al, Cu, Fe, Mn, and Zn. A sixth IM type was found to be rich in Ti and B (suggesting TiB2), and a seventh IM type contained Si. EBSD patterns for the five constituent IM particles containing Al, Cu, Fe, Mn, and Zn indicated that they were isomorphous with four phases in the 2xxx series aluminium alloys including Al6(Fe, Mn), Al13(Fe, Mn)4 (two slightly different compositions), Al37Cu2Fe12 and Al7Cu2Fe. SXES revealed that Li was present in some constituent IM particles. Al SXES mapping revealed an Al-enriched (i.e., Cu, Li-depleted) zone in the grain boundary network. From the EBSD analysis, the kernel average misorientation map showed higher levels of localized misorientation in this region, suggesting greater deformation or stored energy. Proton-induced X-ray emission revealed banding of the TiB2 IM particles and Cu inter-band enrichment.

Observations on the Early Stages of Corrosion on AA2099-T83

“…The GB between the two particles shows some evidence of attack but the attack is more severe where the GB continues on the other side of the Al 6 (Fe,Mn) particle. As shown in a previou paper, the GB in this alloy is decorated by T 1 phase (MacRae et al, 2018). The attack on the GB decorated by the T 1 particle is more severe suggesting that this structure plays a major role in the level of attack, since the attack on the short length of GB between the two Al 6 (Fe,Mn) particles is not as severe.…”

“…All SEM and EBSD data were collected on an FEI Quanta 400, field-emission electron microscope operated at 15 kV accelerating voltage. The conditions for EBSD have been reported previously (MacRae et al, 2018) but are reproduced here for convenience. EBSD was performed at an angle of 70° to the horizontal and data were collected with a Bruker e - flash system.…”

“…In a recent paper, the authors were able to identify seven unique compositions for constituent intermetallic (IM) particles in AA2099-T83 using electron microprobe microanalysis (EPMA) and electron backscatter diffraction (EBSD) (MacRae et al, 2018). Specifically, five compositions containing combinations of Al, Cu, Fe, Mn, and Zn were found.…”

“…A third aspect to this study is the development of a methodology to minimize the influence of carbon deposited under long exposure times to the electron beam during mapping (EPMA or EBSD). In this study, a well-characterized surface of AA2099-T83 [the electron probe microanalysis and EBSD study reported previously (MacRae et al, 2018)] has been repeatedly exposed to 0.1 M NaCl solution for a series of times from 10 to 50 min total exposure. This provides a site-by-site assessment of the development of corrosion as characterized using scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and EBSD.…”

The Impact of Chemical Bonding on Mass Absorption Coefficients of Soft X-rays

“…For the purpose of this study, an additional internal calibration was done based on assigning the Al L3 edge to 72.7 eV (Terauchi et al, 2011; MacRae et al, 2018 b ). The spectral resolution at the Al L α edge was approximately 150 meV as defined by the difference in energy between 75 and 25% of the Al L Fermi edge.…”