Quantitative Characterization of Nanovoids in Quenched Aluminum Using Small Angle X-ray Scattering Studies
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A small angle X-ray scattering (SAXS) study of quenched-in nanovoids in 99.988 and 99.995 at.% aluminum (Al) is presented. Nanovoids are agglomerations of point defects (vacancies) in the metallic host. SAXS is a non-destructive, dynamic probe which is ideally suited to characterize nanoscale electron density inhomogeneities, e.g., electron density difference between void and metal matrix. Previous SAXS studies of nanovoid structures in metals were limited by the presence of multiple scattering from the metal matrix. The first published report of nanovoid characterization in quenched Al using the SAXS method took place in 2008. In the present work, meticulous material processing methods were applied to induce a significant nanovoid concentration in nominally pure Al while minimizing the introduction of multiple scattering artifacts through sample deformation. Absolute intensity calibration with a secondary glassy carbon standard was performed. Data analysis procedures were developed to extract void scattering by subtracting calibrated reference scattering from calibrated quenched sample scattering, independent of their thicknesses. SAXS analyses were used to estimate void size as well as the void-metal interface structure, void number distribution and void volume fraction in quenched aluminum samples, an extension over the previous work where only estimates of void sizes were obtained. This work, including identification of experimental tools that can be readily improved, demonstrates that SAXS studies are capable of providing precise characterization of nanovoid structure in aluminum. SAXS analysis provides statistically averaged parameters from scattering data collected from a macroscopic sample volume, which is an advantage over electron microscopy studies. Also, preliminary aging studies of nanovoids in Al revealed some interesting trends in the kinetic properties of the nanovoids which can be extended to test and develop the existing phenomenological models of void nucleation and growth. This level of information, i.e., at the nanoscale and, ideally, yielding kinetic information about the early stage formation and growth of nanovoids, has the potential to be used to develop novel aluminum alloy materials.