Characterization of Modulated Nanostructure Using Aberration Corrected Scanning Transmission Electron Microscopy (STEM)

Modulated nanostructures are short-range quasiperiodic composition fluctuations observed in metals, semiconductors, and ceramic alloys and have an important effect on the mechanical, electrical, and magnetic properties of the materials. Their presence is often attributed to spinodal decomposition [1]. The typical wavelength of the modulations ranges from 5 nm to 15 nm in metal alloys [2]. In past, such modulations have been analyzed using x-ray diffraction (XRD), electron diffraction, and diffraction/strain contrast imaging techniques [3]. These investigations gave useful information about the wavelength and strain fields in the modulation but nothing about the composition amplitude of the modulated structure. With the development of field emission sources [4], aberration correctors [5], and ADF imaging for STEM microscopes, measuring the amplitude of the modulation directly and examining the diffused interface is possible. Also, to study the reaction kinetics measurement of the composition amplitude is necessary.<br/>In this study, we measure the composition amplitude of the fluctuations directly using STEM HAADF imaging and energy dispersive x-ray (EDS) spectroscopy. Au-Pt alloys with a symmetrical solid-state miscibility gap are used for the investigation of the modulated structure. Three alloys of compositions 60%Pt, 80%Pt, and 40%Pt are analyzed at different temperatures at various aging times for studying the reaction kinetics. Composition variation and the diffused interface across modulations were successfully mapped using atomic resolution EDS spectroscopy. Results show the wavelength, as well as the composition amplitude of the modulations, increases as the alloy is aged for a longer time. As the reaction proceeds towards equilibrium, the amplitude of the fluctuations should increase and reach the equilibrium composition at the end of the tie line at the selected aging temperature. Although the entire specimen region was modulated, variations in both the wavelength and composition amplitude were observed. 1D and 2D modulation were seen along the &lt;100&gt; directions. These results obtained are in good agreement with the spinodal theories and XRD experimental data [2]. Cellular precipitates were observed along the grain boundary of alloys aged for a longer time. Nucleation and Growth mechanism is responsible for such precipitation reaction and is also a competing reaction along with the spinodal decomposition. Analyzing such cellular precipitates at the grain boundary is very important as it influences fracture behaviors in spinodal alloys. A similar analysis is also being carried out in the Cu-Ti alloy system which has an asymmetrical miscibility gap. The results obtained from this study show that analyzing the compositions of modulated structures using aberration-corrected STEM is possible and therefore a comparison between the spinodal rate theory and experimental kinetics of the modulated structures can be obtained.<br/><br/>References:<br/>[1] J. W. Cahn (1962), Acta Met.10, 179-183.<br/>[2] R. W. Carpenter (1967), Acta Met.15, 1567-1572.<br/>[3] J. Bentley (1989), Ultramicros.30, 157-171.<br/>[4] A. V. Crewe et al (1975), p. 47 in Physical Aspects of Electron Microscopy and Microbeam Analysis, ed. by Siegel and Beaman, Wiley, New York.<br/>[5] O. L. Krivanek et al (2008), Ultramicros.108, 179-195.<br/>[6] The authors acknowledge funding from the Division of Materials Research of the National Science Foundation, and the use of facilities within the John M. Cowley Center for High-Resolution Electron Microscopy at Arizona State University.