High-Resolution Composition Wave Characterization in Cu-Ti Alloys using Aberration Corrected STEM

Modulated nanostructures are short-range quasiperiodic composition variations observed in metals, semiconductors, and ceramic alloys. These structures are often formed through a thermodynamic process called spinodal decomposition [1]. The typical wavelength of these fluctuations ranges from 5 to 15 nm [2]. During the early stages of spinodal decomposition, small composition fluctuations occur in the material and grow in both amplitude and wavelength over time. Previous studies [3] using X-ray diffraction and diffraction/strain contrast imaging techniques have focused on analyzing the wavelength of the modulated structures, but less attention has been paid to their amplitude. In order to study the kinetics of spinodal decomposition, it's necessary to directly measure the amplitude of the composition fluctuations. Recent advancements in transmission electron microscopy, including field emission guns [4], aberration correctors [5], and ADF imaging, have improved the resolution of these instruments and now allow for direct measurement of both the wavelength and amplitude of the modulated structures. Here we analyze a Cu-Ti alloy system with an asymmetrical miscibility gap to study the modulated nanostructures' reaction kinetics. A Cu-4%Ti alloy was heat treated at a temperature of 400°C for 5min and 1000min. This provided the necessary driving force for the reaction to proceed and decompose by spinodal decomposition. To analyze the modulated nanostructures, [100] zone axis is necessary as the wave propagation vector is near <100> direction. In order to do that, a two-step approach was incorporated for TEM sample preparation. The electron backscatter diffraction (EBSD) method was used to map the grain orientation. The sample was then made by using the focused ion beam (FIB) lift-out technique such that the orientation normal to the sample surface is close to the [100] zone axis. The samples were then analyzed in an aberration corrected JEOL ARM 200F STEM microscope for STEM Imaging. Electron Energy loss spectroscopy (EELS) and high-resolution STEM Imaging were carried out in an aberration corrected Nion UltraSTEM 100 microscope to map the composition variation along the modulations. All the imaging and EELS analysis were performed along the [100] zone axis. Fig. 1a shows a low magnification high angle annular dark field (HAADF) image of the modulated structure in an alloy aged for 5 min at 400°C. No distinct modulations were observed in the alloy aged for 5min. This is due to the reaction being at a very early stage of decomposition at this aging time. Fig. 1b shows a low magnification HAADF image of the modulated structure in an alloy aged for 1000 min at 400°C. 1D and 2D modulation were observed along the <100> directions. The structure was not uniform across the specimen, and a large variation in the wavelengths and composition amplitude throughout the specimen volume was seen. The typical wavelength of the modulated structure in this alloy is about 15.7±1.4nm. The contrast observed in the HAADF image is a combination of z-contrast and strain contrast coming from the modulated structure. Typically, the dark regions are the Ti-rich region while the bright regions are Cu-rich. In Cu-Ti alloy systems undergoing spinodal decomposition [6], chemical ordering occurs as the reaction proceeds within the Ti-rich region along the {100} planes. This can be seen in Fig. 1c which shows an atomic resolution HAADF STEM image showing an ordering within the Ti-rich region of the modulated nanostructure in alloys aged for 1000min at 400°C. Fig. 1d shows a STEM BF image of the cellular precipitation along the grain boundary of the nanostructure in alloy aged for a longer time. These precipitates are due to nucleation and growth mechanism, which is also a competing reaction mechanism along with spinodal decomposition. These cellular precipitates have a significant influence on the fracture behavior of the alloy. Fig. 2a shows a high magnification HAADF STEM image of the modulated structure along [100] direction in Cu-4%Ti alloys aged for 5min at 400°C. Fig. 2b shows an EELS line scan across the modulated structure along the [100] scan direction in alloy aged for 5min. As seen from the EELS line scan, although we see some composition fluctuation, the reaction is still at an early stage of decomposition. The contrast that we get from the HAADF image is mostly from the strain rather than the atomic number(z) of the elements. Fig. 2c shows a high magnification HAADF STEM image of the modulated structure along [100] direction in Cu-4%Ti alloys aged for 1000min at 400°C. Fig. 2d shows an EELS line scan across the modulated structure along the [100] scan direction in alloy aged for 1000min. From the plot, we see the Cu-rich region has an average composition of about 70At%Cu, and about 30At%Ti is the composition for the Ti-rich region. But a large variation in the amplitude is observed. The diffused interface between the regions has a composition change of about 8.18 At%/nm. From the two line scans it can be clearly seen that as we age for a longer aging time the amplitude of the modulation increases and we observed uphill diffusion. The results of this study demonstrate that the use of an aberration-corrected scanning transmission electron microscope (STEM) allows for the analysis of the structure down to the atomic level. Furthermore, by utilizing electron energy loss spectroscopy (EELS), the amplitude of fluctuations can be directly measured. The primary aim of this research is to compare the composition amplitude with respect to aging time, thus providing a direct comparison between the experimental aging kinetics and the spinodal rate theory [1, 7]. (a) Low Magnification HAADF STEM image of the modulated structure along the [100] zone axis in Cu-4%Ti alloy aged at 400°C for 5min. (b) Low Magnification HAADF STEM image of the modulated structure along the [100] zone axis for alloy aged at 400°C for 1000min. The contrast of the HAADF STEM image is a combination of z-contrast and strain contrast coming from the modulations. The entire specimen volume is modulated, but a large variation in the wavelength and amplitude is observed. (c) Atomic resolution HAADF STEM image showing an ordering within the Ti-rich region of the modulated nanostructure in alloys aged for 1000min at 400°C. (d) STEM BF image showing the cellular precipitation along the grain boundary of the nanostructure for the alloy aged for 1000min. The right side of the grain boundary is along the [100] zone axis orientation. (a) HAADF STEM image of the modulated structure along [100] direction in Cu-4%Ti alloys aged for 5min at 400°C. (b) EELS line scan across the modulated structure along the [100] scan direction as shown in (a). (c) HAADF STEM image of the modulated structure along [100] direction in Cu-4%Ti alloys aged for 1000min at 400°C. (d) EELS line scan across the modulated structure along the [100] scan direction as shown in (c). The vertical axis shows the relative composition of the alloy in At%. Moire fringes were observed at random orientations in the HAADF images, as seen in (a) and (c). These fringes are from a very thin discontinuous polycrystalline layer of copper oxide on the as-thinned surfaces of the specimen formed by reaction with laboratory air. The effect of this film on the quantification is negligible.