Abstract
Elastic constants are among the most fundamental properties of materials. Simulations of microstructural evolution and constitutive/micro-mechanistic modeling of materials properties require elastic constants that are predominately measured from single crystals that are labor intensive to grow. A facile technique is developed to measure elastic constants from polycrystalline samples. The technique is based upon measurements of the surface acoustic wave velocities with the help of a polydimethylsiloxane film grating that is placed on a polished surface of a polycrystalline sample to confine surface acoustic waves that are induced by a femtosecond laser and measured using pump-probe time-domain thermoreflectance. Electron backscatter diffraction is employed to measure the crystallographic orientation along which the surface acoustic wave propagates in each grain (perpendicular to the polydimethylsiloxane grating). Such measurements are performed on several grains. A robust mathematical solution was developed to compute the surface acoustic wave velocity along any crystallographic orientation of any crystal structure with given elastic constants and density. By inputting various starting values of elastic constants to compute the surface acoustic wave velocities to match experimental measurements in several distinct crystallographic orientations using an optimization algorithm, accurate elastic constant values have been obtained from seven polycrystalline metal samples to be within 6.8% of single-crystal measurements. This new technique can help change the current scenario that experimentally measured elastic constants are available for only about 1% of the estimated 160,000 distinct solid compounds, not to mention the significant need for elastic constants of various solid solution compositions that are the base of structural materials.
Highlights
Several experimental techniques have been developed to facilitate the measurement of elastic constants such as the resonant acoustic spectroscopy (RUS),[6, 7] and the Brillouin scattering.[8, 9]
Based on the electron backscatter diffraction (EBSD) orientation map and the indents placed on the pure Ni sample, nine grains were selected for the time-domain thermoreflectance (TDTR)
Large grains were intentionally selected for the measurement since it is easier to locate them in the TDTR system using the indents as references
Summary
Even though significant progress has been made in computing the elastic constants using density functional theory,[5] accurate experimental measurements are still essential until the computed results are fully validated for accuracy. The elastic constants are predominately obtained from measurements on relatively large single crystal samples. Several experimental techniques have been developed to facilitate the measurement of elastic constants such as the resonant acoustic spectroscopy (RUS),[6, 7] and the Brillouin scattering.[8, 9] These methods, albeit simpler in measurements, still require single crystals that are usually labor-intensive and time-consuming to grow. The RUS technique has been applied to measure elastic constants from micro-single-crystal samples of several hundred microns in dimensions and it has the advantage of being able to perform temperature-dependent measurements when the apparatus is enclosed within an inert environment
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