Measurement of coherent surface acoustic wave attenuation in polycrystalline aluminum

Abstract

Attenuation of Rayleigh-type surface acoustic waves induced by grain-boundary scattering is studied experimentally and theoretically by an effective medium approach. A frequency domain opto-acoustic laboratory setup, capable of measuring a coherent Rayleigh wave response by emulating an ensemble average via spatial averaging, is presented. Measurements are conducted on polycrystalline aluminum at ultrasonic frequencies from 10 MHz to 130 MHz. A constant effective phase velocity of 2893 m s−1 is found below 80 MHz. The effective attenuation coefficient varies in the whole frequency range by nearly two orders of magnitude, and shows classical scattering behavior, comprising stochastic and geometric scattering regimes. A semi-analytical attenuation model is presented, valid below the geometric limit. The model incorporates the material’s spatial two-point correlation function obtained from metallurgical micrographs. Comparisons to experimentally obtained attenuation coefficients show good quantitative agreement, with differences in the frequency power-law dependence. This study attempts to elucidate microstructure induced surface acoustic wave attenuation experimentally by means of a statistical approach. The proposed method and the obtained findings contribute to the understanding of wave propagation in heterogeneous media, and promote the use of surface acoustic waves in non-destructive microstructure characterization.

Free Radical Generation from High-Frequency Electromechanical Dissociation of Pure Water

“…We note the placement of the reservoir 5 mm from the interdigitated transducer (IDT) electrodes (Figure a,b) to which an oscillating AC signal with potentials across the range 3–10 V rms was applied in order to generate the SAW, whose amplitude and profile is measured using a laser Doppler vibrometer (UHF-120; Polytec GmBH, Waldbronn, Germany) as seen from the scan in Figure d. The reservoir is thus sufficiently far from the IDT to prevent any electrode contact with it although within reach of the SAW transmission (Figure d) given that the SAW attenuation length in the substrate is approximately 12 mm along its propagation direction for the 10 MHz frequency employed . We also note the measurement probe wires, inserted into the reservoir as described in the Supporting Information, were in contact with the sample only to monitor the current enhancement.…”

“…The Journal of Physical Chemistry Letters sufficiently far from the IDT to prevent any electrode contact with it although within reach of the SAW transmission (Figure 1d) given that the SAW attenuation length in the substrate is approximately 12 mm along its propagation direction for the 10 MHz frequency employed. 11 We also note the measurement probe wires, inserted into the reservoir as described in the Supporting Information, were in contact with the sample only to monitor the current enhancement. The separation distance between the probes was approximately 2 mm, such that any electric field generated across it is weak, thus eliminating the possibility of the participation of the probes in the breakdown of the pure water sample.…”

A far-field scattering model in textured polycrystals with ellipsoidal grains of arbitrary crystal symmetry

“…This study can be further extended to incorporate a genialized spatial TPCF as in [3,29,59], such that one can compare with experimental attenuation data or the finite element modeling of wave scattering in textured polycrystals, which has not been reported so far. The work also presents an advantageous model for ultrasonic determination of microstructures in textured polycrystals, such as titanium alloys, and its application to practical ultrasonic microstructure characterization will be addressed in the future.…”

Attenuation and Phase Velocity of Elastic Wave in Textured Polycrystals with Ellipsoidal Grains of Arbitrary Crystal Symmetry

“…This will not be demonstrated here since details about the determination of effective elastic moduli from ultrasonic phase velocities can be found in Ref. [15].…”

“…Elastic waves propagating in heterogeneous polycrystals are subject to scattering at the grain boundaries due to the misorientation between adjacent grains [9][10][11][12], and this further induces beam attenuation due to energy dissipation from the main ultrasonic beam. Although there are various wave attenuation mechanisms in polycrystalline materials such as viscosity [13] and relaxation [14], in many cases the main source of attenuation is wave scattering [5,11] and it has been experimentally proved by a study [15]. The scattering-induced attenuation in polycrystalline materials has an important implication of microstructure information [10,16,17] and thus has been extensively applied to the nondestructive characterization of grain microstructures [18,19].…”