A resonant ultrasound spectroscopy study of metals, alloys and intermetallics

Abstract

The experimental technique of Resonant Ultrasound Spectroscopy (RUS) has been successfully applied to three different systems in condensed matter physics. RUS can determine the full elastic constant tensor and the ultrasonic loss of a material. The results obtained would have been extremely difficult to achieve using more conventional acoustic techniques. The use of RUS has been expanded to investigate hydrogen motion in intermetallic and quasicrystalline hydrides. The elastic moduli and ultrasonic loss of a C15 Laves-phase intermetallic compound, TaV2Hx, were measured over a temperature range of 2.8 - 345 K. for a series of hydrogen concentrations, x = 0.00, 0.06, 0.10, 0.18, 0.34 and 0.53. Large ultrasonic loss peaks centered at roughly 240 K were observed for all hydrogen containing compounds, for measurement frequencies of about 1 MHz. Shifts in the resonant frequencies of the modes were also observed in the same temperature region. In a novel approach, both the loss and dispersion results were fit simultaneously to a Debye-type relaxation function to obtain parameters of the hydrogen motion, including activation energies, attempt frequencies and relaxation strengths. The results for the activation energies and attempt frequencies were in good agreement with those determined from nuclear magnetic resonance measurements, for the concentration range where the two techniques overlapped. The present study was conducted over an extended concentration regime compared to that of the NMR work and weak concentration dependent effects were revealed. For the first time, evidence of two parallel Arrhenius processes occurring in this system was discovered with one of the processes being suppressed at higher H concentrations. The magnitude of the effects depended linearly on x implying that it is the relaxation of isolated H atoms that are contributing to the mechanical damping. The results appear to be the first to clearly demonstrate the existence of the so-called Snoek effect in intermetallic hydrides. Similar measurements were made on the related compound, TaV2D0.17, and the results compared directly to those of TaV2H0.18. The TaV2D0.17 alloy also showed broad attenuation peaks centered at about 240 K which were interpreted in the same way as for the hydrogenated compounds. There was a weak isotope effect with the H hopping rate being somewhat faster than that of D. More importantly, a second, much weaker attenuation peak centered at approximately 20 K was observed for the deuterated compound which was completely absent in the loss data of TaV2H0.18. This peak was attributed to a rapid localized motion of D atoms. This work provided a particularly clear demonstration of a strong isotope effect in a metal-hydride system. Unlike the NMR results, the peaks attributed to the two frequency scales of D motion were separately resolved in temperature. This should aid in elucidating the physics behind these hopping mechanisms. The existence of a large isotope effect at low temperatures implies that the motion is dominated by quantum processes. The magnitude and temperature dependence of shear elastic modulus G(T) of the TaV2H(D)x system was found to be highly dependent on x. G(T) of TaV2 anomalously increased with increasing temperature from 3 K to 350 K. The effect of adding hydrogen was both to systematically increase the magnitude of G(T) and to completely reverse the temperature dependence for all x ≥ 0.17. G(T) of TaV2H0.53 was 55 % greater than that of the H free material at 20 K, representing an anomalously strong effect. The elastic constant results were quantitatively described by an electronic band structure model and constitute a remarkable and unusual example of a hydrogen-related electronic effect. RUS measurements were made on a Ti-Zr-Ni based icosahedral quasicrystal (QC) and a related 1/1 bcc crystal approximant. The shear and bulk moduli were derived over a temperature range of 15 -345 K. The temperature dependencies resembled those of ordinary metals with the moduli of the QC and the approximant phase being almost the same, providing further confirmation of the similarity of the QC and approximant phases. These results represent the very first elastic moduli measurements on the large class of Ti-based QC systems. Both phases absorb hydrogen in solid solution up to H concentrations of x ≤ 1.7. The ultrasonic loss of the QC and approximant phases for x = 0.79 and 0.20 respectively, was measured. Broad attenuation peaks were observed, centered at roughly 250 K for the hydrogenated materials. The peaks were associated with H motion and were fit with a Debye-type relaxation function. A Gaussian distribution of activation energies was required to fit the loss data, with a mean activation energy of 0.40 eV. The results indicated that the H motion in the QC, in this temperature range, is at least one order of magnitude foster than that of the approximant phase. This was the first time that internal friction measurements have been utilized to derive information about H diffusion in a quasicrystalline system. In a final, more applied study, the texture of rolled sheets of copper and brass was investigated. Texture is related to the non-random orientation of crystallites within a polycrystalline aggregate. Rolled sheets are often assumed to possess macroscopic orthorhombic symmetry. RUS was used to measure all nine elastic moduli of the plates. The elastic constant results were then used to derive the orientation distribution coefficients. The ultrasonically derived values were in good agreement with those obtained by neutron diffraction, as evidenced by the remarkable similarity of the acoustic and neutron pole-figures. This work represented one of the most extensive studies of the suitability of ultrasonic techniques for texture characterization. The results support the idea that acoustic techniques may be used to determine the main features of texture in rolled plates.