Ultrasonic Studies of Metallic Glasses at Low Temperatures.
This thesis presents the results of measurements of ultrasonic attenuation and velocity changes at low temperatures in metallic glasses. The specimens used in the experiments were Pd(,.775)Si(,.165)Cu(,.06), Pd(,.82)Si(,.18) and Pd(,.40)Ni(,.40)P(,.20) which were all bulk samples prepared by rapid quenching of the melt. The frequency and temperature ranges where ultrasonic properties of these metallic glasses were studied are 10 to 600 MHz and 0.27 to 10 K, respectively. The experiments include determinations of the amplitude dependence of attenuation, and the temperature dependence of velocity changes below a few degrees. More specifically, these ultrasonic measurements were intended to investigate the amplitude, frequency and temperature dependences of attenuation and velocity changes in metallic glasses. The results are compared with those obtained on insulating glasses and predictions from existing theory, which seems to explain most of the thermal and ultrasonic properties of insulating glasses and many of the properties of metallic glasses. The saturable amplitude-dependent attenuation changes were measured extensively at temperatures below 2 K and for frequencies up to 600 MHz. The amplitude dependence at a fixed temperature and frequency is observed to have 1/SQRT.(1(' )+(' )J/J(,c) dependence, where J is an acoustic amplitude and J(,c) is a critical amplitude. From this amplitude dependence, the total change of the amplitude -dependent attenuation is obtained. Its frequency and temperature dependences are then compared with the predictions of the current theory, namely, the two-level-systems (TLS) tunneling theory. It is found that the temperature and frequency dependences of total change of the saturable attenuation are quite different from the predictions of the theory; a linear frequency dependence rather than a quadratic dependence is observed over all frequency ranges covered, and at most a very weak temperature dependence rather than 1/T dependence below about 1 K. All three alloys (two ternary alloys and one binary alloy) show similar behavior and thus these ultrasonic properties, which are different from those in insulating glasses, seem to be characteristic of metallic glasses. The acoustic velocity changes were also measured for frequencies below 100 MHz, and their temperature variation shows lnT dependence. However, the slopes of the velocity changes vs. lnT give a noticeable amplitude dependence (more than 25% decrease from the lowest amplitude to the highest amplitude used) and a frequency dependence (more than 40% increase from 10 to 100 MH(,z)), which is again contrary to the predictions of the theory. Although there are some attempts to explain these unexpected ultrasonic properties in metallic glasses, none of them, so far, gives clear explanations. Therefore, it seems that the current TLS tunneling theory may have to be seriously modified, or a new mechanism will have to be considered.
- Pub Date:
- Physics: Acoustics