This survey on the metallurgical effects of ultrasonic waves, including scientific and technological literature, deals with the following: A. The techniques for the treatment of melts by mechanical vibrations. 1. Mechanical vibrators. 2. Electromechanical transducers. 3. Ponderomotive effects of induced electrical currents. B. Metallurgical effects observed after ultrasonic treatment. 1. Reduction in grain size. 2. Degassing of melts. 3. Dispersion of other substances in melts. 4. Acceleration of rate processes. 5. Other metallurgical and related effects. HE idea of improving the quality of cast metal by means of a vibrational or stirring motion during the melting or solidification phase is a very old one. New possibilities of producing mechanical motion in a melt arose from the use of electrical induction furnaces at higher frequencies and from the development of ultrasonic generators. The use of both types of devices has led to the discovery of several physical effects: The degassing of melts, the dispersion of one metal into another, a reduction in grain size and corresponding improvement of metals, and other effects. The interest in these developments is shown by a considerable number of review articles •-• and patents•-7a; original research papers in the field, however, are comparatively few in number 74-•øø and have brought no real understanding of the mechanisms of the observed effects. A review of the present unsatisfactory state of the art may serve to revive interest in the needed fundamental research.
The study of the asymmetry of the light diffracted by ultrasonic waves progressing with moderate amplitudes in liquids is used to detect and measure the finite amplitude distortion in water and carbon tetrachloride. A theory is developed which relates the distortion present to available isothermal compressibility measurements. This development includes the presence of absorption.Measurements of the second harmonic were made in water at 2 Mc and 4 Mc, at distances up to 50 cm, and at peak ultrasonic pressures between zero and 1.5 atmos. More detailed measurements were made in carbon tetrachloride at frequencies of 2 and 3 Mc, at distances up to 21 cm, and at pressures up to 0.7 atmos. For both liquids, good agreement is obtained between the experimentally measured values and the theoretically predicted results.Small amounts of distortion were easily measured. The second harmonic could be measured when the intensity of the harmonic was less than 0.04% of the intensity of the fundamental. Distortion could be measured when the fundamental was much less than 0.2 atmos. I. INTRODUCTIONINCE it was discovered that ultrasonic waves could produce diffraction of light, several authors have offered theoretical explanations of the details of this diffraction. Some of these theories •-a are rigorous but very complex and difficult to apply to practical situations. In many cases it is preferable to use an approximate theory which is much simpler to handle and which can be used in an easily obtainable experimental region. This theory, which was developed by Raman and Nath, 4-6 gives satisfactory results in the region of moderately low amplitudes and frequencies.All of the theories predict that there is symmetry about the zero order of diffraction when this diffraction is produced by light incident at right angles to a plane, sinusoidal, ultrasonic wave. In the usual visual observations, one does not hotice any asymmetry. However Sanders, 7 using a photomultiplier, found that an asymmetry between the negative and positive orders appeared to be present. Sette 8 visually observed slight asymmetry at large distances from the sound source. Rao 9 also observed asymmetry and tried to explain it by the generation of second harmonics in the ultrasonic source. Miller •ø very carefully studied this problem and * Submitted by K. L. Zankel in partial fulfillment of the requirements for a Ph.D. degree at Michigan State University. $ This research was sponsored by the Office of Ordnance Research, U.S. Army. • R. Extermann and G. Wannier, Helv. Phys. Acta. 9, 520-532 (1936). . •0 R. B. Miller, thesis, "A study of the intensity distribution of the light diffracted by ultrasonic waves," Michigan State University, East Lansing (1956). 44found that the asymmetry could not be eliminated by adjustments in the equipment. Breazeale and Hiedemann n showed that a similar asymmetry observed with refraction could be explained by finite amplitude distortion. Mikhailov and Shutilov x2 were able to show that for extremely high intensities (which at times appro...
The rate at which harmonics are developed during the propagation of an initially sinusoidal ultrasonic wave of finite amplitude depends on the nonlinearity of the compressibility of the medium. Thus, determinations of the harmonics as a function of propagation distance may be used to determine the parameter B/A (describing nonlinearity). A method which circumvents the effects of absorption in the medium is developed and used to determine the parameter B/A for water and m-xylene. This method utilizes optical determinations of the second harmonic component of a distorted 3.0 Mc ultrasonic wave at various distances and initial pressure amplitudes. The second harmonic was isolated by use of a calibrated acoustic filter plate. The values obtained for the nonlinearity parameter are B/A = 6.2±0.6 for water and B/A = 9.6±1 for m-xylene.
In order to evaluate the method for measurement of the ultrasonic pressure amplitude in progressive waves which was developed by Loeber and Hiedemann for stationary waves, a comparison between it and two other optical methods was made. Pressure measurements under identical conditions by the three methods are given. Experimental considerations for the use of optical methods for measurement of sound pressure amplitudes are discussed.
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