Ultrasonic Nondestructive Techniques for Materials Characterization
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but a synergism of many scientific and engineering disciplines. Since it would be impractical here to present all the new NDE methodologies with application to materials research, this issue of MRS Bulletin focuses exclusively on those ultrasonic techniques that are increasingly important in materials characterization. Ultrasonic methods are used to characterize elastic properties and microstructural states by introducing low-level, high-frequency stress waves into the material under examination. Ultrasonic waves propagate in the material, interact with the microstructure, and subsequently are detected. The characteristics of the ultrasonic waves are modified as they travel through the material due to reflection, scattering, and attenuation. The detected signal is displayed, processed, and interpreted in terms of the internal structure of the material under investigation based on its relation to the input wave. Although ultrasonic methods have been used for several decades in materials characterization, recent advances have produced high-resolution imaging as well as quantitative elasticproperty measurement capabilities. The articles presented here offer a survey of a few of these advances in ultrasonic methods, as well as the types of problems being addressed by these techniques. The first article of this survey, by S.I. Rokhlin and T.E. Matikas, offers an introduction to various ultrasonic measurement methods for the characterization of materials surfaces and interphases. Ap-
plications discussed in this article include quantitative characterization of thin films and interphases in layered materials and high-temperature composites. Various methods have been described for elastic property determination of thin films and interphases. Imaging techniques for internal mapping of damage have also been presented. These techniques can provide a quantitative description of environmental effects, due to fatigue and oxidation, on interfacial layers and on the adhesion of these layers to the substance. The next article, by G.A.D. Briggs and O.V. Kolosov, is an introduction to acoustic microscopy. This ultrasonic technique provides both imaging and quantification capabilities and can be used for characterizing the near-surface elastic properties of a material. This article summarizes applications of the technique in various areas including imaging of surface/subsurface damage, evaluation of interfaces between a protective coating and its substrate, measurement of small cracks, characterization of interfaces in composites, and quantitative measurements of elastic properties of surfaces. This article also provides a short introduction to Brillouin spectroscopy, which is finding increasing applications in the characterization of surfaces and thin layers of electronic materials. Atomic force microscopy (AFM) has received great attention in the materials community for surface profiling and imaging capabilities. The article by K. Yamanaka is devoted to the description of a new modification of AFM, termed ultrasonic force micro
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