Multilayers: A Research Tool for Mechanical Property and Alloy Phase Stability Investigations
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The magic of multilayer structures — comprised of a regular repeating pattern of alternating layers that vary in thickness from less than 1 nm to more than several micrometers — is present today in nearly every facet of materials science. A technological gamut of issues has been approached through the use of multilayer structures. Material systems have been routinely investigated, ranging from simple binary through complex quaternary and including nearly every element of the periodic table. Applications include thinfilm semiconductor devices in the form of strained-layer superlattices; protective coatings for improved surface hardness and wear resistance of conventional alloys (tribology); and reflective and transmissive x-ray optical devices. Multilayer structures have also provided an experimental means to investigate the interdiffusion and stability of alloy phases; the relationship between the presence, lack, or variations of crystallinity with magnetism (and most recently superconductivity); and the role of interfaces in adhesion and elastic/plastic behavior. Of late, several review articles and lectures have addressed these issues. Magnetic and superconducting properties were addressed by Schuller and Homma1 and Hong, 2 semiconductor and electricaloptical properties by Peercy and Osbourn,3 and x-ray optic applications by Barbee.4 An approach to intercalation in compounds was reviewed by Dresselhaus5 as well. A recent collection of viewpoints6 on artificially layered thin films covers x-ray characterization of amorphous multilayers, transmission electron microscopy of modulated structures, atomic diffusion and phase transformations in multilayers, plastic strain and stress during interdiffusion, the supermodulus effect' in cGrripositionmodulated thin films, and nonlinear diffusion in multilayers. Perhaps the most thorough, historical review of artificially prepared composition-modulated structures appears in Baral's doctoral thesis.7 Rather than duplicate these efforts, the following article will complement them through an informal review emphasizing the use of multilayered structures to study mechanical properties and as a tool for research in alloy phase stability.
Multilayer structures are composite materials. The concerns in this discussion (and those referenced) will be with macroscopic materials consisting of building blocks of microscopic dimension. Of great interest and application are those structures consisting of layers sequenced in a regular repeating pattern. The thickness of the group of layers which comprise the repeating structural unit, on which the multilayer is constructed, is termed the multilayer period (or wavelength) X. In essence, the multilayer structure can best be described by a composition modulation (or wave) which may functionally be sinusoidal, square, sawtooth or some combination of these. The simplest example of a multilayer is a binary system composed of elements A and B. The multilayer consists then of an alternating arrangement of a given number of atomic planes of element
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