Structure and Mechanical Properties of Fe/Zr Multilayers
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STRUCTURE AND MECHANICAL PROPERTIES OF Fe/Zr MULTILAYERS. Joost J. Vlassak*, Takenori Nakayama**, Toyohiko J. Konno* and William D. Nix* *Department of Materials Science and Engineering, Stanford University, Stanford, CA. 94305 **Kobe Steel, Ltd., Japan
ABSTRACT Iron zirconium multilayer films have been prepared by sputter deposition and studied using xray diffraction, high-resolution transmission electron microscopy and Nanoindenter techniques. The composition-modulation wavelength was varied between 0.8 and 92 rim. For modulation wavelengths greater than 4 nm the multilayers are crystalline with amorphous interfaces; for smaller wavelengths the samples are entirely amorphous. It was not possible to obtain layered structures with wavelengths smaller than 0.8 nm. Both the hardness and the elastic modulus were measured as a function of compositionmodulation wavelength by means of continuous indentation testing. The elastic modulus shows some variation with wavelength; the average value being 131 GPa. The hardness increases sharply when the modulation wavelength decreases below 4 nm. We attribute this increase to the crystalline to amorphous transition that occurs in these films at this wavelength.
INTRODUCTION The mechanical properties of multilayer films have received a great deal of attention. Both elastic and plastic properties have been studied extensively as a function of composition-modulation wavelength. Elastic constants have been found to vary for very small wavelengths in some multilayer systems [1-4], whereas yield stress and hardness often display marked increases [5-9]. The variation of the elastic constants at small wavelengths has been attributed to the presence of interfaces in the films. The width of the interfaces has been measured in a number of multilayer systems [4]. As the wavelength becomes very small, the interfaces make up a considerable volume fraction of the film, resulting in a decrease of the film stiffness in the direction perpendicular to the plane of the film. Interfacial stresses have also been suggested as a cause of stiffness variations
[10].
Interfaces also affect the strength of multilayers. The most widely quoted model to explain the high yield strength of multilayers at small wavelengths was developed by J.S. Koehler [111 and is based on the difference in stiffness of the constituents of the multilayers. Dislocations in the more compliant material are repelled from the interface by their images in the stiffer material and a very high resolved shear stress is needed to drive a dislocation across the interface. However, when the thicknesses of the individual layers in the film exceed a critical value, pile-ups of dislocations at the interface are possible and the strength of the multilayer film decreases as the bilayer period increases. In this study we investigate and try to correlate the structure and mechanical properties of Fe/Zr multilayers. We have prepared a series of samples with a wide range of composition-modulation wavelengths by means of sputter deposition. The sa
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