Elastic and nanostructural properties of Cu/Pd superlattices
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J. B. Ketterson Physics and Astronomy Department and the Materials Research Center, College of Arts and Sciences, Northwestern University, Evanston, Illinois 60208
R. Bhadra and M. Grimsditch Materials Science Division, Building 223, Argonne National Laboratory, Argonne, Illinois 60439 (Received 20 December 1990; accepted 6 February 1992)
A series of Cu/Pd superlattices with composition modulation wavelengths (A's) ranging from 1.6 to 3.5 nm and a strong [111] growth texture were prepared by electron beam evaporation. The elastic properties of the films were examined using the methods of uniaxial tension tests [a Young's modulus (1/sn), where Sy is an elastic compliance] with the applied load parallel to the plane of the Cu/Pd interface and Brillouin scattering [a shear modulus (l/s 4 4 ) with the shear waves parallel to the plane of the Cu/Pd interface]. Also, the films were characterized using both x-ray diffraction and high-resolution electron microscopy; this was done to assess the effect of the nanostructure on a possible "supermodulus effect." The films are nanostructurally very similar to the superlattice films employed in previous studies at Northwestern in which a supermodulus effect was reported. But, contrary to previous studies, no anomalous behavior was observed for the measured elastic properties of the thin films. Therefore the present results negate the earlier results and cast a serious doubt on the existence of a supermodulus effect.
I. INTRODUCTION Classical linear elasticity theory predicts that the elastic constants for a material with a fixed composition should be relatively insensitive to macroscopic processing steps.1 In a series of studies on the elastic properties of several metal/metal superlattice systems, Cu/Ni,2-3 Cu/Pd, 4 Ag/Pd, 5 and Au/Ni, 4 containing one-dimensional composition modulations, evidence was presented showing a strong dependence of the elastic constants on the value of the composition modulation wavelength (A). Typically, the various measured elastic moduli reached a maximum value that was two to five times greater than the value predicted for a material that had the same chemical composition, and was either homogeneous or had a value of A approximately equal to the thickness of the thin film. The maximum in the elastic moduli occurred for thin films with a value of A equal to approximately 2 nm. This anomalous behavior, the so-called "supermodulus effect," is the largest reported variation in the elastic properties of any material with a fixed composition. In all of these previous studies,2-4-5 with the exception of chronologically the last study,3 the elastic properties of the films were measured using the bulge test to extract the biaxial modulus. Considerable controversy exists,6 however, with respect to the interpretation of the results obtained using the bulge test. The critical point 1356 http://journals.cambridge.org
J. Mater. Res., Vol. 7, No. 6, Jun 1992
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