Uniformity and interfaces in ion-beam deposited Al/Ni multilayers
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Uniformity and interfaces in ion-beam deposited AlyNi multilayers A. S. Edelstein and R. K. Everett Naval Research Laboratory, Washington, DC 20375-5343
J. H. Perepezko and M. H. da Silva Bassani Department of Materials Science and Engineering, University of Wisconsin, Madison, Wisconsin 53706 (Received 25 April 1996; accepted 21 August 1996)
The uniformity and reaction kinetics of ion-beam deposited AlyNi multilayer samples with the same composition, Al81.8 Ni18.2 , and modulation wavelength, L 20 nm, but with different total film thicknesses were investigated by x-ray diffraction and differential scanning calorimetry measurements. The total film thicknesses varied between approximately 0.5 and 2.0 mm. It was found that the interface widths were approximately 1 nm and the Ni layers are much more disordered than the Al layers. The thicker samples show an increase in disorder on a length scale comparable to L. In other experiments, a change was observed with increasing modulation wavelength from semicoherent interfaces with a low density of misfit dislocations to semicoherent interfaces with a high density of misfit dislocations. The reaction kinetics for forming the Al9 Ni2 phase is independent of the sample thickness.
I. INTRODUCTION
Due to knowledge concerning phase formation1,2 during diffusion in thin films and the availability of ancillary kinetic and thermodynamic data, the AlyNi system is an attractive model system for basic studies of intermetallic phase development. Some AlyNi intermetallics also have technological applications. For example, the compound AlNi has high temperature strength and low density.3 Multilayer samples are well-suited for investigating the heterogeneous nucleation of new AlyNi phases because a sizable fraction of the atoms are near interfaces. In earlier studies4,5 of AlyNi multilayers prepared by ion-beam sputtering containing 25 and 60 at. % Ni with modulation wavelengths approximately 80 nm, the first phase to form was Al9 Ni2 . In other studies by Ma et al.6 of multilayer samples prepared by electron beam evaporation containing 25 at. % Ni, only the Al3 Ni phase was formed. This led us to consider how our multilayer films differed from those of Ma et al. Substantial stresses are known7,8 to exist in multilayers such as the AuyNi system in which there is a 13.6% lattice mismatch. These stresses can affect phase formation. For example, Jongste et al.9 found that in TiySi multilayers the formation of the C49 TiSi2 phase was correlated with an abrupt change in the stress of the film due to heating. To investigate whether stress influenced the reaction pathways or products in our AlyNi multilayers, a series of multilayer samples were prepared by ion-beam sputtering which were as similar as possible except that the total film thickness was varied by a factor of J. Mater. Res., Vol. 12, No. 2, Feb 1997
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four from the thinnest, approximately 0.5 mm, to the thickest, approximately 2.
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