Morphology and Phase Metastability in the Al/Cu Thin Film Reaction
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Morphology and Phase Metastability in the Al/Cu Thin Film Reaction. E. D. McCARTY and S. A. HACKNEY Dept. of Metallurgical and Materials Engineering, Michigan Technological University Houghton, Michigan 49931
Abstract. The initial stages of the reaction between thin Al grains with an average diameter of 2 x 10-5 m and sputter deposited, nanocrystalline Cu films has been studied in plan view using in situ transmission electron microscopy. At high reaction rates, the phase transformation in the Al grain resulting from the interdiffusion process is found to exhibit metastable growth morphologies characterized by negative curvature at the growth interface. The crystal structure of the initial phase formed in the Al grain under relatively high reaction rates is a metastable, orthorhombic distortion of the equilibrium body centered tetragonal 0 phase. The distortion is found to vary with Al grain surface orientation. The degree of metastability can be experimentally correlated with the kinetics of the interdiffusion process as controlled by diffusion barrier thickness.
Introduction. It has recently been shown that the growth of an intermediate phase at the interface between Al and Cu films can result in dendrite-like growth morphologies lying parallel to the Al/Cu boundary plane [1]. The lateral growth instabilities leading to the two dimensional interfacial dendrites are observed to exhibit a wavelength which depends strongly on the temperature and on the nature of the barrier to interdiffusion at the Al/Cu interface [1]. The observations of such growth structures during precipitation in the Al-Cu solid state reactions is unexpected from considerations of bulk alloy behavior as precipitation at the relevant temperatures in bulk, homogeneous solid solutions generally occurs with convex growth shapes (plates, laths, rods) [2] and negative interfacial curvature is rarely observed. It is also evident from the data presented in this paper that a polymorphic distortion of the equilibrium precipitate crystal structure is present. It is the purpose of this note to present the experimental data on phase and morphology metastability observed for this geometry and to explore the correlations between the observed metastability and the kinetics of the transformation.
Experimental Procedure. The experimental procedure is related in detail in reference [1]. Large grain Al foils with a grain diameter of 2 x 10- 5 m are electropolished to perforation and cleaned by ion beam sputtering. Copper is then deposited by sputter deposition onto one surface of the Al film and the bilayer foils are heated in situ in the transmission electron microscope to a specific reaction temperature. An important variation on this experiment is to deposit a thin layer of carbon on the Al surface prior to the deposition of the Cu. This allows the study of the effect of increasing the resistance to interdiffusion at a specific temperature on the transformation dynamics (and thus the relationship to the transformation metastability). All experiments referred
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