Interlamellar Spacing in Directionally Solidified Eutectic Thin Films
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I.
INTRODUCTION
IN the last few decades there has been considerable effort in controlling the composite microstructure of bulk eutectic alloys. Directionally solidified eutectic alloys form a periodic microstructure that consists of either aligned plates or rods. In bulk alloys small regions of parallel plates are oriented along the growth direction but at random about this direction. In nonstructural applications a thin film structure consisting of submicron stripes is desired.~'2 Theories of eutectic growth consider a simpler two-dimensional geometry of parallel plates. We may approach this ideal geometry in sufficiently thin film samples where the surfaces constrain the growth of the plates to be oriented normal to the f i l m s u r f a c e . 3-6 In thin films the perfection of the structure is increased over that of bulk eutectic growth of the same alloy because the surfaces help align the structure. In nonstructural applications a thin film structure consisting of submicron stripes is desired. The growth of perfect eutectic structures depends on the stability of the growth with respect to perturbations in the morphology. Even if the growth conditions are accurately controlled to prevent fluctuations, there are always infinitesimal variations induced by thermal fluctuations. If all possible modes of distortion of the lamellar geometry decrease in amplitude with time, then the growth is stable. There exists a range of lamellar spacings where the growth of the two-dimensional eutectic is stable. A central question is what determines the lamellar spacing. It is appropriate to compare the growth of eutectic thin films with the theory of eutectic growth which for simplicity assumes a two-dimensional geometry.
II.
EUTECTIC GROWTH
chosen because the bulk alloys were previously found to yield lamellar structures. Deposition of the eutectic alloy film was made by deposition of the two component films sequentially. In each case the total film thickness was 2 microns. Thus, the thicknesses of the component films were calculated to give the overall eutectic composition. The components of the thin film are melted together by a moving heat zone to form the eutectic liquid and the film is directionally solidified to form a lamellar structure consisting of the two equilibrium solid phases. Two methods of producing a heat zone were used. First, a 90 watt CW Nd-Yag laser was focused into a line of heat with a cylindrical lens to produce a narrow heat zone with a high thermal gradient necessary for solidification at high speeds (Figure 1). Second, a quartz iodine lamp with a linear filament was focused with an elliptical reflector to yield a well controlled heat zone suitable for producing uniform structures over the entire 5 cm by 7.5 cm substrate. Using the laser beam, the thermal gradient was of the order of 8000 ~ cm -~ while with the lamp heat source the thermal gradient was only 200 ~ cm -~. Since a larger thermal gradient was necessary at high speeds to produce a planar
LASER BEAM
THIN FILM LAYERS
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SOLIDIFIED EU
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