Microstructural Control through Diffusion-Induced Grain Boundary Migration
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MICROSTRUCTURAL CONTROL THROUGH DIFFUSION-INDUCED GRAIN BOUNDARY MIGRATION Carol A. Handwerker and John W. Cahn Institute for Materials Science and Engineering, National Bureau of Standards, Gaithersburg MD 20899 ABSTRACT Diffusion-induced grain boundary migration (DIGM) is a common, but only recently discovered low temperature phenomenon that results in high rates of both chemical mixing (or unmixing) and grain boundary migration. DIGM is found in many situations where chemical heterogeneities lead to diffusion. For example, DIGM is observed during diffusion and compound formation in polycrystalline multilayer contact systems produced by low temperature deposition techniques. The diffusional mixing along the moving grain boundary is high, localized, and results in a distinctive composition profile behind the moving interface. Theory has indicated, and experiments have confirmed, which conditions lead to DIGM and which conditions suppress it. The microstructural changes can result in either a grain refinement as seen in many metallic systems or in enhanced grain growth as seen in polysilicon. In either case these microstructural and compositional changes are controllable in a way that may allow fabrication of unique devices. AN INTRODUCTION TO DIGM Diffusion induced grain boundary motion (DIGM) is a common, but only recently discovered, phenomenon in which otherwise stable grain boundaries move under the influence of a chemical gradient. In DIGM, grain boundary migration and diffusion are coupled processes. Grain boundaries migrate, often with little regard for their curvature, and affect atom transport in an unusual way. In the regions swept by the moving boundary, alloying or dealloying occurs, often deep within a grain, and often at temperatures where little bulk diffusion would occur. As a result of DIGM there is both greatly enhanced atom transport, and a major change in the grain structure. The typical microstructure and composition distribution developed by DIGM are shown in Figure 1, both schematically and on the surface of an Fe foil after exposure to a source of Zn. Diffusion-induced grain boundary migration has been reported in twenty-six metal and five ceramic systems, including for example, Ag diffusing into Au, Cu out of Ni, NiO into MgO, Sr into CaCO 3 and Pb out of PLZT [1,2]. Grain boundaries take on a sinusoidal grain boundary shape and the regions swept by the moving boundary segments have the same crystallographic orientation as the grains on which they form. The kinetics of migration depend on the materials system and the annealing temperature. For Ni diffusing into Cu, DIGM occurs between 350° and 900°C with migration rates ranging between 3.2 x 10-11 and 4 x 0-9 in/s [3]. For Ag diffusing into Au, DIGM is extensive. Using Rutherford backscattering, alloying is observed after 3 minutes at 1500
and after only 5 hours at room temperature [1,4].
DIGM turned out to participate in many processes, but it simply was not recognized as such until recently. It is easy to understand the engineering con
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