Two-dimensional grain growth in rapidly solidified succinonitrile films
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I.
INTRODUCTION
THE microstructure of polycrystalline materials--i.e., the sizes, shapes, and arrangement of grains--essentially determines the mechanical, optical, magnetic, electrical, and other properties of the material. One of the important microstructure-forming processes is grain growth, when migration of grain boundaries under capillary forces causes the coarsening of the grains. Due to this capillary force, grain boundaries migrate, reducing the total free energy of the system. This motion leads to shrinking of some grains and the growth of others. The grain size distributions determine the properties of thin films as well. In this case, it is convenient to assume that the system is two-dimensional (2-D). Grain growth in a 2-D polycrystalline aggregate occurs due to the growth, shrinkage, and annihilation of individual grains. In turn, the growth, shrinkage, and annihilation of individual grains depends upon the overall movement of the grain boundaries. Therefore, the behavior of individual grains depends significantly upon topology. One of the first attempts at examining the behavior of individual grains was advanced by Mullins and Von Neumann 1~'2Mbased on the uniform boundary model. They formulated a correlation between grain growth kinetics and the topological class of the individual grains: dA --
dt
= KN (N-
6)
[11
where A is the grain area, N the topological class of the grain, t the time, and /in a kinetic rate constant. Numerous computer simulations 131 have been based upon the Mullins-Von Neumann relationship. These simulations agree with a large body of experimental work in very pure materials, where solute drag does not effect grain-boundary motion. These data confirm that normal
M. P A L M E R and J. N O R D B E R G , Graduate Students, K. RAJAN and M. G L I C K S M A N , Professors, and V. F R A D K O V , Research Assistant Professor, are with the Materials Engineering Department, Rensselaer Polytechnic Institute, Troy, NY 12180-3590. This article is based on a presentation made in the symposium "Fine Grains And Their Growth in Rapidly Solidified Materials," TMS Materials Week '93, Pittsburgh, PA, October 18-21, 1993.
METALLURGICAL AND MATERIALS TRANSACTIONS A
grain growth should follow parabolic growth law proposed by Burke and Turnbull, 14~to describe the behavior of polycrystalline solids; namely, /52(/) -- / ) 2 ( 0 ) = Kot.
[2]
Here, /5 represents the average grain diameter, t represents time, and Ko is a kinetic rate constant. Examining Eq. [ 1] requires the direct observation of each individual grain during the grain growth process. We have chosen to examine succinonitrile (SCN). Succinonitrile is an easily purified, transparent organic material with a low melting point (58.08 ~ and isotropic properties. In addition, this material has been used to study other solidification phenomenon such as dendritic growth in solids 15} and has well-characterized physical parameters. The primary advantages of this material are its low melting point and the fact that it is transparent. Thu
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