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Initial Crystallization Reactions
Published online by Cambridge University Press: 15 February 2011
Abstract
During the initial stage of phase formation reactions nucleation conditions often apply a controlling influence on the phase selection, product phase number density and grain size. In most cases the operation of nucleation control is also associated with the development of significant undercooling or supersaturation in order to initiate crystallization. Under this constraint it is common to observe different forms of alloy metastability. In fact, a hierarchy of equilibria can be identified based upon the severity of the kinetic constraints that act during transformation. One consequence of the different levels of metastability is the development of precursor reactions before the initial crystallization. In other cases a kinetic competition between different reaction modes or structures can yield a change in the transformation pathway during reaction. The appearance of kinetic transitions is one consequence of the competition between concurrent reactions and provides a valuable opportunity for kinetics analysis and modeling. In cases where sequential reactions are separated by temperature or time intervals other kinetic constraints operate to expose metastability due to diffusional growth limitations. The identification of the reaction control provides the foundation for kinetics analysis and the development of alloy design strategies. These features can be considered in terms of the observed crystallization behavior during rapid solidification and solid state interface reactions.
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- Copyright © Materials Research Society 2000
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