Nucleation-Controlled Solidification Kinetics
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INTRODUCTION
SOLIDIFICATION reactions and the resulting microstructural morphology of the product material can be influenced by a number of externally controlled processing conditions (i.e., extrinsic factors) such as the rate of heat extraction and forced melt convection. Similarly, there are several key internal conditions (i.e., intrinsic factors) that can exert a strong influence on the microstructure, including nucleation conditions, growth kinetics, morphological stability, and interface processes. These features are more characteristic of the material than of the specific processing regimen. During solidification, many transformation processes may be active simultaneously and operate in a general competition. The isolation of a given influence for separate study usually requires some knowledge of the conditions favoring each of the processes so that the experiment may be designed to emphasize the desired influence. For the most part, this approach has been successfully applied and much progress has been achieved in process models and growth kinetics analysis. Throughout the analysis of solidification, it is commonly recognized that nucleation kinetics are an important part of the initial stage of a reactionY .2I However, the effects of nucleation reactions can be masked by other competing processes and subsequent transformations so that special care is required to isolate nucleation events. For example, it is widely accepted that nucleation plays a role in determining which of the eutectic phases starts the solidification reactiont3J and in determining the effectiveness of grain refinement in melts nucleated close to the equilibrium temperature.[4,51 It is also clear that nucleation plays a key role
J.H. PEREPEZKO, Professor, and M.J. UTTORMARK, Postdoctoral Fellow, are with the Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, WI 53706-1595. This article is based on a presentation made at the "Analysis and Modeling of Solidification" Symposium as part of the 1994 Fall meeting o f TMS in Rosemont, Illinois, October 2-6, 1994, under the auspices o f the TMS Solidification Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
in a number of microstructural transitions that occur beyond the initial stage of reaction. For example, nucleation reactions control the solidification pathway for mixed morphology microstructures from processes such as the columnar to equiaxed morphology transition during directional solidification,t61 the dendrite-eutectic transition which occurs with a change in the solidification front velocity,iT] and the phase selection transition between metastable and stable phase formations such as the crystal-glass transition. tSl This control develops under competition with other processes such as dendrite fragmentationL91and directs the course of the microstructural evolution. In each of these cases, an initial reaction has progressed to some extent before an alternate form takes over to complete the solidification. At the transition, nucleation
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