Metastable phase equilibria in the lead-tin alloy system: Part I. Experimental
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I. I N T R O D U C T I O N T H E application o f rapid solidification processes offers a broad range of opportunities to produce microstructures that are unique compared to those available in conventionally processed alloys, t~,21The important attributes o f rapid solidification such as grain refinement, increased homogeneity, uniform dispersion of fine precipitates, extended solid solubility, and metastable phase formation have a significant impact on the properties and structural engineering applications of alloys. Indeed, one of the most interesting applications of the technique is the synthesis of new alloy phases which cannot be obtained either under equilibrium conditions or by quenching in the solid state. Relatively little is known about the solidification .... reactions involved in the synthesis of metastable alloy phases. For an understanding of these transformations, the level of undercooling of the liquid and the nucleation and growth kinetics of the stable or metastable solidification products are of importance. There are only a very limited number of phase diagrams available which include metastable phase boundaries indicating the temperature and concentration range of metastable phases found after quenching to low temperatures. To a large extent, the metastable phase diagrams that are available are based on computer calculations to extrapolate the Gibbs energy data of the stable phases into the metastable region, e.g., for Ag-Cu and Cd-Zn. t3] Most often, though, there exists no experimental determination of the solidification paths for metastable solids by in situ measurements to confirm the theoretical calculation analysi s . One strategy to obtain the metastable phase boundaries is offered by the study of the solidification behavior H.J. FECHT is with the Keck Laboratory, California Institute of Technology, Pasadena, CA 91125. J.H. PEREPEZKO, Professor, is with the Department of Metallurgical and Mineral Engineering, University of Wisconsin-Madison, Madison, WI 53706. Manuscript submitted September 18, 198% METALLURGICAL TRANSACTIONS A
of liquid droplet dispersions which undercool to a large degree during slow cooling. H-71By starting with high purity metal and dispersing the liquid into a large number of small drops within a suitable medium, only a small fraction of the drops may contain potent nucleants, ff droplet independence and separation are maintained by the use of thin, inert coatings, which are not catalytic to nucleation, then the effects of any internal nucleants can be restricted to the few drops in which they are located. As a sample of liquid metal in the form of a stabilized droplet emulsion is undercooled at a slow cooling rate, the droplets containing potent nucleants will freeze at low levels of undercooling, but the majority of the droplets will not freeze until reaching a maximum undercooling nucleation temperature, T,. This allows the undercooled liquid to be maintained for extended periods above Tn for the measurement of the thermodynamic properties of the metastable liquid
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