Interface attachment kinetics in alloy solidification
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I.
INTRODUCTION
IN solidification, the phase selection, growth velocity, chemical composition, long-range order, and microstructure of a growing phase are functions of the local conditions at the solid/liquid interface, e.g., temperature, composition, orientation, curvature, and crystal structure. The objective of the present research is to illuminate how atomic structure and the kinetics of atom movements lead to these functions. It constitutes several steps toward predictive capability for the production of materials under solidification processing and other nonequilibrium conditions. The modeling of an alloy solidification process typically treats the transport of solute and heat through the bulk of one or both of the phases involved in the transformation. To facilitate solution of the transport equations, the assumption of local equilibrium boundary conditions at the crystal/melt interface had been used extensively. Local equilibrium is not maintained during rapid solidification,t~.2J for which models of nonequilibrium interface kinetics abound but reliable measurements, until recently, have been lacking. Substantial deviations from local equilibrium set in for solidification as the interface velocity approaches the diffusive speed vD--the ratio of the diffusion coefficient across the interface to the interatomic spacing.t3~ Measured diffusive speeds for solidification~3m fall in the range 0.1100 rrds. The issue of local interfacial equilibrium often also arises during solid-solid transformations, where the diffusive speed as defined above is expected to be much smaller. The local equilibrium assumption is often used in the analysis of these transformations under circumstances where it is questionable. However, it has not yet been possible to resolve this issue for solid-state transformations as it has been for solidification.
MICHAEL J. AZIZ, Gordon McKay Professor of Materials Science, is with the Division of Applied Sciences, Harvard University, Cambridge, MA 02138. This article is based on a presentation made at the "Analysis and Modeling of Solidification" symposium as part of the 1994 Fall meeting of TMS in Rosemont, Illinois, October 2-6, 1994, under the auspices of the TMS Solidification Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
Rapid solidification following pulsed laser melting is a well-controlled experimental technique for investigating the kinetics of solidification far from local equilibrium. Pulsed laser irradiation creates on the surface of the sample a shortlived liquid layer that resolidifies, due to the high thermal gradients, at speeds measured to be as high as 15 m/s fSJ in Si and 100 m/s [6] in pure elemental metals. Tremendously supersaturated crystalline solid solutions are also observed, with solubility extensions of up to five orders of magnitude beyond the equilibrium solubility.tn With a spatially uniform pulsed excimer laser, we constrain melting and solidification to occur in a one-dimensional geometry, permitting simple but accurate calculations and even measuremen
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