The occurrence and periodicity of oscillating peritectic microstructures developed during directional solidification
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I.
INTRODUCTION
ONE of the microstructural variants that can occur in binary peritectic alloys during directional solidification with a planar solid-liquid interface is alternating layers of primary solid and peritectic solid oriented perpendicular to the growth direction.[1–10] For an alloy system where the solute element is more dense in the liquid state than the solvent element, solidification upward tends to stabilize the liquid against convective mixing in a terrestrial environment. Under these conditions, the layers of primary and peritectic solid can form over large solidification distances, as in the example in Reference 5. Layer formation in this case has been reported for alloy compositions between the primary and peritectic phase compositions at the peritectic isotherm.[2,3,5] The majority of these directional solidification studies on peritectic systems were concerned with establishing the correlation between the morphology of the solid-liquid interface, the phases forming at this interface, the alloy composition, and the ratio of temperature gradient in the liquid to growth velocity (G/V). The development of the layered microstructures was explained only qualitatively; three types of mechanisms intrinsic to the peritectic alloys have been given. (1) Repetitive accumulation and depletion of solute in the liquid ahead of the interface results from and causes alternating nucleation and growth of the two solids.[2,3] (2) The primary solid nucleates in the peritectic solid behind the solid-liquid interface and grows past the KATHRYN L. ZEISLER-MASHL, formerly Postdoctoral Research Associate, Ames Laboratory, United States Department of Energy, Iowa State University, is ASEE Postdoctoral Fellow, Physical Metallurgy Branch, Naval Research Laboratory, Washington, DC 20375. THOMAS A. LOGRASSO, Metallurgist, is with the Ames Laboratory, United States Department of Energy, Iowa State University, Ames, IA 50011. Manuscript submitted September 3, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
peritectic solid until sufficient solute accumulates in the liquid ahead of the interface to cause nucleation of the peritectic solid.[5] (3) Metastable peritectic solid with a planar solid-liquid interface can grow at a higher temperature than primary solid with a planar solid-liquid interface, which suggests that a layer of peritectic solid can form ahead of the planar primary front. However, the peritectic solid is not stable with respect to the primary solid above the peritectic temperature, so the primary solid may nucleate and grow at the interface at the expense of the peritectic solid.[11] A model has recently been developed by Trivedi[12] based on the first mechanism listed, which was originally proposed by Boettinger;[2] it can be summarized as follows. The model assumes that solute transport in the liquid occurs only through diffusion (no convective mixing), solid-state diffusion is insignificant, and solidification conditions are such that a planar solid-liquid interface is stable. This implies that a section
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