The effect of undercooling on the cellular precipitation reaction in Cu-3 Pct Ti
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I.
INTRODUCTION
In the cellular precipitation reaction, also known as the discontinuous precipitation reaction,[1] a mobile grain boundary migrating into a supersaturated matrix (a') forms cells or colonies consisting of lamellae or rods of a precipitate phase (b) within a less supersaturated matrix phase (a).[2–6] The grain boundary is displaced from its original position during the initial stages of the reaction and is subsequently conveyed at the reaction front, where it redistributes the solute from the supersaturated matrix into the two cellular constituents. This reaction front is comprised of both the displaced grain boundary (the a:a' interface) and the precipitate growth interface (the b:a' interface) and is typically viewed as an incoherent boundary.[1] At the base of the colony, where the grain boundary was originally located, the cellular matrix phase is continuous with the grain from which the colony is growing.[1,7] Nucleation of the cellular precipitation reaction is generally described in terms of two phenomenological descriptions. Thirty years ago, Tu and Turnbull[8,9] and Tu[10] proposed the ‘‘pucker’’ mechanism, in which nucleation of a plate-shaped precipitate at a grain boundary results in a deflection of that grain boundary. The subsequent migration of the grain boundary along the high energy face of the precipitate decreases the interfacial energy and favorably orients the grain boundary for the nucleation and growth of R.W. FONDA, formerly a Graduate Student in the Materials Science and Engineering Department of the University of Virginia, is currently a Metallurgist at the Naval Research Laboratory, and M.A. MANGAN, formerly a Graduate Student in the Materials Science and Engineering Department of the University of Virginia, is currently a Postdoctoral Fellow at the Naval Research Laboratory, Washington, DC 20375, G.J. SHIFLET, Professor, is with the Materials Science and Engineering Department, University of Virginia, Charlottesville, VA 22903. This article is based on a presentation made in the symposium ‘‘Kinetically Determined Particle Shapes and the Dynamics of Solid:Solid Interfaces,’’ presented at the October 1996 Fall meeting of TMS/ASM in Cincinnati, Ohio, under the auspices of the ASM Phase Transformations Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
additional precipitate plates parallel to the first. Alternatively, Fournelle and Clark[11] proposed that thermally activated migration of a grain boundary, followed by grain boundary precipitation, would give rise to a concentration difference across the grain boundary. This would induce continued migration of that grain boundary, with solute diffusing along the boundary to the growing precipitates. In dilute Cu-Ti alloys, precipitation of the equilibrium b phase (Cu4Ti) at small undercooling occurs in two morphologies—as Widmansta¨tten plates within the grain interiors and as lamellae in cellular colonies at the grain boundaries.[12,13] In both of these morphologies, the basal plane of the orthorhombic b phase (ordered ne
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