Time-dependent nucleation in partitioning transformations

A model is presented and solved numerically for the time-dependent nucleation of a precipitate, coupling the interfacial and long-range diffusive fluxes. The effect of pre-existing cluster distributions is studied. Results from the numerical calculation are compared with those from the classical theory of nucleation. The introduction of coupled fluxes decreases the steady-state nucleation rate and increases the induction time over that from the classical theory, often by several orders of magnitude. The composition of the parent phase near a cluster of the new phase is shifted toward the new phase composition. Such linked-flux nucleation can be important for microstructural control of solid-state precipitation processes, and may explain the formation of the nano-structured composites in some devitrified metallic glasses. Approximate expressions for the size-dependent cluster growth rate, steady-state nucleation rate and induction time are derived and checked against the results of the numerical calculation. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.