A synthesis of mechanisms for initiation of the cellular (or discontinuous precipitation) reaction

To provide a basis for understanding mechanisms for initiation of the cellular reaction, the junction of an incoherent grain boundary with microfaceted interfaces at both the rationally and the irrationally oriented interfaces of grain boundary allotriomorphs is examined with the Cahn-Hoffman formalism. When relative values of the three xi vectors. are appropriate, the angle between the grain boundary and a facet at the allotriomorph edge can vary from 0 to 90 degrees and still be in equilibrium throughout this range. Also, the same Vector applies everywhere on a given planar facet. With the aid of DIGM, these considerations can permit a grain boundary to migrate part way along the broad face of an allotriomorph to initiate the cellular reaction via the Tu-Turnbull mechanism, or migrate to the allotriomorph end and isolate it a la Hillert-Lagneborg, or entirely fail migration, thereby allowing operation of the Fournelle-Clark mechanism, depending upon relative values of the xi vector components. (C) 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.