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

To provide a basis for understanding mechanisms for initiation of the cellu lar reaction, the junction of an incoherent grain boundary with microfacete d interfaces at both the rationally and the irrationally oriented interface s of grain boundary allotriomorphs is examined with the Cahn-Hoffman formal ism. When relative values of the three xi vectors. are appropriate, the ang le between the grain boundary and a facet at the allotriomorph edge can var y from 0 to 90 degrees and still be in equilibrium throughout this range. A lso, the same Vector applies everywhere on a given planar facet. With the a id of DIGM, these considerations can permit a grain boundary to migrate par t way along the broad face of an allotriomorph to initiate the cellular rea ction via the Tu-Turnbull mechanism, or migrate to the allotriomorph end an d isolate it a la Hillert-Lagneborg, or entirely fail migration, thereby al lowing operation of the Fournelle-Clark mechanism, depending upon relative values of the xi vector components. (C) 1998 Acta Metallurgica Inc. Publish ed by Elsevier Science Ltd. All rights reserved.