Different types of discontinuous precipitation in Cu-15 Wt% in alloy

Abstract

Discontinuous precipitation is a solid-state reaction in which an initially homogeneous solid solution, {alpha}{sub 0}, is decomposed into a lamellar two-phase microstructure, {alpha}+{beta}, in the region behind the migrating reaction front. Two types of discontinuous precipitation can be distinguished: the interfacial type reaction and the dislocation type. For the interfacial type reaction, generally the reaction front is a high-angle grain boundary. However, for the dislocation type reaction, it is highly probable that dislocations play a dominate role in nucleation and growth. Preferred nucleation sites are dislocation rich areas in the matrix. The dislocation type reaction is found principally in systems in which the specific volumes of the initial matrix and of the reaction product strongly differ. It is assumed that in the vicinity of the growing {beta} particles dislocations are produced and act as new nucleation sites. By this means the reaction front can further migrate in an autocatalytic fashion, and with isothermal conditions a constant migration velocity results. The dislocation type reaction does not occur as frequently as the interfacial type reaction. In several solid solutions both reactions can be observed simultaneously, but in Cu-In alloy only the interfacial one can appear.