Grain boundary discontinuous precipitation reactions

Abstract

Discontinuous precipitation results in the formation of a two-phase lamellar structure behind moving grain-boundaries. The reaction is of interest because it generally has a deleterious effect on the mechanical, physical, and chemical properties of many commercial alloys. The combination of heterogeneous boundary precipitation and concurrent migration is complex and has resulted in a diverse range of proposed reaction mechanisms, several models of the growth kinetics, and many empirical observations of the effects of ternary additions and lattice strain. This review summarizes existing theories and attempts to reconcile them with the latest experimental data for reactions which take place from supersaturated solid solutions (eutectoid decomposition is specifically excluded). It is considered that fuller understanding requires increased application of the latest electron -optical techniques such as high-resolution microanalysis and in situ experiments in the high-voltage microscope. Furthermore, to understand the effects of grain-boundary structure on initiation and growth kinetics “investigations should be confined to well characterized boundaries, such as bicrystals.