Kinetics of discontinuous precipitation and type I discontinuous coarsening in Zn-4 at % Ag alloy

Abstract

A detailed study of the kinetics of discontinuous precipitation (DP) and type I discontinuous coarsening (DCI) in Zn-4 at % Ag alloy is reported here for the first time. DCI succeeds DP during prolonged isothermal ageing. Both DP and DCI are characterized by a predominantly lamellar morphology of the precipitate phase, statistically constant interlamellar spacing and steady state reaction front (RF) velocity at a given temperature. The interlamellar spacing increases with temperature. The RF velocity shows a C-curve behavior for DP, but increases monotonically for DCI, as a function of temperature. DCI is distinguished from DP by a 3-5 times larger interlamellar spacing and 1-2 orders of magnitude lower RF velocity than those of DP under comparable conditions. DCI may be initiated from an interface between two DP colonies, a former DP-RF, or the free surface intersecting a DP colony. Kinetic analysis of DP using the models of Turnbull, Cahn, Hillert, and Petermann and Hornbogen, and of DCI using the modified Petermann and Hornbogen model (by Fournelle) have yielded grain boundary diffusivity data in the temperature range 353-573 K. Subsequent Arrhenius analysis shows that the activation energy of the DP and DCI processes lies between 50-66 kJ mol^-1. The latter is comparable with the activation energy of grain-boundary self-diffusion of Zn and is nearly half that of tracer impurity diffusion (volume/bulk) of Ag in Zn. Hence, it is concluded that DP and DCI are grain-boundary diffusion-controlled processes in the present alloy.