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

A detailed study of the kinetics of discontinuous precipitation (DP) and ty pe I discontinuous coarsening (DCI) in Zn-4 at % Ag alloy is reported here for the first time. DCI succeeds DP during prolonged isothermal ageing. Bot h DP and DCI are characterized by a predominantly lamellar morphology of th e precipitate phase, statistically constant interlamellar spacing and stead y state reaction front (RF) velocity at a given temperature. The interlamel lar spacing increases with temperature. The RF velocity shows a C-curve beh avior for DP, but increases monotonically for DCI, as a function of tempera ture. DCI is distinguished from DP by a 3-5 times larger interlamellar spac ing and 1-2 orders of magnitude lower RF velocity than those of DP under co mparable conditions. DCI may be initiated from an interface between two DP colonies, a former DP-RF, or the free surface intersecting a DP colony. Kin etic analysis of DP using the models of Turnbull, Cahn, Hillert, and Peterm ann and Hornbogen, and of DCI using the modified Petermann and Hornbogen mo del (by Fournelle) have yielded grain boundary diffusivity data in the temp erature range 353-573 K. Subsequent Arrhenius analysis shows that the activ ation 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-dif fusion of Zn and is nearly half that of tracer impurity diffusion (volume/b ulk) of Ag in Zn. Hence, it is concluded that DP and DCI are grain-boundary diffusion-controlled processes in the present alloy. (C) 1999 Kluwer Acade mic Publishers.