Mechanism and kinetics of type II discontinuous coarsening in a Zn-4 at %Ag alloy

Discontinuous coarsening (DC) may succeed discontinuous precipitation (DP) either at the same (DCI) or another temperature (DCII). The present study concerns mechanism and kinetics of DCII in a Zn-4 at % Ag alloy in the range 353-513 K following DP at 393 K for 60 h. DCII colonies prefer to initiate either from one or both sides of the interfaces between the former DP colonies. A suitable comparison of the kinetic data reveals that interlamellar spacing (lambda) and steady-state growth velocity (v) values in DCII are significantly different than those in DP. On the other hand, the kinetics of DCI vis-a-vis DCII in terms of lambda and v are comparable to each other, though the calculated values of the driving forces between them differ marginally. A detailed kinetic analysis of DCII through the Livingston-Cahn model leads to an underestimation of the activation energy (Q(b)) of grain boundary chemical diffusion of Ag in Zn-Ag (= 30.7 kJ mol(-1)), whereas the same obtained from the modified Petermann-Hornbogen model (= 61.0 kJ mol(-1)) compares well with that for DP/DCI (reported elsewhere by us), and grain boundary self diffusion of Zn. Considering that Q(b) in DCII is nearly 50% of the activation energy for volume/matrix diffusion of Ag in Zn, it appears that DCII in the present alloy is a boundary diffusion controlled process.