MECHANISM AND KINETICS OF TYPE-II DISCONTINUOUS COARSENING IN A ZN-4 AT PERCENT-AG ALLOY

Discontinuous coarsening (DC) may succeed discontinuous precipitation (DP) either at the same (DCI) or another temperature (DCII). The prese nt study concerns mechanism and kinetics of DCII in a Zn-4 at % Ag all oy in the range 353-513 K following DP at 393 K for 60 h. DCII colonie s prefer to initiate either from one or both sides of the interfaces b etween the former DP colonies. A suitable comparison of the kinetic da ta 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 valu es of the driving forces between them differ marginally. A detailed ki netic analysis of DCII through the Livingston-Cahn model leads to an u nderestimation of the activation energy (Q(b)) of grain boundary chemi cal diffusion of Ag in Zn-Ag (= 30.7 kJ mol(-1)), whereas the same obt ained from the modified Petermann-Hornbogen model (= 61.0 kJ mol(-1)) compares well with that for DP/DCI (reported elsewhere by us), and gra in boundary self diffusion of Zn. Considering that Q(b) in DCII is nea rly 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.