KINETICS OF DISCONTINUOUS PRECIPITATION AND DISSOLUTION IN CU-AG ALLOYS

The kinetics of discontinuous precipitation and dissolution of the cel lular precipitate have been studied in Cu-3 at.% Ag and Cu-4 at.% Ag a lloys. The growth rates of the cells were measured using optical micro scopy. The interlamellar spacings of the primary cells and composition s of the depleted matrix were measured using scanning electron microsc ope and X-ray diffraction, respectively. The Cu-Ag alloys were observe d to decompose into a lamellar structure consisting, of alternate lame llae of the alpha (Cu-rich) and beta (Ag-rich) phases when a solid sol ution of the alloy was aged below the solvus temperature. The rod shap ed morphology of the beta phase dispersed in the matrix of alpha was o bserved at all temperatures. The primary cell growth data were analyse d using the theories of Cahn, Hillert, Sundquist, Turnbull and Peterma nn and Hornbogen. From the diffusivity values, it has been shown that the growth of primary cells occurs by the diffusion of Ag along the gr ain boundaries. The results are consistent with the diffusivity values reported in the literature in the same temperature range. The discont inuous dissolution of the primary cells occurred above the solvus temp erature as well as 30-40 K below it. The dissolution occurred primaril y at the prior positions of grain boundaries as well as at the primary cells intersection, at least in the initial stages of dissolution. Th e discontinuous dissolution occurred by the diffusion of Ag along the interface boundary between the primary and dissolution cells. The diff usivity and mobility values obtained during dissolution are one order of magnitude smaller than those obtained for the discontinuous precipi tation. This has been explained by volume diffusion ahead of the inter face and not all of the free energy being used to drive the interface boundary. (C) 1998 Canadian Institute of Mining and Metallurgy. Publis hed by Elsevier Science Ltd. All rights reserved.