DISCONTINUOUS PRECIPITATION IN A CD-6 AT-PERCENT AG ALLOY

Discontinuous precipitation (DP) in a Cd-6 at.% Ag alloy has been inve stigated for the first time. The precipitate phase maintains a lamella r morphology and statistically constant interlamellar spacing under a given isothermal condition in the temperature range studied (333-523 K ). The interlamellar spacing increases with an increase in isothermal temperature. The reaction front velocity registers a typical C-curve v ariation with the inverse of temperature. The reaction rate is maximum at 470 K. The predicted upper limit of DP occurrence in this alloy is 23 K lower than the concerned equilibrium solvus temperature. Continu ous precipitation accompanies DP at all temperatures, especially beyon d a certain time, and adversely affects the growth kinetics of DP colo nies by reducing the local chemical driving force and/or posing physic al hindrance to the reaction front migration. An extensive kinetic ana lysis of DP using the models by Turnbull, Aaronson and Liu, and Peterm ann and Hornbogen has yielded the grain boundary chemical diffusivity data in Cd-6 at.% Ag for the first time, the activation energy of whic h lies in the range 55-77 kJ/mol. Copyright (C) 1996 Acta Metallurgica Inc.