KINETICS OF DISCONTINUOUS PRECIPITATION IN A ZN-2.5 AT PERCENT CU ALLOY

The morphology and growth kinetics of discontinuous precipitation in a Zn-2.5 at % Cu alloy have been studied in the temperature range 383-5 83 K by optical and scanning electron microscopy. The precipitate phas e has a lamellar morphology, and maintains a statistically constant in terlamellar spacing under isothermal growth conditions. The interlamel lar spacing increases with an increase in temperature. The isothermal growth kinetics in terms of reaction front migration rate is maximum a t 523 K. The upper temperature limit for the occurrence of reaction in this alloy has been predicted to be 643 K. A detailed kinetic analysi s of the experimental data using several analytical models has confirm ed discontinuous precipitation in this system to be a boundary diffusi on controlled reaction, and enabled the determination of the grain bou ndary chemical diffusivity of Cu in a Zn-rich Zn-Cu alloy in the tempe rature range studied. The corresponding activation energy values deter mined in this study, range between 65 to 86 kJ/mol(-1), which compare well with the relevant data in the literature.