Phase formation and reaction kinetics in Co-Zn diffusion couples

Two intermetallic layers were found to grow between cobalt and zinc at 250 to 400 degrees C. The Zn-bordering layer(91.1 to 92.8 at.% Zn) consisted of the gamma(2) phase of the Go-Zn binary system. The Go-bordering layer (76. 0 to 89.1 at.% Zn), in which three sublayers could be distinguished, grew a s a whole and compositionally comprised the gamma and gamma(1) phases. The layer growth kinetics were close to parabolic, although we observed a sligh t decrease in values of the reaction-diffusion constants with increasing an nealing time. No proportionality was found between the width of the homogen eity range of an intermetallic compound and the growth rate of its layer. T he temperature dependence of the reaction-diffusion constants is well descr ibed by the Arrhenius equation k(1) = A exp(- EIRT), where A = 3.8 x 10(-6) m(2) s(-1) for the Go-bordering layer, 2.06 X 10(-8) m(2) s(-1) for the Zn -bordering layer and 4.0 X 10(-7) m(2) s(-1) for the total thickness of bot h layers. The values of E are 104.1, 71.1 and 81.8 kJ mol(-1), respectively . (C) 1999 Elsevier Science S.A. All rights reserved.