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 as a whole and compositionally comprised the gamma and gamma(1) phases. The layer growth kinetics were close to parabolic, although we observed a slight decrease in values of the reaction-diffusion constants with increasing annealing time. No proportionality was found between the width of the homogeneity range of an intermetallic compound and the growth rate of its layer. The temperature dependence of the reaction-diffusion constants is well described 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 both 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.