Phase formation in the nickel-zinc reaction couple

Annealing of Ni-Zn reaction couples at 400 degrees C for 1-25 h results in the formation of intermetallic layers of the gamma- and delta-phases at the interface between nickel and zinc. The width of the homogeneity region for the gamma- phase is 73.0-86.0 at. % Zn, while that for the delta-phase is 87.5-89.0 at % Zn. The chemical formula for the gamma- phase is thus NiZn3-6, while that for the delta-phase is NiZn7-8. The gamma-phase growth kinetics is described by a parabolic equation x(i)(2) - x(0)(2) = 2k(1)t(i)(x(i) and x(0) are the thicknesses of the layer at times t(i) and t(0) = 0, k(1) is the growth rate constant): k(1) = (4.4 +/- 0.2). 10(-14) m(2)/sec. The delta-phase shows a deviation from the parabolic law since its growth rate constant decreases from 9.6 . 10(-14) m(2)/sec at t = 1 h to 6.9 . 10(-14) m(2)/sec at t = 25 h. The thickness ratio for the layers in this time interval chages by about 13%.