Growth kinetics of intermetallic compound layers at the Co-Zn interface

Abstract

The Co-Zn binary system is multiphase, with four intermetallic compounds stable below the melting point of pure Zn (420 ◦C) [1, 2]. All the compounds possess relatively wide ranges of homogeneity (some 1.5 to 10.0 at %). As no data on compound-layer formation have been found in the literature, the aim of the work reported here was to study the growth kinetics of intermetallic layers in the Co-Zn reaction couples, with special emphasis on the comparison of formation rates of the compounds differing by the width of their homogeneity ranges. A relatively low temperature of 250 ◦C was chosen to make it possible to compare the growth rates of intermetallic layers at the CoZn interface with the growth rate of the NiBi3 intermetallic layer at the Ni-Bi interface. The NiBi3 compound is known to have no noticeable range of homogeneity [3]. The growth kinetics of its layer between nickel and bismuth was studied earlier [4]. The materials used were 14× 5× 3 mm3 cobalt plates (99.98 wt % Co) and Zn granules (99.94 wt % Zn). The reaction couples were obtained by the interaction of solid cobalt and molten zinc under a flux at 460–470 ◦C (below the decomposition temperature of the γ2-phase, that being 475 ◦C [1]) in an 11 mm inner diameter graphite crucible with their subsequent joint cooling to the crystallization of the melt. This technique, firstly, ensured good contact between the initial phases everywhere along the Co-Zn interface and, secondly, allowed the nuclei of all the possible phases to be formed. Each of the Co-Zn samples was cut into two pieces using an electric-spark machine, so that the 7× 5× 3 mm3 cobalt plate, with the 5× 3 mm2 surface open, was embedded into the zinc matrix, 11 mm diameter. The surfaces of the samples were first mechanically ground and then electrolytically polished. The Co-Zn reaction couples thus prepared were annealed in sealed glass ampoules, filled with high-purity helium, in an electric-heated furnace for 10 to 60 h at 250 ◦C. During annealing, the temperature was maintained to within ±2 ◦C. After annealing, the reaction couples were rapidly cooled to room temperature, ground and polished again. The cross-sections obtained were studied metallographically and with the help of a CAMECA Camebax SX50 microanalyzer operating at 15 kV. The beam spot diameter and the phase volume analyzed at each point were estimated to be around 1μm and 2μm3, respectively. A typical example of the Co-Zn diffusion zone is given in Fig. 1. As can be seen, two intermetallic layers grow at the interface between cobalt and zinc. The layer adjacent to the zinc phase is much thicker than that adjacent to the cobalt phase. From the electron probe microanalysis (EPMA) data presented in Table I, it follows that the layer bordering the zinc matrix consists of the γ2-phase (CoZn13). Its chemical composition gradually changes from one interface to the other from 90.6 to 92.4 at % Zn, in very good agreement with the values 91.0 to 92.8 at % Zn indicated on the available equilibrium phase diagram [1]. The results obtained for the other layer are less certain probably due to two reasons. First, there is some ambiguity in the position of phase boundaries in the region of γ , γ1 and γ2 phases of the Co-Zn phase diagram [1]. Secondly, EPMA measurements in this layer with a thickness of about 7μm were less precise, though the average chemical composition of the layer fell into the composition range of the γ -phase. Clearly, it was impossible to measure the Co and Zn content at distances less than 1.5μm away from the phase interfaces. Therefore, the boundary values obtained (82.2 to 85.2 at % Zn) are in worse agreement with the width of the homogeneity range of the γ -phase (75.2 to 85.4 at % Zn). Note that at annealing times longer than 60 h, in many specimens there developed a crack along the