On the Superlattices of the Cu-Au System (II)

Abstract

In a previous paper (I), we reported on some physical propertis of the superlattices which correspond to Mg3Cd, MgCd and MgCd3. In this paper we shall present a general review of this Mg-Cd alloy system using the results of our investigations performedon about fourty specimes of various compositions. The transition temperature-vs-composition curve is of a smooth parabolic form, having a maximum (ca. 250°) at the atomic ratio 1:1, but having.r.o maximum at 3:1 and 1:3 (Fig. 1). The specific heat-vs-temperaure curves and expansion coefficient-vs-temperature curves are represented in Figs. 2 and 3. Expansion coefficient-vs-composition curve has three maxima at the compositions 1:3, 1:1 and 3:1, for both ordered and disordered states (Fig. 4). The curves in Fig. 6 depict the electrical resistance measured at various temperature against alloy compositions. Below the Curie point, these curves show three minima in the neighbourhood of the compositions, Mg3Cd, MgCd and MgCd3, just as in the cases of the Cu-Au, Cu-Pt and other systems; while above the Curie point, the appearance of thee curves are very different from the other cases of ordinary superlatties; i.e. the alloys of compositions. rear MgCd have still lower resistance than those of other compositions. Fig. 7 represents graphically the hardness of these alloys as a function of compositions for both annealed and quenched states. It remains unexplained why the alloys near 40 atomic% Cd inaicate singularity in hardress-vs-composition and Curie point-vs-composition curves. It is interesting to note that both specific heat-vs-temperature curves and expansion coefficient-vs-temperature curves cf an alloy corresponding to 44.5 atomic%, Cd, show three peaks (Fig. 10), and also that those of the alloys in region from about 60 to 68 atomic% Cd, have anomalous form, which appear to be originated from a superposition cf two peaks (Fig. 11). These phenomena may be explained by Shockley's theory on the Cu-Au alloy system.