The equilibrium and lattice-spacing relations in the system magnesium-cadmium

The alloys of magnesium and cadmium are of great interest, since both metals have the same valency, and almost the same atomic volume, but the axial ratios of their close-packed hexagonal structures differ markedly, being 1.885(2) for cadmium, and 1.6237 for magnesium. The equilibrium diagram of the system has always attracted great interest, and very conflicting results have been obtained by different investigators. The earlier investigations led to the conclusion that the liquidus and solidus curveswere very close together, and fell continuously from the melting point of magnesium to that of cadmium, so that, at the higher temperatures, a continuous solid solution was formed, which, at low temperatures, underwent a transformation in the region of 50 atomic % of cadmium, owing to the supposed existence of a compound MgCd. The work of Hume-Rothery and Rowell (1927) confirmed the existence of this transformation, and later work (Grube and Schiedt 1930; Dehlinger 1930; Riederer 1937) showed that this transformation corresponded to the formation of a superlattice based on the composition MgCd, and that at low temperatures two other superlattices, based on the compositions MgCd3 and Mg3Cd, were also formed. These low temperature transformations were securely established, and all investigations indicated the existence of wide solid solutions in both magnesium and cadmium, but the remainder of the system has been in great dispute. According to Hume-Rothery and Rowell, magnesium could hold approximately 60 atomic % of cadmium in solid solution, and cadmium could dissolve approximately 24 atomic % of magnesium, whilst, between these two solid solutions, a compound MgCd2 of fixed composition was formed by a peritectic reaction. Grube and Schiedt (1930) qualitatively confirmed the limits of the primary solid solutions, but not the existence of the compound MgCd2, and, apart from the superlattice transformations, their equilibrium diagram showed the two primary solid solutions separated by a two-phase region. Dehlinger (1930) stated that his X-ray investigations confirmed the diagram of Grube and Schiedt, but a careful examination of his powder photographs shows that they are really inconclusive, since, with varying compositions, lines appear and disappear in a way which is consistent with the existence of MgCd2. Riederer (1937) concluded that the diagram of Hume-Rothery and Rowell represented stable equilibrium, and that of Grube and Schiedt metastable equilibrium. The present investigation has succeeded in explaining these apparent inconsistencies, and has also shown interesting relations between the two types of crystal structure, which may be interpreted in terms of the Brillouin zone theories.