Seizo Nagasaki1, Makoto Hirabayashi1, Hideo Nagasu2
The Mg-Cd system has a close-packed hexagonal lattice and possesses ordered structures for concentrations corresponding to Mg3Cd, MgCd and MgCd3. In general the close-packed hexagonal lattice (let the lattice constants of it be denoted by a and c) can be devided into four large hexagonal sublattices (lattice constants of them now become a'=2a and c=c) (Fig. 2). For ordered MgCd, two of these sublattices are occupied by Mg while the other two by Cd; and for the ordered Mg3Cd three sublattices are occupied by Mg and the remaining one is occupied by Cd, for the ordered MgCd3 the relation is just the reverse. We have measured through the order-disorder transformation of these alloys temperature dependencies of several physical properties, such as specific heat (Cp), expansion coefficient (β) and electrical resistivity (ρ). (Figs. 5-13) For Mg3Cd alloy Cp-vs-temperature curve shows anomalous behavior similar to β-brass, while the, presence of latent heats has been confirmed for MgCd and MgCd3 alloys. For Mg3Cd and MgCd alloys, β-vs-temperature curves indicate the close parallelism to their Cp-vs-temperature curves. On the other hand, for MgCd3 alloy, remarkable contraction is associated with its disordering, therefore, coefficient of thermal expansion tend to negative infinity at the Curie point, the ρ-vs-temperature curves for these alloys have characteristic from of the order-disorder transformation.
During the annealing of the previously quenched Mg3Cd and MgCd alloys, their hardness decrease with time (Fig. 14). It is interesting to note that these highly ordered supperlattices possess hardness smaller than the random alloys of the same compositions, contrary to the Cu-Au, Fe-Ni and other cases. The results of these experiments are summarized in Table 1.
(Received 1948/11/19 Published 1949/06/20)
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