On Anisotropy in Contact Melting of Substances Forming a Ternary Eutectic
Abstract Generated abstract
This paper examines whether crystallographic anisotropy persists at high temperatures during contact melting in a ternary eutectic system. Using zinc single crystals contacted with a polycrystalline lead, cadmium binary eutectic, the authors compared melting behavior on the basal plane and on a plane perpendicular to it, with the contact melting temperature determined as 164.4 °C. Melting consistently began along the basal plane, indicating anisotropy in contact melting. The authors suggest that the denser atomic population of the zinc basal plane may facilitate encounters among zinc, cadmium, and lead atoms that form the ternary eutectic.
Full Text
ON ANISOTROPY IN CONTACT MELTING OF SUBSTANCES FORMING A TERNARY EUTECTIC
P. A. Savintsev and L. Ya. Kalachnikova
It is known that many physical properties depend substantially on the crystallographic direction in crystals of a noncubic system. For example, the thermal conductivity of Zn and Cd single crystals, which belong to the hexagonal system, is greater in the direction parallel to the principal crystallographic axis than in the direction perpendicular to it \[1\].
The electrical resistance in these same single crystals depends on the angle $\alpha$ formed by the given direction with the principal crystallographic axis; moreover, a linear dependence of the specific resistance on $\cos^2 \alpha$ is practically observed \[1\].
A. F. Ioffe established anisotropy of electrolytic conductivity in quartz crystals \[2\]. The presence of anisotropy in self-diffusion was found for the isotope of bismuth (ThC) in bismuth \[2\]; moreover, the diffusion coefficient in the direction perpendicular to the base is 60 times smaller than in the direction of the base.
A number of authors \[2, 3, 4\] investigated anisotropy in the diffusion of mercury into Zn and Cd single crystals. They all came to the conclusion that anisotropy is substantial and gave quantitative results, as well as an explanation of the data obtained.
The most plausible explanation \[4\] of this phenomenon is that, along different crystallographic directions, the height of the potential barrier for particles located in the crystal lattice, as well as for foreign particles, is different. This proposition is confirmed by the found values of the energies $U_b$ and $U_p$ (from the relation between the penetration depth of mercury into zinc and the diffusion coefficients) which a mercury atom must have in order for it to be able to move in the crystal lattice perpendicular to the prism of the first order. $U_b$ proved to be greater than $U_p$ by 22%.
In the works listed above, the phenomenon of anisotropy was observed mainly at low temperatures.
Some authors \[3\] believed that with increasing temperature anisotropy decreases and completely disappears at the melting point. Opposite results followed from the experiments of M. A. Bolshanina and F. P. Rybalko \[4\], which showed that anisotropy in diffusion persists up to the melting temperature.
It seemed interesting to verify the presence (or absence) of anisotropy at high temperatures in the phenomenon of contact melting of substances forming eutectics.
As is known \[5\], when two different contacting metals forming eutectics are heated, a liquid appears at a temperature somewhat above the eutectic temperature. In general, contact melting is observed for substances that give binary, ternary, and quaternary eutectics.
We carried out experiments with Zn, Pb, and Cd, which form a ternary eutectic; zinc was taken in the form of a single crystal, while Pb and Cd were taken in the form of a polycrystalline piece of the binary eutectic. In the Zn single crystal, the basal plane was exposed by cleavage, after which it was brought into contact with the polycrystal (Cd—Pb). With the aid of an ultrathermostat, the temperature of contact melting at this boundary was determined with an accuracy of $\pm 0.1^\circ$. It turned out that contact melting begins at a temperature of $164.4^\circ\mathrm{C}$. Next, in the following zinc specimens the basal plane and a plane perpendicular to it were exposed. This specimen was then wrapped in foil made of the binary eutectic (Pb—Cd) in such a way that simultaneous contact of it with the planes of the zinc single crystal that interested us was achieved. On heating to $164.4^\circ\mathrm{C}$, melting began each time along the basal plane. After some time the foil was drawn onto the basal plane, where melting was completed.
Thus, there is no doubt that anisotropy also occurs in contact melting.
The result obtained can apparently be explained by the fact that the basal plane is most densely populated by zinc atoms, which facilitates the encounter of dissimilar atoms Zn, Cd, Pb that form the ternary eutectic.
We express our deep gratitude to the supervisor of the work, Docent A. F. Gorodetskii, for valuable guidance and critical comments in carrying it out.
LITERATURE
- Encyclopedia of Metallophysics, ed. G. Mazing, 1937.
- V. E. Bugakov. Diffusion in Metals and Alloys, 1940.
- V. E. Bugakov and E. I. Bronshtein. ZhETF 5, 9, 1935.
- M. A. Bolshanina and F. P. Rybalko. ZhETF 7, issue 2, 1937.
- D. D. Saratovkin and P. A. Savintsev. DAN SSSR 33, 4, 1941; DAN SSSR 58, 9, 1947; Izvestia TPI 66, issue 3, 1948;