EFFECT OF RADIATION ON CONTACT MELTING OF CRYSTALS

I. G. BERZINA, P. A. SAVINTSEV

At experimental temperatures close to the temperature of contact melting, the process of contact melting reduces to the mutual dissolution of crystals whose solid solutions melt at temperatures lower than the melting temperature of the individual components. This conclusion was reached at the Department of Physics of the Tomsk Polytechnic Institute as a result of investigations of the structure of the contact layer with increasing temperature. Study of the contact layer at low temperatures indicates the absence of any practically noticeable solubility of the crystals [1]. Table 1 gives the melting temperatures of the individual components of various systems and their contact-melting temperatures [2].

Table 1

Usually, alloys with a lowered melting temperature are obtained by fusing a mixture of crystals at temperatures not lower than the melting temperatures of the components, followed by crystallization. The method of contact melting makes it possible to obtain low-melting alloys without heating the components to their melting temperatures. Depending on the holding time at a given temperature, a liquid of known composition may flow out of the contact, or the crystals may fuse together (for a short contact time).

Contact melting has found practical application for joining metals with metals, metals with semiconductors, di-

dielectrics with dielectrics, etc. In chemistry the method of contact melting is used to detect chemical compounds of a system.

One of the important characteristics of the kinetics of contact melting is the rate of contact melting, determined by the relation

\[ v = k \frac{h}{\tau}, \]

where \(h\) is the height of the melting crystal,
\(\tau\) is the time of its complete melting,
\(k\) is a proportionality coefficient.

It has been established [3] that the rate of contact melting is related to temperature according to the law

\[ v = A \exp \left(-\frac{U}{RT}\right), \]

where \(U\) is called the activation energy of contact melting. This quantity depends on the nature and structure of the crystals in contact; \(A\) is a quantity depending on the geometrical characteristics of the crystal.

When radiation acts on metals, a change occurs in the structure of the crystals, which should lead to a change in the rate of contact melting. For this purpose experiments were undertaken on the influence of radiation on the process of contact melting.

We carried out experiments on the contact melting of a number of binary systems in which one of the components was subjected to irradiation. The experiments were arranged so that, simultaneously and under identical conditions, contact melting took place in irradiated and non-irradiated specimens of Sn, Bi, Cd, Pb, Zn, and In.

The contact pair consisted of an upper pellet made of one component (this pellet was subjected to irradiation) and a lower pellet or plate made of the other component. The lower specimen was considerably larger in size than the upper one, so that the liquid formed in the course of the experiment did not run off the surface of the lower specimen.

In the course of the experiment the time of complete melting of the upper specimen was determined, and the difference between the melting times of the irradiated and non-irradiated crystals (\(\tau\), \(\tau_0\), and \(\tau_n\), respectively) was calculated: \(\tau = \tau_n - \tau_0\).

The upper pellets were made from tubes of identical diameter drawn from molten metal. The specimens were carefully ground and selected to be equal in height and weight. The specimens thus prepared were then irradiated with X-rays, electrons and neutrons, and with \(\gamma\)-rays from a Ra–\(\alpha\)–Be source.

For exposure of the crystals to X-rays, the specimens were placed perpendicular to the flux of X-rays at a distance of about 5 mm from the window of a tube with a copper anode (URS-70 apparatus). Under the selected operating conditions the intensity of the X-rays was 130 roentgens/min. The irradiation time varied from 1 minute to 2.5 hours.

For exposure of the crystals to slow neutrons and \(\gamma\)-rays from a Ra–\(\alpha\)–Be source, the specimens were placed at a distance of 15 cm from the source in a paraffin block. The activity of the source was \(4 \cdot 10^{6}\) neutrons per second. The exposure time varied from 10 minutes to 300 hours.

Irradiation with electrons was carried out using an EM-3 electron microscope designed by Academician Lebedev. The microscope operated in the following regime: accelerating voltage—30 kV, high-voltage current—50 μA, gun filament current—55 μA. The specimens were placed on a plate in a photo cassette. The irradiation time was varied from 1 to 30 minutes.

The irradiated tablets, preliminarily heated to the experimental temperature in a glycerin bath, were brought in that same bath into contact with likewise polished specimens of the second component of the system. The lower specimens in the experiments were always unirradiated.

In the glycerin bath, next to the contacting pair—one specimen of which had been irradiated—the same pair, but consisting of unirradiated crystals, was melted simultaneously.

Fig. 1. Diagram of the experimental setup. 1—bath with glycerin; 2—electric furnace; 3—contact thermocouple.

Fig. 2. Dependence of the difference in the melting times of unirradiated and irradiated indium from a Ra-γ-source in a liquid tin–indium melt on temperature.

The diagram of the apparatus [4] on which the experiments on the contact melting of irradiated and unirradiated crystals were carried out is shown [[unclear: sentence continues on next page]].

Fig. 3. Dependence of the difference in the melting times of unirradiated and X-ray-irradiated bismuth in a liquid tin–bismuth melt on irradiation dose.

Fig. 4. Dependence of the difference in the melting times of unirradiated and electron-irradiated bismuth in a liquid tin–bismuth melt on the electron-irradiation time.

is shown in Fig. 1. The bath with glycerin (1) was placed in an electric furnace (2), in which a constant temperature is maintained by means of a thermostatic relay with a contact thermometer (3).

Figure 2 presents the dependence of the difference in melting times of unirradiated and irradiated indium in the tin–indium system on the experimental temperature. Irradiation time: 40 hours. The irradiation was carried out using a Ra–$\alpha$–Be source. As the experimental temperature approaches the temperature at which contact melting begins ($119^\circ$C), the difference $\Delta \tau$ increases sharply. The curves obtained for the systems tin–bismuth, tin–lead, tin–cadmium, cadmium–bismuth, cadmium–indium, zinc–indium, bismuth–indium, and bismuth–lead have the same character.

Figure 3 presents the dependence of the difference in melting times of unirradiated bismuth and bismuth irradiated with X rays for the tin–bismuth pair on the irradiation dose. Experimental temperature: $148^\circ$C. Temperature at which contact melting begins: $139^\circ$C. At an irradiation dose of 2000 r saturation sets in. Similar curves for the other systems listed above have the same character.

Figure 4 presents the dependence of $\Delta \tau$ on the time of irradiation of bismuth by electrons for the cadmium–bismuth pair. Experimental temperature: $156^\circ$C. Temperature at which contact melting begins: $144^\circ$C. The value of $\Delta \tau$ approaches a constant value for irradiation times of about $\sim 5$ min.

The experiments carried out showed that changing the type of irradiation does not alter the character of the dependence of the difference in melting times of unirradiated and irradiated crystals on the experimental temperature and on the irradiation dose. An increase in the number of radiation defects, depending on the radiation energy (Figs. 4, 5), causes only an increase in the difference between the melting times of unirradiated and irradiated crystals at a given temperature. Figure 5 shows the dependence of $\Delta \tau$ on the dose of X-ray irradiation of bismuth for the cadmium–bismuth pair. Experimental temperature: $156^\circ$C.

Fig. 5. Dependence of the difference in melting times of unirradiated bismuth and bismuth irradiated with X rays in a liquid bismuth–cadmium melt on the irradiation dose.

To understand the nature of radiation damage, it is essential to study the rate of contact melting in connection with annealing. When irradiated with neutrons and hard $\gamma$ quanta, various defects are formed in metals, including Frenkel defects. Annealing of defects caused by irradiation of metals has several stages depending on the annealing temperature [5]. Annealing of copper at various temperatures shows that recovery continues approximately up to $900^\circ$K [6]. Annealing of copper in the temperature interval $240$–$300^\circ$K is explained by vacancy migration [7]. However, there is the suggestion that annealing of defects in the temperature interval $240$–$300^\circ$K is associated only with migration of interstitial atoms, while annealing associated with vacancy migration is observed only at temperatures of $100$–$200^\circ$K [8].

The change in Young’s modulus of tin and bismuth [9] at small irradiation doses indicates that, even at temperatures of $300^\circ$K, pro-

there occurs a settling of point defects on the thresholds of dislocations, which leads to blocking of the latter. The pinning of dislocations leads to strengthening of the material. With an increase in the temperature of the experiment or of annealing, the dislocations can be released from the pinned centers that impede their motion. At these temperatures, an even stronger annihilation of Frenkel defects also occurs. Disturbances of other types are removed at higher temperatures.

Fig. 6. Dependence of the difference in the melting times of unirradiated and irradiated indium in a liquid tin–indium melt on annealing at room temperature, 26°C, and on the temperature of the experiment. 1—immediately after irradiation; 2—half an hour after irradiation; 3—four hours after irradiation; 4—7.5 hours after irradiation; 5—10 hours after irradiation.

Figure 6 shows the dependence of \(\Delta \tau\) on annealing at room temperature and on the temperature of the experiment. Indium of the tin–indium system was irradiated with slow neutrons and \(\gamma\)-rays from a Ra–\(\gamma\)–Be source. It is seen from the figure that, with time, there occurs

Table 2

hardening of the irradiated crystals, which can be explained by the settling of vacancies on dislocations with pinning of the latter, and then, as the temperature of the experiment is raised, softening with the manifestation of other types of defects, which apparently are annealed out at higher temperatures. Similar curves were also obtained for other systems: tin--cadmium, tin--indium, and tin--lead.

Table 2 gives the values of the activation energy of contact melting of irradiated and unirradiated crystals, calculated over a certain temperature interval for each pair of crystals immediately after irradiation. The time from the end of irradiation to the measurement temperature did not exceed 30 minutes, with the crystals kept at the temperature of liquid nitrogen.

From the foregoing it follows that:

1) irradiation of crystals by X-rays, neutrons, and gamma rays from a Ra--a--Be source, and by electrons, causes an increase in the rate of contact melting;

2) the greatest change in the rate of contact melting occurs at temperatures close to the temperatures at which contact melting begins;

3) a saturation phenomenon has been recorded in the dependence of the change in the rate of contact melting on the radiation dose;

4) the defects caused by radiation damage have a nonuniform structure and are annealed out in different temperature intervals;

5) the activation energy of contact melting of irradiated crystals immediately after irradiation is less than the activation energy of unirradiated crystals.

Literature

1. I. A. Savintsev, V. E. Averintseva, V. Ya. Zueko. Proceedings of the Academy of Sciences of the USSR, 127, No. 4, 28, 1959.

2. I. A. Savintsev, A. V. Vyatkina. Proceedings of Higher Educational Institutions. Physics, No. 4, 120, 1958.

3. I. A. Savintsev, A. V. Vyatkina. Proceedings of Higher Educational Institutions. Ferrous Metallurgy, No. 11, 1959.

4. A. V. Vyatkina. Proceedings of Higher Educational Institutions. Physics, No. 2, 1961.

5. Eggleston R. R. Acta Met., 1, 679, (1953).

6. McReynolds A. W. et al., Phys. Rev., 98, 418, (1954).

7. H. B. Huntington, Phys. Rev., 91, 1092, (1953).

8. Seeger. Report at the Geneva Conference, No. 998, 1958.

9. I. G. Berzina, A. A. Botaki, I. A. Savintsev. Proceedings of Higher Educational Institutions. Physics, 6, 39, 1961.