APPLICATION OF A COUNTER FOR THE STUDY OF SURFACE LAYERS OF METALS

It is known that the number of dark discharges of a counter (the so-called background) depends on the choice of the cathode metal and on the state of its surface. The author of the work under review*) made use of this dependence to study problems of metallography. This idea is not

) I. Kramer, Z. Physik 125*, No. 11–12 (1949).

generally speaking, new. As early as 1938, the Soviet scientists Pavlova and Shal’nikov (whose work, incidentally, the author of the paper under review does not mention) investigated the dependence of the counter background on the cathode temperature.* On the basis of these investigations, the aforementioned Soviet scientists were able to draw a number of conclusions concerning the structure of the surface layer of an aluminum cathode. As for the paper under review, it gives no physical interpretation of the results obtained. The interest of the work lies in the fact that it points specifically to a whole series of problems in metallography that can be studied with the aid of a counter.

In the paper under review the author, like Pavlova and Shal’nikov in their time, used a counter with a heated cathode. Heating of the flat cathode of the end-window counter was carried out by a filament, and the cathode temperature was measured with a thermoelement. The counter pulses were fed to an electrometer or a loudspeaker and were recorded visually or by ear.

With the aid of a counter one can investigate the establishment of equilibrium in the surface layer of a metal when this equilibrium has been disturbed as a result of mechanical treatment of that surface. Using a freshly processed metal plate as the counter cathode, one can observe the restoration of equilibrium in the surface layer: the number of pulses per unit time, which at first, when equilibrium is disturbed, is very large, decreases as equilibrium is restored. The character of the curves obtained depends to a considerable extent on the kind of metal being studied, its temperature, and the method of treating its surface.

By investigating the dependence of the number of pulses per unit time on the cathode temperature, one can determine the critical temperature of the amorphous phase of the metal under study much more easily and reliably than before.

If the cathode surface is carefully polished or treated by another appropriate method, then, as it is heated, the number of pulses per unit time suddenly begins to increase upon reaching a quite definite temperature, which coincides with the critical temperature of the amorphous phase of the thin layer of the metal under study.

This circumstance is, in the opinion of the author of the paper under review, evidence for the supposition that treatment of the surface layer of a metal leads to the formation of an amorphous phase in that layer. Investigations with the counter have shown, however, that not every treatment of a metallic surface leads to such a result. All the curves obtained were divided into two groups. Curves of the first group indicated a constant increase in the number of pulses per unit time with increasing temperature. Curves of the second group were characterized by the fact that up to the critical temperature the number of pulses per unit time remained insignificant and constant, while beyond the critical temperature it began to increase abruptly. The surface layer of metals belonging to this group has, below the critical temperature, a stable amorphous structure. Curves of the first group indicate the presence in the surface layer of both amorphous and crystalline phases, the equilibrium between them being unstable at any temperature. Since the fraction corresponding to the amorphous phase may be different depending on the method of treating the surface layer, the form and the position of the curves are also very different. In the figure is shown—

* Pavlova and Shal’nikov, ZhETF 8, 183 (1938).

for example, the curves obtained with chromium plates, in the surface layer of which the ratio between the phases is different. Curve 1 was obtained with a plate whose surface had been treated with emery; curve 2 was obtained with a polished plate; curve 3 corresponds to a layer of the pure amorphous phase obtained by electrolysis. On the last curve the critical temperature of the amorphous phase (between 200 and 250° C) was sharply marked.

In an analogous manner the counter can be used to study the transition of a metal from one crystalline modification to another.

The investigations carried out led the author of the paper under review to the conviction that the counter can be successfully applied in the study of all those metallographic processes in surface layers which proceed exothermically.

Along with the questions indicated, the paper under review attempts to use the counter to study the behavior of a metal under static and dynamic loads.

V. Bredel