On the Metallic State of Boron Atoms in Steel

N. D. Tyutëva, A. I. Fal’kov

(Presented by Professor Doctor A. N. Dobrovidov)

The hypothesis of the metallic state of boron atoms dissolved in metals of the transition groups has been proposed in a number of works \[1, 2, 3\].

In the present work, the assumption of the metallic state of boron atoms is confirmed experimentally.

The experiment was as follows: through specimens with a cross section of \(0.15 \times 2\) mm and a length of 20 mm, placed between two electrodes in an atmosphere of inert gas, direct and alternating current was passed. As the electrodes directly adjoining the ends of the specimens under study, Armco iron was used. The current density in all experiments was \(2300\text{--}2700\ \mathrm{A}/\mathrm{cm}^2\).

The test specimens were made from an alloy of Armco iron with 1.5% boron and from Armco iron alone. Specimens made from the iron alloy with 1.5% boron were heated by direct and alternating current to \(1150\text{--}1250^\circ\) with a holding time of 20 to 35 hours. The microstructure of the cast ferroalloy from which the specimens were made is shown in Fig. 1. In the structure, along with ferrite grains, characteristic regions of a eutectic are visible, consisting of boride and the surrounding dark-etching constituent.

A specimen made from Armco iron was heated only by direct current under the same regime. All specimens of the Armco-iron alloy with boron were tested in the cast state. The microstructure of the alloy specimen with 1.5% boron after heating by alternating current differs in no way from the microstructure of a specimen with the same boron content annealed in a furnace at \(1200^\circ\) and cooled in air.

Evidently, under the action of alternating current no transfer of boron occurs in the alloy.

In this case the structure undergoes no changes other than those that take place after the ordinary annealing of the cast alloy.

The microstructure of a specimen made from Armco iron, after a 20-hour holding under direct current, does not change, if one disregards insignificant changes in grain size. This experiment showed that we were dealing with iron sufficiently pure of impurities, and that the carbon usually present in small quantities in Armco iron did not give rise to the phenomena that were obtained in the experiment with the iron–boron alloy.

A specimen of the iron–boron alloy, heated by direct current, showed an entirely different result. The macrostructure (Fig. 2) shows a sharp difference in the structure of the individual parts of the specimen.

Fig. 1

Fig. 2

Fig. 3

Fig. 4

Fig. 5

Fig. 6

Fig. 7

A line of separation between two zones was clearly defined (Fig. 3). In the zone adjacent to the cathode, an increase in the boron concentration is noticeable, accompanied by strong grain refinement and the formation around each of the grains of eutectic regions and of a dark-etching constituent, similar to that which surrounds borides in the eutectic regions of the cast ferroalloy (Fig. 4). In the direction toward the anode, immediately beyond the dividing line there follows a large ferrite grain—up to 2 mm; then the ferrite becomes more fine-grained. The middle part of the specimen consists of ferrite grains free from the presence of borides and the dark-etching constituent (Fig. 5).

At the positive pole of the specimen, at the boundaries and in the centers of the grains, there are predominantly coagulated borides without solid solution—the dark-etching constituent. Only in isolated places did regions of eutectic structure remain (Fig. 6).

It must be remembered that the transfer of boron, like that of carbon, in a constant electric field can take place only when they are in solid solution. Boron atoms entering into a chemical compound—borides—are not transported directly under the action of a direct current.

In the cast ferroalloy, before it was heated by direct current, there were eutectic regions—borides surrounded by a dark-etching constituent (presumably a solid solution of boron in $\alpha$-iron). The disappearance of the eutectic and of its constituents in the middle part of the specimen can be explained only by the transfer of boron under the action of direct current in the direction of the cathode. At the same time, the experiment showed that boron was transferred not only to the end part of the specimen under test, but also to the current-supplying plate of Armco iron. The microstructure of the plate is given in Fig. 7 (a strongly refined ferrite grain surrounded by a dark-etching constituent; borides are absent).

The experimental data obtained make it possible to represent the process of boron transfer under the action of direct current as follows: positively ionized boron atoms of the solid solution are transported under the influence of the electric field toward the cathode. Since the heating temperature reaches 1150–1250°, the boride, owing to diffusion, dissolves in the ferrite, replenishing it with new positively ionized boron atoms. The boron ions newly formed through the dissolution of borides, as long as the electric field exists, are continuously transported to the cathode. The process of electrotransfer of boron will continue until the eutectic regions disappear completely, as was observed in our experiment.

Such a mechanism of boron transfer is observed along the entire length of the specimen, including the anode zone. However, the temperature in the anode zone was considerably lower (750–800°) than in the middle part of the specimen, and therefore the borides did not dissolve completely. The structure in the anode zone is shown in Fig. 6.

In this case, the process of transfer of ionized boron atoms proceeded with a considerable lead in comparison with the diffusion process of dissolution of borides; therefore, in the anode zone of the specimen the borides remained isolated, i.e., without the surrounding solid solution. The microhardness of the dark-etching constituent of the eutectic of the cast ferroalloy is $H_v = 306\ \mathrm{kg/mm^2}$.

The microhardness of the dark-etching constituent of the eutectic at the cathode of the specimen and on the current-supplying plates is $H_v = 284$–$300\ \mathrm{kg/mm^2}$.

Thus, the microhardness data and the nature of the etchability indicate that we are dealing with one and the same structural constituent—

...but in one case it was obtained during crystallization of a cast ferroalloy, and in the other as the result of the transfer of boron under the influence of a direct current.

The arcuate form of the boundary line between the zones (Fig. 2) is noteworthy. This form resulted from the different temperature of the specimen at the edges and in the middle part. On this basis it may be asserted that the mobility of boron during electrotransfer increases sharply with increasing temperature.

Conclusions

1. The present work has experimentally confirmed that boron in iron is in the metallic state, is subject to electrotransfer, i.e., manifests itself as an electropositive element.

2. The mobility of boron ions in the investigated temperature interval of 1150–1250° during electrotransfer increases sharply with increasing temperature.

3. The process of electrotransfer of positively ionized boron atoms under the action of a constant electric field proceeds more rapidly in comparison with the process of diffusional dissolution of ferroalloy borides.

4. The effect of electrotransfer and positive ionization of boron gives grounds to assert that boron forms solid solutions in alloys.

5. The phenomenon of electrotransfer of boron may be used for the deboronizing of alloys in which the presence of boron is undesirable.

6. The use of pure iron as a “receiver” of ionized boron atoms during electrotransfer, with the application of microstructural and accurate quantitative analyses, will make it possible to determine the solubility of boron in $\alpha$- and $\gamma$-iron and the diffusion rate.

References

1. Lebedev T. A. New Aspects in the Theory of Iron–Carbon Alloys, Lenizdat, 1939.

2. Samsonov G. V. and Tseitina N. I. Physics of Metals and Metallography, issue 2, 303, 1955.

3. Frantsevich A. N. and Kalinovich M. K. Problems of Powder Metallurgy and the Strength of Metals, issue 3, 1956.