G. V. SAMSONOV

PHASES OF THE TUNGSTEN—BORON SYSTEM

(Presented by Academician A. N. Frumkin, January 2, 1957)

CHEMISTRY

In the x-ray investigation of the tungsten—boron system, three borides were initially found—tetragonal W₂B and WB, and also hexagonal W₂B₅ (¹). Later studies established the existence of a high-temperature modification of tungsten monoboride, β-WB (², ³). According to (¹), W₂B (γ-phase) has very narrow homogeneity limits, WB (δ-phase) has broader limits from 48 to 51 at. % boron (WB₀.₉₂–₁.₀₂), and W₂B₅ (ε-phase), in its usual defective state, is homogeneous within the range from 66.7 to 68.0 at. % boron (WB₂.₀₀–₂.₁₂). The phase WB₂ established earlier in (⁴) is evidently identical with W₂B₅ with a deficiency of boron atoms.

In order to refine the phase regions of the tungsten—boron diagram, we* carried out a study of alloys of this system by methods of microhardness, x-ray, and metallographic analyses. For the preparation of alloys, tungsten powder with a content of 99.93% W (96% of grains < 3 μ) and a preparation with the nominal formula WB₄.₃₇₅, prepared by the vacuum-thermal method (⁵) in attempts to obtain WBₓ alloys with x > 5/2, were used. The sum of the boron and carbon contents in this preparation is close to 100%. Mixtures W + WB₄.₃₇₅ were compounded with the calculation of preparing samples with the nominal formulas WBₓ (where x = 0.01; 0.02; 0.05; 0.1; 0.2; 0.4; 0.8; 1.0; 1.5; 2.0; 2.5; 3.0) and were sintered by hot pressing followed by prolonged annealing at 1900° and cooling from this temperature over ~10 h. After determination of the apparent specific gravities and hardness and study of the microstructure, the sintered samples were ground into powder and subjected to chemical and phase x-ray analysis. The results obtained are given in Table 1.

The chemical composition of the alloys (see Table 1) practically did not change during sintering, with the exception of samples charged for WB₃ and the sample WB₄.₃₇₅, in which the boron content decreased somewhat. The samples obtained upon sintering are sufficiently dense, and after annealing the density of most of them practically does not change or increases slightly; the exceptions are samples WB₀.₅, WB₀.₈, WB₁.₀, WB₂.₅, WB₃, and WB₄.₃₇₅, upon annealing of which the density decreases rather sharply. This can be explained in accordance with the indication of Raub and Plate (⁶), according to whom the effect of spreading forces (“crystallization pressure”) in the formation of intermetallic compounds is caused by the individual separation of newly forming particles.

Investigation of the microstructure of the samples showed that, already beginning with a boron content of 0.9 at. %, two phases are found on polished sections: one of them, light (“white phase”), occupies the main area of the polished section; the second, darker (“gray phase”), is located along the boundaries of the polyhedra of the white phase. With an increase in boron content, the amount of gray phase increases, and

* S. D. Krasenkova took part in the work.

Table 1

Properties of alloys of the W—B system

At a content of 16.69 at. % B in the alloy, its grains occupy up to 40% of the area of the polished section. The white phase, which apparently represents a solid solution of boron in tungsten, has a hardness of 550–770 kg/mm², practically unchanged with the composition of the alloy up to a content of 28.06 at. % B. The hardness of the gray phase, the boride (W_2B), is (2420 \pm 120) kg/mm². With a further increase in the boron content in the alloys from 33.3 to 50 at. %, the WB phase appears; its hardness rises within these limits of boron concentration from (\sim 280) to (\sim 3700) kg/mm².

Increasing the boron content to 60 at. % and higher causes the appearance of a phase colored dark gray, with a hardness that increases regularly from 1764 kg/mm² (at 66.7 at. % B) to 2599 kg/mm² (at 81.4 at. % B in the alloy). The maximum hardness of this phase agrees well with the hardness of (W_2B_5) (⁷). Apparently, this phase has a wide homogeneity range (from 66.7 to 75–77 at. % B), since the (WB_2) specimen is single-phase, while a second phase, lighter in color, appears only in the specimen with a content of 75.0 at. % B (15.24 wt. %). This latter has a hardness of 3600 kg/mm², which is close to the hardness of boron (⁸).

The data of the metallographic investigation are well confirmed by the results of X-ray analysis. The lattice period of the tungsten powder used in the work was 3.149 Å, which agrees rather closely with the latest literature data (⁹). Upon adding 0.9 at. % B to tungsten, the period decreases somewhat (to 3.133 Å), after which it remains practically unchanged up to a boron content of 28.6 at. %. All these alloys are two-phase—the second phase is the boride (W_2B) with periods (a = 5.564) Å and (c = 4.739) Å, which exactly agrees with the data of Kiessling (¹). The alloys (WB_{0,8}) and (WB_{1,0}) are single-phase, if one does not count the presence, in the X-ray pattern of (WB_{0,8}), of separate very weak lines of (W_2B), and represent the boride WB with periods increasing, for example along the (a) axis, from 3.124 to 3.131 Å. This boride is also observed in the alloys (WB_{1,5}) and (WB_2), but in the form

a small number of weak lines on the X-ray patterns. Beginning with the composition (\mathrm{WB}_{1.5}), the principal phase is the boride (\mathrm{W}_2\mathrm{B}_5) with a maximum period (a = 2.983) Å (according to Kiessling, 2.982 Å). In the specimen with 75.0 at. % B, in addition to the (\mathrm{W}_2\mathrm{B}_5) lines, boron lines are observed, identified by comparing interplanar spacings with the use of data ({}^{(10)}).

On the basis of the data obtained, the following phase regions of the tungsten—boron system (from 1 to 70–80 at. % B) may be outlined:

1) the (\alpha)-region of a very limited solid solution of boron in (\alpha)-W, formed with a decrease in the lattice period and a corresponding distortion, leading to an increase in hardness from 340 to 550–770 kg/mm²;

2) a two-phase region (\alpha + \gamma), the (\gamma)-phase ((\mathrm{W}_2\mathrm{B})) having a very narrow homogeneity range and a hardness of 2420 kg/mm²;

3) a two-phase region (\gamma + \delta) ((\mathrm{W}_2\mathrm{B} + \mathrm{WB}));

4) a homogeneity region of (\mathrm{WB}) (the (\delta)-phase), extending at least from 44.4 to 50–55 at. % B, with hardness varying within these limits from 2080 to 3752 kg/mm²;

5) a two-phase region (\delta + \varepsilon) ((\mathrm{WB} + \mathrm{W}_2\mathrm{B}_5));

6) a homogeneity region of (\mathrm{W}_2\mathrm{B}_5), probably fairly broad—from 68 to 75 at. % B—which does not agree with Kiessling’s earlier data ({}^{(1)}) on the absence, or only a very narrow range, of homogeneity of (\mathrm{W}_2\mathrm{B}_5). Evidently, a phase containing less than 71.4 at. % B is formed by the type of subtraction of boron atoms, and one with more than 71.4% by the type of interstitial incorporation.

The comparatively low hardness of the (\gamma)-phase ((\mathrm{W}_2\mathrm{B})) is associated with the isolated position of the boron atoms in the unit cell, whereas the (\delta)-phase ((\mathrm{WB})), with boron atoms linked into zigzag chains, has the maximum hardness. The hardness of the (\varepsilon)-phase ((\mathrm{W}_2\mathrm{B}_5)) is somewhat lower than the hardness of the (\delta)-phase, probably as a result of the formation of boron-atom layers that make the alloy more prone to shear deformation, especially since the (\varepsilon)-phase may be regarded as having a simple hexagonal lattice with boron atoms in its interstices.

In addition, the (\mathrm{W}_2\mathrm{B}_5) phase does not possess a high degree of ordering, which is manifested, in particular, in its ability to form wide ranges of solid solutions with other borides as the solvent ({}^{(11)}).

Moscow Institute of Nonferrous Metals
and Gold named after M. I. Kalinin

Received
29 X 1955

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