Chemistry

K. I. PORTNOI and G. V. SAMSONOV*

PROPERTIES OF TERNARY ALLOYS OF TITANIUM, CHROMIUM, AND ZIRCONIUM DIBORIDES

(Presented by Academician A. A. Bochvar, 18 VI 1957)

Borides of refractory rare metals, distinguished by high hardness, wear resistance, and resistance to oxidation in various aggressive chemical media, are of substantial interest for modern technology involving high mechanical loads at elevated temperatures. Of particular importance is the study of the properties of boride alloys in systems in which, as in systems of other metal-like compounds (carbides, nitrides), extrema of properties—hardness, electrical resistance, etc.—may be expected. By the present time the results of studies of a number of binary diboride systems have been published: TiB₂—ZrB₂ (¹,²), TiB₂—CrB₂, ZrB₂—CrB₂ (³), and others.

However, data on the properties of ternary boride alloys are almost entirely lacking, including those of such technically most promising borides as those of titanium, zirconium, and chromium. In this connection, the aim of the present work was to clarify certain properties of alloys of titanium, zirconium, and chromium diborides located on the radial section TiB₂—CrB₂ (50 : 50 mol. %)—ZrB₂.

According to (¹–³), it is known that the borides of titanium and chromium, as well as of titanium and zirconium, form continuous series of solid solutions, while the borides of chromium and zirconium dissolve in one another (at 2000°) only to a limited extent: in CrB₂ up to 20 mol. % ZrB₂ and in ZrB₂ up to 5 mol. % CrB₂. It was therefore of interest simultaneously to follow how the solubility of CrB₂ changes in the solid solution TiB₂—CrB₂ in comparison with its limited solubility in CrB₂ and continuous solubility in TiB₂, and vice versa.

For the preparation of alloy specimens we used the method of sintering by hot pressing, which makes it possible to obtain fairly dense compact specimens from nonplastic, brittle compounds of the boride type. Sintering was carried out in graphite press molds; the investigation of sintering conditions was conducted within temperature limits from 2100 to 2550°, pressures from 60 to 330 kG/cm², and holding times from 0.5 to 5 min.

The presence of a minimum at 40–50 mol. % ZrB₂ in the alloys was established on the curve of optimal sintering temperatures; this indicates the absence of a continuous series of solid solutions in the system and the presence of a more complex type of phase diagram.

To relieve internal stresses after sintering by hot pressing, and also to fix the phase state of the alloys, the latter were subjected to annealing in vacuum at temperatures of 1900, 1500, and 1100° with subsequent

* K. I. Frolova and G. V. Moskalik took part in the experimental portion of the work.

with rapid cooling to room temperature. The annealed samples were studied by X-ray, metallographic, and microdurometric methods.

Photographs of microsections of a series of alloys, annealed and quenched from 1900°, are shown in Fig. 1. Up to 40–50% ZrB₂ the alloys are single-phase; beginning with 50–60% ZrB₂ a second phase begins to appear, which is visible on the microsection of the alloy with 60 mol.% ZrB₂ in the form of rather fine constituents, and on the microsections of the alloy with 80% ZrB₂—in the form of large inclusions.

Measurement of the microhardness of the phases shows the presence of a maximum microhardness of 3900 kg/mm² at 20 mol.% ZrB₂, after which there follows an approximately linear course of the microhardness of the principal phase (2800 kg/mm²). The hardness of the second phase can be measured with sufficient reliability only beginning with 70 mol.% ZrB₂ in the alloys; it increases from 1900 for ZrB₂ to 2100 kg/mm² for solutions in it of the double boride (Ti, Cr)B₂.

Fig. 2. Change in lattice periods of (Ti, Cr)B₂–ZrB₂ alloys as a function of composition

X-ray investigation of the alloys, performed by the Debye method in a normal camera of diameter 57.4 mm, showed (Fig. 2) an increase in the lattice period of the phase based on (Ti, Cr)B₂ with increasing ZrB₂ content in alloys up to 40 mol.%, after which the lattice period of the solid solution of ZrB₂ in (Ti, Cr)B₂ remains practically constant and, beginning with 60 mol.% ZrB₂, zirconium boride—or, more precisely, a solid solution of (Ti, Cr)B₂ in ZrB₂ with a lattice period somewhat smaller than that of pure ZrB₂—is observed on the X-ray diffraction patterns. Determination of the electrical resistivity of the alloys showed the presence of a low maximum at 10 mol.% ZrB₂ in the alloy with (Ti, Cr)B₂. The specific electrical resistivity of ZrB₂ and (Ti, Cr)B₂ is, respectively, 10.8 and 18.8 µΩ/cm.

Fig. 3. Microbrittleness of the phases TiB₂, CrB₂, and the solid solution (Ti, Cr)B₂

Thus, it may be assumed that at 1900° the solubility of ZrB₂ in (Ti, Cr)B₂ is of the order of 40 mol.% and the solubility of (Ti, Cr)B₂ in ZrB₂ is less than 10 mol.%; moreover, since the atomic radius of zirconium is larger than the atomic radii of titanium and chromium, the increase in the period upon formation of the first of these solutions indicates dissolution by substitution, and of the second—by interstitial dissolution.

The limited solubility of ZrB₂ in the double boride (Ti, Cr)B₂ is natural, but nevertheless it rises to 40 mol.%, which may be explained by the influence of TiB₂, which forms a continuous series of solid solutions with ZrB₂.

The alloys studied are of interest in that one of the components (ZrB₂) has in the other, (Ti, Cr)B₂, a solubility that changes little with temperature over a wide interval from 1100 to 1900°.

Consequently, in accordance with the principles of designing heat-resistant alloys (*), one may expect, for an alloy with a ZrB₂ content somewhat greater than 40%, the precipitation of excess zirconium boride in the form of a finely dispersed constituent along grain boundaries, preserved at high temperatures and providing high resistance of the entire alloy to creep. At the same time, the main component of the alloy—the solid solution (Ti, Cr)B₂—is formed according to the principle of alloying the more corrosion-resistant, but also more brittle, titanium boride with the less brittle chromium boride (⁵, ⁶), as is shown by comparison of the microbrittleness of these three phases (Fig. 3).

Conclusions

The solubility of ( \mathrm{ZrB_2} ) in ( (\mathrm{Ti,Cr})\mathrm{B_2} ) is about 40 mol.%, while that of ( (\mathrm{Ti,Cr})\mathrm{B_2} ) in ( \mathrm{ZrB_2} ) is less than 10 mol.%. The solubility range of ( \mathrm{ZrB_2} ) in ( \mathrm{CrB_2} ) increases when ( \mathrm{ZrB_2} ) is dissolved in the binary boride ( (\mathrm{Ti,Cr})\mathrm{B_2} ), one of whose components, ( \mathrm{TiB_2} ), forms a continuous series of solid solutions with zirconium boride.

In the single-phase region of the solid solution of ( \mathrm{ZrB_2} ) in ( (\mathrm{Ti,Cr})\mathrm{B_2} ), in specimens quenched from (1900^\circ), there is a maximum in microhardness of (3900\ \mathrm{kg/mm^2}) (at 20 mol.% ( \mathrm{ZrB_2} )) and in electrical resistivity ((216\ \mu\Omega/\mathrm{cm}) at 10 mol.% ( \mathrm{ZrB_2} )).

All-Union Scientific Research Institute
of Aviation Materials

Received
7 VI 1957

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