CHEMISTRY

I. I. Kornilov, K. I. Shakhova, P. B. Budberg,
N. A. Nedumov

PHASE DIAGRAM OF TiCr₂—NbCr₂

(Presented by Academician I. I. Chernyaev, January 8, 1963)

Metallic compounds are characterized by a high melting point, high hardness, and low electrical resistance; some of them are used as semiconductor and antifriction materials. More than 4000 metallic compounds are known, whereas the type of crystal lattice has been determined for approximately 800 binary compounds \(^{(1)}\). The conditions governing the interaction of binary metallic compounds were formulated by one of the authors as early as 1951 \(^{(2)}\). However, up to the present time only a very limited number of works are known on the experimental investigation of this question.

The compounds TiCr₂ and NbCr₂, by their crystal-chemical nature, belong to the Laves phases and possess a polymorphic transformation.

The low-temperature modification of the compounds has the MgCu₂ lattice \((C_{15})\), i.e., a complex cubic lattice with 24 atoms per unit cell. In the titanium—chromium system this modification exists up to \(1300^\circ\) \(^{(3)}\), and in the niobium—chromium system up to \(1590^\circ\) \(^{(4)}\).

The lattice parameter of TiCr₂ is 6.943 kX \(^{(5)}\), and that of NbCr₂ is 6.95 kX \(^{(6)}\).

The high-temperature modification of the compounds has the MgZn₂-type lattice \((C_{14})\), i.e., a hexagonal lattice with 12 atoms per unit cell. The lattice parameters of TiCr₂ are \(a = 4.921\) kX, \(c = 9.45\) kX \(^{(3)}\); for NbCr₂, \(a = 4.92\) kX, \(c = 8.11\) kX \(^{(4)}\).

The same lattice type, the small difference in parameters, and the atomic similarity of the elements forming them make it possible to assume the formation of a continuous series of solid solutions both between the high-temperature and between the low-temperature modifications of the compounds under study.

Table 1

Temperatures of phase transformations on heating, °C

To construct the TiCr₂—NbCr₂ phase diagram, the optical method for determining the temperatures at which melting begins, the method of high-temperature contactless thermal analysis \(^{(7)}\), and the method of X-ray phase analysis, carried out by the Debye method using unfiltered vanadium radiation, were used. The compositions of the alloys studied by the thermal-analysis method are given in Table 1. In the same table are presented

data on the temperatures of the beginning and end of melting of the alloys and on the temperatures of phase transformations. The values of the melting-onset temperatures obtained by the optical method and by contactless thermal analysis coincide practically completely. The fusibility diagram constructed from these data has the form characteristic of systems with a continuous series of solid solutions.

Fig. 1. Phase diagram of the TiCr₂—NbCr₂ system: 1—high-temperature thermal analysis, 2—optical method, 3—X-ray phase analysis

The results of thermal analysis confirm the existence of polymorphic transformations in the compounds TiCr₂ and NbCr₂. The temperature of the polymorphic transformation in the compound TiCr₂ was determined to be 1220 ± 10°, and in the compound NbCr₂, 1585 ± 10°. It also follows from the results of thermal analysis that all the alloys studied have a polymorphic transformation, the temperatures of the beginning and end of which increase monotonically from the TiCr₂ compound to the NbCr₂ compound. This indicates that the compounds TiCr₂ and NbCr₂ form a continuous series of solid solutions with one another over the entire concentration range, not only between the high-temperature modifications but also between the low-temperature modifications.

Fig. 2. a—line diagram of TiCr₂ after quenching from 1300°; b—the same for an alloy of 90% TiCr₂ + 10% NbCr₂; c—the same for an alloy of 20% NbCr₂ + 80% TiCr₂

The phase diagram of the system constructed from the thermal-analysis data is shown in Fig. 1. The solid solution based on the high-temperature modification is denoted by the δ phase, and that based on the low-temperature modification by the γ phase. The TiCr₂ compound in the binary titanium—chromium system is formed from a solid solution with a body-centered cubic lattice (β phase). This explains the appearance in the phase diagram, Fig. 1, of the single-phase β region, the two-phase regions β + δ and β + Ж, and also the three-phase region Ж + β + δ. On the NbCr₂ side, no additional phase regions are observed in the system studied, owing to the congruent formation of the NbCr₂ compound in the niobium—chromium system.

Table 2

* Lattice period according to data [5, 6].

In addition to thermal analysis, X-ray phase analysis was carried out on alloys after quenching from 1300° and prolonged annealing at 1000°. The compositions of the alloys studied and their phase constitution according to X-ray phase analysis are given in Table 2. X-ray phase analysis of alloys quenched from 1300° shows that at this temperature the compound TiCr₂ exists in the form of a hexagonal modification, Fig. 2a. The alloy,

containing 90% TiCr₂ + 10% NbCr₂, has, on the X-ray pattern, two systems of lines corresponding to the hexagonal and cubic modifications of the solid solutions, Fig. 2б. Alloys with a higher NbCr₂ content, including the compound NbCr₂ itself, are characterized only by the lines of the cubic modification of the solid solutions (Fig. 2в).

In the X-ray patterns of alloys annealed at 1000° for 200 h, there is only one system of lines, corresponding to the cubic modification (Fig. 3). The results of the X-ray analysis unambiguously indicate the formation of a continuous series of solid solutions between the cubic modifications of the compounds.

Fig. 3. Line diagrams of alloys of the TiCr₂—NbCr₂ system after annealing at 1000°:
а — TiCr₂, б — alloy 20% NbCr₂ + 80% TiCr₂, в — alloy 80% NbCr₂ + 20% TiCr₂, г — NbCr₂

Fig. 4. Change in the lattice parameter of alloys of the TiCr₂—NbCr₂ system after annealing at 1000°

The calculation of the lattice parameter of the γ-phase showed that a negative deviation from Vegard’s law is observed (Table 2). The character of the change in the lattice parameter shown in Fig. 4 is typical of systems with a continuous series of solid solutions of the components.

Considering the TiCr₂—NbCr₂ system as a section of the ternary titanium—niobium—chromium system, it may be asserted that this section is quasibinary up to the temperature at which the compound decomposes into a solid solution of titanium and chromium, i.e., up to 1340°.

Institute of Metallurgy
named after A. A. Baikov

Received
19 XII 1962

CITED LITERATURE

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5. P. Duwez, Trans. ASM, 44, 495 (1952).
6. В. П. Елютин, В. Ф. Функе, Изв. АН СССР, ОТН, No. 3, 321 (1956).
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