Chemistry

I. I. Kornilov and T. T. Nartova

Continuous Solid Solutions of the Metallides Ti₃Al–Ti₃Sn in the Ti–Al–Sn System

(Presented by Academician I. I. Chernyaev, 20 IV 1961)

The study of chemical interaction between metallic compounds is of considerable interest. Solid solutions based on these compounds have been called metallide solutions. In work (¹), the principal considerations determining the formation of continuous solid solutions between compounds with metallic bonding were examined theoretically. These investigations were developed in (¹–⁷). In the literature (⁸–¹²) there are indications of the possibility of formation, in the Ti–Al system, from α-solid solutions of titanium, of the compounds Ti₃Al and Ti₆Al. These assumptions are based on data obtained in studying various properties of the binary Ti–Al system. According to work (¹⁰), an x-ray investigation of alloys established the presence of an ordered phase of composition Ti₃Al. In work (¹²), in a study of alloys by the galvanomagnetic method, the compositions of the compounds Ti₆Al and Ti₃Al were established from the inflection points on the composition—Hall-effect diagram. Thus it may be considered that one of these compounds, Ti₃Al, is formed in the Ti–Al system from α-solid solutions of titanium with an ordered structure. The lattice of this compound is hexagonal and is isomorphous with the structure of the compound Ti₃Sn (45.24 wt. % Sn) (see (¹³)). It melts with an open maximum; its melting temperature is 1663° (¹⁴), and it has a hexagonal close-packed lattice. In an investigation of the equilibrium diagram of the ternary titanium–aluminum–tin system (¹⁵), carried out by microstructural and x-ray methods, the formation of a considerable region of ternary solid solutions based on α-titanium and the compound Ti₃Sn, having a hexagonal lattice, was established.

In the present work, the phase diagram and properties of alloys of the ternary Ti–Al–Sn system corresponding to compositions of alloys located on the Ti₃Al–Ti₃Sn section were studied. The alloys were investigated by methods of thermal, microstructural, and x-ray analyses and by measurements of electrical resistivity and hardness.

For preparation of the alloys, ductile titanium of grade TG-00 (with a tensile strength of ~38 kg/mm²), aluminum, and tin of high purity were used. The alloys were prepared by melting in an arc furnace with a nonconsumable tungsten electrode in an argon atmosphere. To study crystallization and phase transformations in the solid state of alloys of the Ti₃Al–Ti₃Sn section, the method of contactless thermal analysis on the apparatus of N. A. Nedumov (¹⁶) was used, by recording heating and cooling curves. The thermographing rate was 23–25°/min. The investigation was carried out on specimens annealed according to the regime given below.

Analysis of the thermograms made it possible to construct the liquidus and solidus lines of alloys of the Ti₃Al–Ti₃Sn section, and also to indicate the transformation points in the solid state for alloys having polymorphic or other types of transformation. According to thermal-analysis data, the constructed melting diagram shows that the alloys crystallize according to the eutectic type with mutually limited solid solutions. The liquidus and solidus temperatures decrease both from the side of crystallization of the β-solid solution of the Ti–Al system and from the side of Ti₃Sn. In the interval 1590–1615°, alloys containing 40–80% Ti₃Sn (~16–36% Sn) crystallize as eutectic mixtures consisting of two phases: a β-solid solution based on

of the Ti—Al system and a ternary $\delta$-solid solution based on the compound $\mathrm{Ti}_3\mathrm{Sn}$. The eutectic point corresponds to 45% $\mathrm{Ti}_3\mathrm{Sn}$ ($\sim 20$ wt.% Sn).

Most of the alloys studied undergo a transformation in the solid state caused by the polymorphism of titanium. For an alloy of composition $\mathrm{Ti}_3\mathrm{Al}$, the onset of the transition from the $\alpha(\delta)$ region to the $\beta+\alpha(\delta)$ region corresponds to a temperature of 1120°. Addition of $\mathrm{Ti}_3\mathrm{Sn}$ to $\mathrm{Ti}_3\mathrm{Al}$ initially causes a certain decrease in the temperature of the $\alpha \rightleftarrows \beta$ transformation, down to a minimum of 990° at 60% $\mathrm{Ti}_3\mathrm{Sn}$, followed by a slight increase to 1035° (see Fig. 1).

Fig. 1. Phase diagram of the $\mathrm{Ti}_3\mathrm{Al}$—$\mathrm{Ti}_3\mathrm{Sn}$ section

In the thermograms of alloys of the $\mathrm{Ti}_3\mathrm{Al}$—$\mathrm{Ti}_3\mathrm{Sn}$ section adjacent to the $\mathrm{Ti}_3\mathrm{Al}$ composition, at temperatures below 960° additional thermal effects appear, denoted in Fig. 1 by crosses. The presence of these effects is associated with the formation of the compound $\mathrm{Ti}_3\mathrm{Al}$ from $\alpha(\delta)$ solid solutions. According to thermal-analysis data, the temperature of formation of this compound in the binary Ti—Al system is 960°. This temperature gradually decreases as $\mathrm{Ti}_3\mathrm{Sn}$ is added, as shown by the dotted line.

The microstructure of the alloys of this section was studied in the cast state and after the following heat-treatment regimes: 1) homogenizing and step annealing at temperatures of 1200, 1100, 1000, 800, and 600° for, respectively, 100, 50, 200, 300, and 500 h, followed by cooling with the furnace; 2) quenching in water from temperatures of 1500, 1200, and 1000°, with holding at each temperature for 3, 50, and 100 h. Quenching from 1500° was carried out in a vacuum quenching furnace, and from 1200 and 1000° in evacuated quartz ampoules. To accelerate the attainment of equilibrium, almost all alloy compositions were deformed by upsetting in a press with a small degree of deformation, 10—15%.

Etching of the alloy specimens was carried out with a mixture of acids: hydrofluoric and concentrated nitric acid with glycerin (1 : 1 : 2).

Microexamination of the cast alloys confirms the eutectic character of alloy crystallization. In this state, alloys with a content of up to 30% $\mathrm{Ti}_3\mathrm{Sn}$ (13.5 wt.% Sn) are solid solutions based on $\beta$-titanium. In alloys with a content of more than 30% $\mathrm{Ti}_3\mathrm{Sn}$, precipitation of a second phase—$\delta$—takes place. The two-phase $\beta+\delta(\alpha)$ region of this section has a considerable extent. With a further increase in the $\mathrm{Ti}_3\mathrm{Sn}$ content in the alloys, the amount of eutectic decreases and the amount of the excess phase (based on the compound $\mathrm{Ti}_3\mathrm{Sn}$) increases. Alloys containing more than 80% $\mathrm{Ti}_3\mathrm{Sn}$ are solid solutions based on the compound $\mathrm{Ti}_3\mathrm{Sn}$. Study of the microstructure of quenched alloys shows a certain increase in the solubility limit of the compound $\mathrm{Ti}_3\mathrm{Sn}$ in the $\beta$-solid solution of titanium as the quenching temperature is raised.

Study of the microstructure of annealed and slowly cooled alloys showed that all alloys have a single-phase structure, which indicates the formation of a continuous series of solid solutions based on the $\alpha$-solid solution of the Ti—Al system and the compound $\mathrm{Ti}_3\mathrm{Sn}$. As an example, Fig. 2 shows the change in the microstructure of an alloy containing 50% $\mathrm{Ti}_3\mathrm{Sn}$, 50% $\mathrm{Ti}_3\mathrm{Al}$ during annealing. The structure of the cast alloy consists of excess precipitates of $\delta$-phase crystals, etched white, and a $\beta+\delta(\alpha)$ eutectic (Fig. 2a). The same alloy after annealing has a single-phase structure—polyhedra of the $\alpha(\delta)$ solid solution are visible (Fig. 2b).

To the article by I. I. Kornilov and T. T. Nartova, p. 829

Fig. 2. Microstructures of alloys.
a — 50% $\mathrm{Ti}_3\mathrm{Al}$, 50% $\mathrm{Ti}_3\mathrm{Sn}$, as-cast;
b — 50% $\mathrm{Ti}_3\mathrm{Al}$, 50% $\mathrm{Ti}_3\mathrm{Sn}$, annealed.
$200\times$

To the article by T. P. Gondareva, G. I. Nemkov, and V. I. Samodurov, p. 892

Fig. 1. 1–3, 5–7 — Nummulites prestwichianus Jones; 4, 8, 9 — N. ex gr. variolarius (Lamarck); 10, 11 — Operculina sp.; 12 — Nummulites tchavannesi de la Harpe; 13 — N. orbignyi Galeotti; 14 — Discocyclina sp. $10\times$

Prolongedly annealed alloys were subjected to X-ray structural analysis. X-ray photographs of the powders were taken with copper radiation. The X-ray photographs of the annealed alloys of this section are identical for all the alloy compositions studied and have only one system of lines, corresponding either to $\alpha$-titanium or to the compound $\mathrm{Ti}_3\mathrm{Sn}$. Superstructure lines on the X-ray photographs of the ternary alloys, as well as of the binary alloy of composition $\mathrm{Ti}_3\mathrm{Al}$, have not yet been detected. This question requires further investigation.

Fig. 3. Specific electrical resistance and hardness of alloys of the $\mathrm{Ti}_3\mathrm{Al}$—$\mathrm{Ti}_3\mathrm{Sn}$ section

The continuous series of solid solutions of the annealed alloys of the section studied, $\mathrm{Ti}_3\mathrm{Al}$—$\mathrm{Ti}_3\mathrm{Sn}$, is confirmed by the hardness and electrical-resistance curves presented in Fig. 3. Both these curves have a sympathetic character and, on the composition—property diagram, pass through a broad maximum, characteristic of systems with continuous solid solutions.

Thus, on the basis of the data of thermal, microstructural, and X-ray structural analyses, a phase diagram was constructed for the section of the ternary system $\mathrm{Ti}$—$\mathrm{Al}$—$\mathrm{Sn}$ passing through the compositions of the metallides $\mathrm{Ti}_3\mathrm{Al}$—$\mathrm{Ti}_3\mathrm{Sn}$ (Fig. 1). The alloys of the indicated section crystallize as mutually limited solid solutions based on the $\beta$-solid solution of the $\mathrm{Ti}$—$\mathrm{Al}$ system and on the compound $\mathrm{Ti}_3\mathrm{Sn}$, with formation of a eutectic mixture. Upon slow cooling, owing to phase transformations, these mixtures pass into a continuous series of $\alpha(\delta)$-solid solutions. At $960^\circ$, during prolonged annealing, the compound $\mathrm{Ti}_3\mathrm{Al}$ is formed from the $\alpha$-solid solution of the $\mathrm{Ti}$—$\mathrm{Al}$ system; this compound, in turn, gives solid solutions with the compound $\mathrm{Ti}_3\mathrm{Sn}$. In this state the alloys of the $\mathrm{Ti}_3\mathrm{Al}$—$\mathrm{Ti}_3\mathrm{Sn}$ section represent continuous solid solutions with a hexagonal crystal lattice.

The alloys investigated in this section of the $\mathrm{Ti}$—$\mathrm{Al}$—$\mathrm{Sn}$ system are an example of a complex phase equilibrium that changes considerably depending on temperature and on transformations in the solid state.

Metallide continuous solid solutions in alloys of the $\mathrm{Ti}_3\mathrm{Al}$—$\mathrm{Ti}_3\mathrm{Sn}$ section exist in prolongedly annealed alloys. The formation of such solid solutions should be regarded as the continuous substitution in the $\mathrm{Ti}_3\mathrm{Sn}$ lattice of tin atoms by aluminum atoms without a change in the type of crystal lattice.

Institute of Metallurgy named after A. A. Baikov
Academy of Sciences of the USSR

Received
27 III 1961

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