Ti-Al alloy

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DOI: 10.1007/s11663-017-1129-z

© The Minerals, Metals & Materials Society and ASM International 2017

Communication

Chemical Vapor Synthesis of Titanium Aluminides by Reaction of Aluminum Subchloride and Titanium Tetrachloride

ROMAN A. ZAKIROV, OLEG G. PARFENOV, and LEONID A. SOLOVYOV are with the Institute of Chemistry and Chemical Technology of SB RAS, Akademgorodok 50/24, Krasnoyarsk 660036, Russia. Contact email: zakirow.roman@gmail.com

Manuscript submitted on June 6, 2017.

Article published online November 14, 2017.

A new process for developing titanium aluminides (TiAls) using chemical vapor synthesis was investigated in a laboratory experiment. Aluminum subchloride (AlCl) was used as the reducing agent in the reaction with TiCl4 and the source of aluminum for Ti-Al alloy. Two types of products, with large crystals and ?ne particles, were fabricated. The large crystals were determined to be TiAl, with small amounts of Ti and Ti3Al phases. The composition of ?ne particles, on the other hand, varied in wide range.

titanium aluminide chemical synthese

Titanium aluminides (TiAls) are an important class of materials for the automotive and aerospace industries.[1,2] Intermetallic compounds of titanium (TiAl, Ti3Al, TiAl3) are characterized by superior mechanical and chemical properties: low density, high-temperature oxidation, and corrosion resistance. However, these alloys also have low ductility at room temperature, and mechanical postprocessing of ingots, therefore, is complicated. Powder metallurgy (PM), including three-dimensional printing technology, is considered an e?ective method for automotive components production. Currently, much investigation has been carried out on techniques for the production of TiAl powder.[3-5] However, combining aluminum and titanium powders for the production of TiAl alloys has limitations due to the di?erences in the densities and melting temperatures of these metals. Several strategies have been proposed to overcome these limitations, for example, the use of additional techniques such as hydrogenation of titanium, milling, and prealloying.

The main objective of this study is to develop a facile technique for the fabrication of TiAl alloys powder, which involves the preparation of aluminum subchloride (AlCl) as the reducing agent and its reaction with titanium tetrachloride (TiCl4). A similar metallothermic approach has been reported previously.[6] In that report, TiAls were produced via the co-reduction of TiCl3 and AlCl3 by magnesium. It is known that metallic aluminum has very low vapor pressure and, therefore, cannot be used as a gas-phase reducing agent. On the other hand, aluminum subhalides are highly stable in the gas phase at temperatures ‡ 1173 K (900 °C). In the 20th century, the subhalide method for aluminum extraction attracted the attention of researchers as an alternative route for aluminum production.[7,8]

We have published several articles concerning the application of AlCl as the reducing agent.[9-11] In addition, the possibility of silicon tetrachloride reduction by AlCl has been investigated.[12,13] Thermodynamic calculations for the application of AlCl to the synthesis of aluminum alloys have been reported,[14] but the results have not yet been validated experimentally. Therefore, in this report, we demonstrate the feasibility of using the AlCl method for the synthesis of TiAl alloys in a laboratory experiment.

Aluminum chloride (98 pct, puri?ed by sublimation), metallic aluminum (> 99 pct), and TiCl4 (99.9 pct) were used as the starting reagents. Argon was additionally puri?ed using P2O5 and magnesium. The experimental setup is shown in Figure 1. The reactor consisted of two coaxial ceramic tubes (1, 2), a ceramic boat (3), gas-tight ?anges (4), chloride vaporizers (5, 6), and a powder collector (7). The outer ceramic tube (1000-mm length and 50-mm inner diameter) was placed in the tubular furnace. Subsequently, the ceramic boat with metallic aluminum for AlCl synthesis was placed in the inner tube.

The supply of aluminum and titanium chlorides into the reaction zone was controlled by the argon ?ow rate and the temperature in the vaporizers. The temperature in the aluminum chloride vaporizer was set at 443 K (170 °C), while that in the titanium chloride vaporizer was set at 383 K to 403 K (110 °C to 130 °C). Argon was used for transporting the reagents into the reaction zone, as well as for maintaining an inert atmosphere in the reactor. Aluminum chloride was passed over the liquid aluminum surface for synthesizing AlCl. The coaxial scheme of the reactor prevents direct contact of metallic aluminum with TiCl4 vapor.

The AlCl ?ow was calculated based on the aluminum weight change, according to Eq. [1]. After the experiment, the furnace was cooled, and the product was washed with water to remove soluble chlorides and dried. Chemical composition, morphology, and phase analyses were performed by a scanning electron micro- scope (SEM) Hitachi TM-3000 system and a PANalytical X'Pert PRO X-ray di?ractometer with Co Ka radiation.

Prior to the synthesis of aluminides, experiments were carried out to prepare AlCl per the following reaction:

2Al ю AlCl3 ј 3AlCl Ѕ1]

The experiments were carried out at temperatures of 1273 K, 1373 K, and 1473 K (1000 °C, 1100 °C, and 1200 °C) at a pressure of 0.1 MPa. The reaction yield and conversion of aluminum chloride into subchloride (PAlCl3) seemed particularly dependent on the reaction temperature (Table I). In general, the reaction yield and conversion increased with an increase in the temperature, and the maximum subchloride formation rate was observed at 1473 K (1200 °C).

The TiAl alloy was synthesized by mixing TiCl4 and AlCl per the generalized equation

TiCl4 ю 7=2AlCl ј TiAl ю 5=2AlCl3 Ѕ2]

In preliminary proof-of-concept experiments, it was found that TiAls were formed only in the presence of AlCl3 vapor. Therefore, the product formation proceeded via reduction of TiCl4 by AlCl.

As intensive formation of titanium subchlorides (TiCl2, TiCl3) occurred at temperatures above 1373 K (1100 °C) in the mixing zone, aluminides were synthesized at 1173 K and 1273 K (900 °C and 1000 °C). Temperature in the AlCl synthesis zone was 1373 K (1100 °C). During a 1-hour experiment, approximately 2 g of aluminum was used in the reaction with AlCl3 to obtain 6.9 g of AlCl (~ 0.1 mol). The rate of TiCl4 evaporation was in the range 3 to 10 g per hour and, as mentioned previously, was regulated by the temperature in the vaporizer and the argon ?ow rate. Thus, the molar ratio [AlCl]:[TiCl4] in the aluminide synthesis experiments varied from 2.4 to 7.8.

Two types of products were formed (Figure 2) in these reactions, with large crystals in the mixing zone and ?ne powder on the inner surface of the reactor and in the powder collector. Upon the initial contact of AlCl with TiCl4, dendrite crystals (Figure 2(a)) were formed on the edge of the inner ceramic tube. On the other hand, ?ne metal particles with sizes ranging from 1 to 3 lm (Figure 2(b)) were deposited in the collector along with titanium subchlorides (TiCl2, TiCl3). Experimental data for the di?erent [AlCl]:[TiCl4] molar ratios used in this study are listed in Table II.

The titanium content in the products as a function of [AlCl]:[TiCl4] molar ratio is shown in Figure 3. In the large crystals, the titanium content was nearly constant (70 to 81 wt pct). XRD analyses indicated TiAl as the dominant phase, with a small admixture of Al, Ti, and Ti3Al phases (Figure 4). In contrast, the composition of ?ne particles strongly depended on the [AlCl]:[TiCl4] molar ratio. The titanium content was 27 wt pct at the ratio of 7.8, and it increased to 56 wt pct at the ratio of 2.4. The ?ne powder composition, as a function of the [reducing agent]:[TiCl4] ratio, was similar to the results obtained in a previously reported two-stage aluminothermic reduction process.[15] The temperature regime of the synthesis did not have any e?ect on the titanium content of the product. Figure 5 shows the X-ray di?ractograms of the ?ne powder product at the molar ratios of (a) 2.4 and (b) 7.8. From the patterns, it can be seen that the powder is a mixture of di?erent phases. At the ratio of 2.4, the main phase was TiAl (54 wt pct), with an admixture of Ti3Al (12 pct), Ti2Al5 (19 pct), and TiAl3 (15 pct). On the other hand, at the ratio of 7.8, the dominant phases were TiAl3 (62 wt pct) and Al (28 pct); i.e., an increase in the aluminum content in the powder was observed with an increase in the AlCl concentration in the reagent mixture.

The crystal yield considerably decreased when the temperature in the mixing zone exceeded 1373 K (1100 °C), and at temperatures ‡ 1423 K (1150 °C), no crystals were formed. Based on these observations, we suggest that the crystal growth proceeds via AlCl disproportion at temperatures lower than 1273 K (1000 °C), and a heterogeneous reaction between metallic aluminum and TiCl4 vapor occurs on the reactor surface.

The ?ne powder product is trapped in the collector along with titanium chlorides (II, III). The formation of titanium subchlorides constitutes an undesired side

Fig. 1--Schematic diagram of laboratory reactor for TiAl synthesis.

Table I - Aluminum Mass Loss and AlCl3 Conversion

Fig. 2--SEM images of reaction products: (a) large crystals and (b) ?ne particles.

Table II - Experimental Data for Synthesis of TiAls

*Temperature in the mixing zone.

**CTi(LC) = titanium content in large crystals, and CTi(FP) = titanium content in ?ne particles.

Fig. 3--Ti concentration in products as a function of temperature and [AlCl]:[TiCl4] ratio.

reaction, because it decreases the ultimate yield of titanium in the aluminides. This side reaction occurs due to the incomplete reduction of TiCl4 and the secondary reaction of residual tetrachloride and metallic phases. This process can be suppressed by the conversion of TiCl4 to TiCl3 in the initial reaction mixture by hydrogen.[16] It was qualitatively veri?ed in the experiment at a [H2]:[TiCl4] molar ratio of 5. In this case, the amount of titanium subchlorides in the product was signi?cantly lower and the powder yield increased by 1.5 times.

Fig. 4--XRD pattern of aluminide crystals.

The conversion of TiCl4 to aluminides (crystals and ?ne powder in the collector) in experiments was about 25 pct. The mixture of chlorides condensed in the cold part of the reactor was dark colored and apparently contained ?ne metallic inclusions and titanium subchlorides. The mixture was removed from the cold part of the reactor and placed in a ceramic boat under argon atmosphere, as TiCl2 in the mixture readily oxidizes upon exposure to air. A two-step separation process was carried out, beginning with the removal of TiCl4 and aluminum chlorides at 413 K and 473 K (140 °C and 200 °C), respectively. In the second stage at 700 K (427 °C), violet-colored deposits of TiCl3 condensed on

Fig. 5--Phase composition of powder samples at [AlCl]:[TiCl4] ratios of (a) 2.4 and (b) 7.8.

Fig. 6--XRD pattern of pressed and sintered ?ne powder ([AlCl]:[- TiCl4] ratio 2.4).

the cold part of the tube. As the temperature was

Figure 6. The number of phases was distinctly reduced, and the peaks were mainly attributed to TiAl.

AlCl was used for synthesizing TiAls via a reaction with TiCl4. Two types of products, with large crystals and ?ne powders, were obtained. The large crystals contained 70 to 81 wt pct titanium and were represented by the TiAl phase with low contents of other phases. The composition of this phase showed no signi?cant dependence on the experimental conditions (temperature and molar ratio [AlCl]:[TiCl4]). The powder pro- duct was, as a rule, a mixture of di?erent Ti-Al phases, with the titanium content varying between 27 and 56 wt pct. The titanium content in the powder was found to be a function of AlCl concentration in the initial gas mixture of the reagents. The total yield of TiAls in a laboratory experiment was ~ 25 pct, but it increased upon improving the reactor design and optimizing the synthesis conditions. Hydrogen addition to the reaction mixture suppressed the formation of titanium subchlorides and, thus, increased the yield of TiAls.

elevated to 873 K (600 °C), a notable amount of aluminum chloride deposit was formed, probably due to the reaction of titanium subchlorides and aluminum in the Ti-Al alloy. Upon cooling and washing the powder in distilled water, approximately 50 pct yield of Ti in the aluminides was realized.

The aluminide crystals obtained in our experiments are highly suitable for PM applications, as they have a block structure (crystallites), with the average block size ranging from 40 to 60 lm (Figure 2(a)). The product can be easily ground in a laboratory grinder to obtain a coarse powder. The main shortcoming is the number of phases present in the powder product, which can be overcome by choosing the appropriate experimental conditions and by improving the reactor design. In addition, we demonstrate that homogenization sintering can be applied by pressing the powder in a tablet form and subsequently sintering the tablets at 1273 K (1000 °C) for 1 hour in an inert atmosphere. The XRD pattern of the sintered specimen is shown in

References

1. M.S. Chu and S.K. Wu: Oxid. Met., 2005, vol. 63 (1-2), pp. 1-13.

2. Y.W. Kim: JOM, 1995, vol. 6, pp. 39-41.

3. H.W. Liu and K.P. Plucknett: Adv. Powder Technol., 2017, vol. 28, pp. 314-23.

4. S.K. Vajpai and K. Ameyama: Intermetallics, 2013, vol. 42, pp. 146-55.

5. R.K. Gupta, B. Pant, and P.P. Sinha: Trans. Ind. Inst. Met., 2014, vol. 67 (2), pp. 143-65.

6. H.Y. Sohn and S. Paldey: Metall. Mater. Trans. B, 1998, vol. 29B, pp. 457-64.

7. T. Kikuchi and T. Kurosawa: Mater. Trans. JIM, 1964, vol. 5, pp. 122-27.

8. T. Kikuchi and T. Kurosawa: Mater. Trans. JIM, 1972, vol. 13, pp. 365-70.

9. R.A. Zakirov, O.G. Parfenov, and G.L. Pashkov: Dokl. Chem., 2009, vol. 425 (2), pp. 77-79.

10. O.G. Parfenov, G.L. Pashkov, and R.A. Zakirov: Theor. Found. Chem. Eng., 2008, vol. 42 (5), pp. 684-87.

11. R.A. Zakirov and O.G. Parfenov: Chem. Sustain. Dev., 2007, vol. 15, pp. 711-13.

12. O.G. Parfenov and G.L. Pashkov: Dokl. Chem., 2008, vol. 422 (2), pp. 225-26.

13. 13. K. Yasuda and T.H. Okabe: JOM, 2010, vol. 62 (12), pp. 94-101.

14. J. Myrick: Production of Metals and Their Alloys, WO 01/45906 A2, 2001.

15. J. Haidar, S. Gnanarajan, and J.B. Dunlop: Intermetallics, 2009, vol. 17, pp. 651-56.

16. D.H. Yean and J.R. Riter: Metall. Trans., 1974, vol. 5, pp. 2473- 74.

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