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Titanium Aluminides
Titanium Aluminides Victoria V. Kurbatkina
TiAl is a highly promising material for engine elements in aircraft and aerospace engineering. However, TiAl alloys are difficult-to-form materials [1–4], and hard to obtain a homogeneous melt and gravity segregation (liquation). Activities performed for years by GfE Metalle und Materialien (Germany) [1] showed that the most applicable, traditional technology for producing TiAl alloy ingots is vacuum arc remelting. The disadvantages of TiAl alloys are a high impurity level, primarily with interstitial impurities (oxygen, nitrogen, etc.), and difficult formation of a nonporous material, which have a negative impact on mechanical properties. The potential means of producing TiAl intermetallic compounds are SHSconsolidation [3,4], vacuum SHS rolling [5,6], and centrifugal SHS-molding [7,8]. Force SHS-pressing technology enables the production of TiAl alloys with a dispersed no-liquation structure in a very short cycling process that prevents segregation, which is important for systems with significant differences in the density of their basic components. From the perspective of practical synthesis of TiAl, the most suitable SHS schemes are titanium oxide reduction reaction because the TiAl formation heat is 73.6 kJ/mole and the adiabatic combustion temperature is 1246°C, which is lower than titanium (1668°C) and TiAl (1447°C) melting temperatures. SHS in this system is possible only when the reaction mixture is heated to a temperature higher than the aluminum melting point (660°C). Fig. 1 shows schematically the TiAl phase composition and melt structure formation mechanism. The initial structure formation stage involves aluminum melting and its further spread along capillary-porous-medium channels. Further diffusion of aluminum atoms into the titanium lattice leads to the formation of a TiAl3 intermetallic compound layer (Fig. 1A). Gradual growth of the TiAl3 layer leads to depletion of the aluminum melt and further formation of TiAl titanium monoaluminide. When the process spreads inside the titanium particle (Fig. 1B), the concentration of aluminum decreases leading to formation of a Ti3Al intermetallic compound (Fig. 1C). The last structure formation stage is adjustment of the composition of intermetallic layers by the recrystallization of Ti3Al into TiAl (Fig. 1D). Full completion of this stage depends on multiple factors, such as the composition of the initial reaction mixture, modes of cooling under SHS, and so on.
392
Copyright © 2017 Elsevier Inc. All rights reserved.
Concise Encyclopedia of Self-Propagating High-Temperature Synthesis http://dx.doi.org/10.1016/B978-0-12-804173-4.00158-7
Titanium Aluminides
Fig. 1 Main stages of structure formation in Al-Ti system.
To produce molded titanium aluminides using centrifugal molding technology, Ti +Al or TiO2 + Al hybrid mixtures and CaO2 + Al high-energy additive are used. The combustion temperature of 3CaO2 + 2Al is 3700 K. The addition of 40% (CaO2 + Al) into Ti + Al mixtures increases the combustion temperature to 2200–2300 K and produces a melt of TixAly and 3CaOxAl2O3 combustion products. A small difference in the specific gravities of the metal and oxide phases of the combustion products with a high content of Al in TixAly should be noted. Therefore, in the centrifugal field, at an overload exceeding 250 g, molded products easily segregate into two layers, a metal layer and an oxide layer. Variation of aluminum content in the basic mixture allows control over the chemical composition of the metal phase and production of Ti3Al, TiAl, and TiAl3 phases. Oxide combustion products, CaO-Al2O3, are inert in relation to the metal melt. Calcium oxide formed as a combustion product causes a decrease in the viscosity of the oxide (slag) phase, which allows almost full recovery of the target components as a metal ingot.
REFERENCES [1] Guther V, et al. In: Kim YW, Clemens H, Rosenberger AH, editors. Proceedings of gamma titanium aluminides, USA: Minerals Metals and Materials Society; 2003. p. 241–7. [2] Cao J, Feng JC, Li ZR. J Mater Sci 2006;41:4720–4. [3] Agote I, Coleto a J, Gutierrez M, Sargsyan A, Garcı´a de Cortazar M, Lagos MA, et al. Intermetallics 2008;16:1310–6. [4] Bertolino N, Monagheddu M, Tacca A, Giuliani P, Zanotti C. Intermetallics 2003;11:41–9. [5] Osipov EE, Levashov EA, Chernyshov VN, Merzhanov AG, Borovinskaya IP. Int J SHS 1992; 2(1):314–7. [6] Chernyshov VN, Osipov EE, Levashov EA, Merzhanov AG, Biyachi L. Int J SHS 1993;3(2):315–21. [7] Andreev DE, Sanin VN, Yukhvid VI. Inorg Mater 2009;45(8):867–72. [8] Sanin V, Andreev D, Ikornikov D, Yukhvid V. Cast intermetallic alloys by SHS under high gravity. Acta Phys Pol A 2011;(2):331–5.
393
TiAl is a highly promising material for engine elements in aircraft and aerospace engineering. However, TiAl alloys are difficult-to-form materials [1–4], and hard to obtain a homogeneous melt and gravity segregation (liquation). Activities performed for years by GfE Metalle und Materialien (Germany) [1] showed that the most applicable, traditional technology for producing TiAl alloy ingots is vacuum arc remelting. The disadvantages of TiAl alloys are a high impurity level, primarily with interstitial impurities (oxygen, nitrogen, etc.), and difficult formation of a nonporous material, which have a negative impact on mechanical properties. The potential means of producing TiAl intermetallic compounds are SHSconsolidation [3,4], vacuum SHS rolling [5,6], and centrifugal SHS-molding [7,8]. Force SHS-pressing technology enables the production of TiAl alloys with a dispersed no-liquation structure in a very short cycling process that prevents segregation, which is important for systems with significant differences in the density of their basic components. From the perspective of practical synthesis of TiAl, the most suitable SHS schemes are titanium oxide reduction reaction because the TiAl formation heat is 73.6 kJ/mole and the adiabatic combustion temperature is 1246°C, which is lower than titanium (1668°C) and TiAl (1447°C) melting temperatures. SHS in this system is possible only when the reaction mixture is heated to a temperature higher than the aluminum melting point (660°C). Fig. 1 shows schematically the TiAl phase composition and melt structure formation mechanism. The initial structure formation stage involves aluminum melting and its further spread along capillary-porous-medium channels. Further diffusion of aluminum atoms into the titanium lattice leads to the formation of a TiAl3 intermetallic compound layer (Fig. 1A). Gradual growth of the TiAl3 layer leads to depletion of the aluminum melt and further formation of TiAl titanium monoaluminide. When the process spreads inside the titanium particle (Fig. 1B), the concentration of aluminum decreases leading to formation of a Ti3Al intermetallic compound (Fig. 1C). The last structure formation stage is adjustment of the composition of intermetallic layers by the recrystallization of Ti3Al into TiAl (Fig. 1D). Full completion of this stage depends on multiple factors, such as the composition of the initial reaction mixture, modes of cooling under SHS, and so on.
392
Copyright © 2017 Elsevier Inc. All rights reserved.
Concise Encyclopedia of Self-Propagating High-Temperature Synthesis http://dx.doi.org/10.1016/B978-0-12-804173-4.00158-7
Titanium Aluminides
Fig. 1 Main stages of structure formation in Al-Ti system.
To produce molded titanium aluminides using centrifugal molding technology, Ti +Al or TiO2 + Al hybrid mixtures and CaO2 + Al high-energy additive are used. The combustion temperature of 3CaO2 + 2Al is 3700 K. The addition of 40% (CaO2 + Al) into Ti + Al mixtures increases the combustion temperature to 2200–2300 K and produces a melt of TixAly and 3CaOxAl2O3 combustion products. A small difference in the specific gravities of the metal and oxide phases of the combustion products with a high content of Al in TixAly should be noted. Therefore, in the centrifugal field, at an overload exceeding 250 g, molded products easily segregate into two layers, a metal layer and an oxide layer. Variation of aluminum content in the basic mixture allows control over the chemical composition of the metal phase and production of Ti3Al, TiAl, and TiAl3 phases. Oxide combustion products, CaO-Al2O3, are inert in relation to the metal melt. Calcium oxide formed as a combustion product causes a decrease in the viscosity of the oxide (slag) phase, which allows almost full recovery of the target components as a metal ingot.
REFERENCES [1] Guther V, et al. In: Kim YW, Clemens H, Rosenberger AH, editors. Proceedings of gamma titanium aluminides, USA: Minerals Metals and Materials Society; 2003. p. 241–7. [2] Cao J, Feng JC, Li ZR. J Mater Sci 2006;41:4720–4. [3] Agote I, Coleto a J, Gutierrez M, Sargsyan A, Garcı´a de Cortazar M, Lagos MA, et al. Intermetallics 2008;16:1310–6. [4] Bertolino N, Monagheddu M, Tacca A, Giuliani P, Zanotti C. Intermetallics 2003;11:41–9. [5] Osipov EE, Levashov EA, Chernyshov VN, Merzhanov AG, Borovinskaya IP. Int J SHS 1992; 2(1):314–7. [6] Chernyshov VN, Osipov EE, Levashov EA, Merzhanov AG, Biyachi L. Int J SHS 1993;3(2):315–21. [7] Andreev DE, Sanin VN, Yukhvid VI. Inorg Mater 2009;45(8):867–72. [8] Sanin V, Andreev D, Ikornikov D, Yukhvid V. Cast intermetallic alloys by SHS under high gravity. Acta Phys Pol A 2011;(2):331–5.
393