Temperature titanium aluminides

P/M processing of titanium aluminides results in more consistent product quaHty than the conventional casting process, and offers novel alloy/microstmcture possibiHties and improved ductiHty. Processing trends include use of high (1200—1350°C) temperature sintering to improve mechanical properties of steel and stainless steel parts. 

There are two important titanium aluminides Tig A1 which has a hexagonal structure with a density of 4.20 g/cm and a melting point of 1600°C and Ti A1 which has a tetragonal structure with a density of 3.91 g/cm and a melting point of 1445°C. As do all aluminides, they have excellent high temperature oxidation resistance owing to the formation of a thin alumina layer on the surface. They have potential applications in aerospace structures. 

The melting point of titanium is 1670°C, while that of aluminium is 660°C.142 In kelvins, these are 1943 K and 933 K, respectively. Thus, the temperature 625°C (898 K) amounts to 0.46 7melting of titanium and 0.96 melting of aluminium. Hence, at this temperature the aluminium atoms may be expected to be much more mobile in the crystal lattices of the titanium aluminides than the titanium atoms. This appears to be the case even with the Ti3Al intermetallic compound. The duplex structure of the Ti3Al layer in the Ti-TiAl diffusion couple (see Fig. 5.13 in Ref. 66) provides evidence that aluminium is the main diffusant. Otherwise, its microstructure would be homogeneous. This point will be explained in more detail in the next chapter devoted to the consideration of growth kinetics of the same compound layer in various reaction couples of a multiphase binary system. 

Titanium aluminides are ordered intermetallics and hence have lower diffusivity and high elastic modulus. These compounds are stronger than the conventional titanium alloys and are suitable for high temperature applications. But these compounds have low ductility due to the planarity of slip in these compounds. 

The NMR parameters for titanium metal deduced in earlier studies by field sweeping techniques (Narath 1967, Ebert et al. 1986) have been confirmed by more recent room temperature FT NMR (Bastow et al. 1998a). The value of Xq deduced from the ( /2, 2) satellite transitions was used in an accurate simulation of the central transition, which required an axial Knight shift of 70 10 ppm. The Ti NMR spectra of a number of titanium aluminide alloys and TiAg have also been reported... 

Eq. (11) is not written as a simple exchange reaction, but rather includes the stable titanium aluminide and aluminum boride phases expected to be in equilibrium with molten Al. Consideration of the formation of inter-metallics must not be overlooked during analysis of reinforcement stability because these, not the free element, are the most likely phases to form (presuming, of course, they exist at the growth temperature). 

Y. Nishiyama, T. Miyashita, S. Isobe, T. Noda Development of Titanium Aluminide Turbo-Charger Rotors. In S. H. Whang, C. T. Liu, D. P. Pope et al. High-Temperature Aluminides and Intermetallics. TMS, Warrendale (1990) 557-584. 

There has been relatively little work published on the reaction of titanium aluminides in atmospheres other than air or oxygen. Niu et al. [96] studied the reaction of Ti-25Al-llNb in a simulated combustion atmosphere (N2+1%02+ 0.5%SO2) with and without surface deposits of Na2S04-t- NaCl at temperatures between 600 and 800°C. Exposures in the absence of surface deposits resulted in reaction rates similar to those described above for simple oxidation. The rates in the presence of the deposits at 600 and 700 °C were initially rapid and then slowed markedly after 25 to 50 hours exposure. The rate at 800°C remained rapid with the kinetics being essentially linear. The major difference in the corrosion morphology at 800 °C was the presence of copious amounts of sulfides below the oxide scales. The authors postulate a mechanism of attack involving a combination of sulfidation-oxidation and scale-fluxing. 

F. -C. Dary, T. M. Dollock Effects of High-Temperature Air or Vacuum Exposures on the Room Temperature Tensile Behavior of the (0+B2) Titanium Aluminide , submitted to Mater. Sci. and Eng. 

Titanium aluminide alloys based on Ti3 A1 and TiAl are of interest as construction material for high temperature components particularly in aerospace industry. Good mechanical properties can be attained with alloys consisting of y-TiAl with 3 to 15 vol% a2-Ti3Al. The disadvantages are the low ductility and the inadequate oxidation resistance at service temperatures of 700-900°C [1]. A fundamental understanding of the oxidation behaviour is necessary in order to improve the corrosion resistance. The formation of the oxides on the alloy surface depends on the temperature, the oxygen partial pressure of the corrosive atmosphere, and the thermodynamic activities of Ti and A1 in the alloys. 

N.S. Choudhury, H.C. Graham, J.W. Hinze Oxidation Behavior of Titanium Aluminides, Z.A. Foroulis, F.S. Pettit, Eds., Proceedings of the Symposium on Properties of High Temperature Alloys (Tlte Electrochemical Society, Princeton, N.J.), (1976), pp. 668. 

In industrial applications the environments usually contain more than one reactant. For example high temperature oxidation occurs in air by the combined attack of oxygen, nitrogen and quite frequently water vapour. However, most of the studies concerning the oxidation resistance are performed in dry oxygen or dry air. The oxidation behaviour of the intermetallic phases of theTi-Al system has recently received considerable attention. The influence of water vapour on the oxidation of titanium aluminides has not been studied intensively. There are only a few studies of the high temperature corrosion of titanium and its alloys. 

New structural intermetallic alloys for high-temperature applications are at the center of the present interest in intermetallics, which is still growing. A few developments, which are based on the classic phases NijAl, TijAl and TiAl, and which are known as the nickel aluminides and the titanium aluminides, are on the brink of commercialization, but even these developments are still at an early stage compared with other developments of advanced materials, e.g. the modern engineering ceramics. Much more experimental and theoretical work is necessary to solve the processing problems and to ad-... 

The much advanced nickel aluminides and titanium aluminides can be used only up to about 1000 °C because of their limited strength or oxidation resistance or both at higher temperatures, as has been stated before (Sauthoff, 1994). For applications significantly above 1000 °C other less-com--mon phases with higher melting temperatures have to be used. Such phases are available, and examples are shown in Fig. 34 (Sauthoff, 1992). In comparison to the nickel aluminides and titanium alu-... 

Recently, a great interest has been placed on intermetallic titanium aluminides the two systems under development are based on Tis A1 and TiAl compounds, which promise temperature capabilities of 800 °C and 980 C, but great efforts have to go into the achievement of ductility, toughness and oxidation resistance above 650 C. 

One candidate is a titanium alioy that is reinforced with iarge diameter SiC/C filaments (see Chapter 4) and is fabricated by superplastic forming/ diffusion bonding. This MMC is suited to the fabrication of bladed compressor rings, shafts, ducks, fan components or structural rods for jet engines. Their use for parts submitted to still higher temperatures is limited by tiber/matrix reaction and environmental considerations. Titanium aluminide TisAI (or y-TiAl) matrices could permit an increase in the service temperature of the composites. 

Ti is an attractive material with many industrial uses. The various alloys of Ti have a multitude of applications in the aerospace sector due to their high strength, low density, and durability [2-4]. Titanium aluminide alloys are used in the aircraft and automotive industries due to their high-temperature stability, resistance to oxidation, and attractive strength-to-weight ratio [5]. 

Metal-Ceramic Composites. Metals such as aluminum, titanium, copper and the intermetallic titanium aluminide, which are reinforced with silicon-carbide fibers or whiskers show an appreciable increase in mechanical properties particularly at elevated temperatures. These composites are being considered for advanced aerospace structures.1 1... 

Figure 6-20. Schematic Ti-Al phase diagram (after Kattner et al. (1992) and Zhang et al. (1997)). There is no overlap between protective AI2O3 scale formation in air and room-temperature ductility (with alloying additions) in the Ti-Al system. The intermixed AI2O3 + Ti02 scale that forms on titanium aluminides can provide adequate protection from scaling up to ca 750-800 C.

Of the titanium aluminide intermetallic phases, only TiAl2, TiAl3, and certain t phase compositions can form a protective AI2O3 scale over a wide range of temperatures in air. They will be treated first, followed by the y, oui, and orthorhombic phases. 

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