Titanium aluminide

Fig. 5. Comparison of crack growth rates of titanium aluminides, composites, and IN-100 tests at 650°C, R = 0.1, v = 0.2 Hz except for the composite...

Table 15. Properties of Titanium Aluminides, Titanium-Base Conventional Alloys, and SuperaHoys ...

The market for lighter weight P/M materials such as aluminum and titanium aluminides is expected to grow, especially for uses in automobiles. 

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. 

It was observed, under conditions when the nickel-aluminide mixtures of the same ratio were fully reacted, that the titanium aluminides were essentially unreacted reactions were only localized. Because the products were of such small size, it was difficult to identify them, but they were thought to be TiAlj or ordered superstructures TiQAl23 or TigAl24. No further studies have been carried out on these samples. 

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. 

Titanium aluminides are produced by CVD from the halides by the following sequence of reactions... 

Reinforcement for metal-matrix composites with such metals as titanium, titanium aluminide, aluminum, magnesium, and copper. Applications are found mostly in advanced aerospace programs and include fan blades, drive shafts, and other components. 

Saunders, N. (1997a) in Light Metals, ed. Huglen, R. (TMS, Warrendalc, PA), p. 911. Saunders, N. (1997b) Phase diagram modelling of TiAl alloys , presented at the Symp. Fundamentals of y Titanium Aluminides, TMS Armual Meeting, Orlando, Florida, 10-13 February 1997. 

Clement. T.P., Parsonage, T,Band M.B, Kuxhaus Ti2AlNb = Based. Alloys Outperform Conventional Titanium Aluminides, Advanced Materials < Processes, 37 (March 1992). 

Jha, S.C., et al. Titanium-Aluminide Foils, Advanced Materials Processes, 87 (April 1991). 

Nickel aluminide (NiAl, Ni3AI), titanium aluminide (TiAl, Ti3AI), molybdenum disilicide (MoSi2)... 

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. 

Note that in the framework of purely diffusional considerations any diffusing atoms are assumed to be available for any growing compound layer. In other words, the existence of any interface barriers to prevent diffusion of appropriate atoms is not recognised. From this viewpoint, it would be more logical to compare the diffusion coefficients of aluminium, as the more mobile component, in all the titanium aluminides. In such a case, the absence of most aluminide layers becomes quite unexplainable. It is highly unlikely that the diffusion coefficients of aluminium in different titanium aluminides are so different as to exclude the formation, say, of the TiAl2 layer. 

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. 

Intermet allies Nickel aluminide (NiAl, NijAl) Titanium aluminide (TiAl, TijAl) Molybdenum disilicide (MoSi )... 

Ferreira, A., Meyers, M. A., and Thadani, N. N., Dynamic compaction of titanium aluminides by explosively generated shock waves Microstructure and mechanical properties. Metall. Trans. A, 23A,3251 (1992). 

Hahn, Y.-D., and Song, I.-H., Microstructural characteristics of titanium aluminides synthesized by using the wave propagation mode. Int. J. SHS, 4,293 (1995). 

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). 

Fig. 5 Bright-field and dark-field imaging (A) BF image of lamellar y/ 2 titanium aluminide (B) corresponding SAD pattern (see Fig. 4) (C) and (D) DF images of the reflections marked in (B). Each of these reflections is characteristic for one twin variant of tetragonal y-TiAl appearing with high intensity in the corresponding DF image. (View this art in color at www.dekker.com.)...

Fig. 7 High-resolution transmission electron microscopy. HRTEM micrograph of lamellar y/ot2 titanium aluminide. From top to bottom, first twin variant of tetragonal y-TiAl, hexagonal 2-Ti3Al, second twin variant of y-TiAl and again ot2-Ti3Al. Incident beam direction for the tetragonal phases is 1 1 0, for the hexagonal phase 1 1 0. (View this art in color at www.dekker.com.)...

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