Structure, titanium aluminides

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 aluminide alloys with two compositions, and differing structures, were used as substrates. The a-2 Ti3Al was a commercial alloy (Heat T8991), fabricated as 1 mm thick sheet, by the Titanium Metals Corporation of America and contained (as w/0)... 

Ti3Al has an extended composition range, as is visible in the commonly used Ti-Al phase diagram (Fig. 10a), which, however, is still in discussion and has recently been revised, as shown in Fig. 10b (Hellwig etal., 1992 Kainuma etal., 1994). It forms stable equilibria with the two disordered Ti phases, a-Ti with a hexagonal close-packed A3 structure and P-Ti with a b.c.c. A2 structure, and with the other important titanium aluminide TiAl,... 

The titanium aluminide TiAl - often designated as y phase - crystallizes with the tetragonal LIq structure (CuAu-type) which is shown in Fig. 1. The LI o structure results from ordering in the f.c.c. lattice (Al), i.e. it is basically a cubic structure which is tetragonally distorted because of the particular stacking of the atom planes, as is seen in Fig. 1. The ratio of the lattice parameters c and a is cja = 1.015 at the stoichiometric composition and the density is 3.76 g/cm (Kim and Dimiduk, 1991), whereas for TiAl-base alloys the range 3.7-3.9 g/cm is given (see Table 2). This density is still lower than that of TijAl and has made the titanium aluminides most attractive for materials developments. 

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 present monograph was first written as a chapter for Volume 8 of the series Materials Sdence and Technology A Comprehensive Treatment , edited by Robert W. Cahn, Peter Haasen, and Edward J. Kramer (Volume Editor Dr. Karl Heinz Matucha). Its aim is to give an overview of intermetallics, which is both detailed and comprehensive and which includes the fundamentals as well as applications. The result is an extended, critical review of the whole field of intermetallics with an emphasis on those intermetallic phases which have already been applied as functional or structural materials or which are currently the subject of materials developments. A historical introduction and a discussion of the relationship between atomic bonding, crystal structure, phase stability and properties is followed by a discussion of the major classes of intermetallics. The titanium aluminides, nickel aluminides, iron aluminides, copper phases, A15 phases. Laves phases, beryllides, rare earth phases, and siliddes are reviewed. In particular, the crystal structures, phase diagrams, and physical properties as well as the mechanical and corrosion behavior are treated. The state of developments as well as prospects and problems are discussed in view of present and future applications. The publisher has decided to publish the review as a separate monograph in order to make it accessible to a wider audience. 

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. 

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... 

Berger S, Tsuchiya H, Schmuki P (2008) Transition from nanopores to nanotubes self-ordered anodic oxide structures on titanium—aluminides. Chem Mater 20(10) 3245-3247... 

At higher niobium levels, the 02 phase evolves to a new ordered orthorhombic structure that is based on the composition Ti2AlNb (O phase), which has been observed in titanium aluminides with compositions finm Ti-25Al-12.5Nb to Ti-25A1-30Nb (Ref 1-4). The crystal structures of the o2 and ordered orthorhombic phases are compared in the accompanying figure, which shows the basal planes and atomic positions in the lanes above and below the plane of sheet. 

The titanium aluminide Ti2AlNb with an ordered orthorhombic crystal structure rather than the ordered hexagonal DOiq structure of Ti3Al was stronger and has higher fractvire toughness than... 

Dur2] Durlu, N., Inal, O.T., Z,l2-iype Ternary Titanium Aluminides as Eleetron Concentration Phases , J. Mater. Sci., 27(12), 3225-3230 (1992) (Assessment, Eleetronic Stracture, Crys. Structure, 41)... 

Up to 4 wt% H can be dissolved in titanium alloys at elevated temperatures. This hydrogen can then be used to improve processibility, and final mechanical properties are enhanced after its removal. Removal of the hydrogen can be easily achieved by vacuum annealing (Ref 7, 8). This thermomechanical processing technique allows ti-tanium aluminides to be processed at reduced temperatures (Ref 7-10) and results in a finer micro-structure (Ref 7,8,10,11). 

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