Phases, titanium aluminides

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. 

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. 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.)...

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. 

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 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-... 

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. 

The selective oxidation of an alloy component, e.g., A1 or Si, requires the alumina or silica to be more stable than the oxides of the other components in the alloy. Figure 2.5 indicates this condition would be met for compounds such as nickel aluminides and molybdenum silicides. However, in the case of Nb- or Ti-base compounds the oxides of the base metal are nearly as stable as those of A1 or Si. This can result in conditions for which selective oxidation is impossible. This situation exists for titanium aluminides containing less than 50 at% A1 as illustrated in Figure 5.27. In this case a two-phase scale of intermixed AI2O3 and I1O2 is generally observed. It should be emphasized that the determination of which oxide is more stable must take into account the prevailing metal activities. 

Alloys based on y-TiAl, also called gamma titanium aluminides, excel due to their high strength per unit density. These alloys contain the Q 2-phase Ti3Al as a second phase and are further alloyed with other elements for property optimization. The composition range is Ti-(45 8) Al-(0-2)(Cr, Mn, V)-(0-5)(Nb, Ta, W)-(0-2)(Si,B, Fe, N) (at.%). Further components such as Hf,... 

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