TiAl alloy

Fig. 3.35. HF-plasma SNMS survey signals coming from adsorbed and respectrum of a powder metallurgically- sputtered plasma gas contamination, produced TiAl alloy sample. In brackets ...

A review of alloy and process development of the TiAl alloy has been published by Wu (2006). The main conclusions are that TiAl development is now relatively mature the major requirement is the promotion of low cost processing coupled with the supply of high-quality raw material the future application of net shape powder process routes could be cost effective both for complex large components and for some small components. Special attention is dedicated in this review to alloys based on Ti-(45—46)Al-(5-8)Nb (at.%) and Ti-48Al-2Cr-2Nb (at.%). 

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. 

The alloy y-TiAl is a tetragonal solid in which face-centered Ti + Al (001) planes alternate, with a relative displacement of 1/2 along the a (or, equivalently, b) axis (Fig. 11.10). The TiAl alloy is considered a covalent intermetallic compound, compared with the more ionic character of NiAl. Lu et al. (1994) compared the... 

Al, Brass, Ag, Cu, Pt, Ni, Stainless and C-Steel, AI2O3, Fiber Tows, Glass, Graphite, LaAlOs, MgO, NAFION , NiCr, Optical Fibers, Polycarbonate, Silica, Si, Si-Ti/Pt Wafers, SiC, Si3N4, Superalloys, Teflon , Ti, TiAl alloy, YSZ. 

A. Bartels, C. Koeppe, K. Wurzwallner, P. Schretter, H. Clemens Microstructure and Mechanical Properties of TiAl Alloys after Thermomechanical Processing. In H. Bildstein and R. Eck (eds.) Plansee Proceedings - Proc. 3th Interntl. Plansee Seminar, Vol. 3. Plansee Metall AG, Reutte (1993) 564-577. 

The formation of A1C13 is thermodynamically more favoured.The A1C13 and TiCl2 that are formed diffuse outward through the same pores and channels, where they react in the outer part of the scale, or at the scale surface. Any hydrogen that is formed diffuses away into the -y-TiAl alloy. As was identified for high aluminium containing titanium alloys, the a2-phase is preferentially attacked [27-29]. 

The presence of small amounts of salt, typically 37 xg/cm2, result in accelerated oxidation rates for a 7 + a2 TiAl alloy over the temperature range 500-700 °C. 

Cover picture Metallographic section of the oxide scale and the subsurface zone of a two-phase TiAl alloy after oxidation in air at 1350 °C for 1 h (interference layer metallography)... 

Figure 17. Fracture toughness, tensile strength, and tensile elongation at room temperature, and reciprocal secondary creep rate measuring the creep resistance as a function of microstructure (schematic) for two-phase TiAl alloys (Kim and Dimiduk, 1991).

The oxidation resistance of TiAl alloys can be improved by special pre-oxidation treatments (Suzuki etal., 1991). Alternatively, it can be improved by alloying with Nb, Ta, and W which, however, reduces the ductility whereas V, Cr, and Mn, which are used for increasing ductility, reduce the oxidation resistance (Kim, 1989). Ti Al-based alloys with high contents of Nb have been studied recently with respect to the conditions for protective scale formation, and indeed protective oxidation has been found at 1400°C for an Al content of 50 at.% (Brady et al., 1993). In view of the oxidation problems of the titanium alu-minides a coating has been proposed for providing sufficient oxidation protection, and various approaches have been studied (Nishiyama etal., 1990 Taniguchi etal., 1991 b Yoshiharaet al., 1991 Wuand Lin, 1993). 

H. Qemens, S. Mayer, Design, processing, microstructure, properties, and applications of advanced intermetaUic TiAl alloys, Adv. Eng. Mater. 15 (2013) 191—215. 

Physical Properties. Some thermal data for single crystalline and polycrystalline TiAl alloys are shown in Tables 3.1-30 and 3.1-31. 

Table 3.1-31 Thermal expansion coefficient a, thermal conductivity X and specific heat Cp at various temperatures T for polycrystalline TiAl alloys [1.62]...

Chemical Properties. The oxidation behavior of TiAl alloys is complex as both AI2O3 and/or Ti02 are formed... 

Ranges of yield strength arvl tensile elongalion as functions of test temperature for y-TiAl alloys. BDT, briWe-duclile transition. Source >ISMHteidboal< Vol2, lOthed., 1990,p929... 

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