Alloy TiAl-based

J. M. Rakowski, F. Dettenwanger, E. Schumann, G. H. Meier, F. S. Pettit, M. Riihle "The Effect of Surface Preparation on the Oxidation Behavior of Gamma TiAl Base lntermetallie Alloys , submitted to Scripta Mater. 

The high-temperature oxidation behaviour of y-TiAl based compounds implanted with various elements was studied and compared with materials in which some of these elements were added by alloying.The results presented have shown that... 

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. 

TiAl-base alloys are in the range 160-180 GPa which is only 10-20% lower than that of the superalloys (see Table 2). Recently, it has been found by ab initio calculations that deviations from stoichiometry are due to accommodated antistructure atoms, i.e. constitutional disorder, instead of vacancies in the sublattices, and that the concentration of thermal vacancies is comparatively low because of the high formation energy (Fu and Yoo, 1993). The self-diffusion of Ti in TiAl has been studied (Kroll etal., 1992). 

The intermetallic compounds Tis A1 and TiAl are studied for high-temperature materials developments. Extensive accounts [1.52,57] are the sources of the data presented here. In Table 3.1-21 the property ranges of TisAl- and TiAl-based alloys are compared to those of conventional Ti alloys and Ni-based superalloys. 

Various jet engine and car engine components have been identified for application of TiAl-based alloys. It is particularly noteworthy that large sheets, which are suitable for superplastic forming and joining by diffusion bonding, can be produced from these intrinsically brittle intermetallic compound alloys. 

Mechanical Properties. Elastic properties of TiAl-based materials are compiled in Table 3.1-32. Strength, ductility, and toughness of the TiAl-based alloys are sensitive functions of both composition and microstructure which is controlled by prior processing [1.59]. Characteristic data are shown in Table 3.1-33 for various alloys. Different property data for the same alloy composition indicate the effect of different prior thermo-mechanical treatments. It is noted that TiAl-based alloys are prone to hydrogen/environmental embrittlement depending on the amount of q 2 from Ti3Al [1.60]. Creep and fatigue data are available [1.59,61]. 

Table 3.1-28 Standard microstructures of TiAl-based alloys [ 1.59] ...

Table 3.1-3tr Characteristic oxidation data for some TiAl-based alloys of Table 3.1-29. Apparent paraboUc rate constant...

Usually y-TiAl-based alloys contain Q 2-Ti3Al as a second phase and are, therefore, subject to hydrogen uptake and hydrogen/environmental embrittlement depending on the amount of Q 2-Ti3Al. 

Ti-based alloys have long been known to be susceptible to hot-salt stress corrosion (e.g. Gray, 1973). Recent work (Nicholls et al., 1997) indicates that TiAl-based alloys suffer from accelerated hot-salt corrosion attack in the temperature range 650 to... 

S.M.L.Sastiy and H.A Lipsitt, Fatigue Deformation of TiAl Base Alloys, MetaU. Thins. A, Vol 8,1977, p 299... 

Property Ti-based alloys TisAl-based intermetallic materials TiAl-based intermetallic materials Ni-based superalloys... 

Copland E H, Gleeson B and Yonng D J (1999), Formation of Z-Ti5o-Al3o-O2o in the suboxide zones of y-TiAl based alloys during oxidation at 1000°C , Acta Mater, 47 (10), 2937-2949. 

Kawaura H, Nishino K and Saito T (1999), New surface treatment using a fluidized bed for improving oxidation resistance of TiAl-base alloys , J Japan Inst Met, 63 (12), 1584-1590. 

Li X Y, Zhu Y C, Fujita K, Matsunaga Y, Nakagawa K and Taniguchi S (2001b), Improvement of oxidation resistance of y-TiAl based alloy by high temperature Nb implantation . Mater High Temp, 18 (S), 379-384. 

Li X Y and Taniguchi S (2004), Oxidation behavior of TiAl based alloys in a simulated combustion atmosphere , IntermetaUics, 12 (1), 11-21. 

Sunderkbtter J D, Gobel M, HaanappelVAC, Perez P and StroosnijderM F (1999), The niobium effect on the oxidation behaviour of TiAl-based alloys An approach to decouple relevant parameters . Gamma Titanium Aluminides 1999, San Diego, TMS. 

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