Alloying TiAl-base materials

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]. 

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. 

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. 

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

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.%). 

Titanium aluminide alloys based on Ti3 A1 and TiAl are of interest as construction material for high temperature components particularly in aerospace industry. Good mechanical properties can be attained with alloys consisting of y-TiAl with 3 to 15 vol% a2-Ti3Al. The disadvantages are the low ductility and the inadequate oxidation resistance at service temperatures of 700-900°C [1]. A fundamental understanding of the oxidation behaviour is necessary in order to improve the corrosion resistance. The formation of the oxides on the alloy surface depends on the temperature, the oxygen partial pressure of the corrosive atmosphere, and the thermodynamic activities of Ti and A1 in the alloys. 

Briefly reviewing the technical structural intermetallic alloys, a commercial alloy based on NisAl with small B, Zr, Hf, and Cr additions is now being commercialized for propulsion applications TiAl- and TiaAl-based alloys are considered especially promising as matrix materials for metal-matrix composites that are given high-temperature strength by SiC or AI2O3 fibers. ... 

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-... 

M. J. Blackburn and M.P. Smith, The Understanding and E loitation of Alloys Based on the Compound TiAl (Gamma Pharc), Tfechni-cal Report AFML-TR-79-4056, U.S. Air Force Materials Laboratory, 1979... 

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