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http://gltrs.grc.nasa.gov/reports/1999/TM-1999-209071.pdf

Titanium aluminide
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Titanium aluminide, TiAl, is an intermetallic chemical compound. It is lightweight and resistant to oxidation and heat, however it suffers from low ductility. The density of gamma TiAl is about 4.0 g/cm³. It finds use in several applicationsincluding automobiles and aircraft. The development of TiAl based alloys began about 1970; however the alloys have only been used in these applications since about 2000.
Titanium aluminide has three major intermetallic compounds: gamma TiAl, alpha 2-Ti3Al and TiAl3. Among the three, gamma TiAl has received the most interest and applications. Gamma TiAl has excellent mechanical properties andoxidation and corrosion resistance at elevated temperatures (over 600 degrees Celsius), which makes it a possible replacement for traditional Ni based superalloy components in aircraft turbine engines.
TiAl based alloys have a strong potential to increase the thrust-to-weight ratio in the aircraft engine. This is especially the case with the engine's low pressure turbine blades and the high pressurecompressor blades. These are traditionally made up of Ni based superalloy, which is nearly twice as dense as TiAl based alloys.
General Electric has recently announced that gamma TiAl low pressure turbine blades will be used on its GEnx engine, which powers the Boeing 787 and Boeing 747-8 aircraft. This is the first large-scale use of this material on a commercial jet engine. The TiAl LPT bladesare cast by Precision Castparts Corp.
http://en.wikipedia.org/wiki/Titanium_aluminide

Titanium Aluminides - Intermetallics |
Background |
Aluminides are intermetallic compounds. A wide range of potentially useful alloys of these have been investigated with attention now concentrated on gamma (Ti-Al) and orthorhombic alloys. Alpha and super-alpha compositions (TiAl3 based) are no longer ofsignificant interest.Aluminide base alloys offer superior high temperature performance with low weight and non-burn. Gamma aluminide turbine blades based on Ti47Al2Cr2Ni have been tested and are strong as nickel based alloys up to 760°C (1400°F) and half the weight.Aluminides have generally proved difficult to process, have limited heat treatability, and generally low ductility at roomtemperature. Work continues however in an attempt to optimise these characteristics and deliver useful alloys for specific applications.Alloys based on orthorhombic phase Ti2Nb are candidate materials for use in future aeroengine gas turbines, due to their low density, high strength retention at elevated temperatures and their potential as matrix materials for fibre reinforced matl matrix composites(MMC’s). They have a desnity of 5.5g/cm3 (0.2lb/in3), which makes them lighter than steels or nickel based alloys they would be likely to replace. They do not suffer from the poor ductility and fracture toughness of gamma nased alloys, and possess better burn resistance compared to traditional titanium alloys. The most significant weakness appears to be poor oxidation and corrosion resistance, especiallywith the presence of some alloying elements. Research continues to fcus on understanding phase equilibria and stability; optimising composition, mechanical properties and processing routes; and enhancing the oxidation and corrosion resistance. |

http://www.azom.com/details.asp?ArticleID=1548

Very Long Term Oxidation of Titanium Aluminides Investigated
Titanium aluminides (TiAl) are ofgreat interest for intermediate-temperature (600 to 850 °C) aerospace and power-generation applications because they offer significant weight savings over today's nickel alloys. TiAl alloys are being investigated for low-pressure turbine blade applications, exhaust nozzle components, and compressor cases in advanced subsonic and supersonic engines (ref. 1).
Significant progress has been made in...