Titanium aluminide

Titanium aluminide, TiAl, commonly gamma titanium, is an intermetallic chemical compound. It is lightweight and resistant to oxidation [1] and heat, however it suffers from low ductility. The density of γ-TiAl is about 4.0 g/cm³. It finds use in several applications including aircraft, jet engines, sporting equipment and automobiles. The development of TiAl based alloys began circa 1970. The alloys have been used in these applications only 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.

Applications of gamma-TiAl
Pole figures displaying crystallographic texture of gamma-TiAl in a rolled sheet of alpha2-gamma alloy, as measured by high energy X-rays.[2]

Gamma TiAl has excellent mechanical properties and oxidation 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 potential to increase the thrust-to-weight ratio in aircraft engines. This is especially the case with the engine's low-pressure turbine blades and the high-pressure compressor blades. These are traditionally made of Ni-based superalloy, which is nearly twice as dense as TiAl-based alloys. Some gamma titanium aluminide alloys retain strength and oxidation resistance to 1000 °C, which is 400 °C higher than the operating temperature limit of conventional titanium alloys.[3][4]

General Electric uses gamma TiAl for the low-pressure turbine blades on its GEnx engine, which powers the Boeing 787 and Boeing 747-8 aircraft. This was the first large-scale use of this material on a commercial jet engine[5] when it entered service in 2011[6]. The TiAl LPT blades are cast by Precision Castparts Corp. and Avio s.p.a.. Machining of the Stage 6, and Stage 7 LPT blades is performed by Moeller Manufacturing.[7] An alternate pathway for production of the gamma TiAl blades for the GEnx and GE9x engines using additive manufacturing is being explored.[8]

In 2019 a new 55g lightweight version of the Omega Seamaster wristwatch was made, using gamma titanium aluminide for the case, backcase and crown, and a titanium dial and mechanism in Ti 6/4 (grade 5). The retail price of this watch at £37,240 was nine times that of the basic Seamaster and comparable to the top of the range platinum-cased version with a moonphase complication.[9]

Alpha 2-Ti3Al

TiAl3
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References
  1. Voskoboinikov, R.E.; Lumpkin, G.R.; Middleburgh, S.C. (2013). "Preferential formation of Al self-interstitial defects in γ-TiAl under irradiation". Intermetallics. 32: 230–232. doi:10.1016/j.intermet.2012.07.026.
  2. Liss KD, Bartels A, Schreyer A, Clemens H (2003). "High energy X-rays: A tool for advanced bulk investigations in materials science and physics". Textures Microstruct. 35 (3/4): 219–52. doi:10.1080/07303300310001634952.
  3. Book chapters on TiAL
  4. Fundamentals of Creep in Metals and Alloys (Third Edition) Table 7
  5. Bewlay BP, Nag S, Suzuki A, Weimer MJ (2016). "TiAl alloys in commercial aircraft engines". Materials at High Temperatures. 33 (4–5): 549–559. doi:10.1080/09603409.2016.1183068.
  6. "GE Aviation Rolls Out its 1,000th GEnx Engine". AviationPros. 21 October 2015. Retrieved 10 August 2017.
  7. Moeller Manufacturing, Aerospace Division, in Wixom, Michigan, USA
  8. Heidi Milkert (18 August 2014). "GE Uses Breakthrough New Electron Gun For 3D Printing – 10X's More Powerful Than Laser Sintering". 3D Print.com.
  9. Tim Barber (31 August 2019). "The new Omega Seamaster Aqua Terra is made of titanium and weighs just 55g". Wired.
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