MTU Aero Engines announced in March that its internal experts and industry partners have jointly developed a new class of intermetallic, high-temperature materials for highly stressed engine components. Named titanium aluminide (TiAl), this new lightweight material is designed for application for turbine blades and combines the advantages of metallic and ceramic materials.
According to MTU COO, Dr. Rainer Martens, “While the introduction of a new material used to take 20 years or so, we’ve succeeded in coming up with an entirely new material class and maturing it for production within a mere seven years.”
The hardware is already flight worthy and in late September 2014 a development Airbus A320neo was the first aircraft ever to take to the skies with custom-made TiAl blades installed in its engines–the new P&W Pure Power geared turbofans (GTFs), which subsequently received certification in December. The blades in the new material are fitted to the third rotor stage of the three-stage, high-speed low-pressure turbine developed by MTU for the GTF engine for the A320neo and other new and re-engined aircraft.
Continuing research is underway and the company’s materials experts are busy developing an enhanced TiAl alloy aimed at manufacturing more turbine stages from the new material. An environmental bonus of the new material is that TiAl allows engines to be built that use up fewer resources, burn less fuel, and are cleaner and quieter than today’s engines.
MTU specialists have been thinking of ways to tap the immense potential afforded by TiAl-based intermetallic materials for aero engine applications for many years. In terms of mechanical properties, it is almost equivalent to the nickel alloys in use today, although its density is much lower, but it has a high melting point and a considerably higher creep strength than titanium alloys. These properties are attributable to the specific composition of the alloy and to the multiple heat treatments especially developed for the purpose.
Turbine blades in TiAl are about half the weight of comparable nickel-alloy components but boast the same reliability and durability. Also, the high aluminum content makes the material resistant to oxidation and corrosion. According to MTU, this is why TiAl is the ideal candidate for applications under extreme conditions–high temperatures and pressures—such as those to be found in a high-speed low-pressure turbine.
“We’ve been mulling the use of titanium aluminides ever since we started work on this unique low-pressure turbine for the geared turbofan,” said Dr. Wilfried Smarsly, a specialist in advanced materials at MTU.
TiAls are seen as enablers to open up new horizons for design engineers, helping to reduce the weight of other engine components. The high centrifugal forces acting on turbine disks and shafts required these components to be made from heavy nickel alloys to have sufficient mass. Thanks to the use of TiAl blades, these centrifugal forces are now much lower. As a result, the disk design can be optimized for appreciably lighter weight, and each reduction in weight will assist in improving fuel economy and CO2 emissions.
The biggest hurdle that stood in the way of the use of the lightweight material in the GTF was the fact that it is extremely difficult to form. Previously, it turned out impossible to forge turbine blades using conventional, affordable methods.
“We performed thermodynamic calculations to determine the optimum temperature range and phase configuration for forging,” said Prof. Dr. Helmut Clemens, who leads the Department of Physical Metallurgy and Materials Testing at the University of Leoben in Austria. Last year, Clemens, an MTU development partner, was honored in Japan with the Honda Award for his groundbreaking research work.
“With the TiAl alloy now developed, forging can be carried out on conventional forming machines—that’s what makes things so radically different,” he said.
It seems that TiAl is going to feature increasingly as new materials roll out of the realms of advanced R&D into production on new generation powerplants.