g-TiAl is an attractive new material for aerospace applications because of its low density and high specific strength in comparison to currently used titanium and nickel-base alloys. However, this intermetallic is inherently brittle, and long-life durability is a potential problem. As part of the aviation safety goal to reduce the aircraft accident rate, the potential for g-TiAl to be used for robust low-pressure turbine blades has been thoroughly studied. The effect of impact damage and casting porosity on the fatigue life of cast Ti-48Al-2Cr-2Nb alloys was assessed. The Al content of TiAl can vary slightly because of the casting process, so the effect of a lower Al content on the impact resistance and resulting fatigue strength was also studied. Ti-48Al-2Cr-2Nb is one of the original cast TiAl alloys to show promising mechanical properties. However, more recently developed alloys have been developed for enhanced mechanical properties. One of these alloys, ABB-2, has a much higher strength but only half the ductility of Ti-48Al-2Cr-2Nb. The ballistic impact resistance and remnant fatigue strength of ABB-2 was determined and compared with Ti-48Al-2Cr-2Nb. In addition, the effect of fretting damage on the fatigue life of cast Ti-48Al-2Cr-2Nb was studied at the NASA Glenn Research Center.
Fatigue strength of Ti-48Al-2Cr-2Nb and ABB-2 decreased with increasing surface crack length.
Ballistic impacts resulted in two forms of cracks, the severity of which depended on the impact energy. At lower impact energies, the specimens failed in fatigue from backside cracks that were perpendicular to the specimen axis. At higher impact energies, the fatigue failure initiated from the hertzian cracks on the front side of the specimen (ref. 1). Hertzian and backside crack lengths were similar for a particular impact condition for both high and low Al content Ti-48Al-2Cr-2Nb alloys as well as for the ABB-2 alloy. Defect size played a large role in determining the critical fatigue loads. Increasing the defect size, regardless of whether the flaws resulted from casting porosity or from impact cracks, led to a decrease in the fatigue strength according to the 1/(sqrt a ) relationship described by fracture mechanics (ref. 2). The fatigue strength of Ti-48Al-2Cr-2Nb and ABB-2 showed a similar dependence on external crack length, and this was due to the fact that both materials had the same fatigue threshold stress intensity. Therefore, to improve damage tolerance, an alloy with a higher fatigue threshold is required. Within the limits of the test program, the fatigue strength of Ti-48Al-2Cr-2Nb was not affected by fretting damage (ref. 3), indicating its excellent fretting resistance. Synthesis of the entire data set in regards to its impact, chemistry, processing, fatigue, and fretting has demonstrated that TiAl has sufficient durability to allow the design of a robust low-pressure turbine blade.
ABB-2 fatigue fracture surfaces showing backside crack initiation for low-energy impacts and front side, or hertzian, crack initiation for high-energy impacts.
Glenn contact:
Susan Draper, 216-433-3257, Susan.L.Draper@grc.nasa.gov
Authors: Susan L. Draper, Dr.
Bradley A. Lerch, Dr. Kazuhisa Miyoshi, and Dr. J. Michael Pereira
Headquarters program office: OAT
Programs/Projects: Ultra Safe, RTA
Last updated: June 2002
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