A team consisting of GE Aircraft Engines, Precision Cast Parts, Oremet, and Chromalloy were chosen for a NASA-sponsored Aerospace Industry Technology Program (AITP) to develop a design and manufacturing capability that will lead to an engine test demonstration and the eventual implementation of a titanium-aluminide (TiAl) low-pressure turbine blade into commercial service. One of the main technical risks of implementing TiAl low-pressure turbine blades is the poor impact resistance of TiAl in comparison to the currently used nickel-based superalloy. The impact resistance of TiAl is being investigated at the NASA Lewis Research Center as part of this program.
The overall objective of this work is to determine the influence of impact damage on the high-cycle fatigue life of TiAl simulated low-pressure turbine blades. To minimize the quantity of high-cycle fatigue tests required, a design of experiments was initiated to determine the level of importance of various impact variables based on a microstructural analysis. The variables studied included energy, specimen thickness, hardness of projectile, and impact temperature.
Impact specimens were cast to size in a dog-bone configuration and given a typical processing sequence followed by an exposure at 650 °C for 20 hr to simulate embrittlement at service conditions. The specimens were impacted at 260 °C under a 69-MPa load. Steel projectiles with diameters of 1.6 and 3.2 mm and Rockwell hardnesses of 20 and 60 Rc were impacted at 90 to the leading edge. Backside crack lengths were emphasized in the analysis, but cracks on the front (impact) side were also characterized.
The experimental impact conditions chosen produced a spectrum of damage from minor denting to major cracking. The backside crack length was strongly dependent on the projectile energy, but the projectile hardness had no effect on crack length. Thin specimens and medium thickness specimens exhibited similar crack resistance for low and medium energies, but the crack lengths increased substantially for thin specimens impacted at high energies. Thick specimens exhibited improved impact resistance at all energy levels. Temperature had no effect on the backside crack length when either hard or soft projectiles impacted the specimens at the medium energy.
This study is being used by the industry-NASA team to determine the technical risks associated with impact issues, as well as the design and processing methods that could minimize such risks. The actual damage tolerable for the low-pressure turbine blade application will be determined by a combination of fatigue testing and consideration of actual engine conditions. Current evaluations indicate that Ti-47Al-2Nb-2Cr possesses the level of damage tolerance required for implementation. These results were incorporated into the selection of an airfoil design that balanced impact resistance with component weight.
Lewis contact: Susan L. Draper, (216) 433-3257,
Susan.L.Draper@grc.nasa.gov
Authors: Susan L. Draper, Dr. J. Michael Pereira, and Dr.
Michael V. Nathal
Headquarters program office: OASTT
Programs/Projects: Propulsion Systems R&T, HITEMP,
AITP
Research and TechnologyLast updated April 14, 1998, by Nancy.L.Obryan@nasa.gov
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