Fatigue and Fracture of Titanium Aluminides Program

 

Program Highlights

The efficiency of high-performance gas turbines is limited by the temperature capabilities of materials used for engine components. These applications demand materials with higher specific stiffness, strength and damage tolerance at higher operating temperatures. In this regard, gamma-TiAl based intermetallic alloys show particular promise due primarily to their lower density compared to the Ti- and Ni-base alloys currently used in engines. In addition, due to their adequate creep resistance and improved strength retention at elevated temperatures, gamma-TiAl alloys have become attractive candidates to replace conventional titanium alloys in the compressor sections of engines. Although various microstructures have been developed, optimal mechanical properties have been obtained to date by modifying the composition and thermomechanical processing to achieve a two phase, (TiAl + Ti3Al), structure. The two phases can be distributed as fine or coarse lamellar microstructures, with ~100-2000 micron-sized colonies of alternating TiAl and Ti3Al layers, or as duplex microstructures with fine equiaxed (~10-40 micron-sized) TiAl and Ti3Al grains. Accordingly, in the present program of study, ambient and high temperature fatigue-crack growth behavior in titanium aluminide alloys is studied with consideration of the role of environment and microstructure. Specifically, microstructural effects are examined in terms of the role of extrinsic toughening mechanisms on crack-growth resistance, principally uncracked ligament bridging and roughness-induced crack closure. Additionally, the role of crack size is examined by comparing the growth-rate behavior of "small" (~70-500 micron) surface cracks and "long" (> 5 mm) through-thickness cracks.

As gamma-TiAl alloys are being developed for applications requiring temperatures up to 800 C, it is important to evaluate the fatigue properties at these high temperatures. This graph shows the fatigue-crack growth rate vs. the applied stress intensity range for Ti-47Al-2Nb-0.8Mn + 1 vol. % TiB2 particles at three temperatures: 25, 600, and 800 C.
 
 
 
 

Researchers

Josh Campbell
Jay Kruzic
Aindrea McKelvey
K.T. Rao
Prof. Robert Ritchie

Publications