Evolution of fatigue crack growth and fracture behavior in gamma titanium aluminide Ti-43.5Al-4Nb-1Mo-0.1B (TNM) forgings

The effects of thermal and thermomechanical processing, sample orientation, and load ratio on the room temperature fatigue crack growth and fracture behavior of a third-generation gamma titanium aluminide Ti-43.5Al-4Nb-1Mo-0.1B (TNM) were determined in this study. Bend bar specimens, excised from TNM in as-cast, cast + hot isostatic pressing, and forge + heat treat conditions in the longitudinal and transverse directions, were tested at room temperature in lab air. Load ratio in the range 0.1-0.9 was used in fatigue testing to determine its effect on the fatigue threshold, Paris regime, and stress intensity at overload. Microscopy, fractography, and X-ray diffraction were used to understand the effects of process and testing variables on the fatigue crack path and morphology. The effects of microstructure length scales and microconstituents are discussed in relation to the properties measured.