MICROSTRUCTURAL EFFECTS ON AMBIENT AND ELEVATED-TEMPERATURE FATIGUE CRACK-GROWTH IN TITANIUM ALUMINIDE INTERMETALLICS

The role of microstructure in influencing the fatigue crack growth characteristics of a Ti-Al-Nb alloy at room temperature and at 700-degrees-C is investigated. Specifically, duplex microstructures composed of different proportions of alpha-2 and beta phases are produced by systematic heat treatments. The mechanisms of fatigue fracture in the dual-phase microstructures are examined, with particular attention devoted to the path of the fatigue crack through the microstructure and to the estimation of an intrinsic resistance to fatigue failure. The effects of test temperature, cyclic frequency, load ratio, environment, and crack closure on the fatigue behavior are also discussed. The results indicate that the different alpha-2 + beta microstructures investigated in this work exhibit fatigue crack growth rates which differ by as much as a factor of 500 at room temperature. Furthermore, microstructures with approximately the same overall yield strength and fracture toughness show variations in rates of fatigue crack propagation which differ by two orders of magnitude at the same imposed value of stress intensity factor range. Such differences, however, are drastically reduced in the elevated temperature environment. Such differences, however, are drastically reduced in the elevated temperature environment. Furthermore, microscopic mechanisms of fatigue crack growth at room temperature are distinctly different from those observed at the elevated temperature, with the test temperature having either a deleterious or beneficial effect on fatigue resistance, depending on the microstructural constitution. A quantitative description of the inherent effects of microstructure on the rates of fatigue crack growth is presented for different heat-treatment conditions in an attempt to provide guidelines for the development of titanium aluminide alloys with superior resistance to cyclic fracture.