ELSEVIER Materials Science and Engineering A192/193 (1995) 457-464 Effects of microstructure and temperature on fatigue crack growth in the TiAl alloy Ti-46.5A1-3Nb-2Cr-0.2W Stephen J. Balsonea, James M. Larsen”, David C. Maxwellb, J. Wayne Jonesc “ Air Force W right Laboratory, MaterialsDirectorate, W L/MLLN, W right-Patterson AFB, OH 45433, USA bUnivers‘ty f I o Da y ton Research Institute,300 College Park, Dayton, OH 45469, USA cDepartment of M aterialsScience and Engineering, University of Michigan, Ann Arbor, MI 48109, USA Abstract The fatigue crack growth behavior of forged Ti-46.5AI-3Nb-2Cr-0.2W ( t “/ ) a . o was investigated. Heat treatments were used to generate both a nearly fully lamellar microstructure (grains of a,-y lamellae) and a duplex microstructure (equiaxed y and lamellar grains) to span the wide range of microstructural conditions available through thermomechanical processing of these alloys. Fatigue crack growth tests using load-shedding threshold and constant-load-amplitude techniques were conducted at room temperature, 600 “C (below the ductile-to-brittle transition temperature (DBTT)) and 800 “C (above the DBTT). Results show that the fatigue crack growth resistance of the lamellar microstructure is superior to that of the duplex microstructure. The nature of fatigue crack advance depends strongly on microstructure, which explains, at least in part, the differences observed in crack growth rates for the lamellar and duplex microstructures. Fractography was conducted to identify the dominant crack growth mechanisms in both the lamellar and duplex microstructures. Keywords: Titanium; Aluminium; Niobium; Chromium; Tungsten; Fatigue 1. Introduction y-Ti aluminides, alloys based on the ordered TM phase, are being developed for selected gas turbine engine applications because of their attractive high temperature, density-corrected properties. In fracture- critical applications, adequate knowledge of the damage tolerance of these materials is essential [l]. Therefore interest in the fatigue crack growth resist- ance of this class of intermetallics has been increasing in recent years and a number of studies exploring the influence of microstructure, temperature and environ- ment on fatigue crack growth behavior have been conducted [2-121. A number of these studies have shown that fatigue crack propagation depends significantly on micro- structure. In alloys processed to produce a lamellar microstructure, experimental results have shown substantial improvements in crack growth resistance and levels of threshold stress intensity factor range AK, when compared with the behavior of alloys with a duplex or predominantly y microstructure. The poorer crack growth resistance in the duplex microstructures is generally attributed to brittle modes of crack advance during fatigue. The improvement in fatigue crack growth resistance in the lamellar microstructure may arise in part from the anisotropic nature of the microstructure and from crack-tip-shielding mechan- isms that result from crack deflection and limited crack bridging in such microstructures. In the present study the influence of microstructure and temperature on fatigue crack propagation behavior is examined in a relatively new TiAl alloy processed to produce both a nearly fully lamellar and a duplex microstructure. Fatigue crack growth behavior is examined using load-shedding threshold and constant- load-amplitude techniques at room temperature, 600 and 800 “C. The ductile-to-brittle transition tempera- ture (DBTT ), as defined by tensile ductility, lies between 600 and 800 “C for the duplex microstructure and is approximately 800 “C for the lamellar micro- structure. The dominant fatigue crack growth modes in both microstructures were examined in this study by scanning electron fractography. 0921-5093/95/$9.50 0 1995 - Elsevier Science S.A. All rights reserved SSDI 0921-5093(94)03262-9