Mechanisms of ambient temperature fatigue crack growth in Ti-46.5Al-3Nb-2Cr-0.2W
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INTRODUCTION
IN recent years, near-g titanium aluminides, alloys based on the ordered TiAl phase, have attracted considerable interest as candidate materials for selected gas turbine engine applications because of their attractive high-temperature mechanical properties and relatively low density. In fracture critical applications, adequate knowledge of the damage tolerance of these materials is essential,[1] and an increasing effort is being devoted to understanding the influence of microstructure, temperature, and environment on cyclic properties that determine service lifetime. It has been demonstrated that tensile strength, ductility, creep resistance, fracture toughness, and fatigue crack growth resistance depend sensitively on microstructure in g-TiAl alloys.[2,3,4] In particular, significant differences in properties are observed between the lamellar microstructures (near-lamellar and fully lamellar) and the microstructures comprised mainly of equiaxed g grains (near-g and duplex). For the specific case of fatigue crack growth, a number of studies[3,5–18] have shown that g-TiAl alloys processed to produce a lamellar microstructure show substantial improvement in crack growth resistance and an increase in the threshold stress intensity factor range, DKth, when compared with the behavior of alloys with a duplex or preB.D. WORTH, Associate Research Engineer, is with the Structural Integrity Division, University of Dayton Research Institute, Dayton, OH 45419-0128. J.M. LARSEN, Research Group Leader, is with the United States Air Force, Wright Laboratory, Materials Directorate, WL/MLLN, Wright-Patterson Air Force Base, OH 45433-7818. S.J. BALSONE, formerly with the United States Air Force, Wright Laboratory, Materials Directorate, is an Engineer with the Physical Metallurgy Laboratory, General Electric Company, Schenectady, NY 12301. J.W. JONES, Professor, is with the Department of Materials Science and Engineering, The University of Michigan, Ann Arbor, MI 48109-2136. Manuscript submitted February 29, 1996. METALLURGICAL AND MATERIALS TRANSACTIONS A
dominantly equiaxed g microstructure. This fatigue crack growth behavior parallels the significantly higher monotonic fracture toughness of g-TiAl alloys with lamellar microstructures, as compared to those with duplex or near-g microstructures. Chan and co-workers[18–21] have examined the effect of microstructure on toughness in detail and attribute the increase in fracture toughness and the accompanying R-curve behavior of the lamellar microstructure to microcracking in the process zone ahead of the crack tip and shear ligament bridging mechanisms in the crack wake. The lower toughness in the duplex alloys is attributed to the absence of extrinsic toughening mechanisms such as bridging. Although it is tempting to use this bridging and ligament toughening concept to explain the observed influence of microstructure on fatigue crack growth behavior, the situation is somewhat more complicated. It does not appear that the fracture processes that lead to extrinsic to
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