High-temperature fracture and fatigue-crack growth behavior of an XD gamma-based titanium aluminide intermetallic alloy
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INTRODUCTION
IN recent years, the aerospace industry has shown considerable interest in titanium aluminides based on the g -TiAl phase for potential use in aircraft engines. Compared to Ti-6Al-4V and Ni-based superalloys, which are presently used, g -TiAl alloys offer several advantages. Gamma-based TiAl alloys have a low density and retain strength and good oxidation resistance at elevated temperatures due to the formation of a passivating layer of titania and alumina, which are stable up to 1870 8C and 2054 8C, respectively.[1,2] However, their toughness and ductility are significantly lower than conventional Ti- and Ni-based alloys. It is, therefore, critical that fracture toughness and fatigue-crack growth resistance be both understood and improved before the widespread use of titanium aluminides for structural applications is implemented. Whereas several mechanisms have been identified to date for improving the ambient-temperature fracture resistance of g -TiAl alloys, including crack deflection,[3–7] slip-band decohesion,[8,9] twin toughening,[9,10,11] and, particularly, crack bridging,[3,10–18] there is far less understanding of the A.L. McKELVEY, formerly Graduate Student, Department of Materials Science and Mineral Engineering, University of California at Berkeley, is Technical Specialist with the Materials Science Department, Ford Research Laboratory, Dearborn, MI 48121-2053. K.T. VENKATESWARA RAO, formerly Research Associate, Department of Materials Science and Mineral Engineering, is Director for Research, Advanced Cardiovascular Systems, Guidant Corporation, Santa Clara, CA 95052-8167. R.O. RITCHIE, Professor, is with the Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720-1760. Manuscript submitted December 15, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
fracture resistance at higher temperatures, i.e., in the 600 8C to 800 8C range. Indeed, it is unclear whether the lowtemperature toughening mechanisms actually are beneficial or detrimental at such elevated temperatures. Moreover, there have been even fewer studies to characterize the corresponding fatigue-crack propagation of g -TiAl alloys, especially at high temperatures.[18–23] The effects of lamellar orientation,[16,19–21,24] twin deformation,[25,26,27] load ratio,[18,22,27] crack bridging,[18] and microstructure[18,22] on growth rates have been analyzed, although, again, the majority of studies have been focused on room-temperature behavior. Based on fatigue-crack growth measurements in hightemperature air, several studies[22,23,28–31] have reported that the fastest crack growth rates and lowest fatigue thresholds in g -TiAl alloys are seen at intermediate temperatures, between 540 8C and 650 8C, whereas the slowest growth rates and highest thresholds are seen at elevated temperatures, ,760 8C to 800 8C; the properties at 25 8C are intermediate. Balsone et al.[29] and Larsen and Balsone[32] have found that this apparently anomalous temperature effect is less striking in vacuo than in air,
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