Fatigue-Crack Propagation in Gamma-Based Titanium Aluminide Alloys at Large and Small Crack Sizes
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Most evaluations of the fracture and fatigue-crack propagation properties of y + a 2 titanium aluminide alloys to date have been performed using standard "large-crack" samples, e.g., compact-tension specimens containing crack sizes which are on the order of tens of millimeters, i.e., large compared to microstructural dimensions. However, these alloys have been targeted for applications, such as blades in gas-turbine engines, where relevant crack sizes are much smaller ( 5 mm) and small (c - 25-300 gm) cracks in a y-TiAl based alloy, of composition Ti-47A1-2Nb-2Cr-0.2B (at.%), specifically for duplex (average grain size -17 gm) and refined lamellar (average colony size -150 gim) microstructures. It is found that, whereas the lamellar microstructure displays far superior fracture toughness and fatigue-crack growth resistance in the presence of large cracks, in small-crack testing the duplex microstructure exhibits a better combination of properties. The reasons for such contrasting behavior are examined in terms of the intrinsic and extrinsic (i.e., crack bridging) contributions to cyclic crack advance. INTRODUCTION
Two-phase gamma-TiA1 based intermetallic alloys have received considerable attention in recent years as candidate materials for high-temperature aerospace and automotive applications, in particular as possible replacements for conventional nickel and titanium alloys in gas turbines [1-4]. Two conditions have been prominent: a duplex microstructure, consisting of equiaxed grains of y (TiAl) with small amounts of X2 (Ti3 AA)grains, and a lamellar microstructure, consisting of lamellar colonies containing alternating y and O2 platelets. In general, duplex structures display better elongation and strength properties, whereas lamellar structures show better creep resistance, toughness, and (large-crack) fatigue-crack growth resistance [1,2,5-8]. Although duplex structures have somewhat higher 'smooth-bar' fatigue limits [4,9], fatiguecrack growth properties, conventionally measured using large-crack specimens containing >5 mm long cracks, are clearly superior in lamellar structures [4,8,10-13]. However, preliminary indications are that this benefit is lost when tests are performed on small (5 mm) cracks [4,8,10]. Such superior performance of lamellar structures is evident in Fig. 2 which shows the relative crack-growth rates of large (through-thickness) cracks in several duplex and lamellar alloys, including those studied herein [11,12]. It is clear that the lamellar alloys have the higher threshold (AKTH) values and lower Paris power-law exponents. KK2.3.2
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Figure 2. A comparison of the fatiguecrack growth resistance of a fine nearly-lamellar microstructure Ti-47.7AI2.ONb-0.8Mn (XD) alloy, both duplex and fine fully-lamellar microstructure Ti-47AI-2Nb-2Cr-0.2B (MD) alloy, both duplex and coarse lamellar microstructures in a Ti-47.5AI-2.3Nb-1.5Cr0.4V (G7) alloy and a single phase y alloy (Ti-55AI with traces of Nb, Ta, C, and 0) [11,12].
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