Fatigue-Crack Propagation in Gamma-Based Titanium Aluminide Alloys at Large and Small Crack Sizes J. J. Kruzic, J. P. Campbell and R. O. Ritchie Materials Sciences Division, Lawrence Berkeley National Laboratory, and Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720-1760 ABSTRACT Most evaluations of the fracture and fatigue-crack propagation properties of γ + α 2 titanium alu- minide alloys to date have been performed using standard “large-crack” samples, e.g., compact-ten- sion specimens containing crack sizes which are on the order of tens of millimeters, i.e., large com- pared 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 (<500 μm) and where the small-crack fatigue threshold may be the most relevant design parameter. In this study, we com- pare and contrast the cyclic crack-growth behavior of both large (a > 5 mm) and small (c ~ 25–300 μm) cracks in a γ-TiAl based alloy, of composition Ti-47Al-2Nb-2Cr-0.2B (at.%), specifically for duplex (average grain size ~17 μm) and refined lamellar (average colony size ~150 μm) microstruc- tures. 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-TiAl 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 γ (TiAl) with small amounts of α 2 (Ti 3 Al) grains, and a lamellar microstructure, consisting of lamellar colonies containing alternating γ and α 2 platelets. In general, duplex structures display better elonga- tion 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], fatigue- crack 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 (<500 μm) surface cracks [13]. As many potential applications for γ-TiAl alloys may involve fatigue design based on a small-crack threshold, the current work is focused on a comparison of large- and small-crack growth behavior in γ-TiAl based alloys and on the specific role of microstructure in influencing such behavior. EXPERIMENTAL PROCEDURES Duplex and lamellar microstructures (~10 vol.% α 2 ) were studied in a γ-based alloy of compo- sition Ti-47Al-2Nb-2Cr-0.2B (at.%); the B additions resulted in ~0.5 vol.% of needle-like TiB 2 parti- cles (~ 2-10 μm in length, ~1 μm in diameter). The lamellar microstructure (Fig. 1a) was obtained by forging and subsequent heat treating at 1370°C in flowing argon for 1 hr, air cooling and then holding KK2.3.1 Mat. Res. Soc. Symp. Proc. Vol. 552 © 1999 Materials Research Society