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Fatigue-Crack Propagation in Gamma-Based Titanium Aluminide Alloys at Large and Small Crack Sizes

Published online by Cambridge University Press:  10 February 2011

J. J Kruzic ,
J. P Campbell  and
R. O. Ritchie
J. J Kruzic
Affiliation:
Materials Sciences Division, Lawrence Berkeley National Laboratory, and Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720-1760
J. P Campbell
Affiliation:
Materials Sciences Division, Lawrence Berkeley National Laboratory, and Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720-1760
R. O. Ritchie
Affiliation:
Materials Sciences Division, Lawrence Berkeley National Laboratory, and Department of Materials Science and Mineral Engineering, University of California, Berkeley, CA 94720-1760
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Abstract

Most evaluations of the fracture and fatigue-crack propagation properties of γ + α2 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 (<500 µm) and where the small-crack fatigue threshold may be the most relevant design parameter. In this study, we compare 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-47A1–2Nb-2Cr-0.2B (at%), specifically for duplex (average grain size ∼17 µm) and refined lamellar (average colony size ∼150 µm) 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.

Type
Research Article
Information
MRS Online Proceedings Library (OPL) , Volume 552: Symposium KK – High-Temperature Ordered Intermetallic Alloys VIII , 1998 , KK2.3.1
Copyright
Copyright © Materials Research Society 1999

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