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1128-U05-14

Microscale Fracture Toughness Testing of TiAl PST Crystals Daisuke Miyaguchi1, Masaaki Otsu1, Kazuki Takashima1 and Masao Takeyama2 1 Department of Materials Science and Engineering, Kumamoto University, 2-39-1, Kurokami, Kumamoto, Japan 2 Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, 2-12-1, Ookayama Meguro-ku, Tokyo, Japan ABSTRACT A microscale fracture testing technique has been applied to examine the fracture properties of lamellar in TiAl PST crystals. Micro-sized cantilever specimens with a size ≈ 10×20×50 µm3 were prepared from Ti-48Al two-phase single crystals (PST) lamellar by focused ion beam (FIB) machining. Notches with a width of 0.5 µm and a depth of 5 µm were also introduced into the specimens by FIB. Two types of notch directions (interlamellar and translamellar) were selected when introducing the notches. Fracture tests were successfully completed using a mechanical testing machine for micro-sized specimens at room temperature. The fracture toughness (KQ) values of the interlamellar type specimens were obtained in the range 1.5–3.6 MPam1/2, while those of the translamellar specimens were 5.0–8.1 MPam1/2. These fracture toughness values are lower than those having been previously reported in conventional TiAl PST samples. For macro-sized specimens, extrinsic toughening mechanisms, including shear ligament bridging, act in the crack wake, and the crack growth resistance increases rapidly with increasing length of crack wake for lamellar structured TiAl alloys. In contrast, the crack length in microsized specimens is only 2–3 µm. This indicates that extrinsic toughening mechanisms are not activated in micro-sized specimens. This also indicates that intrinsic fracture toughness can be evaluated using microscale fracture toughness testing. INTRODUCTION TiAl based alloys are promising for gas turbine engine components. This is because of their high specific strength and modulus coupled with their relatively good elevated temperature strength [1, 2]. Their mechanical properties, however, depend strongly upon microstructure, which varies widely with differing heat-treatments [3]. The microstructures of these alloys can be roughly divided into lamellar and duplex microstructures. The lamellar microstructure is expected to practical use, because it especially has good balance of elevated temperature properties, crack growth resistance and fracture toughness [4]. This increased fracture toughness, relative to duplex microstructure, is attributed to crack tip shielding by extrinsic toughening mechanisms, including shear ligament bridging. These extrinsic toughening mechanisms are dominated by microscopic lamellar structures including lamellar orientation [5, 6], colony size [7] and plate thickness [8]. It is therefore important to investigate the fracture properties of lamellar materials on the micro meter scale. In particular, the measurement of the interlamellar fracture toughness is required for designing of these alloys. However, it is extremely difficult to meas