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20/1/04

2:52 PM

Page 439

Study on Notch Fracture of TiAl Alloys at Room Temperature J.H. CHEN, R. CAO, G.Z. WANG, and J. ZHANG In-situ observations of fracture processes combined with one-to-one observations of fracture surfaces and finite-element method (FEM) calculations are carried out on notched tensile specimens of twophase polycrystalline TiAl alloys. The results reveal that most cracks are initiated and propagated along the interfaces between lamellae before plastic deformation. The driving force for the fracture process is the tensile stress, which is consistent with a previous study.[1] In specimens with a slit notch, most cracks are initiated directly from the notch root and extended along lamellar interfaces. The main crack can be stopped or deflected into a delamination mode by a barrier grain with a lamellar interface orientation deviated from the direction of crack propagation. In this case, new cracks are nucleated along lamellar interfaces of grains with favorable orientation ahead of the barrier grain. The main crack and a new crack are then linked by the translamellar cleavage fracture of the barrier grain with increasing applied load. In order to extend the main crack, further increases of the applied load are needed to move the high stress region into the ligament until catastrophic fracture. The FEM calculations reveal that the strength along lamellar interfaces (interlamellar fracture) is as low as 50 MPa and appreciably lower than the strength perpendicular to the lamellae (translamellar fracture), which shows a value higher than 120 MPa. This explains the reason why cracks nucleate and preferably extend along the lamellar interfaces.

I. INTRODUCTION

MANY studies have been done that include in-situ observation to reveal the fracture processes of TiAl alloys.[2–6] The main conclusions about the fracture mechanism of lamellar microstructure of TiAl alloys can be drawn as follows. 1. The microcracks are formed either at colony boundaries, along lamellar interfaces, or at equiaxed
grains located at colony boundaries.[2] The crack propagates either along lamellar interfaces (interlamellar fracture) or skew to the lamellae (translamellar fracture), depending on the direction of main crack relative to the lamellar.[3,9] Deflection of the main crack by the lamellae, formation of a diffuse zone of microcracks and ligaments ahead of the crack tip, and the linkage of the microcracks with the main crack by shear fracture of the near-tip ligaments dominate the fracture process in the lamellar microstructure.[2] 2. The coarse lamellar microstructure shows an inferior tensile strength and ductility relative to that of the fine duplex microstructure. However, it shows superior fracture toughness in terms of initiation-toughness (KIC) and fracture toughness-resistance curves, i.e., the saturation toughness (Kmax). In addition, tensile ductility and fracture toughness depend upon colony or grain sizes in opposite manners.[2] The inverse ductility/KIC relationships are caused by the difference in samplin