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Evaluation of fracture toughness of alpha-Nb5Si3 by micro-sized cantilever beam testing Shiori Suzuki*1; Nobuaki Sekido2; Takahito Ohmura2; Seiji Miura1; 1Division of Materials Science and Engineering, Faculty of Engineering, Hokkaido University; 2National Institute for Materials Science; *Graduate student ABSTRACT A micro-sized fracture testing method has been applied to investigate fracture toughness of alpha-Nb5Si3. Chevron-notched single crystal specimens with a size of 3 x 3 x 15 μm3 were prepared in a grain of polycrystalline alpha-Nb5Si3 by focused ion beam, FIB, technique. Fracture tests were conducted using a nanoindenter at room temperature and linear load-displacement curves and smooth fracture surfaces were obtained. This fracture behavior was presumed to be brittle fracture similar to bulk alpha-Nb5Si3. The average of fracture toughness KQ is 3.45 ± 0.29 MPa√m under a small-scale yielding condition. INTRODUCTION Among various refractory materials, many investigations have been conducted on alloys with Nb/Nb-silicide two-phase microstructure because of its high melting point and low density [1-11]. However, these alloys still show poor ductility at ambient temperatures. The microstructure control based on “Ductile-phase toughening” with fine brittle Nb5Si3 particles dispersed in ductile bcc-Nb phase matrix is a way to improve their poor toughness [5, 9-11]. In this microstructure the small brittle phase tends to be a single crystal and the fracture toughness of single crystal alpha-Nb5Si3 is needed to understand the fracture behavior of bulk two-phase alloys. However, it is difficult to prepare single crystal or coarse grained alpha-Nb5Si3 for traditional fracture testing methods, such as bending test for macro specimens or indentation fracture method, because alpha-Nb5Si3 is formed from a high melting point beta-Nb5Si3 through a phase transformation. Recently, the investigations of mechanical properties using micro-sized testing have been actively conducted for the development of microelectromechanical systems devices [12-15]. The micro-sized fracture testing method can be applied for the investigation of the

mechanical properties of grain boundary, interface, and grain interior of brittle materials because the size effect caused by local plastic deformation is quite small. The brittle Nb5Si3 seems to fulfill such a condition. Therefore, in this study, we attempt to conduct fracture tests on alpha-Nb5Si3 single crystal by preparing the micro-sized specimen from a grain of alphaNb5Si3. Micro-sized fracture testing method The chevron notch bend test is advantageous to evaluating the fracture toughness of brittle materials, because no fatigue pre-crack is required to be introduced before testing [16]. This characteristic notch geometry facilitates the initiation of a natural sharp crack, and induces a stable crack growth during loading. Figure 1 shows a schematic drawing of the micro-sized specimen used in this study. The specimen dimensions are B=3, W=3, L=15 (μm), and a chevron notch with the notch ang