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The evaluation of the composition dependence of fracture toughness of Al3Nb alloys by using micro-size fracture testing Nobuhiro Matsuzaki 1*, Ken-ichi Ikeda 1, Seiji Miura1, Nobuaki Sekido2, Takahito Ohmura3 1 Division of Materials Science and Engineering, Graduate School of Engineering, Hokkaido University, Sapporo, Hokkaido 060-8628, Japan. *Graduate Student 2 Department of Materials Science, Tohoku University, Sendai, Miyagi 980-8579, Japan. 3 National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan. ABSTRACT Al3Nb is known as a high oxidation resistant material, while it is quite brittle. As the fracture toughness of Al3Nb single crystal and its dependence on the composition are not obtained, the micro-sized fracture testing proposed by Suzuki et al. was performed. Al3Nb single crystal micron-order size cantilevers with a chevron-notch were fabricated in a grain of two-phase polycrystalline alloys by using FIB (Focused Ion Beam). From the load-displacement curves during the bending by a nanoindenter, the average value of fracture toughness of Nb-rich Al3Nb is evaluated to be 2.90 MPam1/2, while the fracture toughness of Al-rich Al3Nb is also evaluated to be 2.82 MPam1/2. From this result, the fracture toughness of Al3Nb is less dependent on its Al/Nb ratio. Furthermore the fracture toughness of Al3 (Nb, V) was evaluated to be 2.82 MPam1/2.The fracture toughness of Al3Nb is seemingly insensitive to V addition. INTRODUCTION Nb alloys have been investigated as a new heat resistant material to replace the Ni-based superalloy [1]. However, Niobium shows low strength and poor oxidation resistance at high temperatures [2-3]. Now we focused on Al3Nb, which is known to have a good oxidation resistance [4], suitable for oxidation resisting coating. Also Al3Nb is expected as a dispersion strengthening phase for Nb-based bcc alloys. Therefore, mechanical properties of Al3Nb are crucial. Al3Nb is known to be brittle and its fracture toughness is reported to be as low as 1.8 MPam1/2 [5] and 2.5 MPam1/2 [6], however, it is not easy to obtain a large single phase Al3Nb sample for mechanical testing because of its narrow single phase region. Therefore, previous experiments have been conducted on two-phase alloys composed of FCC-Al or AlNb2. So, fracture toughness of its single-phase alloy has not been evaluated. Naturally, fracture toughness of its single crystal also has not been evaluated. Moreover, it has not been confirmed whether the difference of the fracture toughness of previous studies was caused by the effect of Al/Nb ratio of Al3Nb. Recently micro-size fracture testing methods were proposed [7-9], with which the properties of single crystal are evaluated by using small cantilever with a notch fabricated from polycrystalline alloys. This method has a large advantage that the single crystals specimens can be obtained from two-phase materials such as Al3Nb+Al. In other words, the properties of single crystal in equilibrium with other phases can be evaluated by the methods. Therefore, another advantage