Fracture Behavior of Micro-Sized Specimens Prepared from a TiAl Thin Foil
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Fracture Behavior of Micro-Sized Specimens Prepared from a TiAl Thin Foil K. Takashima, T. P. Halford, D. Rudinal, Y. Higo and P. Bowen1 Precision and Intelligence Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama, 226-8503, Japan 1 Department of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham B15-2TT, United Kingdom ABSTRACT Fracture tests have been carried out on micro-sized specimens prepared from a fully lamellar γ-TiAl based alloy thin foil. Micro cantilever beam type specimens with dimensions ≈ 50 x 10 x 20 µm were prepared from one lamellar colony of the thin foil by focused ion beam machining. Notches with a width of 0.5 µm and a depth of 10 µm were also introduced into the micro-sized specimens by focused ion beam machining. Notch directions were introduced into samples in order to select the trans- and inter-lamellar directions, respectively. Fracture tests were carried out using a mechanical testing machine for micro-sized specimens. Fracture tests for the micro-sized specimens were performed successfully, showing the fracture behaviour to be dependent upon the notch orientation. The fracture toughness of specimens with a notch direction perpendicular to the lamellar direction was 4.7 – 6.9 MPam1/2, while that with a notch direction in the inter-lamellar direction was 1.4 – 2.7 MPm1/2. This indicates that the orientation of the lamellar microstructure greatly affects the fracture properties of micro-sized components prepared from fully lamellar γ-TiAl based alloy thin foils. It is required to consider the results obtained in this investigation when designing actual micro scale structures using TiAl thin foils. INTRODUCTION Fully lamellar γ-TiAl based materials, such as Alloy 7 (Ti-46Al-5Nb-1W), have been developed by the aerospace industry in order to replace the Ni-based alloys currently used in gas turbine engine compressors. This is intended to gain a significant weight advantage [1]. These materials are being considered for applications of this type due to the high yield stress, creep limit and Young’s modulus available from them, in combination with a low density and the retention of these properties at temperatures up to 700°C [2]. The same properties also make γTiAl based materials an attractive possibility for the Microelectromechanical systems (MEMS) market. MEMS devices are currently being implemented into applications including sensors and actuators in automotive and other fields [3]. Intended future applications, however, include aeronautics and space travel, where significant advantages may be derived from miniaturization, leading to cascading benefits such as reduced fuel requirements and costs. In order to apply MEMS devices in these areas an increased understanding of materials that are capable of providing the necessary mechanical properties at elevated temperatures is needed. For this reason an increased understanding of the fracture properties of microsized samples of lamellar γTiAl based materials is highly desirable. Thes
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