Compression of Micropillars of TiAl Coexisting with Ti 3 Al
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Compression of Micropillars of TiAl Coexisting with Ti3Al Kazuki Fujimura1, Kyosuke Kishida1, Katsushi Tanaka2 and Haruyuki Inui1 1 Department of Materials Science and Engineering, Kyoto University Sakyo-ku, Kyoto 606-8501, Japan 2 Department of Mechanical Engineering, Kobe University 1-1 Rokkoudai-cho, Nada-ku, Kobe 657-8501, Japan ABSTRACT Square-shaped micropillar of nearly stoichiometric TiAl single crystals with various loading axis orientations were prepared from TiAl PST crystals by focused ion beam (FIB) technique and deformed in compression using a micro hardness testing machine equipped with a flat diamond tip in order to investigate the values of CRSS for the three types of operative deformation modes, namely {111}1/2 ordinary slip, {111} superlattice slip and {111} twinning. The selective activation of the three types of deformation modes was confirmed to be achieved by compression tests of [¯211]-, [¯110]- and [¯1¯12]-oriented single crystalline micropillars, respectively. The average CRSS values for ordinary slip, superlattice slip and deformation twinning obtained for micropillars with an initial side length between 3.8 and 7.9ȝm were estimated to be about 145, 284 and 113 MPa, respectively. INTRODUCTION TiAl/Ti3Al two-phase alloys with a lamellar structure are one of the most promising materials for high temperature structural applications due to their excellent mechanical properties such as good fracture toughness and creep resistance [1]. We have systematically studied the intrinsic properties of the two-phase alloys using our polysynthetically twinned (PST) crystals composed of the lamellar structure with a single lamellar orientation over the entire crystal and also those of component TiAl and Ti3Al phases [1-7]. In TiAl phase with the tetragonal L10 structure, three different deformation modes, namely {111}1/2 ordinary slip, {111} superlattice slip and {111} twinning, have been known to be operative [4,6,7]. Previous studies on mechanical properties of Al-rich TiAl using single crystals have revealed the temperature and composition dependences of critical resolved shear stresses (CRSS) for the three deformation modes [4]. However, the values of CRSS for these deformation modes in nearly stoichiometric TiAl phase coexisting with Ti3Al phase have not been directly determined mainly because large-scale single crystals, which are large enough to carry out standard uniaxial compression or tension tests, have not been obtained. Instead, the approximate CRSS values for bulk TiAl crystals have been deduced to be about 45, 59 and 38 MPa for ordinary slip, superlattice slip and deformation twinning, respectively, the values of which are estimated from our previous results of PST crystals and Al-rich TiAl single crystals [4,7]. The exact values of CRSS for the three different deformation modes operative in nearly stoichiometric TiAl are essential factors in understanding not only the inherent characteristics of the three different deformation modes but also the deformation behavior of TiAl/Ti3Al two-phase
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