Mechanical Properties of Cr 5 Si 3 with the D8 m Structure
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Mechanical Properties of Cr5Si3 with the D8m Structure Yuji Ochiai1, 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 Elastic properties, thermal expansion and deformation behavior of Cr5Si3 with the D8m structure were investigated using single crystals. From the values of Cauchy pressures as well as the ratio of the polycrystalline bulk modulus (B) to shear moduls (G) estimated from singlecrystal elastic constants (cij), deformation behavior of Cr5Si3 is expected to be relatively brittle compared to Mo5Si3 with the same crystal structure. However, plastic deformation of Cr5Si3 is confirmed above 900 ~ 1100 °C depending on the loading axis orientations. INTRODUCTION Transition-metal silicides with a general formula of TM5Si3 (TM = transition-metal) have become the subject of extensive studies because of their very high melting temperatures. However, TM5Si3 are expected to exhibit poor fracture toughness and ductility at room temperatures in the monolithic form because of their very complex crystal structures of the D8m, D8l and D88-types. In fact, most studies on TM5Si3 have been focused on utilizing them as strengthening phases in two phase composite. Hence, almost nothing is known about the inherent deformation mechanisms of TM5Si3 in the monolithic form. Among various types of TM5Si3 with different crystal structure, most of TM5Si3 including Mo5Si3, HT-Nb5Si3 have a complex tetragonal structure of the W5Si3-type (the D8m structure) with the space group of I4/mcm and therefore, it is quite important to systematically understand the mechanical properties of TM5Si3 with the D8m structure (figure 1). In the present study, we prepared single crystals of Cr5Si3 (melting point: 1680ÛC) and investigated its mechanical properties such as elastic constants, thermal expansion and deformation behavior. EXPERIMENTAL Single crystals of binary stoichiometric Cr5Si3 with a nominally composition of Cr-37.5 at%Si were grown using an optical floating zone (FZ) furnace at a growth rate of 7 mm/h under an Ar gas flow. Rectangular parallelepiped specimens having three orthogonal faces parallel to the (100), (010) and (001) planes for measurement of elastic constants and thermal expansion were cut from the single crystals by electric discharge machining. The specimen surfaces were mechanically polished and finished with 0.3 ȝm alumina abrasive powders. Measurements of the elastic constants were carried out by the rectangular parallelepiped resonance (RPR) method at room temperature [1]. Thermal expansion along a- and c-axes was measured at a heating rate of 10 ÛC / min up to 800 ÛC.
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Figure 1. Crystal structure of TM5Si3 of the D8m-type: (a) [001] and (b) [100] projections.
Figure 2. Standard stereographic projection of the loading axis orientations.
Table I. The highest Schmid factor values for possible deformat
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