Mechanical and thermal properties of single crystals of ZrB 2

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Mechanical and thermal properties of single crystals of ZrB2 N. L. Okamoto, M. Kusakari, K. Tanaka, H. Inui, M. Yamaguchi and S. Otani1 Department of Materials Science and Engineering, Kyoto University Sakyo-ku, Kyoto 606-8501, Japan 1 Advanced Materials Laboratory, National Institute for Materials Science, 1-1, Namiki, Tsukuba, Ibaraki 305-0044, Japan ABSTRACT Coefficients of thermal expansion (CTE), elastic constants and plastic deformation behaviors of single crystals of ZrB2, which possesses a hexagonal layered structure where pure Zr and pure B atomic planes stack alternatively along the c-axis, have been investigated in wide temperature ranges. While the observed elastic constants indicate highly anisotropic nature of atomic bonding being consistent with the layered _structure, the observed CTE values are rather _ _ isotropic. Two operative slip systems, (0001) and on {1100}, are identified in compression tests. The observed plastic behaviors are discussed in the light of the deduced anisotropy in atomic bonding. INTRODUCTION Many transition-metal diborides with the hexagonal AlB2 structure (space group: P6/mmm) exhibit attractive properties such as high melting temperature, high stiffness and hardness and high electrical and thermal conductivity [1-3]. ZrB2 is one of these diborides and is in practical use as refractory crucibles and sheaths in steel making industries because of its high corrosion-resistance in addition to these attractive properties [1-3]. In recent years, ZrB2 is being considered for the use in the thermal protection system of hypersonic vehicles in the form of composites reinforced with SiC and C because of the excellent heat and thermal shock resistance [4]. On top of that, ZrB2 is also recently considered for the use as a substrate for heteroepitaxial growth of GaN in the form of single crystal because of the small lattice mismatch [5-7]. The quality of the grown GaN crystal is recently reported to be much higher on the ZrB2 substrate than on α(6H)-SiC [7,8] and sapphire [9-11] substrates, which have been usually used to date. Single crystals of ZrB2 may exhibit highly anisotropic physical properties when its crystal structure is considered. Its hexagonal structure is basically a layered structure in which pure Zr and pure B atomic planes stack alternatively along the c-axis. However, only a little is known about physical properties of single crystals such as thermal expansion, elastic constants, electrical and thermal conductivity, although these data are available for the polycrystals [1-3,12]. This may stem from the fact that the very high melting temperature (3060°C) makes single-crystal growth difficult. However, one of the authors has recently succeeded in growing large-size single crystals of ZrB2 by the RF heated floating-zone method [13,14]. In the present study, we investigate thermal expansion, elastic constants and plastic deformation behaviors of single crystals of ZrB2 as a function of crystal orientation and temperature for the first time and discuss the results o

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