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The crystallographic thermal expansion coefficients of Ti5Si3 from 20 to 1000 °C as a function of B, C, N, O, or Ge content were measured by high-temperature x-ray diffraction using synchrotron sources at Cornell University (Cornell High Energy Synchrotron Source; CHESS) and Argonne National Laboratory (Advanced Photon Source; APS). Whereas the ratio of the thermal expansion coefficients along the c and a axes was approximately 3 for pure Ti5Si3, this ratio decreased to about 2 when B, C, or N atoms were added. Additions of O and Ge were less efficient at reducing this thermal expansion anisotropy. The extent by which the thermal expansion was changed when B, C, N, or O atoms were added to Ti5Si3 correlated with their expected effect on bonding in Ti5Si3.

I. INTRODUCTION

Ti5Si3 displays a high melting point, low density, and with certain interstitial additions,1 excellent oxidation resistance. However, the large thermal-expansion anisotropy of Ti5Si3 severely limits its practical use. This large anisotropy unavoidably causes the development of strain and microcracks during high-temperature synthesis and processing. Four previous studies have quantified the thermal-expansion anisotropy of Ti5Si3, three by hightemperature x-ray diffraction2–4 and one by lengthchange measurements of a single crystal.5 All studies measured a significantly larger expansion along the c axis compared to the a axis. The larger anharmonic vibration along the c axis was attributed to weak metallic bonding along this axis compared to strong covalent bonding along the a axis. This explanation is partly corroborated by electrical conductivity measurements that show the conductivity is twice as large along the c axis than along the a axis.5 Although all studies reported similar relative thermal expansions, the absolute values varied considerably (see Table I). Specifically, the standard deviation of measurement between the four studies was 9.7% for the coefficient of thermal expansion along the c axis (␣c) and 27% for the coefficient of thermal expansion along the a axis (␣a). One reason for the differences may be due to the presence of oxygen and nitrogen impurities. Based on reported lattice parameters, the Ti5Si3 synthesized by Ikarashi et al.3 must have had approximately 1.0 wt% of oxygen, and the study by Thom et al.4,6 suggests approximately 0.1 to 0.4 wt% of oxygen. The study by Williams et al.,7 which systematically measured the change in lattice parameters of Ti5Si3 as a function of 1780

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J. Mater. Res., Vol. 15, No. 8, Aug 2000 Downloaded: 16 Mar 2015

various interstitial additions, was used to estimate the impurity content. Due to similar effects on the lattice, nitrogen impurities may also be present. Regarding the remaining two studies, the lattice parameters reported by Zhang and Wu2 were consistent with approximately 0.3 wt% excess silicon, and Nakashima and Umakoshi5 did not report any lattice parameters. One purpose of this study is to compare the effects that oxygen, nitrogen, carbon, and boron