Thermal expansion behavior of intermetallic compounds in the Mo–Si–B system

The binary tetragonal intermetallic compound Mo5Si3 has significant thermal expansion anisotropy (TEA) with the coefficient of thermal expansion (CTE) of the c-axis (αc) more than double that of the a-axis (αa). Since boron has been shown to improve the oxidation stability of alloys near this composition, we explore the effect of adding boron on the thermal expansion anisotropy of Mo5+ySi3−yBx (T1) and subsequently the adjoining phases in thermal equilibrium. Powder X-ray diffraction was conducted at high temperatures using high energy (>40 keV) synchrotron X-rays. All of the intermetallics in this system (Mo5SiB2, MoSi2, MoB and Mo3Si) have moderate CTE values ranging from 7.17 ppm/°C for the cubic Mo3Si to nearly 14 ppm/°C for the c-axis of Mo5Si3B. The nearly isotropic TEA for Mo5B2Si was confirmed. The variability of the TEA of Mo5Si3B from 1.83 to 2.43 is explained in terms of the site occupancy and the change of the inter-atomic spacing of the Mo atom chains along the c-axis.