Controlling the thermal expansion anisotropy of Mo5Si3 and Ti5Si3 silicides

The coefficients of thermal expansion (CTE) of the tetragonal silicides Mo5Si3 and Ti5Si3 are highly anisotropic. The CTE anisotropy values, defined as the ratio of the CTEs in the c and a directions, CTE(c)/CTE(a), are 2.0 and 2.7 for Mo5Si3 and Ti5Si3, respectively. Guided by first-principles calculations by Fu et al. (Acta Mater 51 (2003) 5033), ternary and quaternary alloying additions for reducing the anisotropy were selected. The smallest anisotropy, namely, 1.16, was found for a Mo–25Nb–12.5V–37.5Si (at.%) alloy. The experimentally determined reductions in CTE(c)/CTE(a) were not always as large as predicted because the measured site occupations of the ternary alloying elements were not as ideal as those calculated by Fu et al. Suitable alloying also reduces the CTE anisotropy of Ti5Si3, but the reduction is not nearly as large as that claimed in previous publications.