Effects of substitution of Al for Si on the lattice variations and thermal expansion of Mo(Si,Al)2

The lattice variations and thermal expansion of Mo(Si1−x,Alx)2 with x=0 to 0.6 were investigated, and the results were discussed in terms of the substitution ratio of Al–Si. The crystal structure of Mo(Si,Al)2 alters from C11b to C40, and further to orthorhombic C54 with the increasing Al substitution. The lattice of each structure generally expands with the increasing substitution ratio due to the larger atomic radius of Al compared to Si. To clarify the lattice expansion in a wide composition range including C11b, C40 and C54, new parameters corresponding to the nearest neighbor interatomic distance were considered, and a relation between the parameters and the substitution ratio was indicated. The mean values of the coefficients of thermal expansion (CTE) in the temperature range from 298 to 1723 K vary from 9.2×10−6 K−1 for MoSi2 to 10.4×10−6 K−1 for Mo(Si0.4,Al0.6)2. However, the CTE variation is mostly attributed to the phase constitutions, and not to the Al substitution ratio. There is only a slight change in the C40 single-phase region.