Reducing the thermal expansion anisotropy in Mo5Si3 by Nb and V additions:: theory and experiment

Mo5Si3 exhibits a high anisotropy of its coefficients of thermal expansion (CTEs) in the a and c directions, i.e., CTE(c)/CTE(a) = 2.0. The high CTE anisotropy is due to the existence of prominent atom chains along the c-axis. Ternary alloying additions that effectively stretch the atom chains are likely to reduce the bonding directionality/strength and CTE anisotropy. First-principles local-density-functional calculations were carried out for two alloy systems, MoNb4Si3 and Mo4VSi3. Indeed, we find a significant reduction of the CTE ratio to a value of 1.5 for MoNb4Si3 and a value close to 1.0 for Mo4VSi3. The decrease in the CTE anisotropy mainly comes from the reduction in CTE in the c-direction. While a conventional approach to reduce the CTE would involve an increase in lattice rigidity by increasing the bond strength, our strategy focuses on the reduction of driving force for thermal expansion by decreasing the bond directionality. (C) 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.