Low temperature deformation of MoSi${}_2$ and the effect of Ta, Nb and Al as alloying elements

Abstract

Molybdenum disilicide (MoSi${}_2$) is a very promising material for high temperature structural applications due to its high melting point (2030 °C), low density, high thermal conductivity and good oxidation resistance. However, MoSi${}_2$ has limited ductility below 900 °C due to its anisotropic plastic deformation behaviour and high critical resolved shear stresses on particular slip systems. Nanoindentation of MoSi${}_2$ microalloyed with aluminium, niobium or tantalum showed that all alloying elements cause a decrease in hardness. Analysis of surface slip lines indicated the activation of the additional {1 1 0}<1 1 1> slip system in microalloyed MoSi${}_2$, which is not active below 300 °C in pure MoSi${}_2$. This was confirmed by TEM dislocation analysis of the indentation plastic zone. Further micropillar compression experiments comparing pure MoSi${}_2$ and the Ta-alloyed sample enabled the determination of the critical resolved shear stresses of individual slip systems even in the most brittle [0 0 1] crystal direction.