  Molybdenum disilicide (MoSi$_2$) is a very promising material for high
temperature structural applications due to its high melting point (2030
{\deg}C), low density, high thermal conductivity and good oxidation resistance.
However, MoSi$_2$ has limited ductility below 900 {\deg}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 {\deg}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.
