  We present results of extensive theoretical studies of
Co$_2$FeAl$_{1-x}$Si$_x$ Heusler alloys, which have been performed in the
framework of density functional theory employing the all-electron
full-potential linearized augmented plane-wave scheme. It is shown that the
Si-rich alloys are more resistive to structural disorder and as a consequence
Si stabilizes the $L2_1$ structure. Si alloying changes position of the Fermi
level, pushing it into the gap of the minority spin-band. It is also shown that
the hyperfine field on Co nuclei increases with the Si concentration, and this
increase originates mostly from the changes in the electronic density of the
valence electrons.
