  Molecular dynamics simulations have been used to study the mechanism of ion
beam mixing in metal bilayers. We are able to explain the ion induced
low-temperature phase stability and melting behavior of bilayers using only a
simple ballistic picture up to 10 keV ion energies. The atomic mass ratio of
the overlayer and the substrate constituents seems to be a key quantity in
understanding atomic mixing. The critical bilayer mass ratio of $\delta < 0.33$
is required for the occurrence of a thermal spike (local melting) with a
lifetime of $\tau > 0.3$ ps at low-energy ion irradiation (1 keV) due to a
ballistic mechanism. The existing experimental data follow the same trend as
the simulated values.
