We have studied by transmission electron microscopy the amorphization of silicon-germanium (SiGe) alloys by Ge+ implantation. We show that when implanted with the same amorphization dose, the resulting amorphous layers get narrower when the Ge content increases. The experimental results can be simulated using the critical damage energy density model assuming that the amorphization threshold rises linearly with the Ge content from 3 eV/at for pure Si to 5 eV/at for pure Ge. These results and simulations are needed to optimize the fabrication of highly doped regions in SiGe alloys.