By comparing results on ion induced amorphization of metals and metal alloys at liquid nitrogen temperature it is possible to separate the effects of atomic collisions from the effects due to the compositional changes. The crystal to amorphous transition in ion-implanted metals can be interpreted by a statistical model in which an amorphous cluster develops when a given concentration is reached inside a small critical volume ν c. The same transition occurs at much lower fluences in the case of metal alloys, demonstrating the overwhelming influence of chemical short range order. Quasi-universal amorphization curves are then obtained as a function of the concentration of displacement collisions whatever the incoming ion mass is. It is hence reasonable to assume that an amorphous cluster is formed when the number of displaced atoms inside a given volume ν c exceeds a critical value. Experimental results indicate that the size of ν c for both type of experiments corresponds to that over which structural correlations exist in a glass.