The relative stability of crystalline and amorphous phases in metallic structures during ion irradiation is discussed in terms of the free energy changes and point defect mobility. The increase in free energy due to radiation induced defects is predicted to be larger for line compounds than compounds with wide solubility, which can explain the greater tendency toward amorphization for line compounds. Low defect mobility is also necessary to ensure a high total defect concentration. Nucleation of dislocation structure is an alternate way to relieve the high internal free energy and therefore may preclude subsequent amorphous transformation. Ion irradiation can also enhance the crystallization kinetics of the amorphous phase. A critical temperature region can be defined which separates the regions of crystalline stability from amorphous stability during irradiation.