Amorphization mechanisms in ion-bombarded metallic alloys.

Abstract

A model of amorphous phase formation in ion-bombarded metallic alloys is proposed and compared to experimental results recorded with various techniques. The elementary mechanism is the formation of small amorphous clusters in the bombarded layer as soon as a favorable short-range order (threshold ion concentration) and a given topological disorder (threshold defect concentration) are locally established. The influence of the bombardment temperature is also discussed. Statistical considerations concerning the final spatial distributions of ions and defects allow us to account for the results obtained at low temperature, where implanted ions and radiation defects are immobile in the bombarded layer. A sigmoidal shape of the amorphization kinetics is obtained in implantation experiments or irradiation experiments with very light ions, while a nearly linear fluence dependence of the amorphous fraction is observed in irradiation experiments with heavy ions. At temperatures where implanted ions and radiation defects become mobile in the bombarded layer, the statistical description fails and ion trapping as well as defect recombinations have to be considered to reproduce experimental data. There is evidence that the amorphization model described in this paper for metallic systems also holds for other crystalline materials such as semiconductors or insulators.