GENERALIZED MELTING CRITERION FOR BEAM-INDUCED AMORPHIZATION

Recent studies have shown that the mean-square static atomic displacements provide a generic measure of the enthalpy stored in the lattice in the form of chemical and topological disorder, and that the effect of the displacements on the softening of shear elastic constants is identical to that of heating. This finding lends support to a generalized form of the Lindemann phenomenological melting criterion and leads to a natural interpretation of crystalline-to-amorphous transformations as defect-induced melting of metastable crystals driven beyond a critical state of disorder where the melting temperature falls below the glass transition temperature. Application of the generalized Lindemann criterion to both the crystalline and the amorphous phases indicates that the enthalpies of the two phases become identical when their shear moduli become equal. This thermoelastic rule provides a basis for predicting the relative susceptibility of compounds to amorphization in terms of their elastic properties as measured by Debye temperatures. The present approach can explain many of the basic findings on beam-induced amorphization of intermetallic compounds as well as amorphous phase formation associated with ion implantation, ion beam mixing and other solid state processes.