GENERALIZED MELTING CRITERION FOR BEAM-INDUCED AMORPHIZATION

Recent studies have shown that the mean-square static atomic displacem ents provide a generic measure of the enthalpy stored in the lattice i n the form of chemical and topological disorder, and that the effect o f the displacements on the softening of shear elastic constants is ide ntical 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 criti cal state of disorder where the melting temperature falls below the gl ass transition temperature. Application of the generalized Lindemann c riterion to both the crystalline and the amorphous phases indicates th at the enthalpies of the two phases become identical when their shear moduli become equal. This thermoelastic rule provides a basis for pred icting the relative susceptibility of compounds to amorphization in te rms of their elastic properties as measured by Debye temperatures. The present approach can explain many of the basic findings on beam-induc ed amorphization of intermetallic compounds as well as amorphous phase formation associated with ion implantation, ion beam mixing and other solid state processes.