ION-BEAM-INDUCED AMORPHIZATION IN THE INTERMETALLIC COMPOUNDS NITI AND NIAL

The mechanisms of ion beam induced crystal to amorphous transition in intermetallic compounds are analysed through amorphization kinetics results recently obtained from two ordered alloys, NiTi and NiAl. It is shown that the deposited energy density, which depends on the mass and the energy of the incident particle, as well as on the target, controls the transformation mechanisms. Heavy ion irradiation induces a quasi-direct amorphization in the displacement cascades, whereas damage accumulation is required when light ion is used. The influence of the material characteristics is also studied. The temperature effect on amorphous phase formation is investigated. When the temperature increases, reordering effects via radiation-enhanced diffusion processes compete with ballistic disordering so that the transformation is never complete. Finally, the influence of the amorphization kinetics on nitride precipitation in N+ implanted NiTi alloy is discussed. At high fluences (10(17) ions cm-2) TiN0.8 fine precipitates, embedded in an amorphous or in a crystalline matrix (depending on the temperature), are observed. These precipitates all have the same orientation, even in an amorphous matrix, when epitaxial nucleation occurs in the crystalline matrix before noticeable amorphization. They are disoriented if the nucleation starts when the amorphization is almost complete.