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 re sults 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, contro ls the transformation mechanisms. Heavy ion irradiation induces a quas i-direct amorphization in the displacement cascades, whereas damage ac cumulation is required when light ion is used. The influence of the ma terial characteristics is also studied. The temperature effect on amor phous 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 comple te. Finally, the influence of the amorphization kinetics on nitride pr ecipitation 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 observe d. These precipitates all have the same orientation, even in an amorph ous matrix, when epitaxial nucleation occurs in the crystalline matrix before noticeable amorphization. They are disoriented if the nucleati on starts when the amorphization is almost complete.