EVIDENCE OF AMORPHIZATION IN MOLECULAR-DYNAMICS SIMULATIONS ON IRRADIATED INTERMETALLIC NIAL

Molecular-dynamics (MD) computer simulations on three intermetallic co mpounds, NiAl, Ni3Al, and Cu3Au have been performed to investigate the kinetics of the disordering and amorphization processes. These system s were chosen because the embedded atom-type potentials work well for these materials and also because they have experimentally different am orphization behavior. Previous simulations of collision cascades with 5-keV Ni and Cu primary knockout atoms (PKA) have shown a complete los s of the crystalline structure but only partial chemical disorder in t he core of the cascade. Dynamical melting simulations of the liquid ph ase provided significant differences in the short-range order between the three intermetallics, namely: (i) Cu3Au is close to an ideal mixtu re, Ni3Al is the most ordered liquid, and the disordering level of NiA l lies between the two A(3)B intermetallics, and (ii) NiAl has the fas test and Cu3Au has the slowest kinetics in the disordering process aft er a sudden increase of temperature. For details see Spacer et al. [Ph ys. Rev. B 50, 13 204 (1994)]. In the present paper we look for the co nditions to induce amorphization in MD cascades in NiAl by 5 and 15 ke V PKA's. The kinetic energy of the atoms in the simulated systems is r emoved on different time scales as a way to mimic strong or weak coupl ing between electrons and phonons. No evidence of amorphization is fou nd at the end of the cascades created by 5 keV recoils. However, the 1 5 keV PKA events show that (i) in the no-coupling case the system evol ves to a highly disordered state, (ii) an amorphous region with about 100 nonlattice atoms is found in the case of weak coupling, (iii) the locally molten and recrystallized region collapses to a small cluster containing 25 atoms when medium coupling is used, and (iv) a highly or dered state results in the case of strong coupling. Amorphization in M D cascades is reported. A 15 keV recoil event with weak electron-phono n coupling is also shown for Ni3Al. The final structure of the Ni3Al s ystem shows no amorphous cluster formation in agreement with experimen tal results.