Evidence of Amorphization in Molecular-Dynamics Simulations on Irradiated Intermetallic Nial
Molecular-dynamics (MD) computer simulations on three intermetallic compounds, NiAl, Ni3Al, and Cu3Au have been performed to investigate the kinetics of the disordering and amorphization processes. These systems were chosen because the embedded atom-type potentials work well for these materials and also because they have experimentally different amorphization behavior. Previous simulations of collision cascades with 5-keV Ni and Cu primary knockout atoms (PKA) have shown a complete loss of the crystalline structure but only partial chemical disorder in the core of the cascade. Dynamical melting simulations of the liquid phase provided significant differences in the short-range order between the three intermetallics, namely: (i) Cu3Au is close to an ideal mixture, Ni3Al is the most ordered liquid, and the disordering level of NiAl lies between the two A(3)B intermetallics, and (ii) NiAl has the fastest and Cu3Au has the slowest kinetics in the disordering process after a sudden increase of temperature. For details see Spacer et al. [Phys. Rev. B 50, 13 204 (1994)]. In the present paper we look for the conditions to induce amorphization in MD cascades in NiAl by 5 and 15 keV PKA's. The kinetic energy of the atoms in the simulated systems is removed on different time scales as a way to mimic strong or weak coupling between electrons and phonons. No evidence of amorphization is found at the end of the cascades created by 5 keV recoils. However, the 15 keV PKA events show that (i) in the no-coupling case the system evolves 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 ordered state results in the case of strong coupling. Amorphization in MD cascades is reported. A 15 keV recoil event with weak electron-phonon coupling is also shown for Ni3Al. The final structure of the Ni3Al system shows no amorphous cluster formation in agreement with experimental results.