Molecular dynamics simulation of displacement cascades in Cu and Ni: Thermal spike behavior
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R. Benedek Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439 (Received 9 September 1988; accepted 27 December 1988) Molecular dynamics simulations of energetic displacement cascades in Cu and Ni were performed with primary-knock-on-atom (PKA) energies up to 5 keV. The interatomic forces were represented by the Gibson II (Cu) and the Johnson-Erginsoy (Ni) potentials. Our results indicate that the primary state of damage produced by displacement cascades is controlled basically by two phenomena: replacement collision sequences during the ballistic phase, and melting and resolidification during the thermal spike. The thermal-spike phase is of longer duration and has a more marked effect in Cu than in Ni. Results for atomic mixing, defect production, and defect clustering are presented and compared with experiment. Simulations of "heat spikes" in these metals suggest a model for "cascade collapse" based on the regrowth kinetics of the molten cascade core.
I. INTRODUCTION The characterization of energetic displacement cascades1 is an important focus of research in the field of radiation effects. A displacement cascade is initiated when an atom in a solid receives an energy greater than about 100 eV in a collision with a fast incident particle, such as a neutron or an ion. Point defects and defect clusters are created and rearrangements of atomic configurations (e.g., disordering in an ordered alloy) occur as a consequence of the ensuing atomic collisions in the surrounding medium. The problem of component degradation under irradiation in fission and fusion reactors2 provided the main technological impetus to radiation-damage research for many years. Recently, the growing importance of ion-implantation technology and ion-beam materials processing3 has generated renewed interest in cascade phenomena. Displacement cascades also pose challenging scientific problems in the areas of defect physics, kinetics, and phase stability. We will see that cascades give rise to an ultrafast solidification process, with quench rates of the order of 1015 K/s. Because of the inhomogeneous, nonlinear, and manybody character of cascades, developing a satisfactory theoretical model has been difficult. As a first approximation, one may treat the short-time, ballistic phase separately from the longer-time "thermal spike" regime. One such approach would be to treat the early part of the cascade as a sequence of independent binary collisions (binarycollision approximation4'5) and the slowly-decaying thermal spike via the classical heat-transport equation in conjunction with chemical rate theory.6 Unfortunately, developing a)
Also with the Department of Physics, SUNY-Albany, Albany, New York 12222. Current address: Lawrence Livermore National Laboratory, Livermore, California 94550. J. Mater. Res., Vol. 4, No. 3, May/Jun 1989
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a satisfactory unified theory of cascades spanning both short- and long-time regimes has proven difficult. In recent years, however, mo
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