Coherent displacement of atoms during ion irradiation

Ion irradiation is a common technique of materials processing, as well as b eing relevant to the radiation damage incurred in nuclear reactors. Early m odels of the effects of ion irradiation typically assumed that particles un dergo two-body elastic collisions', like billiard balls colliding in three dimensions. Later descriptions invoked such phenomena as localization of ki netic energy, thermalization and localized melting(2-4), In all these descr iptions, the displacement of atoms is chaotic in that slight variations in the ion's trajectory produce completely different, unpredictable sets of at omic displacements(5). Here we report molecular-dynamics simulations of hig h-energy self-bombardment of copper and nickel, in which we see collective displacements of atoms. The high pressures developed in collision cascades centred well below the surface can cause a coherent displacement of thousan ds of atoms, over tens of atomic planes, in a shear-induced slip motion tow ards the surface. The mechanism leads to a significant increase in damage p roduction near the surface, characterized by well-ordered islands of adsorb ed atoms. Our findings suggest an explanation for some features of radiatio n damage, as well as for differences between ion and neutron irradiation(6) .