Coherent displacement of atoms during ion irradiation

Ion irradiation is a common technique of materials processing, as well as being relevant to the radiation damage incurred in nuclear reactors. Early models of the effects of ion irradiation typically assumed that particles undergo two-body elastic collisions1, like billiard balls colliding in three dimensions. Later descriptions invoked such phenomena as localization of kinetic energy, thermalization and localized melting2,3,4. In all these descriptions, the displacement of atoms is chaotic in that slight variations in the ion's trajectory produce completely different, unpredictable sets of atomic displacements5. Here we report molecular-dynamics simulations of high-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 thousands of atoms, over tens of atomic planes, in a shear-induced slip motion towards the surface. The mechanism leads to a significant increase in damage production near the surface, characterized by well-ordered islands of adsorbed atoms. Our findings suggest an explanation for some features of radiation damage, as well as for differences between ion and neutron irradiation6.