Aiding the Design of Radiation Resistant Materials with Multiphysics Simulations of Damage Processes

The design of metals and alloys resistant to radiation damage involves the physics of electronic excitations and the creation of defects and microstructure. During irradiation damage of metals by high energy particles, energy is exchanged between ions and electrons. Such "nonadiabatic" processes violate the Born-Oppenheimer approximation, on which all conservative classical interatomic potentials rest. By treating the electrons of a metal explicitly and quantum mechanically we are able to explore the influence of electronic excitations on the ionic motion during irradiation damage. Simple theories suggest that moving ions should feel a damping force proportional to their velocity and directly opposed to it. In contrast, our simulations of a forced oscillating ion have revealed the full complexity of this force: in reality it is anisotropic and dependent on the ion velocity and local atomic environment. A large set of collision cascade simulations has allowed us to explore the form of the damping force further. We have a means of testing various schemes in the literature for incorporating such a force within molecular dynamics (MD) against our semi-classical evolution with explicitly modelled electrons. We find that a model in which the damping force is dependent upon the local electron density is superior to a simple fixed damping model. We also find that applying a lower kinetic energy cut-off for the damping force results in a worse model. A detailed examination of the nature of the forces reveals that there is much scope for further improving the electronic force models within MD. INTRODUCTION A radiation damage cascade begins when an ion in a solid is set in motion by collisions with incoming particulate radiation or the products of radioactive decay processes. This primary knock-on atom (PKA) then goes on to collide with other atoms of the solid creating damage to the material at the atomic level. The initial disruption occurs rapidly on a picosecond timescale and over a region hundreds of nanometers in size. Following the point of maximum damage there is a period of hundreds of ps, during which many of the defects recombine and the lattice largely heals itself to leave a residual defect distribution that forms the initial conditions for the long-term micro-structural evolution of the material [1]. As radiation damage takes place on length and timescales at or beyond the resolution of experimental investigation, much of what we know about cascade evolution has been established via simulation. Classical molecular dynamics (MD) simulations can now handle sufficiently large systems to allow the direct simulation of high energy collision cascades [2]. However, it is well established [3] that the swift motion of ions during a collision cascade can excite the electrons out of their ground state. The cumulative effect of many small electronic excitations is to raise the electronic temperature, away from equilibrium with the ionic temperature [17]. This provides a mechanism of ionic energy loss if the electronic thermal conductivity is high then energy is rapidly transferred out of the cascade region. These effects are ignored by standard classical MD simulations. Several augmented MD schemes [4-6] have attempted to model the effect of electrons on Mater. Res. Soc. Symp. Proc. Vol. 1229 © 2010 Materials Research Society 1229-LL03-06