In computer simulations, the LANL scientists have observed a surprising mechanism that allows copper nanocrystalline materials to heal themselves after suffering radiation-induced damage. [Bai et al., Science (2010) 327, 1631]. The phenomenon exploits the mix of grains and the grain boundaries between them in such materials.

An important considering in the design, construction and operation of nuclear reactors is the effect of high levels of radiation on the materials from which the reactor is constructed. In addition to constant bombardment, reactor materials might also be exposed to high temperatures, physical stresses, and corrosive conditions. Radiation itself can cause dislocations and interstitial atoms and vacancies to form that insidiously weaken the structure at the nanoscale. The accumulation of defects leads to swelling and hardening leading to brittleness and the potential for catastrophic failure.

Nanocrystalline materials contain a large fraction of grain boundaries, which can absorb and remove defects, and so scientists presumed that such materials might be more radiation tolerant than equivalent materials with larger grain sizes. However, few specific details have been obtained regarding the complex behaviour of such solids under different conditions.

Xian-Ming Bai and colleagues explain that they have now used three atomistic simulation methods to investigated defect-grain boundary interactions in copper on the picosecond to microsecond timescales. They found that grain boundaries have a surprising “loading-unloading” effect in which irradiation leads to interstitials being loaded, or trapped, in the boundary on the short timescale. The loaded boundary then acts as a source unloading, or emitting on the long timescale, interstitials back into the bulk, which eradicates vacancies. The simulations reveal that this recombination mechanism, “has a much lower energy barrier than conventional vacancy diffusion and is efficient for annihilating immobile vacancies in the nearby bulk”. The net effect is the surprising self-healing, the efficient annealing, in other words, of material defects caused by radiation exposure.

The team adds that this “loading-unloading” role of grain boundaries might explain the behaviour of irradiated nanocrystalline materials that runs counter to expectations, but more importantly could provide new opportunities for engineering nanocrystalline materials to be self-healing in high-radiation environments.

Team member Blas Pedro Uberuaga told Materials Today that the researchers have compared their simulated data with literature experiments, which he says explains very nicely those results. “In terms of more direct validation, we have some experiments in progress on Au and on metal composites (Cu/Nb), that will indirectly connect to these results, but may or may not be able to directly validate these simulation results. So, we have ongoing experimental activities in related areas,” he adds.