Haiyan Wang

Nanocrystalline metals have shown enhanced radiation tolerance as grain boundaries serve as effective defect sinks for removing radiation-induced defects. However, the thermal and radiation stability of nanograins are of concerns since radiation may induce grain boundary migration and grain coarsening in nanocrystalline metals when the grain size falls in the range of several to tens of nanometers. In addition, prior in situ radiation studies on nanocrystalline metals have focused primarily on single heavy ion beam radiations, with little consideration of the helium effect on damage evolution. In this work, we utilized in situ single-beam (1 MeV Kr++) and dual-beam (1 MeV Kr++ and 12 keV He+) irradiations to investigate the influence of helium on the radiation response and grain coarsening in nanocrystalline Cu at 300 °C. The grain size, orientation, and individual grain boundary character were quantitatively examined before and after irradiations. Statistic results suggest that helium bubbles at grain boundaries and grain interiors may retard the grain coarsening. These findings provide new perspective on the radiation response of nanocrystalline metals.

Significant microstructural damage, in the form of defect clusters, typically occurs in metals subjected to heavy ion irradiation. High angle grain boundaries (GBs) have long been postulated as sinks for defect clusters, like dislocation loops. Here, we provide direct evidence, via in situ Kr ion irradiation within a transmission electron microscope, that high angle GBs in nanocrystalline (NC) Ni, with an average grain size of ~55 nm, can effectively absorb irradiation-induced dislocation loops and segments. These high angle GBs significantly reduce the density and size of irradiation-induced defect clusters in NC Ni compared to their bulk counterparts, and thus NC Ni achieves significant enhancement of irradiation tolerance.

Metallic materials subjected to heavy ion irradiation experience significant radiation damage. Free surface is a type of effective defect sinks to improve the radiation resistance in metallic materials. However, the radiation resistance of metallic nanowires (NWs) is largely unknown. Here we show, via in situ Kr ion irradiations in a transmission electron microscope, Ag NWs exhibited much better radiation resistance than coarse-grained Ag. Irradiation-induced prominent surface roughening in Ag NWs provides direct evidence for interaction between defect clusters and free surface. Diameter dependent variation of the surface roughness in irradiated Ag NWs has also been observed. This study provides insight on mechanisms of enhanced radiation resistance via free surfaces in metallic NWs.