Jin Li

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.

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.

Mechanical behavior of metallic multilayers has been intensively investigated. Here we report on the study of magnetron-sputtered highly textured Al/Ti multilayer films with various individual layer thicknesses (h = 1–90 nm). The hardness of Al/Ti multilayers increases monotonically with decreasing layer thickness without softening and exceeds 7 GPa, making it one of the strongest light-weight multilayer systems reported to date. High-resolution transmission electron microscopy and X-ray diffraction pole figure analyses confirm the formation of high-density nanotwins and 9R phases in Al layers. The density of nanotwins and stacking faults scales inversely with individual layer thickness. In addition, there is an HCP-to-FCC phase transformation of Ti when h ≤ 4.5 nm. The high strength of Al/Ti multilayers primarily originates from incoherent layer interfaces, high-density twin boundaries, as well as stacking faults.