Influence of irradiation damage on stress-assisted grain growth in ultrafine grained Au thin films using in situ transmission electron microscopy mechanical testing

Ultrafine-grained (UFG) materials exhibit unique mechanical properties due to their high surface-to-volume ratio and the dominance of grain boundary plasticity. These materials are promising for radiation tolerant materials applications. This study centers around how ion irradiation affects mechanical properties, grain size evolution, and defect behavior in UFG Au thin films under mechanical loading. Irradiation levels (0.1 dpa, 0.7 dpa, 1 dpa and 5dpa) were examined, with mechanical properties assessed through in situ stress-strain nanomechanical testing in conjunction with Transmission Electron Microscopy (TEM) observations. Results showed thar irradiation up to 1 dpa had little effect on yield stress, but at 5 dpa, yield stress increased significantly, accompanied by a loss of ductility (failures at ~1% strain compared to ~5% for as deposited films). These observations align with the general trend in irradiated materials, where irradiation tends to enhance the strength but reduces ductility due to irradiation induced defect barriers to dislocation motion. The microstructural analysis revealed that irradiation had a significant impact on the grain size distribution. Irradiated samples exhibited a shift toward larger grains, with the fraction of small grains (<100nm) decreasing dramatically, especially at higher dpa levels. At 5dpa, nearly all grains were larger than 100nm.
Grain growth was further exaggerated with mechanical loading, where the fraction of small grains continued to decrease. At the highest irradiation dose (5dpa), grain growth was limited by the absence of small grains to facilitate coarsening. Additionally, irradiation-induced defects were observed to play a role in enhancing grain boundary migration, which in turn contributed to the overall grain growth. This effect is thought to be due to the creation of high-energy, nonequilibrium grain boundaries that are more susceptible to migration. The result of this study shows that ion-irradiation significantly affects mechanical properties and microstructure of UFG Au thin films. At moderate irradiation levels, the material undergoes a transition in behavior. At high irradiation doses, the grain boundaries reach a more stable equilibrium structure. This work advances and contributed to the understanding of complex relationship between irradiation damage, grain size evolution and mechanical properties, critical for the development of radiation -resistant materials.