Influence of irradiation damage on stress-assisted grain growth in ultrafine grained Au thin films using in situ transmission electron microscopy mechanical testing
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Daza Llanos, Lina Vanessa
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Abstract
Ultrafine-grained (UFG) materials such as gold (Au) thin films exhibit unique mechanical properties due to their high surface-to-volume ratio and the dominance of grain boundary plasticity. However, the effects of irradiation on the mechanical behavior and microstructure of UFG materials are yet to be understood. This study investigates the influences of ion irradiation on the mechanical properties, grain size evolution, and defect evolution under mechanical loads in UFG Au thin films. Specifically, this study explores how different irradiation damage levels quantified in displacement per atom (dpa), affect yield stress, ductility and microstructural changes during mechanical loading and relaxation of the material. The UFG Au thin films were irradiated to multiple damage levels (0.1 dpa, 0.7 dpa, 1 dpa and 5dpa), and mechanical properties were studied through in situ stress-strain nanomechanical testing in conjunction with Transmission Electron Microscopy (TEM) observations. The results show that low irradiation levels up to 1 dpa, do not significantly alter the yield stress, but irradiation to 5dpa leads to a pronounced increase in yield stress compared to their as-deposited counterparts. This increase in yield stress is accompanied by a decrease in ductility, with all irradiated specimens failing at strains near 1%, compared to approximately 5% in as-deposited specimens. 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 post mechanical loading and relaxation, where the fraction of small grains continued to decrease. These findings suggest that ion irradiation leads to significant grain boundary migration and coarsening, which is a result of thermal spikes associated with ion impacts that provide sufficient energy for grain boundary motion. At the highest irradiation dose (5dpa), most small grains were eliminated, and further 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. This study also noted a transition period at 1 dpa, where a combination of irradiation-induced grain growth and defect concentration led to a shift in mechanical behavior, suggesting that irradiation induced grain boundary stabilization only occurs at higher damage levels. 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.
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