Effect of Temperature on the Nano/Microstructure and Mechanical Behavior of Nanotwinned Ag Films

In situ and ex situ annealed nanotwinned (NT) Ag thin films have been investigated by TEM and tensile testing to reveal the thermal stability of the twin boundaries, grain boundaries, dislocation densities, and their respective influence of the macroscopic yield stress. The NT Ag films synthesized by magnetron sputtering form both coherent (CTB, Σ3{111}) and incoherent (ITB, Σ3{112}) twin boundaries that are thermally stable up to 473 K (200 °C), i.e., no obvious changes in grain size, twin spacing, and yield stress. In situ TEM observations show the dislocations become mobile at 453 K (180 °C) resulting in dislocation annihilation primarily at twin and grain boundaries. Rotation of grains with low-angle grain boundaries was observed during in situ heating, resulting in the growth of columnar grains above 453 K (180 °C). However, no noticeable changes in the spacings of CTBs were observed during the entire in situ and the ex situ annealing [up to 873 K (600 °C)]. The increase in grain size and concomitant decrease in yield stress following annealing at various temperatures can be described by the Hall-Petch relationship, demonstrating that grain size rather than twin spacing is most sensitive to thermal annealing and plays a dominant role in the deformation of NT Ag films.