Study On Size Effect And Mechanical Properties Of Nanocrystalline Metal Films With Multiple Twins

Thin metal films are widely used in the field of Micro-Electro-Mechanical Systems due to their outstanding properties, which makes more attention on the mechanical properties of thin metal films. Classical methods for strengthening materials rely on strategies that control the generation of internal defects to impede dislocation motion, including solution strengthening, dispersion strengthening and fine crystal strengthening, etc. However, these approaches invariably suffer from the undesirable consequence that an increase in strength also causes reduced ductility. Recent studies have shown that the controlled introduction of nanoscale twins will increase in strength and not accompanied by a sharp reduction in ductility. The nanoscale twins used for studying formation mechanism and grain size effect are usually related to parallel twins, in the experiment, multiple twins with various morphologies have also been observed frequently in nanocrystalline fee metals, such as twofold (V-typed)、threefold、fourfold and fivefold multiple twins.However, the formation mechanism of multiple twins and its grain size effect is still controversial. Based on these considerations above, a series of Cu、Ag and Ag/Cu thin films with different annealing temperature were prepared by direct-current magnetron sputtering method. The microstructures of the films were characterized by X-ray diffraction, scanning electron microscope and transmission electron microscope, and the mechanical properties of the films were tested by nanoindentation. The grain size effect and formation mechanism of V-shaped multiple twins in annealed Cu and Ag films were investigated, and the relationship between mechanical properties and multiple twins in as-deposited and annealed Ag/Cu multilayers was also discussed. The main conclusions are as follows:1. The number of V-shaped multiple twins increases initially and then decreases with decreasing grain size in annealed nanocrystalline Cu, reaching the maximum at a critical grain size of 40 nm and exhibiting an inverse grain size dependence.2.In annealed nanocrystalline Ag films, the number of V-shaped multiple twins also follow the inverse grain size effect, but there are some different details compared with annealed nanocrystalline Cu. Nanocrystalline Ag has a higher peak size 60 nm and a larger maximum grain size 155 nm of V-shaped twins formation, because Ag has a lower stacking fault energy.3. From energy point of view, a new formation mechanism of V-shaped multiple twins has been proposed. The mechanism is based on cooperative grain rotation and grain boundary migration, which comprises twin nucleation and propagation stages.4. According to the critical grain size of V-shaped multiple twins in Cu and Ag films, we designed the novel Cu/Ag multilayer films whose modulation ratio was 2. With decreasing modulation period in as-deposited Ag/Cu multilayers, the hardness is nonmonotonic, reaching the maximum value of 3.42 GPa at modulation period of 45 nm. After annealing at 100℃, the hardness of Ag/Cu multilayers is further improve than as-deposited multilayers, reaching 3.59 GPa, which is attributed to the multiple twins in annealed Ag/Cu multilayers can impede the dislocation motion from various directions. This design provided a new method on improving the ultra-high strength of the multilayer films.