**2. Synthesis, microstructural characterizationw and mechanical tests**

The synthesis of NS metallic thin films can be achieved by several bottom-up techniques, such as physical vapor deposition (PVD) and electrodeposition (ED), in which the choice of deposition conditions has a tremendous influence on the microstructural features and mechanical properties of these NS metallic films/foils.

PVD is the most common approach to fabricate metallic thin films/foils, including evaporation, sputtering, and less commonly molecular beam epitaxy [16, 17]. Compared with other methods, magnetron sputtering (MS) can clean the substrate by "backsputtering" and generate greater impact angles of the sputtered atoms onto the substrate, resulting in smaller surface roughness of the film by covering the defects and/or step on the substrate [17]. Although, MS increases the possibility of crystal damage due to high impact energies of sputtered atoms, it is still the most widely used method to prepare thin films.

ED is a technique within the broader group of electrochemical synthesis methods and uses an electric current to deposit pure metals from an aqueous, electrolytic solution [18, 19]. Compared with PVD, ED offers a lower cost and faster low-temperature deposition method. It displays remarkable advantages to synthesize highly dense NC materials with (1) few size and shape limitations, (2) tunable microstructural size parameters, and (3) hierarchical structures, e.g., a bimodal grain size-distribution [20] and NT grains [21], providing potential benefits to mechanical performance. Especially, these nanotwins improve both the mechanical strength and ductility, yet maintain high electric conductivity [22].

The crystalline structure, orientation, and grain boundaries within metallic thin films could be experimentally probed by suitable techniques, including X-ray diffraction (XRD), scanning and transmission electron microscopy (SEM and TEM), combining with other more superior appurtenances, such as the electron backscattered diffraction (EBSD) system and the precession-enhanced electron diffraction (PED) system. The chemical conuration of the materials can be characterized by the energy dispersive X-ray (EDX) and the powerful 3-D atom probe tomography (APT).

Due to the difficulty in performing the mechanical tests on the free-standing metallic thin films often with thickness of roughly 1 µm or less, researchers put great emphasis on the substrate-supported thin films. For example, the tensile ductility and fatigue lifetime of metallic thin films on flexible substrates, both of which are characterized by the critical strain to nucleate microcracks [23, 24], can be determined by a Micro-Force Test System (MTS® Tytron 250) at RT. By contrast, the strength/hardness and modulus of thin films on rigid substrates can be measured using instrumented nanoindenter apparatus (e.g., TI950 TriboIndenter, Nano XP) often equipped with a standard Berkovich tip and a diamond flat punch. In what follows, we will mainly concentrate on the mechanical properties of substrate-supported metallic (alloyed) NS thin films.
