**4. Grain boundary segregation in nanocrystructured metallic materials**

During the past two decades, NS metallic materials have received considerable attention owing to their unique, often desirable properties for engineering applications, whereas they manifest two adverse properties: low ductility and microstructural instability as mentioned earlier. This is because the high energy GBs associated with high mobility can absorb abundant dislocations, resulting in low dislocation storage inside grains [2, 3]. Therefore, a universal strategy to remarkably enhance/improve the mechanical properties and thermal stability of these NS materials is to manipulate their multihierarchical microstructures by embedding atoms/clusters or nanoparticles in grain interiors to increase dislocations storage and at GBs to prevent grain growth by reducing GB mobility [74]. Fortunately, alloying opens an available avenue to achieve such an idea about microstructure-sensitive design to improve materials' properties by tuning solute distributions, in particular, GB segregation, in NS thin films to achieve thermodynamically stable or metastable states [75–81]. The addition of an alloying element has fundamental thermodynamic implications for NC metals, which can explain the unique ability of alloyed systems to exhibit fine-grained structures [8–13, 75–81]. Specifically, Schuh's group [11–13] recently developed a theoretical framework for a regular NC solution (RNS) that incorporates GB segregation and further built an insightful nanostructure stability map for design alloys with positive enthalpy.

In what follows, we mainly address alloying effects on microstructural evolution on the one hand, and on the mechanical properties on the other in three categories of typical binary Cu-based film systems, i.e., Cu-Zr, Cu-Al, and Cu-W. This division of three typical binary systems is based on the consideration of mixing enthalpy (*H*mix) and the conventional bulk binary diagram under equilibrium states (at RT), and can be extended to other systems like Ni-based binary alloy.
