**4. Summary and future work**

As mentioned in the introduction section, the high volume fraction of atoms resided at GB makes the microstructural stability problem intrinsic to NC materials. In general, two approaches are used to stabilize the grain structure: by kinetically hindering the GB mobility or by thermodynamically lowering the GB energy through solute segregation [62]. Studies have shown that providing a short annealing treatment to allow for grain boundary relaxation can increase the fatigue life of Cu films [49]. HRTEM investigation on the detailed GB characters pre- and post-annealing processes will give more clues about the underlying mechanism. Recently, methods for stabilizing NC materials by the control of interface structure was reviewed by Lu [63]. It has been argued that nanostructures with a high-density of coherent twin boundaries (CTB), which are low energy, low mobility boundaries with a high degree of crystallographic ordering, would provide the required resistance to the thermal coarsening while enhancing the strength. The addition of solute atoms (alloying) can pin the GB and/or lower the GB energy, improving the microstructural stability of NC [64, 65]. However, due to the complexity induced by alloying, more experimental and theoretical investigations on the structural characteristics need to be done in the future.
