**5. Summary**

terms of several strengthening mechanisms. They concluded that the hardening of the Cu-Cr films is mainly caused by grain size refinement whereas the effects of solid solution hardening can be neglected. Nevertheless, they did not consider the contributions from the global effect

Numerous atomistic simulations have demonstrated that GB segregation can remarkably stabilize the grains and enhance the strength/hardness of alloyed systems, such as Cu-Ta [15] and Cu-Nb [84]. Using molecular dynamics simulations with an angular-dependent interatomic potential, Frolov et al. [15] investigated the Ta doping effect on the barrier for grain coarsening and robust performance of NC Cu-6.5 at.% Ta alloys. It is found that Ta segregation at GBs notably increases structural stability and mechanical strength, compared with their siblings with a uniform distribution of the same amount of Ta. With increasing temperature, the Ta atoms agglomerate and segregate at GBs in the form of nanoclusters. These nanoclusters effectively pin GBs and thus prevent grain growth. Vo et al. [84] also revealed that alloying additions that lower GB energy were found to dramatically increase the yield strength of the alloy, with dilute Cu–Nb alloys approaching the theoretical strength of Cu. Their findings indicate the strength is not controlled by the grain size alone, but rather by a combination of both the molar fraction of GB atoms and the degree of GB relaxation, as captured via a new strengthening model for the NC materials. Based on the finding that strength increases with

**Figure 15.** Cross-sectional (a) and planar-view (b) HAADF-TEM images of the annealed Cu(W)-14 thin film. The planar-view TEM image shows the prevailing columnar morphology with relatively broad grain (column) boundary regions between the columns. EDS analyses (c, d) performed by TEM reveal an inhomogeneous solute atom distribution

indicating the nanoscale decomposition process. Figure is taken with permission from Ref. [104].

of solute atoms on the matrix [83] and GB segregation [84] to the measured hardness.

100 Study of Grain Boundary Character

The metallic thin films become essential structural materials in micro- and nanodevices and refining grain size into nanoscale indeed can notably increase their strength and strain-rate sensitivity, whereas they undergo the strength-ductility tradeoff on the one hand and suffer from unstable microstructure, i.e., grain growth, on the other. How to defect the conflict between strength and ductility and simultaneously retain highly stable microstructure is a grand challenge in the material community. The GB segregation engineering seems to open a promising avenue for the design of alloyed thin films with superior property combinations by tuning their multihierarchical structures utilizing alloying additions. The twinning-mediated grain growth is a novel and effective method to toughen the NS FCC metals and alloys with exceptionally high values of both strength and ductility. The effects of GB complexions on static and dynamic properties are far different in the alloyed thin films, and more works require to be performed in the future.
