**4.2. Alloying effects on microstructure and mechanical properties in the Cu-Al model system**

In this miscible system with *H*mix close to zero, solute (Al) has a significant solid solubility in the host metal (Cu), and consequently, only a weak segregation tendency. The miscible solutes inside grains, in principle, allowing certain material properties to be finely

**Figure 11.** (a) A representative planar TEM image showing the grains in Cu–0.5 at.% Zr film stretched to 18% to demonstrate the increase in grain size. (b) Statistical results on the grain size evolution with applied strain in the pure Cu, Cu-0.5 at.% Zr and Cu-2.0 at.% Zr films. (c) Representative color-coded inverse pole figure maps from the Cu-0.5 at.% Zr film before deformation (left) and after stretching to *εC* (right), respectively, with color coding at the lower right corner. Grain growth can also be observed by comparing the images before and after deformation. (d) Corresponding orientation distributions presented as an inverse pole plot, which can be used to show the variation in orientation. Figure is taken with permission from Ref. [82].

tuned: The nucleation of partial dislocations, for instance, is stimulated by miscible solutes through markedly decreasing the SFE of the host metal [91]. For example, as NS Cu films is alloyed with Al, more nanotwins with thinner thickness are observed in the as-deposited Cu-Al thin films with lower SFE or higher fraction of Al [92, 93].

**Figure 12.** Dependence of the fatigue lifetime (*Nf* ) on the strain range (Δ*ε*) for pure Cu and Cu-Zr alloyed thin films. Figure is taken with permission from Ref. [82].

Recently, NS Cu films with different Al additions (0, 1, 5, and 10 at.%) were prepared by MS to investigate the effect of lowering SFE on microstructures and mechanical properties by Zhang et al. [92]. It is found that the Al addition motivates nanotwin formation, and promotes (111) but depresses (100) texture. With increasing Al contents, along with the refinement of grains, the morphologies of nanotwins transformed from parallel nanotwins in pure Cu to multiple nanotwins in Cu-5 at.% Al and to intersected nanotwins network in Cu-10 at.% Al, as shown in **Figure 13** as insets. Concomitantly, these Cu-Al alloyed thin films exhibit increased strength/hardness and reduced ductility with Al contents, namely, the Cu-Al films suffer from the strength-ductility tradeoff. Nevertheless, a good combination of hardness/ductility (6.2 GPa/6.3%) is achieved in the Cu-5 at.% Al film, which can be ascribed to the combined effect of texture and nanotwins [92]. At the same time, Heckman and coworkers [93] synthesized fully NT Cu-Al alloyed thin films with columnar grains and showed an increased strength of up to ~1.5 GPa that was closely related to the decrease in grain size or increase in Al content. Moreover, the ductility could be improved with decreasing the nanotwin thickness [93]. Except for the amorphous phase reinforced effect, all the strengthening mechanisms mentioned in the Cu-Zr model system play important roles in the strength of Cu-Al system. Also, Schäfer et al. [91] suggested that the details of the element distribution in the GBs are of great importance for the yield strength of the miscible alloy. The initial energetic state of the GB controls the barrier for the onset of deformation mechanisms, which is correlated to the maximum strength. Specifically, the formation of stacking faults and coherent TBs leads to material softening at high strains, because they provide additional dislocation sources. This is similar to that in pure Cu [21] and Ni [37] with very thin twins.

tuned: The nucleation of partial dislocations, for instance, is stimulated by miscible solutes through markedly decreasing the SFE of the host metal [91]. For example, as NS Cu films is alloyed with Al, more nanotwins with thinner thickness are observed in the as-deposited

**Figure 11.** (a) A representative planar TEM image showing the grains in Cu–0.5 at.% Zr film stretched to 18% to demonstrate the increase in grain size. (b) Statistical results on the grain size evolution with applied strain in the pure Cu, Cu-0.5 at.% Zr and Cu-2.0 at.% Zr films. (c) Representative color-coded inverse pole figure maps from the Cu-0.5 at.% Zr film before deformation (left) and after stretching to *εC* (right), respectively, with color coding at the lower right corner. Grain growth can also be observed by comparing the images before and after deformation. (d) Corresponding orientation distributions presented as an inverse pole plot, which can be used to show the variation in orientation. Figure

Cu-Al thin films with lower SFE or higher fraction of Al [92, 93].

is taken with permission from Ref. [82].

96 Study of Grain Boundary Character

**Figure 13.** The strength-ductility tradeoff in Cu-Al alloyed thin films from literatures [92, 93]. Insets are the corresponding internal feature of Cu-Al films prepared by Zhang et al. [92].

In order to investigate the GB character evolution associated with the observed grain growth in this binary system, Brons and Thompson [94] carried out the *in situ* TEM observations on a sputter-deposited Cu-20 at.% Ni alloyed film that was annealed within a TEM equipped with the PED. It is found that alloying leads to a preferential evolution of particular grains. The onset of annealing resulted in multiple textures and grain growth evolution. It is clear from these results that significant differences in coincidence site lattice (CSL) boundary evolution occurred with the addition of Ni to the Cu thin film. The boundary fraction of Σ9, for the alloy film, was a factor of five larger than the pure Cu film and these boundaries were notably bounding several (but not all) of these larger grains. Additionally, Σ11 boundaries showed an increase in their fraction as compared with the elemental Cu grain growth evolution. They proposed that the Ni additions occupied subinterface sites as a result of higher surface tension of Ni which impedes other CSL boundaries motion [94]. Though solute segregation has been proposed as an efficient and effective way to stabilize NC grain structures (e.g., in Ni-W films [83]), solute preference to specific CSL boundaries can result in abnormalities in grain growth and lead to destabilization of the grain structure (e.g., in Cu-Ni films [94]). These results suggest that the GB segregation is much more complex. However, most previous numerical studies arbitrarily place solute atoms at GBs to determine their effect on GB energy [95–98], only some limited studies have been conducted to study the correlation between the CSL boundaries and solute segregation [94, 99, 100]. Therefore, further works are urgently needed to focus on how GB character is influenced with solute alloying.
