**4.3. Alloying effects on microstructure and mechanical properties in the Cu-W model system**

It is well recognized that the Cu-W system is an essentially immiscible one characterized with a quite positive *H*mix of about +22 kJ/mol. There does not exist any Cu-W compound in its equilibrium phase diagram. Actually, for the most studied immiscible binary alloyed films (such as Cu-W [101], Cu-Cr [102], and Cu-Ta [103]), nonequilibrium NS alloys, i.e., supersaturated solid solutions (SSSs), can be obtained by MS. Compared with the elemental (Cu) thin films, it is normally expected that the marked solute segregation that alters GB characters could occur and the solute drag effect could yield smaller grains [8, 12], both of which affect the propensity of nanotwin formation in binary Cu-based thin films.

Vüllers and Spolenak [101] recently prepared the "immiscible" Cu-W thin films with different W contents on silicon substrates using MS, and clearly demonstrated that these NC Cu-W thin films transit from the SSSs in a metastable as-deposited state to fully phase separated interpenetrating networks after annealing at 750°C, as shown in **Figure 14**. The W additions notably change the microstructural configurations of crystalline Cu thin film that has a distinctive columnar superstructure consisting of large numbers of partially even equiaxed grains and occasionally occurring twins, in the as-deposited state. While the columnarity dominant for pure Cu is still present in a 5 at.% W film, the subordinate structure making up the single columns in the pure Cu cross-section cannot be observed any longer. Films of higher W contents up to and around 30 at.% W do not exhibit a distinct crystalline structure in the as-deposited state. Subsequently, they measured the hardness and modulus of these Cu-W thin films as function of the W content at different states. It can be deduced that W content strongly influences the film's mechanical performance. As a whole, both hardness and modulus increase with increasing W contents, as shown in **Figure 14**. However, they did not perform quantitatively calculation of the strength of Cu-W thin films. Harzer et al. [102] quantitatively evaluated the hardness of metastable Cu-Cr alloyed thin films which are stable below ~170°C, and further correlated it with respect to film compositions and grain sizes in

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

**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].

98 Study of Grain Boundary Character

with solute alloying.

**Figure 14.** (a) Cross-sectional SEM images (BSE) of as-deposited and annealed Cu-W thin films (left). (b) Hardness and reduced Young's modulus as function of compositional fraction of W with standard deviation error bars for the as-deposited and annealed states (right). Figure is taken with permission from Ref. [101].

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

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].

increasing atomic volume of the solute, they also predicted the possibility of achieving a theoretical strength in Cu by doping suitable solute atoms [84].

In parallel, Csiszár et al. [104] investigated the stability of NT Cu-W alloyed films during annealing in the range of 30–600°C, compared with their Ni-W and Ag-W NT siblings. A major, microstructural difference observed for all films upon annealing is the redistribution of the alloying element (W) content. In the case of Cu-14 at.% W, a significant redistribution of W was detected by TEM and EDS (see **Figure 15**), similar to the case of Ag-13 at.% W film but far different from that of Ni-12 at.% W film associated with a redistribution of the W atoms on an apparently very fine spatial scale. Their TEM analysis shows that an obviously nanoscale phase separation emerges throughout the Cu-14 at.% W film (see **Figure 15**). The size and the composition of the nanoinclusions at the GBs and in the grain (column) interiors are different, see **Figure 15(d)**. At the GBs, the average precipitates (rich in W) have dimensions of about 5–6 nm in diameter and in the grain interiors the precipitates (rich in Cu) are twice as large, see **Figure 15(d)**. Interestingly, the TBs are largely preserved in Ag-W and Ni-W films, whereas they completely disappear in Cu-W films. They attributed this unique phenomenon to an altered faulting energy, due to change in the amount of W segregated at TBs and to the evolution of nanosized precipitates [104]. This systematical, representative study of W-alloyed, heavily faulted NS thin films not only provides deep insights into understanding the atomic interactions in the binary alloyed films with high positive *H*mix like Cu-W system, but also benefits us to tune their microstructural stability and mechanical properties in future.
