**4.1. Alloying effects on microstructure and mechanical properties in the Cu-Zr model system**

*Nf*

92 Study of Grain Boundary Character

 decreases with further reducing *h*. This is caused by the good combination of high strength (~1050 MPa) and suitable ductility (~5.5%). Luo et al. [48] recently also pointed out that in addition to the potential contribution from the high strength of nanograins (of Au), notable improvement in fatigue properties may be closely associated with twinning-mediated grain

, where *ε<sup>f</sup>*

and exponent, respectively, as shown in **Figure 8(b)**. Accordingly, with reduction in *h* from 700 nm with *d* = 220 nm to *h* = 60 nm with *d* = 20 nm, the surface damage morphologies change from extrusion/intrusion to intergranular cracks, due to the transition of deformation mechanism from dislocation-based to GB-mediated. In other words, with decreasing *size parameters* the localized accumulation of plastic strains within grains is hindered and the GBs take over as the preferred site for damage formation, implying the availability and activation of bulk dislocation sources become more limited in NC metals. This is consistent with the postmor-

**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

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

(2 *Nf* ) *C* of the Cu thin film. Moreover, all Cu

and *C* are the fatigue ductility coefficient

on Δ*ε* that could be well described by the well-known

growth. For a given *h*, a higher Δ*ε* leads to a smaller *Nf*

thin films exhibit the dependence of *Nf*

tem TEM observations by Zhang et al. [73].

map for design alloys with positive enthalpy.

Ni-based binary alloy.

Coffin–Manson relationship: (Δ*ε*/2 ) <sup>=</sup> *<sup>ε</sup><sup>f</sup>*

In such system that has a very negative enthalpy of mixing, only elemental Cu and intermetallic Cu-Zr phases coexist at room temperature under equilibrium state. However, nonequilibrium MS can result in the coexistence of solute (Zr) atoms/clusters, Cu-Zr intermetallic particles, and Cu-Zr amorphous phase in the as-deposited alloyed thin films to achieve multihierarchical microstructures, thereby facilitating the combination of high strength and ductility.

Zhang et al. [82] systematically investigated the microstructural evolution, mechanical properties, and deformation mechanisms of NS Cu thin films alloyed with Zr. It is found that Zr addition significantly changes the microstructures of NS Cu thin films. A strong (100) texture observed in the pure Cu film is strongly suppressed while the (110) texture is favorably promoted in the Cu-0.5 at.% Zr and Cu-2.0 at.% Zr films. When the Zr content is up to 8.0 at.%, the (100) and (110) peaks disappear and the (111) peak is also highly weakened, associated with an obvious amorphization tendency. The underlying reason for the change of crystallographic orientations of Cu-Zr alloyed thin films can be attributed to the effect of reduced GB energy caused by GB segregation on the competition between surface energy and strain energy [82].

Along with the crystallographic orientations change, the GB microstructures of Cu-Zr alloyed thin films also change with Zr doping, as displayed in **Figure 9**. Zhang et al. [82] uncovered that in the Cu-0.5 at.% Zr film, some nanosized Cu10Zr<sup>7</sup> precipitates occasionally observed at the GBs, as indicated in **Figure 9(a)** and **(b)**, associated with notable GBs segregation of Zr,

**Figure 9.** Representative TEM and HRTEM images demonstrating the architectured microstructures in the Cu-0.5 at.% Zr (a, b), Cu-2.0 at.% Zr (c, d), and Cu-4.0 at.% Zr (e, f) films. (g) The 3DAP image of the Zr segregation at the GB and the variation of concentration of Zr at different position along the line. Figure subparts (a–f) are taken with permission from Ref. [82].

see **Figure 9(g)**. Actually, besides having an important role in reducing GB energy, GB segregation can drive the formation of new interfacial structures at the GBs. In the Cu-2.0 at.% Zr film, discontinuous amorphous phases are frequently observed at the GBs, as indicated in **Figure 9(c)** and **(d)**. When the Zr addition is up to 4.0 at.%, GBs are unclear and continuous amorphous phase is distributed along the GBs, as shown in **Figure 9(e)** and **(f)**. Their TEM findings are consistent well with the XRD results mentioned above that Zr addition in host metal of Cu tends to induce amorphization.

Apart from the amorphization tendency and grain refinement, another significant change in microstructure caused by the Zr addition is the twinnability in the Cu films. Somewhat soluble Zr atoms reduce the SFE and thus increase twinning propensity, while excessive Zr addition induces sharply reduced twinning propensity. The dependence of twinnability on Zr addition was rationalized from the mechanisms of annealing twins by these authors [82], including (i) the successive and random emission of Shockley partials from GBs, and (ii) the GB migration mechanism accompanied with twins formation. However, the twin thickness monotonically decreases with increasing Zr contents in a fashion as same as the grain size. Furthermore, Zhang et al. [82] unambiguously demonstrated that the architectured microstructures, in particular, the GB complexions, significantly influence the mechanical properties, such as strength/hardness, ductility, and fatigue lifetime of NS materials, addressed below.

The most striking finding in their experiments [82] is that Zr addition offers exceptionally high values of both strength and ductility for the NS Cu thin films and both the strength/hardness and tensile ductility reach peak values at 0.5 at.% Zr addition, as shown in **Figure 10**. With further increasing Zr contents, the hardness shows slow reduction whereas the ductility exhibits sharp reduction. The high strength stems from various contributors, including solid solution (clusters) strengthening [83], GB solute segregation [84], Zener drag effect [85, 86], and GB/TB strengthening [3], in addition to the contribution from amorphous phase in high Zr contents samples [87–90]. The remarkable enhancement in ductility of the Cu-0.5 at.% Zr film stems from the stress-driven grain growth via twinning mechanism, displayed in **Figure 11**, like that in the pure ED Ni foils [37, 71] mentioned in Section 3. This is an indirect effect of Zr doping that benefits the emergence of (110)-oriented grains, leading to random crystallographic orientations, i.e., coexistence of (111), (100), and (110) grains, whose cooperative interaction is known to facilitate grain coarsening. This new finding in Cu-0.5 at.% Zr thin film challenges the conventional wisdom that improving the strength of a metal alloy is always a tradeoff that results in a loss of ductility—the property that allows a metal to deform without fracture.

Also, Zhang and his colleagues [82] explored the mechanical fatigue properties of these deposited Cu-Zr alloyed thin films. The NS Cu-Zr thin films were cyclically strained under different total strain ranges and the strain range Δε versus lifetime *Nf* curves were experimentally determined for the Cu films with different Zr addition, as shown in **Figure 12**. All these films exhibit the dependence of *Nf* on Δ*ε* that could be well described by the Coffin-Manson relationship: (*Δε*/2 ) <sup>=</sup> *<sup>ε</sup><sup>f</sup>* (2 *Nf* ) *C* , where *ε<sup>f</sup>* and *C* are the fatigue ductility coefficient and exponent, respectively. At the strain range of 0.5–3.0, the Cu-0.5 at.% Zr film always displays

see **Figure 9(g)**. Actually, besides having an important role in reducing GB energy, GB segregation can drive the formation of new interfacial structures at the GBs. In the Cu-2.0 at.% Zr film, discontinuous amorphous phases are frequently observed at the GBs, as indicated in **Figure 9(c)** and **(d)**. When the Zr addition is up to 4.0 at.%, GBs are unclear and continuous amorphous phase is distributed along the GBs, as shown in **Figure 9(e)** and **(f)**. Their TEM findings are consistent well with the XRD results mentioned above that Zr addition in host

Apart from the amorphization tendency and grain refinement, another significant change in microstructure caused by the Zr addition is the twinnability in the Cu films. Somewhat soluble Zr atoms reduce the SFE and thus increase twinning propensity, while excessive Zr addition induces sharply reduced twinning propensity. The dependence of twinnability on Zr addition was rationalized from the mechanisms of annealing twins by these authors [82], including (i) the successive and random emission of Shockley partials from GBs, and (ii) the GB migration mechanism accompanied with twins formation. However, the twin thickness monotonically decreases with increasing Zr contents in a fashion as same as the grain size. Furthermore, Zhang et al. [82] unambiguously demonstrated that the architectured microstructures, in particular, the GB complexions, significantly influence the mechanical properties, such as strength/hardness, ductility, and fatigue lifetime of NS materials, addressed

The most striking finding in their experiments [82] is that Zr addition offers exceptionally high values of both strength and ductility for the NS Cu thin films and both the strength/hardness and tensile ductility reach peak values at 0.5 at.% Zr addition, as shown in **Figure 10**. With further increasing Zr contents, the hardness shows slow reduction whereas the ductility exhibits sharp reduction. The high strength stems from various contributors, including solid solution (clusters) strengthening [83], GB solute segregation [84], Zener drag effect [85, 86], and GB/TB strengthening [3], in addition to the contribution from amorphous phase in high Zr contents samples [87–90]. The remarkable enhancement in ductility of the Cu-0.5 at.% Zr film stems from the stress-driven grain growth via twinning mechanism, displayed in **Figure 11**, like that in the pure ED Ni foils [37, 71] mentioned in Section 3. This is an indirect effect of Zr doping that benefits the emergence of (110)-oriented grains, leading to random crystallographic orientations, i.e., coexistence of (111), (100), and (110) grains, whose cooperative interaction is known to facilitate grain coarsening. This new finding in Cu-0.5 at.% Zr thin film challenges the conventional wisdom that improving the strength of a metal alloy is always a tradeoff that results in a loss of ductility—the property that allows a metal to deform

Also, Zhang and his colleagues [82] explored the mechanical fatigue properties of these deposited Cu-Zr alloyed thin films. The NS Cu-Zr thin films were cyclically strained under

mentally determined for the Cu films with different Zr addition, as shown in **Figure 12**. All

exponent, respectively. At the strain range of 0.5–3.0, the Cu-0.5 at.% Zr film always displays

curves were experi-

on Δ*ε* that could be well described by the Coffin-

and *C* are the fatigue ductility coefficient and

different total strain ranges and the strain range Δε versus lifetime *Nf*

, where *ε<sup>f</sup>*

(2 *Nf* ) *C*

metal of Cu tends to induce amorphization.

94 Study of Grain Boundary Character

below.

without fracture.

these films exhibit the dependence of *Nf*

Manson relationship: (*Δε*/2 ) <sup>=</sup> *<sup>ε</sup><sup>f</sup>*

**Figure 10.** Dependence of yield strength (3*σ<sup>y</sup>* = Hardness) (a) and ductility characterized by the critical strain to nucleate microcracks (*εC*) (b) on Zr content. Four regimes (I, II, III, and IV) are divided which correspond to four different microstructures in the Cu, Cu-0.5 at.% Zr, Cu-2.0 at.% Zr, and Cu-Zr (Zr > 4.0 at.%) films, respectively. Figure is taken with permission from Ref. [82].

the greatest fatigue lifetime among all the films while the pure Cu film manifest the shortest one. The fatigue resistance of NS Cu films is notably enhanced via either GB segregation of Zr atoms/precipitates or amorphous phase formation of Cu-Zr, resulting in retardation of fatigue damages. Different from the uniaxial tension test, the amorphous phase plays a crucial role in the prolonged fatigue lifetime through depressing microcrack nucleation and through the arrest of intergranular cracks. Although previously atomistic simulations [87] showed that the nanoscale amorphous intergranular phase as a structure feature plays a critical role in toughening NS materials, as verified in crystalline/amorphous Cu/Cu-Zr nanolaminates [88–90], the presence of amorphous phase in Cu-Zr alloyed thin films apparently deteriorate their tensile ductility, as least at high Zr additions. This GB complexion effect on the mechanical properties is quite interesting and requires further work to reconcile the current discrepancy. Still, postmortem TEM observations in the fatigued Cu-Zr thin films verified that the occurrence of stress-driven grain growth under fatigue conditions, as same as that in tensile deformation. These findings show that the fatigue of thin metallic films remains a very attractive field of research due to the possible complex interplay of the possible deformation and fracture mechanisms.
