Electrodeposition

Electrodeposition techniques were first developed two centuries ago. In recent years, they have been found to be effective for introducing high-density nanoscale growth twins in metals with appropriately low (i.e., below approximately 100 mJ/m^{Reference Lu, Shen, Chen, Qian and Lu2}) γ_sf,^{Reference Lu, You and Lu5} such as Cu,^{Reference Lu, Chen, Huang and Lu1,Reference Lu, Shen, Chen, Qian and Lu2,Reference You, Lu and Lu14} Cu alloys,^{Reference Lu, Lu and Suresh4} Ni,^{Reference Meng, Shao, Zhang, Zhang and Wang8} and Ni alloys.^{Reference Wu, Ferreira and Schuh15} The species to be deposited are initially chemically dissolved in an electrolyte solution. An electrical potential is applied, and the metal is deposited onto an electrode by means of an electrochemical reaction. Some parameters that influence twin formation, such as γ_sf, are controlled by material selection. Others depend on deposition conditions, with the main controllable parameters being temperature, chemistry, and current characteristics of the deposition process.

The simplest case is direct-current electrodeposition (DCED), in which a constant current is applied, resulting in a mostly constant growth rate of the film. Here, after an initial nucleation step, growth processes favoring strong {111} textures dominate, and twins form randomly due to the low γ_sf. The typical microstructure of NT metals prepared by DCED is characterized by columnar grains filled with high-density nanoscale CTBs, most of which are perpendicular to the growth direction,^{Reference You, Lu and Lu14} (see Figure 1a).

Figure 1.

Transmission electron microscope micrographs (TEM) of nanotwinned (NT) microstructures resulting from different deposition methods and parameters. Note: GD, growth direction. (a) Columnar-grained polycrystalline NT Cu formed by direct-current electrodeposition.^{Reference You, Lu and Lu14} (b) Equiaxed-polycrystalline NT Cu formed by pulsed electrodeposition.^{Reference Shen, Lu, Lu, Jin and Lu3} (c) Sputtered columnar-grained polycrystalline NT Ag.^{Reference Bufford, Wang and Zhang24} (d) Sputtered columnar-grained 330 stainless steel. Reproduced with permission from Reference Reference Zhang, Misra, Wang, Swadener, Lima, Hundley and Hoagland31. © 2005 American Institute of Physics. (e) Coherent-twin boundary (CTB) propagation across fully coherent interfaces in Ag/Al multilayer films.^{Reference Bufford, Liu, Zhu, Bi, Jia, Wang and Zhang32} (f) Inclined misfit stacking faults in Cu/Co multilayer films.

A variation to this technique is pulsed electrodeposition (PED),^{Reference Chandrasekar and Pushpavanam16} in which the power is repeatedly switched on and off with respective time intervals t _on and t _off. During t _on, the high-current density induces a high transient deposition rate, and the deposition process momentarily halts during t _off. Each pulse thus provides new nucleation opportunities, decreasing the strength of the {111} growth texture, and the higher transient deposition rate increases twin density. In contrast to DCED foils, PED NT Cu samples often have {110}-preferred orientations, with CTBs parallel to the growth direction,^{Reference Shen, Lu, Lu, Jin and Lu3} as shown in Figure 1b.

During electrodeposition, the nucleation and growth rates of twins can be engineered by controlling the deposition parameters, such as peak and average current density, deposition rate, and bath chemistry. Current density plays an important role in NT Cu deposition; increasing current density from 10 mA/cm^{Reference Lu, Shen, Chen, Qian and Lu2} to 30 mA/cm² decreased the average grain size in NT Cu foils from ∼10 μm to 4 μm, while the twin spacing remained in the range of 30–50 nm.^{Reference Jin, Pan and Lu17} This is because with an increase in current density, overpotential of the cathode increases and grain refinement (i.e., reduction of grain size) occurs.

Effects of the average deposition rate on the twin density are also significant. Increasing the average deposition rate from 10 to 30 Å/s decreased the twin spacing from 90 nm to 15 nm in PED-deposited Cu foils, while further increasing the deposition rate to 80 Å/s achieved an average twin thickness as small as 4 nm.^{Reference Lu, Chen, Huang and Lu1}

Different ions may be added to the electrolyte solution to influence various aspects of film growth, however, chloride (Cl^–) ions influence texture, particularly strongly in Cu.^{Reference Vasiljevic, Wood, Heard and Schwarzacher18} With increasing chloride concentration in the electrolyte, DCED-deposited Cu films showed a monotonically strengthening {110} crystallographic texture, while PED films underwent a transition from {111} to {110} texture when chloride concentration decreased from 10^–4 to 10^–5 M.^{Reference Chan, Chueh and Liao19} The effect of chloride concentration on the crystallographic texture of the Cu films is attributed to differences in exchange current densities for different crystallographic planes.^{Reference Chan, Chueh and Liao19}

Varying amounts of bath components often end up in films during electrodeposition, including both those deliberately added to influence the deposition process and unanticipated impurities. These impurities may interfere with studies of the properties of twins in pure materials, or may cause undesirable behaviors such as embrittlement, however, there may be benefits to deliberately introduced chemical impurities. For example, certain impurities stabilize GBs at elevated temperatures^{Reference Koch, Scattergood, Saber and Kotan20} and may enhance thermal stability of twins.

Magnetron sputtering

In sputter deposition, a large potential accelerates ions from a plasma into the surface of the target material. These collisions eject atoms from the target, which travel across the vacuum chamber and condense on a substrate. The selected target material, substrate, and deposition parameters (including sputter power density, sputter gas pressure, substrate bias voltage, substrate temperature, and substrate-gun distance/orientation) control the film growth process.^{Reference Ohring21} As with electrodeposition techniques, dense TBs are often oriented perpendicular to the growth direction in metals with appropriately low γ_sf, for example, Cu,^{Reference Hodge, Wang and Barbee22,Reference Zhang, Wang, Chen, Lu, Lu, Hoagland and Misra23} Ag,^{Reference Bufford, Wang and Zhang24–Reference Ott, Geng, Besser, Kramer, Wang, Park, LeSar and King26} and alloy systems.^{Reference Zhang, Misra, Wang, Lima, Hundley and Hoagland27–Reference Velasco, Polyakov and Hodge29} Different microstructures can be obtained from magnetron sputtering by changing substrates; columnar grains with strong {111} texture are common with fcc metals on most substrates,^{Reference Hodge, Wang and Barbee30} however, NT microstructures with different orientations have been achieved by epitaxial growth.^{Reference Bufford, Wang and Zhang24}

The tendency to spontaneously form growth twins has been explained by a thermodynamic vapor nucleation model in which the critical radii of formation for twinned and untwinned nuclei are considered.^{Reference Zhang, Misra, Wang, Shen, Nastasi, Mitchell, Hirth, Hoagland and Embury28} According to this model, twin-boundary energy, γ_tb, and deposition rate are the two most important variables for controlling twin nucleation. Several studies have found qualitative agreement with this model, as NT microstructures appear frequently in low-γ_tb metals and alloys, such as Cu,^{Reference Hodge, Wang and Barbee22} Ag (Figure 1c),^{Reference Bufford, Wang and Zhang24} and 330 austenitic stainless steels (Figure 1d),^{Reference Zhang, Misra, Wang, Swadener, Lima, Hundley and Hoagland31} while metals with high γ_tb form twins sparsely under similar deposition conditions.^{Reference Bufford, Liu, Zhu, Bi, Jia, Wang and Zhang32} Also in agreement with the model, deposition rate was found to change twin density in Cu.^{Reference Anderoglu, Misra, Wang, Ronning, Hundley and Zhang33}

In practice, film growth during magnetron sputtering is kinetically controlled, making the process difficult to fully describe by models derived from thermodynamics alone. Although the thermodynamic model discussed in the previous paragraph predicts that higher deposition rates should increase twin density, several experimental studies have indicated that this is not always the case.^{Reference Ott, Geng, Besser, Kramer, Wang, Park, LeSar and King26,Reference Zhang, Anderoglu, Misra and Wang34} In addition, the change in twin spacing is often accompanied by a change in grain size. These issues stem from the fact that adatom kinetic energy and flux affect the film growth process and twin-formation rates,^{Reference Dahlgren, Nicholson, Merz, Bollmann, Devlin and Wang35} but these aspects are intrinsically tied to the same tunable sputtering parameters, making them difficult to control independently.^{Reference Ohring21,Reference Freund and Suresh36} Additional systematic studies in both the experimental and modeling realms could refine the understanding of the influence of these deposition parameters on the twin-formation process.^{Reference Zhang, Anderoglu, Hoagland and Misra37}

The sputtering process itself typically leaves only small amounts of incorporated sputter gas, but impurities may also originate in impure targets, surface contaminants (from targets or substrates), or vacuum contaminants (e.g., oxygen, water vapor, and hydrocarbons). As with electrodeposition, it is desirable to minimize unwanted contaminants, however, deliberate inclusions may sometimes be useful, for example, to tailor γ_sf and hence to manipulate twin density^{Reference Velasco, Polyakov and Hodge29} or as second-phase particles for additional hardening.^{Reference Ma, Zou, Sologubenko and Spolenak38}

In contrast to low-γ_sf metals, growth twins are rarely observed in high-γ_sf metals, such as Al, synthesized by either vapor- or electrodeposition.^{Reference Bufford, Liu, Zhu, Bi, Jia, Wang and Zhang32} However, fractions of growth twins were reported in nanocrystalline Pd prepared by electron-beam evaporation,^{Reference Idrissi, Wang, Colla, Raskin, Schryvers and Pardoen39} but not sputtering.^{Reference Wang, Idrissi, Shi, Colla, Michotte, Raskin, Pardoen and Schryvers40} The nature of film growth during evaporation differs substantially from that of sputtering,^{Reference Thornton41} and results in a comparatively weaker {111} texture and lower-density twins with more random orientations than in other NT metals. The twin-formation mechanisms here are not yet completely understood. The unfavorable energetics of introducing twins spontaneously in high-γ_sf metals during deposition has led to the development of approaches using engineered microstructures, some of which will be discussed in the following paragraphs.

Recent studies have found mechanisms by which twins may form as a result of interfaces in a multilayer film structure. Interface coherency enables twin propagation from one layer to another, while in some multilayer films without coherent interfaces, misfit twins may form instead of dislocations to release the misfit strain. In multilayer film systems where the individual layer thickness, h, is small enough to maintain coherent growth, twins may propagate freely from one layer to the next,^{Reference Bufford, Liu, Zhu, Bi, Jia, Wang and Zhang32,Reference Liu, Bufford, Wang, Sun and Zhang42,Reference Liu, Bufford, Rios, Wang, Chen, Zhang and Zhang43} as shown in Figure 1e.

Recent studies of other multilayer films without fully coherent interfaces suggest that layer-to-layer twin propagation can also occur between dissimilar planes, provided a point of local coherency along a twinning plane exists.^{Reference Yu, Bufford, Chen, Liu, Wang and Zhang44} These studies suggest two criteria for formation of nanotwins by layer-to-layer propagation: (a) Twin nucleation in one layer, and (b) a coherent interface to ensure propagation. Mismatch or misfit twins refer to twins that form in only one layer to release misfit strain as an alternative to misfit dislocations. Such twins were first reported in semiconductors,^{Reference Wegscheider, Eberl, Abstreiter, Cerva and Oppolzer45} but have since been observed in several metallic systems,^{Reference Dynna, Marty, Gilles and Patrat46,Reference Zhang, Liu and Zhang47} including Cu/Co multilayer films, as shown in Figure 1f.^{Reference Liu, Chen, Yu, Wang, Chen and Zhang48} These twins typically appear only at semi-coherent interfaces (at large h), and propagate at an angle away from the interface.

Σ3 {111} and Σ3 {112} TBs

The two most commonly discussed TBs in fcc NT metals are Σ3{111} CTBs that appear as straight lines in transmission electron microscope (TEM) observations, and Σ3{112} incoherent TBs (ITBs) that can be identified by high-resolution TEM or inverse pole figure orientation mapping (IPFOM),^{Reference Rauch, Veron, Portillo, Bultreys, Maniette and Nicolopoulos49} as shown in Figure 2a–b. CTBs have an ordered and symmetrical boundary structure, and typically exhibit much lower energy than conventional high-angle GBs; in Cu for example, CTBs, ITBs, and high-angle GBs have respective energies of 24–39 mJ/m², 590–714 mJ/m², and 625–710 mJ/m².^{Reference Zhang, Anderoglu, Hoagland and Misra37,Reference Wang, Anderoglu, Hirth, Misra and Zhang50} This has strong implications for both the thermal and mechanical stability of NT materials, as low-energy boundaries are often more resistant to migration processes.^{Reference Lu, Lu and Suresh4,Reference Zhang, Anderoglu, Hoagland and Misra37} CTBs are also known to scatter electrons less than GBs and thus help to maintain high electrical conductivity in NT metals.^{Reference Chen, Lu and Lu6}

Figure 2.

Example of twin- and grain-boundary (GB) structures in Cu. (a) Coherent twin boundary (CTB) and incoherent twin boundary (ITB) structures revealed by inverse pole figure orientation mapping in the transmission electron microscope (TEM). Color scheme: Σ3 boundaries in red; high-angle GBs in black; low-angle GBs in yellow. Image credit: T. LaGrange (Lawrence Livermore National Laboratory). (b) TEM micrograph of CTB-ITB junctions in nanotwinned (NT) Cu. Reproduced with permission from Reference Reference Wang, Anderoglu, Hirth, Misra and Zhang50. © 2009 American Institute of Physics. (c) Defect structures (i.e., kinks) observed on CTBs. White arrows indicate several kinks.^{Reference Wang, Sansoz, LaGrange, Ott, Marian, Barbee and Hamza52} (d) High-resolution TEM image showing faceted GBs in an equiaxed–polycrystalline NT Cu sample. (Courtesy of N. Lu.)

The Σ3{112} ITB structure can be described as three {111}〈112〉 partial dislocations occurring on three successive {111} planes, with the sum of their Burgers vectors = 0.^{Reference Wang, Anderoglu, Hirth, Misra and Zhang50,Reference Xu, Xu, Tu, Cai, Wang, Dixit, Pang and Miao51} These dislocations may dissociate to a width determined by γ_sf and local stress to create a small volume of 9R phase (i.e., a periodically faulted structure with a stacking sequence of abcbcacab).^{Reference Wang, Anderoglu, Hirth, Misra and Zhang50} ITB energies tend to be closer to high-angle GB than CTB energies. In polycrystalline NT materials, ITBs mainly bound CTBs terminated inside grains (Figure 2a). However, subnanometer resolution IPFOM recently revealed that many as-grown CTBs contain ITBs that appear as steps along CTBs^{Reference Wang, Sansoz, LaGrange, Ott, Marian, Barbee and Hamza52} (Figure 2c). The length of these ITBs in the 〈111〉 direction (perpendicular to the CTBs) is typically small (∼2–5 nm), as seen in Figure 2c. The formation mechanisms of these ITBs along otherwise straight CTBs are poorly understood, but are expected to be linked to processing parameters. The existence of these defect structures along CTBs is consistent with the observations of partial dislocations impinged on CTBs in as-deposited material with twins spaced on the order of a few nm.^{Reference Lu, Chen, Huang and Lu1}

Due to the low density of ITBs in typical NT metals, their influence on mechanical properties was initially thought to be inconsequential. However, the relatively high energy of ITBs leads to a tendency to migrate, where mobility is inversely proportional to the ITB length in the 〈111〉 direction.^{Reference Wang, Sansoz, LaGrange, Ott, Marian, Barbee and Hamza52} It is clear from both simulations and experiments that these shorter ITBs are susceptible to migration under stresses, leading to annihilation of adjacent CTBs (i.e., detwinning).^{Reference Xu, Xu, Tu, Cai, Wang, Dixit, Pang and Miao51–Reference LaGrange, Reed, Wall, Mason, Barbee and Kumar54} ITBs may thus play critical roles in determining the maximum strength of NT metals,^{Reference Lu, Chen, Huang and Lu1} and microstructure stability at elevated temperatures,^{Reference Bufford, Wang and Zhang55} in radiation environments^{Reference Chen, Yu, Liu, Shao, Wang, Kirk, Wang and Zhang56} and in fatigue-loading conditions.^{Reference Shute, Myers, Xie, Li, Barbee, Hodge and Weertman57,Reference Yoo, Boles, Liu, Zhang, Schwaiger, Eberl and Kraft58} However, it should be noted that longer ITBs are less mobile and may participate in strengthening and work hardening via dislocation interactions.^{Reference Li, Wang, Zhang and Misra59,Reference Bufford, Liu, Wang, Wang and Zhang60} In light of these different behaviors, ITBs remain an active area of research interest, as it is clear that controlling ITB density is important for tailoring the overall material properties.

GBs in nanotwinned metals

The types of GBs in NT metals vary depending on the deposition procedure. GB orientation mapping of magnetron sputtered NT Cu in the TEM^{Reference Wang, Sansoz, LaGrange, Ott, Marian, Barbee and Hamza52,Reference LaGrange, Reed, Wall, Mason, Barbee and Kumar54} suggested that the GB distribution in NT metals is far from the Mackenzie distribution of boundary misorientations, which is characteristic of a polycrystal with completely randomly oriented grains. This deviation indicates a strong preference for certain GBs, as opposed to a more random distribution. Epitaxial sputtered films^{Reference Bufford, Wang and Zhang24,Reference Anderoglu, Misra, Wang, Ronning, Hundley and Zhang33} (Figure 1c and Figure 2b) may exhibit only ITBs in the growth direction, to the point that defining individual grains is difficult. In columnar-grained specimens obtained by electrodeposition or sputtering (Figure 1a and d), low-angle boundaries may join adjacent columns that are tilted or rotated with respect to the growth direction.^{Reference Anderoglu, Misra, Wang and Zhang61}

A particularly interesting case is that of faceted GBs in PED Cu (Figure 2d). Here, GBs preferentially assume only a small number of low-energy crystallographic planes, and a high enough degree of faceting may lead to multifold twins.^{Reference Zhang, Wang, Shan and Zhao62} The formation process for these GB-TB structures is not completely clear. It is possible that twinning and faceting reduce the total concentration of GB defects;^{Reference Lu, Shen, Chen, Qian and Lu2,Reference Chen, Lu and Lu6} however, the associated high growth rate deposition processes are far from equilibrium, so energy minimization arguments are not fully satisfactory. Further work in unraveling the relationships between the growth process and these GBs is clearly needed.

GB-TB intersections may improve thermal stability by mutually pinning each other.^{Reference Saldana, Murthy, Shankar, Stach and Chandrasekar63,Reference Zhao, Furnish, Kassner and Hodge64} GBs are also considered to play important roles in mediating plasticity in NT metals.^{Reference You, Lu and Lu14,Reference Wang, Sansoz, LaGrange, Ott, Marian, Barbee and Hamza52} Deformation anisotropy has been observed in highly textured NT materials,^{Reference Ye, Wang, Barbee and Hamza65–Reference Jang, Li, Gao and Greer67} as the ability of CTBs to block dislocation motion in the directions parallel and perpendicular to the boundary plane differs. As discussed earlier, CTBs have a strong tendency to align perpendicular to the growth direction in some deposition conditions, so a {111} texture is commonly observed in NT materials. There has been some success with PED and evaporation techniques in fabricating materials with more randomly oriented CTBs^{Reference Lu, Chen, Huang and Lu1,Reference You, Lu and Lu14,Reference Idrissi, Wang, Colla, Raskin, Schryvers and Pardoen39,Reference Wang, Idrissi, Shi, Colla, Michotte, Raskin, Pardoen and Schryvers40} or multifold twins;^{Reference Zhang, Wang, Shan and Zhao62} such microstructures may contribute to less intense textures and more isotropic mechanical behavior.