Microhardness and tensile strength of electrochemically synthesized nickel-cobalt binary alloy sheets exfoliated from a dumbbell-shaped titanium cathode

Nanocrystalline nickel–cobalt (Ni–Co) binary alloy sheets were fabricated through electroforming in an acidic aqueous bath using exfoliation from a metallic titanium cathode. Cobalt content in Ni–Co alloy sheets ranged from 28.8 at% to 72.0 at% depending on experimental parameters, such as cathodic overpotential and bath composition. The surface roughness (R a) of the electroformed alloy sheets significantly decreased down to 1.5 μm as saccharin sodium dihydrate was added as an additive to the acidic aqueous solution bath. X-ray diffraction profiles and transmission electron microscopy images indicated that the electroformed Ni–Co alloy sheets have a nanocrystalline structure (grain size ≈ 30 nm). The lattice constant of the electroformed Ni–Co alloy sheets increased with an increase in cobalt content (i.e. solute atom concentration). The mechanical properties were significantly improved because of the synergistic effects of crystal grain refinement and solid solution strengthening. The microhardness and tensile strength of the electroformed Ni–Co alloy sheets reached 609 kgf mm−2 and 1757 MPa (X Co = 49.9 at%), respectively. The tensile strength of the electroformed Ni–Co alloy sheets in this study significantly exceeded that of solidified Ni–Co alloys (approximately 370 MPa). Therefore, this study offers a technique to enhance the mechanical properties of electroformed Ni–Co alloy sheets.

method (e.g. electrodeposition and plasma electrolysis) can be optimised by controlling electrolysis conditions, such as the cathode potential and bath composition [12][13][14]. In previous works, nanocrystalline Ni alloys (crystal grain size: d  100 nm), such as Ni-W alloys [15,16], Ni-Zn alloys [17] and Ni-P alloys [18], were synthesised by electrodeposition. Dehestani et al synthesised Fe-Co-Ni-Mo-W high-entropy alloys using a pulsed current electrodeposition technique [19]. The alloy composition and surface morphology were controlled by adjusting the current density (2 to 10 A dm −2 ) during electrodeposition. They revealed that when the alloys were electrodeposited with a current density of 6 A dm −2 , the structure of the resulting alloys was a mixture of FCC nanocrystalline (29%) and amorphous (71%), based on the XRD and SEAD patterns. Hasegawa et al synthesised Ni-W alloys using electrodeposition with an average current density of −10 to −25 mA cm −2 in an ammoniacal citrate bath (70°C, pH = 8) [15]. They revealed that the average grain size of electrodeposited Ni-W alloy films decreased with increasing tungsten content (from 0 at% to 5 at%) and reached approximately 21 nm. Some researchers have revealed that the physical performance, including mechanical strength and magnetic properties, of electrodeposited nanocrystalline alloys (e.g. pure Co [20], Co-W alloys [21], Ni-Mo alloys [22,23]) can be improved through crystal grain refinement. Su et al revealed that the microstructural characteristics of electrochemically synthesised Co films, such as surface morphology and grain size, can be controlled by adjusting the experimental conditions, specifically the direct current, unipolar pulse, reverse pulse, and bipolar pulse of the electrodeposition [20]. According to their report, the microhardness of electrochemically synthesised Co films increased to approximately 725 kgf mm −2 with a decrease in mean grain size from 50 to 10 nm. They also found that the microhardness of electrochemically synthesised Co films was quite larger than that of pure solid Co (250-300 kgf mm −2 [24,25]). Liu et al electrochemically synthesised nanocrystalline Ni-Mo alloy films through an induced co-deposition process [22]. They found that the microhardness of electrochemically synthesised Ni-Mo alloy films increased with increasing molybdenum content. They also revealed that the microhardness was improved up to 633 kgf mm −2 (crystalline size = 7.9 nm) as a result of the synergistic effects of crystal grain refinement and solid solution hardening. Ledwig et al investigated the microstructure and hardness of nanocrystalline Ni-Co-Fe alloys electrodeposited from a sulphate-citrate bath [26]. They reported that Ni-Co-Fe alloys have a single FCC phase with a grain diameter of less than 30 nm. The microhardness of the electrodeposited alloys reached up to 4.2 ± 0.4 GPa (ca. 430 kgf mm −2 ) due to its nanocrystalline structure.
However, alloy electrodeposition still faces a number of technical difficulties. In several cases, the surface smoothness of electrochemically synthesised alloy films is less than ideal. The addition of brighteners to the electrolytic bath is an effective approach for reducing the surface roughness of electrodeposited alloy films. In the electrodeposition of pure Ni or Ni-based alloys, the throwing power can be enhanced using brighteners, such as sodium saccharin (saccharin) [27,28]. Saccharin is typically adsorbed on the electrode surface during electrodeposition [29]. In this case, crystal nucleation can be promoted because the diffusion of metal atoms on the electrode is inhibited. Consequently, the surface smoothness of the electrodeposited metal films is improved due to the formation of fine crystal grains. Furthermore, it is difficult to exfoliate an electrodeposited metallic film from a substrate because of the strong adhesion between the electrodeposit and the cathode substrate, which depends on the metal combination. The structure, corrosion resistance and physical properties (e.g. magnetic properties) of electrodeposited films integrated with a substrate have been investigated in previous works [30][31][32]. In contrast, there is a gap in the literature regarding the mechanical properties (e.g. tensile strength) of electrodeposited alloy films (hereafter called 'electroformed alloy sheets') exfoliated from a substrate. Recently, electroforming for the synthesis of alloy films (with the same shape as their substrates) has garnered attention as a method for producing small-sized and complicated-shaped metallic materials. The peelability of the electroformed alloy sheets can be improved by using a metallic plate with a thin oxide film on its surface as a cathode. The tunnelling current can flow through a thin metal oxide layer with a thickness of 10 nm or smaller [33]. Metallic titanium forms a stable oxide layer (TiO 2 ) with a thickness of several nanometres in the atmosphere [34]. Hence, a titanium electrode is a potential material for the electroforming of Ni alloy sheets.
Additionally, Cr alloys, such as stainless steels, have high strength and heat resistance; hence, they are widely used in various industrial products (e.g. machine structures and automobile components). Although Cr alloys can also be synthesised through an electrodeposition process, the use of electrolytic baths containing Cr 6+ ions is restricted due to their harmful effects on the environment [35]. Instead, the electroforming technique for Nibased alloys can be applied to the production of novel high-strength materials. In addition, electroforming is a suitable method for imparting various functionalities, such as corrosion resistance, through the surface treatment on inexpensive materials (e.g. carbon steels). It seems that improving the mechanical properties of electroformed alloys is an effective approach for maintaining surface functionalities. In the present study, Ni-Co alloy sheets were fabricated by electroforming using a sulphate bath containing saccharin sodium dihydrate. In addition, the surface morphology, crystal structure, microhardness and tensile strength of the synthesised alloy sheets were investigated.

Methods
Ni-Co alloy sheets (thickness ≈100 μm) were synthesized using an electroforming process in an acidic aqueous bath (40°C, pH show the shape and dimensions of the dumbbell-shaped titanium electrode. In the present work, the original specimens for tensile test were prepared refering to ASTM E08/E8M-11 standard [36,37]. Here, the dimensions were adjusted in order to electrodeposit small specimens in the electrolytic bath (300 ml). Moreover, as an anode and a reference electrode, we selected a soluble metallic plate (pure Ni or Co plate) and a silver/silver chloride (Ag/AgCl) electrode which was saturated by potassium chloride (KCl), respectively. Cathode potential conditions for electroforming of Ni-Co alloy sheets were determined by the cathodic polarization curves with ranging from −1.1 to −1.4 V versus Ag/AgCl sat. KCl. After electroforming, Ni-Co alloy sheets were mechanically exfoliated from titanium substrates using a cutter knife. Subsequently, chemical composition of electroformed alloy sheets was analyzed by EDX (EDX-800HS, Shimadzu Corp., Kyoto, Japan). Surface morphology of electroformed alloy sheets was characterized using SEM (JCM-5700, JEOL Ltd., Tokyo, Japan). The surface roughness was evaluated using Surftest SJ-210 (Mitutoyo, Kanagawa, Japan). The crystal structure (e.g., crystallinity and constituent phase) of the electroformed alloy sheets was analyzed using XRD (Miniflex 600-DX, Rigaku Corp., Tokyo, Japan) and TEM (JEM-2010-HT, JEOL Ltd., Tokyo, Japan). Before the TEM observations, the sample thickness was reduced to less than 100 nm using an ion slicer (EM-09100IS, JEOL Ltd., Tokyo, Japan). The microhardness of the electroformed Ni-Co alloy sheets was measured using a hardness testing machine (HM-211, Mitutoyo, Kanagawa, Japan). The tensile strength was measured using a universal testing machine (Autograph 50 kN, Shimadzu Corp., Kyoto, Japan) at room temperature. The crosshead speed used in the tensile test of the electroformed alloy sheets was set to 0.5 mm min −1 .  .
] is the activity of metallic Ni and Co (1). Bockris et al reported that metal ions such as Fe 2+ , Co 2+ and Ni 2+ can be electrochemically reduced to atoms through multistep reactions [38]. When these metal ions are electrochemically reduced from an aqueous bath, the pH in the vicinity of the cathode rises as H + ions are consumed in the vicinity due to the hydrogen evolution, which occurs as a side reaction. In these instances, M 2+ ions can form metal hydroxide ions (MOH + ), which can be adsorbed on the cathode surface. These MOH + ions can be electrochemically reduced to their metallic state. These metal atoms subsequently gather and nucleate on the cathode. Hence, the electrodeposition of iron-group metal ions requires a large overpotential [39]. As shown in the cathodic polarization curves (figures 2(a)-(e)), the cathodic current density (CCD) sharply increased at a cathode potential of approximately −0.75 V. This increase in the CCD may be attributed to metal deposition (i.e. pure Ni, Ni-Co alloy, or pure Co). To realise an effective electrodeposition process with a high growth rate, the optimum cathode potential range for Ni-Co alloy electroforming was determined to be from −1.
show the EDX profiles of the electroformed Ni-Co alloy sheets ((a) X Co = 28.8 at%, (b) X Co = 49.9 at%, (c) X Co = 66.6 at%). In the profiles, the peaks associated with NiK α and NiK β were observed at around 7.5 keV and 8.3 keV, respectively. In addition, the peaks derived from CoK α and CoK β were confirmed at around 7.0 keV and 7.6 keV, respectively. The chemical composition of the electroformed Ni-Co alloy sheets was evaluated based on the EDX profiles. Figure 4(a) shows the effect of cathode potential on cobalt content (X Co ) in the electroformed Ni-Co alloy sheets. Cobalt content in the alloy sheets increased with increasing concentrations of Co 2+ ions in the electrolytic bath. Moreover, the cobalt content decreased by polarising the cathode potential to a less noble region. The slopes of the Tafel plots (figures 2(b)-(d)) revealed that mass transfer was the rate-limiting process when the cathodic overpotential increased during the electroforming of the Ni-Co alloy sheets. In this work, the concentration of the Co 2+ ions ([Co 2+ ] = 0.05, 0.1 and 0.2 M) was lower than that of Ni 2+ ions ([Ni 2+ ] = 0.8, 0.9 and 1.0 M) in the electrolytic bath. Thus, the cobalt content in the electroformed alloy sheets decreased when the rate-limiting process involved mass transfer. The effect of bath composition on the cobalt content in the electroformed Ni-Co alloy sheets is shown in figure 4(b). The cobalt content in the electroformed Ni-Co alloy sheets reached approximately 30 at%-70 at%, whereas the proportion of Co 2+ ions in the electrolytic bath (r Co ) was 5-20 mol%. As shown in figure 4(b), the cobalt content in the electroformed Ni-Co alloy sheets exceeded that indicated by the composition reference line. Therefore, electrochemically less noble Co 2+ ions preferentially deposited in the Ni-Co system rather than the noble Ni 2+ ions. These results are in line with those of previous reports on the electrodeposition of Ni-Co alloys [40,41]. The electrodeposition behaviour of Ni-Co alloys can be categorised as an 'abnormal co-deposition type' according to the Brenner's report [42]. Based on the formation constants of MOH + ions (Ni 2+ + OH − AE NiOH + (1.26 × 10 4 M −1 ), Co 2+ + OH − AE CoOH + (2.0 × 10 4 M −1 ) [43,44]), the concentration of CoOH + ions can be higher than that of NiOH + ions in the vicinity of the cathode. Hence, in this work, metallic cobalt preferentially deposited during the electroforming of Ni-Co alloy sheets. Figures 5(a)-(e) show the shape of the electroformed metallic sheets ((a) X Co = 0 at%, (b) X Co = 28.8 at%, (c) X Co = 49.9 at%, (d) X Co = 66.6 at%, (e) X Co = 100 at%). The electroformed specimens were dumbbell-shaped -the same shape as the titanium electrode ( figure 1(a)). Figures 6(a)-(d) show the SEM images of the metallic sheets electroformed using an electrolytic bath containing saccharin. As shown in the images, the surface appearance of the electroformed Ni-Co alloy sheets was quite smooth, with no microcracks or nodule-like deposits. Figure 7 shows the effect of cobalt content on the surface roughness of the electrodeposited pure Ni and Ni-Co alloys. The surface roughness was greatly reduced by the addition of saccharin to the electrolytic bath (average roughness, R a < 1.5), which significantly improved the smoothness of the electroformed metallic sheets. Figure 8 shows the XRD profiles of the electroformed Ni, Ni-Co alloy and Co sheets. Peaks associated with the FCC-Ni (111) and (200) planes were observed in the XRD spectra of the electroformed Ni and Ni-Co alloy sheets. The peaks shifted to lower angles as the cobalt content in the electroformed metallic sheets decreased. The peak associated with the hexagonal close packed (HCP)-Co (002) plane was observed in the XRD spectra of the electroformed cobalt sheets. Based on the binary alloy phase diagram, Ni-Co alloys possess an FCC structure at room temperature when the Co content is less than ca. 70%. In the present study, a diffraction peak derived from the FCC-Ni (111) plane was observed in the XRD profile of an electroformed Ni-Co alloy sheet with X Co = 66.6 at%. It seems that the diffraction peak shifted to a lower angle because the lattice constant increased due to the solid solution of Co atoms in the Ni crystal lattice. In contrast, only the diffraction peak associated with the HCP-Co (002) plane was observed in the XRD profile of the electroformed Co sheet. In previous works, Pangarov et al reported that the crystal orientation of electrodeposited metals depends on the cathodic overpotential because each crystal plain has a different formation work of the two-dimensional nucleus [45,46]. Therefore, other diffractions (e.g. HCP-Co (100)) can also be observed by changing the overpotential during the electrodeposition of cobalt. The effect of cobalt content in the electroformed Ni-Co alloy sheets on the lattice constant is shown in figure 9(a). The lattice constant increased with increasing cobalt content in the solid solution alloys. Generally, a substantial lattice strain is introduced into a metallic crystal when a solid solution phase is formed. The amount of lattice strain depends on the difference in the atomic radii of the solvent atoms (i.e. Ni atoms (117 pm)) and solute atoms (i.e. Co atoms, 118 pm). In addition, the amount of introduced strain increases as the solute atom concentration increases. Due to the close radii of Ni and Co atoms, the effect of lattice strain is expected to be small in Ni-Co alloys with low Co content. In the present work, the cobalt content in the electroformed Ni-Co alloy sheets ranged from 28.8 at% to 72.0 at%, which was achieved by controlling the cathodic overpotential and bath composition. Hence, the diffraction peak seems to be shifted to a lower angle because the lattice constant increased as cobalt content increased. The crystallite size, D, of the electroformed metallic sheets was calculated using equation (2) (Scherrer's equation). where K, λ, and β are the shape factor (0.94), x-ray wavelength (Cu: K α = 0.15418 nm), and full width at half maximum of the diffraction peaks, respectively. The effect of cobalt content on crystallite size is shown in figure 9(b). The crystallite size of the electroformed metallic sheets was estimated to be 22-37 nm. Figure 10 shows a TEM bright field image ( figure 10(a)) and an electron diffraction pattern ( figure 10(b)) of an electroformed Ni-Co alloy sheet (X Co = 66.6 at%). In the electron diffraction profile, the concentric pattern (i.e. Debye-Scherrer ring) associated with the FCC-Ni (111) plane was clearly confirmed as the main phase. This finding is consistent with the results of the XRD profile. In addition, the ring patterns derived from the FCC-Ni (200), (220), (311) and (222) planes were also confirmed. The average grain size was estimated based on the TEM image and was found to be consistent with the crystallite size calculated from the XRD profile of the electrodeposited Ni-Co alloy sheet. Moreover, the electron diffraction pattern indicated that the electroformed Ni-Co alloy sheet had a nanocrystalline structure. A cathode potential region between −1.1 and −1.4 V was selected for the electrodeposition of the Ni-Co alloy sheets in this work. As previously mentioned, the equilibrium potentials E eq Ni and E eq Co were estimated to be around −0.43 and −0.47 V, respectively, under the stated experimental conditions. In metal electrodeposition with a large cathode overpotential, a nanocrystalline structure can be achieved with an increase in nucleation frequency [47]. In addition, brighteners, such as saccharin, that are added to the electrolytic bath are adsorbed on the cathode surface. The brighteners adsorbed on the cathode prevent the diffusion of metal atoms on the electrode (electrodeposit) surface, which can decrease the crystal grain size. In a previous work, Wasekar et al investigated the effect of saccharin addition to an electrolytic bath on the crystallite size of electrodeposited Ni films [48]. In their report, the crystallite size decreased to approximately 25 nm when electrodeposition was conducted using an aqueous solution containing saccharin. Therefore, the crystallite size results in this study are in good agreement with their reports.

Mechanical properties of electroformed Ni-Co alloy sheets
The effect of cobalt content on the microhardness of electroformed Ni-Co alloy sheets is shown in figure 11(a). The measurement of microhardness was performed five times for each Ni-Co alloy sheet. Here, the standard error of microhardness was calculated to be 8.20 kgf mm −2 or less. The microhardness was evaluated based on the test force (0.3 kgf) and indentation size ( figure 11 (b)), and it was found to increase with increasing cobalt content. The microhardness of the electroformed Ni-Co alloy sheet, which is close to an equiatomic alloy, reached 609 kgf mm −2 . The microhardness of pure Ni electrodeposited from an aqueous solution without Figure 5. Appearance of the electroformed Ni-Co alloy sheets that were exfoliated from the dumbbell-shaped titanium electrode ((a) X Co = 0 at% (pure Ni), (b) X Co = 28.8 at%, (c) X Co = 49.9 at%, (d) X Co = 66.6 at%, (e) X Co = 100 at% (pure Co)).
brighteners (e.g. saccharin) was reported to be approximately 300 kgf mm −2 [4]. When cobalt atoms are substituted into nickel crystals, lattice strain due to the different atomic radii of Co and Ni occurs in the electroformed Ni-Co alloy sheets. The strength of the metallic sheets increases as a result of the interaction between the dislocation and stress field generated near the solute atoms (i.e. solid solution strengthening). In this work, the average grain size of electroformed metallic sheets decreased to approximately 30 nm. When the grain size decreases (i.e. crystal grain refinement), the volume fraction of the grain boundaries and triple points in the metallic material increases. In this case, mechanical strength of the metallic sheets increases because the dislocation movement along sliding crystal planes is restricted by crystal grain boundaries. Hence, the microhardness of the electroformed metallic sheets is improved as a result of the synergistic effects of crystal grain refinement and solid solution hardening. Figures 12(a)-(i) show the effect of cobalt content on the stressstrain curves of the electroformed Ni-Co alloy sheets ((a) X Co = 0 at% (pure Ni), (b) X Co = 28.8 at%, (c) X Co = 43.5 at%, (d) X Co = 44.8 at%, (e) X Co = 45.6 at%, (f) X Co = 49.9 at%, (g) X Co = 65.4 at%, (h) X Co = 66.6 at%, (i) X Co = 100 at% (pure Co)). The ultimate tensile strength (σ max ) was estimated based on the stress-strain curves.
The effect of cobalt content on the tensile strength of electroformed Ni-Co alloy sheets is shown in figure 13. The tensile strength increased with increasing cobalt content. In the electroformed Ni-Co alloy sheets with X Co = 49.9 at%, the tensile strength reached 1757 MPa. In a previous work, the tensile strength and elongation of Ni-Co alloys (X Co = 50 wt%) synthesised using arc melting were 367 MPa and 18.7%, respectively [49]. The tensile strength of the electroformed Ni-Co alloy sheets significantly increased through the grain refinement mechanism. As shown in figure 12, the elongation of the electroformed metallic sheets was 8% or lower. The XRD peaks attributed to the FCC-Ni (111) and HCP-Co (002) planes were observed in the XRD profiles of the Figure 6. Effect of cobalt content on the SEM images of electroformed Ni-Co alloy sheets ((a) X Co = 0 at% (pure Ni), (b) X Co = 28.8 at%, (c) X Co = 49.9 at%, (d) X Co = 69.6 at%, (e) X Co = 100 at% (pure Co)).
electroformed Ni-Co alloy sheets with high cobalt content (figure 8). The elongation of the electroformed metallic sheets increased due to the preferential orientation of these sliding planes. The ductility of metallic materials decreases when the grain size is significantly reduced to tens of nanometres (e.g. nanocrystalline Ni-W alloy [50] and pure Co [51]). The strength of the material increased because the formation of stable grain boundaries suppressed the dislocation release. In contrast, ductility significantly decreases when the grain size decreases to 1 μm or less [52,53]. In addition, no dislocations form inside the crystal grains when the grain size is less than 20 nm [54]. These factors caused the embrittlement of the electroformed metallic sheets. In this work, a clear elastic deformation region was not observed in the stress-strain curves ( figure 12). The Young's modulus (elastic modulus, σ/ε) of pure Ni is approximately 150 GPa. The slope of the stress-strain curves of the electroformed Ni-Co alloy sheets was several tens of GPa. According to previous studies, electrodeposited metals (e.g. pure Ni [55], Ni-Fe-P alloy [56]) have low Young's modulus values. Schaefer et al investigated the structure of nanocrystalline pure Fe using positron lifetime spectroscopy and concluded that the interatomic distance within the crystal grain boundaries is wider than that within the grains [57]. The elastic deformation of metallic materials is due to interatomic bonds. The bond energy between metal atoms decreases as interatomic    distance increases. Thus, elastic deformation can be suppressed by increasing the volume ratio of crystal grain boundaries, which occurs due to the decrease in the grain refinement. Therefore, the significant decrease in the apparent Young's modulus of the electroformed Ni-Co alloy sheets is due to a mixture of elastic and plastic  Stress-strain curves obtained from the dumbbell-shaped Ni-Co alloy sheets ((a) X Co = 0 at% (pure Ni), (b) X Co = 28.8 at%, (c) X Co = 43.5 at%, (d) X Co = 44.8 at%, (e) X Co = 45.6 at%, (f) X Co = 49.9 at%, (g) X Co = 65.4 at%, (h) X Co = 66.6 at%, (i) X Co = 100 at% (pure Co)).
deformations. In general, elastic deformations are initially observed when stress is applied to metallic materials. The Young's modulus can be calculated from the slope of the stress-strain curve in an elastic deformation area. In the present work, an initial elastic deformation was suppressed because the interatomic distance increased as the volume fraction of crystal grain boundaries in the electroformed Ni-Co alloy sheets increased. Consequently, it seems that the apparent Young's modulus significantly decreased due to a mixture of elastic and plastic deformations. As mentioned previously, the ductility of metallic materials (i.e. breaking elongation) decreases when the grain size is reduced to tens of nanometres because the dislocation release is inhibited. Based on the XRD profiles (figure 8) and TEM images (figure 10), the crystal grain size of electroformed Ni-Co alloy sheets was estimated to be 30 nm. Therefore, it seems that the breaking elongation of electroformed Ni-Co alloy sheets (ca. 8% or less) is lower than that of the solidified Ni-Co alloys (18.7%).

Conclusions
Ni-Co alloy sheets were electrochemically fabricated using a sulphate bath containing a small amount of saccharin as an additive. The cobalt content (X Co = 28.8 at%-72.0 at%) in the alloy sheets was varied by controlling bath composition and cathode potential during electroforming. The proportion of Co 2+ ions in the electrolytic bath (r Co = [Co 2+ ]/([Ni 2+ ] + [Co 2+ ])) was determined to be 5 mol% to 20 mol%. In addition, Ni-Co alloy sheets were prepared using a potentiostatic electrodeposition technique with a cathode potential range of −1.1 to −1.4 V versus Ag/AgCl sat. KCl. During the electroforming of Ni-Co alloy sheets, electrochemically less noble Co 2+ ions preferentially deposited rather than the noble Ni 2+ ions (i.e. abnormal co-deposition). The surface roughness (R a ) of the electroformed Ni-Co alloy sheets was greatly reduced when saccharin was added to the electrolytic bath (R a < 1.5). XRD profiles revealed that the lattice constant increased with increasing cobalt content due to the formation of a solid solution phase. In addition, TEM bright field images and electron diffraction patterns revealed that the alloy sheets exhibited a nanocrystalline structure with a crystal grain size of approximately 30 nm. The crystal grain size can be attributed to the large cathode overpotential and the addition of saccharin to the electrolytic bath. As cobalt content in the electroformed Ni-Co alloy sheets increased, the microhardness and tensile strength reached 609 kgf mm −2 and 1757 MPa, respectively (X Co = 49.9 at%). The tensile strength of electroformed Ni-Co alloy sheets was improved by the synergistic effects of crystal grain refinement and solid solution hardening and greatly exceeded that of solidified Ni-Co alloys (approximately 370 MPa).