2017 Volume 58 Issue 7 Pages 1038-1041
In this study, we fabricated bulk nanocrystalline Ni–W–B alloys by electrodeposition, using trimethylamine-borane (TMAB) as a boron source, showing how B doping affects tensile properties. In electrodeposition, the TMAB concentration was varied from 0 to 5.0 g/L. Adding TMAB to the deposition bath increased the B content of the electrodeposited alloys up to 0.36 at%, but did not significantly change the W content or grain size. Adding more TMAB than 0.1 g/L drastically decreased the material's tensile elongation. Cross-sectional hardness tests on alloys with poor ductility revealed non-uniform hardness and that the initial layer had a high hardness of 6.3–8.7 GPa. This result indicated that the TMAB had immediately decomposed and that the boron decomposition product was mixed into the initial electrodeposited layer. In contrast, the alloys electrodeposited with 0.01–0.05 g/L TMAB showed good tensile elongation of 11%. Our results reveal the appropriate amount of added TMAB in order to produce electrodeposited bulk samples with good ductility.
Electrodeposition is a fabrication process able to produce bulk nanocrystalline metals and alloys with grain sizes of less than 100 nm1–3). Electrodeposition is attractive because its processing parameters—such as electrolyte composition, bath temperature, pH, and applied current density—can be easily adjusted to finely control the final material's solute content, microstructure, and mechanical properties. Additionally, electrodeposition can now produce high-density bulk samples with nanocrystalline structures suitable for mechanical testing. Thus, several papers have reported electrodeposited bulk nanocrystalline metals and alloys with high strength and high ductility4–7). However, experimental results suggest that the tensile behavior of nanocrystalline Ni and Ni alloys is sensitive to segregation of metalloids and nonmetals at grain boundaries8–11). In particular, grain boundary segregation of sulfur reduces grain boundary cohesion12) and decreases ductility10). Thus, electrodeposited bulk nanocrystalline Ni alloys processed with annealing, which promotes segregation, have not exhibited good ductility, even though they exhibit high ductility before annealing13,14). These results suggest that doping with a grain-boundary enhancer such as boron15) could improve the material's tensile properties.
In electrodeposition of Ni and Ni alloys, dimethylamine-borane and trimethylamine-borane (TMAB) are often used as the boron source16,17). Studies have reported on the relationships between the concentration of boron compounds in the deposition bath and the B content of the resultant electrodeposited materials, and they have discussed how the B content affects grain size and hardness. However, few studies have shown how B doping affects the tensile properties of bulk nanocrystalline Ni and Ni alloys. Therefore, in this study, we used electrodeposition, with TMAB as a B source, to produce bulk nanocrystalline Ni–W–B alloys and examine their tensile properties.
One bath of bulk nanocrystalline Ni–W alloys and six baths of bulk nanocrystalline Ni–W–B alloys were prepared by electrodeposition. Table 1 shows the composition of the basic bath. TMAB was used as the B source, and the amount of TMAB added was varied from 0 to 0.5 g/L. We named the samples according to their TMAB concentrations: 0 TB, 0.01 TB, 0.05 TB, 0.1 TB, 1.0 TB, 3.0 TB, and 5.0 TB. All samples were deposited onto copper substrates of commercial purity by using two counter-electrodes of nickel plates (99.98%) and tungsten rods (99.95%). Electrodeposition was performed for 96 h using 1-L deposition systems at a current density of 25 mA/cm2, bath temperature of 50–55℃, and pH of 4.0 ± 0.1. The pH values of the solutions during electrodeposition were maintained by adding drops of 1.0 mol/L sulfamic acid and 5.0 mol/L sodium hydroxide. The details of these deposition systems are described in a previous paper18).
| Chemicals | Amount (g/L) | Purpose |
|---|---|---|
| Nickel sulfamate tetrahydrate | 300.0 | Ni source |
| Sodium tungstate dihydrate | 6.4 | W source |
| Trimethylamine-borane (TMAB) | 0–5.0 | B source |
| Nickel chloride hexahydrate | 5.0 | Passivation inhibitor |
| Sodium propionate | 20.0 | Complexing agent |
| Sodium gluconate | 6.3 | Complexing agent |
| Saccharin sodium dihydrate | 1.0 | Stress reliever |
| Sodium lauryl sulfate | 0.3 | Surface acting agent |
After electrodeposition, we performed the following analyses. The W contents of the electrodeposited samples were determined by using energy-dispersive X-ray spectroscopy (EDX, Shimadzu EDX-8000). The B content was quantified by inductively coupled plasma atomic emission spectroscopy (ICP-AES), and the C and S contents were quantified by IR absorption after combustion in a high-frequency induction furnace. The grain sizes were estimated by X-ray diffraction (XRD, Philp X'Pert-MPD) using Cu Kα radiation. Transmission electron microscopy (TEM) specimens were prepared by ion milling. The microstructures of the TEM specimens were examined with a JEOL JEM-2010 operated at 200 kV. To conduct the tensile tests, three dog-bone specimens were prepared for each condition. The specimens, having a gauge length of 12 mm, width of 3.0 mm, and thickness of ～1.0 mm, were machined by using electrical discharge machining. Note that the copper substrate was removed by surface grinding. Tensile tests were performed at a strain rate of 1 × 10−3 s−1 at room temperature using a universal testing machine (Shimadzu AUTOGRAPH AG-X plus).
Seven bulk samples were electrodeposited, with TMAB concentration varying from 0 to 5.0 g/L. Figure 1 shows the current efficiency of the deposition bath with 0.01–5.0 g/L TMAB. Each deposition bath with TMAB exhibited a high current efficiency of ～91%, while the deposition bath without TMAB exhibited a current efficiency of 90%. TMAB did not affect the current efficiency. Table 2 summarizes the W, B, C, and S contents of the electrodeposited samples. All of the samples had W, C, and S contents of ～1.58 at%, ～0.04 at%, and ～0.06 at%, respectively. With increasing TMAB concentration, the B content increased from 0 to 0.36 at%. Note that the B content was not measured by ICP for the 0 TB sample, and the B content was counted to 0 at%.
Current efficiency of deposition bath as a function of the TMAB concentration in the deposition bath.
| TMAB (g/L) | W (at%) | B (at%) | C (at%) | S (at%) |
|---|---|---|---|---|
| 0 | 1.65 | 0 | 0.06 | 0.05 |
| 0.01 | 1.65 | 0.002 | 0.03 | 0.05 |
| 0.05 | 1.69 | 0.006 | 0.03 | 0.04 |
| 0.10 | 1.89 | 0.014 | 0.04 | 0.05 |
| 1.0 | 1.23 | 0.145 | 0.04 | 0.06 |
| 3.0 | 1.44 | 0.162 | 0.05 | 0.07 |
| 5.0 | 1.48 | 0.363 | 0.05 | 0.08 |
Figure 2 shows the XRD pattern for each sample. These patterns show a single face-centered cubic (fcc) structure. The grain size of each sample was estimated from the XRD peak width. We used a single-line method to separate the size and strain broadening19,20). The estimated grain sizes were 25–31 nm, as calculated and shown in Fig. 2. Figure 3 shows examples of the bright- and dark-field TEM images and the corresponding diffraction pattern for sample 3.0 TB. The TEM diffraction pattern (Fig. 3 (c)) displays characteristics of fcc materials, and the TEM images (Fig. 3 (a) and 3(b)) indicate a nanocrystalline structure with a grain size around 30 nm. These results agree with the XRD results. Several studies21,22) have reported that measuring X-ray line broadening for a grain size is an accurate way to estimate grain sizes of less than approximately 30 nm. The grain size of each sample was approximately 30 nm. The grain size of electrodeposited alloys depends on their solute and impurity contents11,23). In the present study, the samples had the same W content. Also, the present samples have insufficient B, C, and S contents to affect their grain size.
XRD patterns of each sample. T is the concentration of TMAB, and d is the grain size calculated from the (111) peak width from XRD.
(a) Bright-field and (b) dark-field TEM images of sample 3.0 TB, and the (c) corresponding diffraction pattern.
Figure 4 presents the typical stress–strain curves of (a) samples 0.01 TB and 0.05 TB and (b) samples 0.1 TB, 1.0 TB, 3.0 TB, and 5.0 TB, along with results of sample 0 TB. The 0.01 TB and 0.05 TB samples, as well as sample 0 TB, exhibited a tensile strength of 1.5–1.6 GPa and good tensile elongation of 11% (Fig. 4 (a)). In contrast, the tensile elongation decreased as the TMAB concentration increased from 0.1 to 5.0 g/L. We could not improve the tensile properties by B doping.
Typical stress–strain curves of (a) samples 0.01 TB and 0.05 TB and (b) samples 0.1 TB, 1.0 TB, 3.0 TB, and 5.0 TB, along with a result of sample 0 TB.
Several papers have addressed how light elements such as B, C, and S can affect the mechanical properties of electrodeposited nanocrystalline metals8,9,24). For example, Yin and Whang24) compared the tensile behaviors of electrodeposited nanocrystalline Ni and Ni–B alloys. The tensile elongation in both nanocrystalline Ni and Ni–B alloys is approximately 2–4%. In this study, boron doping did not improve ductility, which agrees with our present results. However, the ductility of nanostructured metals is most often related to processing defects, as discussed in the most recent overview article25). In examining how alloying affects ductility in nanocrystalline metals, we must address the effect of processing defects.
In an acid solution, TMAB is decomposed by hydrolysis26). Because of this decomposition, we are concerned that a uniform bulk sample cannot be obtained from electrodeposition with TMAB. To verify this concern, cross-sectional hardness tests were conducted on electrodeposited samples, as shown in Fig. 5 (a). The error bars shown in Fig. 5 (a) correspond to one standard deviation. The hardness was measured at five points, at intervals of approximately 0.2 mm from the interface to the surface, as shown in Fig 5(b). The 0 TB sample, electrodeposited with no TMAB, showed uniform hardness of approximately 4.6 GPa. In contrast, the samples electrodeposited with TMAB showed variable hardness. The hardness of the initial layer increased from 6.3 to 8.7 GPa as the concentration of TMAB increased from 0.1 to 5.0 g/L. The hardness converged to 4.7 GPa while approaching the surface layer. The high hardness of the initial layer in the samples electrodeposited with TMAB is likely related to the fact that a high B content, as a decomposition product, decreases the grain size17). These results indicate that hydrolyss of TMAB immediately occurred during electrodeposition. In addition, the hydrolysis of TMAB in the acid solution generated hydrogen gas26). Repeatedly trapping and desorption of hydrogen gas introduces high internal stress and defects7,27,28). Therefore, we conclude that the decrease in the tensile ductility in the sample electrodeposited with 0.1–5.0 g/L TMAB came from the hydrogen gas generated by TMAB hydrolysis. We also conclude that the appropriate TMAB content needed to produce electrodeposited bulk nanocrystalline Ni–W–B alloys with good ductility is 0.01–0.05 g/L. However, a small amount of TMAB cannot produce a high B content in electrodeposited samples and only affects the initial layer. To experimentally determine how B doping affects electrodeposited nanocrystalline Ni alloys, we will focus future work on developing a novel method for B doping that avoids processing defects and produces a high B content.
(a) Cross-sectional hardness for samples electrodeposited with varying TMAB concentrations from 0 to 5.0 g/L. Each data point represents the average of 3 indentations. (b) Schematic of the measurement point: the hardness was measured at 3 lines with 5 points at intervals of approximately 0.2 mm from the interface to the surface as a line.
We produced bulk nanocrystalline Ni–W–B alloys with electrodeposition. By varying the concentration of TMAB from 0 to 5.0 g/L, we produced electrodeposited alloys with B contents of 0–0.36 at%. The bulk nanocrystalline Ni–W–B alloys electrodeposited with 0.1–5.0 g/L TMAB showed poor ductility in tensile tests. This low ductility was caused by the hydrolysis of TMAB: the decomposition generated hydrogen gas, which produced high internal stress and defects. In contrast, the bulk nanocrystalline Ni–W–B electrodeposited with 0.01–0.05 g/L TMAB exhibited good tensile elongation of 11%, along with a tensile strength of 1.5–1.6 GPa. We also determined the appropriate TMAB content needed to produce bulk nanocrystalline Ni–W–B alloys with electrodeposition.
