Mechanics of Materials
Effect of Boron and Tungsten Addition on Superplasticity and Grain Growth in Electrodeposited Nickel Alloys
2022 Volume 63 Issue 7 Pages 1001-1005
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2022 Volume 63 Issue 7 Pages 1001-1005
The effect of W and B addition on the superplastic deformation and grain growth of electrodeposited nanocrystalline Ni alloys with a crystal grain size of approximately 20 nm was investigated with the aim of improving the mechanical properties by maintaining fine grain sizes during superplastic deformation. A maximum elongation of 12% was recorded for electrodeposited Ni–1.8 at% W at a temperature of 350°C and a strain rate of 1.0 × 10−4 s−1. The electrodeposited Ni–W failed to exhibit superplasticity because the segregated W at the grain boundaries increased the energy required for grain boundary sliding. In contrast, the electrodeposited Ni–0.06 at% B exhibited superplasticity with a recorded elongation of 362% at a temperature of 450°C and a strain rate of 1.0 × 10−4 s−1. With the addition of B, the optimal superplastic strain rates of the electrodeposited Ni–B shifted to lower values than that of the electrodeposited Ni. The grain size and hardness of the electrodeposited Ni–B after superplastic deformation were smaller and higher, respectively, than those of the electrodeposited Ni. The addition of B successfully suppressed grain growth and improved the mechanical properties after superplastic deformation.
The ability of materials to exhibit significant elongation to failure of greater than 200%, and thus superplasticity,1,2) generally requires fine grain sizes of a few microns at elevated temperatures. Superplastic behavior is industrially desirable mainly because it produces excellent deformability at fast strain rates and/or relatively low processing temperatures. As a result, the use of superplasticity for the forming and joining of metal parts has become increasingly widespread.3,4) It has been established that the superplasticity temperature decreases as the grain size decreases.5) One well-known process for constructing nanocrystalline metals is electrodeposition.6) Low-temperature superplasticity has been previously described in electrodeposited ultrafine-grained nickel (Ni).4,7)
However, a critical loss in strength occurs after superplastic deformation in electrodeposited Ni owing to grain growth associated with superplastic deformation8) and the segregation of sulfur (S), which is a general embrittlement element for Ni and other ductile metals and alloys.9–11) Electrodeposited Ni and its alloys typically contain S originating from the use of crucial reagents in the electrodeposition process.12) The relative concentration of S at the grain boundaries increases when the grain sizes are increased by thermal treatment, resulting in a transition from ductile to brittle failure of such alloys.10) To suppress this thermal embrittlement, it is important to reduce the S content in electrodeposited Ni. It has been shown that thermal embrittlement can be suppressed by adding tungsten (W) and boron (B) to electrodeposited Ni.13) A tensile elongation of 0.1% was reported for Ni after annealing at 350°C for 2 h, while higher tensile elongations of approximately 6% were recorded for both Ni containing 1.8 at% W after annealing at 375°C for 2 h and Ni containing 0.06 at% B annealed at 350°C for 2 h. This was attributed to the fact that W at the grain boundaries enables the strengthening of grain boundary cohesion, and B suppresses the grain growth of the nanocrystalline structure.13) There is a further positive impact in that these added elements contribute to better thermal stability of electrodeposited Ni during annealing and deformation at elevated temperatures. In general, grain growth is suppressed by the dragging effect of segregated elements at the grain boundaries, which suggests that the addition of both W and B alloying elements may be effective for suppressing the grain growth of electrodeposited Ni. Accordingly, the present study aims to investigate the significance of the addition of W and B for improving the superplastic characteristics while maintaining ultrafine grain sizes during superplastic deformation. This study examines electrodeposited Ni–W and Ni–B alloys separately, and these are compared with electrodeposited Ni, thus allowing the specific role of each individual solute element to be described during high-temperature deformation.
Ni, Ni–1.8 at% W, and Ni–0.06 at% B were produced using a sulfamate acid bath, which can produce samples with less S than the Watt’s bath method.14)
The compositions of the alloys were the same as those reported in our previous study,13) which were selected to provide a balance of room-temperature tensile ductility and grain size stability at high temperature. The sample preparation, including the electrodeposition processes for the Ni–B and Ni–W alloys, followed the procedures described in our previous studies.15,16) The compositions of the deposition baths are given in Table 1. Electrodeposition was performed using 5 L deposition systems at a current density of 20 mA/cm2, bath temperature of 55°C, and pH of 4.0.
Tensile tests were performed in air at temperatures of 350–450°C and a constant strain rate in the range of 1.0 × 10−5–1.0 × 10−2 s−1. Tensile specimens of the Ni, Ni–W, and Ni–B alloys were produced using electric discharge machining. The gauge section was 10 × 3 mm (length × width) with a thickness of approximately 0.7 mm. The Vickers hardness within the gauge length was measured after the tensile tests.
The average grain sizes of the as-deposited samples were estimated using X-ray powder diffraction (XRD). The deposited surfaces were examined by XRD using Cu Kα radiation in the Bragg–Brentano geometry. This estimation was based on the peak widths and Scherrer’s equation. The deformed microstructure and grain sizes were examined using electron backscatter diffraction (EBSD) analysis.
Taking the full width at half maximum of the 200 peak of the XRD peak profiles and applying Scherrer’s equation, the estimated grain sizes (or domain sizes) of the electrodeposited Ni, Ni–W, and Ni–B alloys are approximately 17, 14, and 19 nm, respectively, which are in agreement with previous transmission electron microscopy (TEM) analyses.15–17)
Representative true stress–strain curves after tensile testing are shown in Fig. 1 for the electrodeposited metals of (a) Ni, (b) Ni–W, and (c) Ni–B tested at combinations of 350–450°C and 1.0 × 10−5–1.0 × 10−2 s−1. The slopes of the stress–strain curves are small because an inlay-type jig was used. The flow stress increases with increasing strain rate under a consistent testing temperature, and it decreases with increasing deformation temperature under a consistent strain rate for all materials. Specifically, Ni and Ni–B exhibit large plastic deformation with a plateau flow stress at a temperature of 450°C and lower strain rates of ≤1.0 × 10−3 s−1. Accordingly, both Ni and Ni–B samples exhibit superplasticity with elongations of over 300% at 450°C and 1.0 × 10−4 s−1. In contrast, the electrodeposited Ni–W alloy exhibits limited plastic strain with relatively high flow stresses under the testing conditions, e.g., an elongation of 12% at 350°C and 1 × 10−4 s−1. Figure 2 shows the appearance of the Ni and Ni–B tensile test specimens before and after superplastic deformation, with elongations to failure of 324% and 362%, respectively.
Representative true stress–strain curves after tensile testing of (a) Ni, (b) Ni–W, and (c) Ni–B.
Appearance of Ni and Ni–B tensile test specimens before and after superplastic deformation with the elongations to failure of 324% and 362%, respectively.
Figure 3 shows scanning electron microscopy (SEM) images taken at the fracture surfaces of the (a) Ni and (b) Ni–B samples after exhibiting superplasticity at 450°C and 1 × 10−4 s−1. Both fracture surfaces show intergranular fractures with equiaxed grain shapes. Thus, the equiaxed grains suggest that superplastic ductility tends to be demonstrated by Rachinger grain boundary sliding18) in these two electrodeposited samples.
SEM images of the fracture surfaces of the (a) Ni and (b) Ni–B samples after exhibiting superplasticity at 450°C and 1 × 10−4 s−1.
The EBSD images at the grip regions (left column) and near the fracture tips (right column) are shown for Ni (upper row) and Ni–B (lower row) specimens after superplastic flow at 450°C and 1.0 × 10−4 s−1 in Fig. 4. The grain sizes obtained from the EBSD analysis and the Vickers hardness of the Ni and Ni–B samples after superplastic deformation are summarized in Table 2. The estimated grain sizes of the deformed regions within the gauges are larger than those of the grip regions for both Ni and Ni–B, suggesting that the occurrence of deformation enhances the grain growth during superplastic deformation.19) The grain size of Ni–B is finer than that of Ni in both the deformed and grip regions. As a result, the hardness of Ni–B after superplastic deformation is higher than that of Ni.
EBSD images of (a), (b) Ni and (c), (d) Ni–B samples after exhibiting superplasticity at 450°C and 1 × 10−4 s−1; (a), (c) show the grip regions and (b), (d) show the area near the fracture tips.
The mechanism of superplastic flow has been discussed using a constitutive equation describing the relationship between the strain rate ($\dot{\varepsilon }$) and flow stress (σ):
| \begin{equation} \dot{\varepsilon} = A\left(\frac{\sigma^{n}}{d^{p}}\right)\exp \left(-\frac{Q}{RT}\right), \end{equation} | (1) |
Figure 5 shows the relationship between the strain rate and flow stress of the electrodeposited Ni, Ni–W, and Ni–B specimens. For comparison, the flow stress of Ni electrodeposited using a Watt’s bath is also shown.7,20) In this study, the flow stress is defined as the stress at a true strain of 0.1. For the Ni–W deformed at 350°C and strain rates of 1 × 10−5 to 1 × 10−3 s−1, the stress exponent, n, is 7, which is consistent with previous Ni creep results.21) The flow stress of Ni–B at 350°C and a strain rate of 1 × 10−4 s−1 is not markedly different from that of Ni–W. Assuming that the deformation mechanism of Ni–B at 400°C and 1 × 10−4 s−1 is also the same as that of Ni–W and Ni–B at 350°C, the activation energy is calculated as Q = 295 kJ/mol based on the results of Ni–B deformed at 350°C and 400°C using constitutive eq. (1) with n = 7 and p = 0. The line showing the dislocation creep of Ni–B at 450°C can be estimated using constitutive eq. (1) and the obtained activation energy. The lines showing the dislocation creep of Ni–B at 350, 400, and 450°C are shown in Fig. 5. As shown in Fig. 5, the dislocation creep properties of Ni–W and Ni–B do not depend largely on the composition. In Ni–W, the flow stress may be increased in comparison with Ni because of the decrease in the stacking fault energy caused by the solution of W. The following constitutive equation considering the stacking fault energy has been reported:22)
| \begin{equation} \dot{\varepsilon} = A\left(\frac{\gamma}{Gb}\right)^{3} \left(\frac{Gb}{kT}\right)\left(\frac{\sigma}{G}\right)^{5}D_{\text{eff}}, \end{equation} | (2) |
Relationship between the flow stress at a strain of 0.1 and strain rate of Ni, Ni–W, and Ni–B.
As shown in Fig. 5, because the flow stress of Ni–B deformed at 450°C is lower than the dislocation creep stress, the deformation mechanism must be changed. In this study, the stress exponent of Ni–B deformed at 450°C and strain rates of 1 × 10−5 to 1 × 10−2 s−1 is n = 2, and the stress exponent of Ni deformed at 450°C and 1 × 10−5 to 10−3 s−1 is also n = 2. Based on these results, the main deformation mechanism of Ni and Ni–B at 450°C is considered to be grain boundary sliding with n = 2.
The electrodeposited Ni–W failed to exhibit superplasticity, as shown in Figs. 1 and 5. Because the grain growth in the Ni–W alloy is suppressed compared to that in Ni and comparable with that of the Ni–B alloy, this cannot be attributed to the effect of grain growth. It has been reported that W segregates at the grain boundary, and the W at the grain boundary strengthens the cohesion of the grain boundary,13) which increases the energy required for grain boundary sliding. This is likely the reason why the electrodeposited Ni–W failed to exhibit superplasticity.
The optimal superplastic strain rates of Ni electrodeposited from a sulfamate acid bath at 450°C shift to the lower values than those of Ni electrodeposited in Watt’s bath. The role of S in superplasticity has been attributed to the promotion of grain boundary sliding through weakened Ni–Ni bonds across S-enriched grain boundaries.24) The shift in optimal superplastic strain rates could be due to the lower S content of Ni electrodeposited in the sulfamate acid bath than in Watt’s bath.13,20)
The optimal superplastic strain rates of Ni–B also shift to lower values than those of Ni electrodeposited in a sulfamate acid bath. The shift in the optimal superplastic strain rates due to the addition of B can be attributed to two factors: one is the increase in activation energy due to the segregation of B at the grain boundaries, and the other is the change in the deformation mechanism due to the formation of fine intermetallic compounds of Ni3B. In Ni–B, B easily segregates at the grain boundaries.25) It has been previously reported that the flow stress of high-temperature deformation caused by grain boundary sliding changes due to grain boundary segregation.26) The activation energy of the grain boundary sliding of Ni is 156 kJ/mol, and that of Ni–B calculated from eq. (1) increases by 10 kJ/mol using the test results obtained at 1 × 10−4 s−1 and 450°C. This increase in activation energy may be the reason for the transition of the flow stress to the lower strain rate side in Ni–B compared to that in Ni.
Another possibility is that B may form Ni3B, which is a very fine intermetallic compound.13,27) The following equation has been reported for the presence of particle reinforcements:28)
| \begin{equation} \dot{\varepsilon} \propto \left(\frac{\sigma - \sigma_{th}}{G}\right)^{2}\left(\frac{b}{d}\right)^{2}\left(\frac{\lambda}{b}\right)^{q}D_{L}, \end{equation} | (3) |
Although the optimal superplastic strain rates of the electrodeposited Ni–B shift to a lower value than that of the electrodeposited Ni, the addition of B successfully suppresses the grain growth and improves the mechanical properties of Ni after superplastic deformation.
In this study, electrodeposited nanocrystalline Ni, Ni–W, and Ni–B were produced using a sulfamate acid bath, and their high-temperature tensile properties and grain growth were investigated. Electrodeposited Ni exhibited superplasticity with an elongation of 324% at a temperature of 450°C and strain rate of 10−4 s−1. The electrodeposited Ni–W failed to exhibit superplasticity because the segregated W at the grain boundaries increased the energy required for grain boundary sliding. Electrodeposited Ni–B exhibited superplasticity with a recorded elongation of 362% at a temperature of 450°C and a strain rate of 1.0 × 10−4 s−1. With the addition of B, the optimal superplastic strain rates of the electrodeposited Ni–B shifted to a lower values than that of electrodeposited Ni. The addition of B successfully suppressed grain growth and improved the mechanical properties after superplastic deformation.
This study was supported in part by KAKENHI (19K05101). We would like to thank Prof. M. Kawasaki (Oregon State University) for her helpful comments on this manuscript.
