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Tensile deformation and microstructure of a nanocrystalline Ni–W alloy produced by electrodeposition
Scripta Materialia 50 (2004) 395–399 www.actamat-journals.com
Tensile deformation and microstructure of a nanocrystalline Ni–W alloy produced by electrodeposition Hajime Iwasaki a, Kenji Higashi b, T.G. Nieh
c,*
a
Department of Materials Science and Engineering, Himeji Institute of Technology, 2167 Shosha, Himeji, Hyogo 671-2201, Japan b Department of Metallurgy and Materials Science, Osaka Prefecture University, Sakai 599-8531, Japan c Lawrence Livermore National Laboratory, Chemistry and Materials Science, L-350, P.O. Box 808, Livermore, CA 94551, USA Received 15 August 2003; received in revised form 29 September 2003; accepted 30 September 2003
Abstract Deformation and fracture characteristics of the electrodeposited nanocrystalline Ni–W alloy with a grain size of 8.1 nm were investigated. Tensile tests were carried out at room temperature with specimen of 25–30 lm in thickness. The fractured surface was examined using SEM and high-resolution TEM was used to study the microstructure of deformed specimens. Based on these observations we propose a deformation mechanism and fracture process for nanocrystalline Ni–W during tensile deformation are initiated by grain boundary sliding. 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Nanocrystalline material; Electrodeposition; Mechanical property; High resolution TEM; Fracture surface
1. Introduction The technological development of nanocrystalline alloys has been driven by the promise of having exceptional properties. Scientifically, the study of nanocrystalline materials is also of great interest because the potential breakdown of classical scaling laws and the accompanying need for new materials physics in the nanostructured state. It has been shown that the hardness of nanocrystalline Ni increased with decreasing grain size down to at least 14 nm before the Hall–Petch breakdown occurred [1]. For electrodeposited Ni–W alloys, a similar phenomenon was observed below grain size of 8 nm [2]. Computational simulations also indicated that in the grain size of about 10–15 nm grain boundary sliding and dislocation glide were predicted, whereas at a grain size of about 5 nm no dislocation activity is observed [3]. The reasons for this behavior are still under debate as dislocation sources within grains are not expected to operate at these grain sizes. In fact, there is no documented evidence of dislocation pile-ups in deformed specimens, and any dislocation activity is primarily believed to originate and terminate at grain
boundaries. Furthermore, it has been reported that the nanocrystalline metals are less ductile than their microcrystalline counterparts [4]. Recently, the mechanisms of deformation and damage evolution in electrodeposited nanocrystalline Ni with an average grain size of 30 nm have been investigated by conducting tensile tests performed in situ in a transmission electron microscope and microscopic observations made at high-resolution following ex situ deformation [5]. It was shown that dislocation-mediated plasticity played a dominant role during the deformation of nanocrystalline Ni and fracture surface exhibited dimpled feature with the scale of the dimples being several times larger than the grain size. Previously [6], we studied the grain and grain boundary structures of nanocrystalline Ni–W alloys with grain sizes below 10 nm produced by direct current (DC) electrodeposition. The present paper is devoted to the investigation of the microstructural characteristics of deformed nanocrystalline Ni–W alloy using high-resolution electron microscopy.
2. Material and experimental procedure *
Corresponding author. Tel.: +1-925-4239802; fax: +1-9254238034. E-mail address: [email protected] (T.G. Nieh).
Electrodeposition of Ni–W alloys has been discussed described elsewhere [7,8], and a similar method was used
1359-6462/$ - see front matter 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2003.09.056
396
H. Iwasaki et al. / Scripta Materialia 50 (2004) 395–399
Table 1 Composition of the plating bath for Ni–W electrodeposition Nickel sulfate (NiSO4 Æ 6H2 O) Citric acid (Na3 C6 H5 O7 Æ 2H2 O) Sodium tungstate (Na2 WO4 Æ 2H2 O) Ammonium chloride (NH4 Cl) Sodium bromide (NaBr) Saccharin (C7 H5 NO3 S) Sodium dodecyl sulfate (CH3 (CH2 )10 CH2 OSO3 Na) pH
0.06 0.50 0.14 0.50 0.15 0.60 0.60
mol/l mol/l mol/l mol/l mol/l g/l g/l
7.6
to produce foils about 30 lm thick; the composition of the plating bath is given in Table 1. A new plating bath was made of analytical reagent-grade chemicals and pure water for each experiment. The cathode was an electro-polished copper sheet of 0.2 mm thick having the shape of a tensile specimen with 10 mm in gage length and 5 mm in gage width. The anode was high purity platinum. During deposition citric acid and ammonium chloride were introduced to form complexes with Ni and W, while sodium bromide was used to improve conductivity. A 500 ml bath was maintained at 72 C, and the applied DC current density was kept constant at 0.05, 0.07, 0.085, and 0.10 A/cm2 . The plating time was 60 min. After plating, all electrodeposited foils were degassed in vacuum at 80 C for 24 h. The Cu substrate was dissolved in an aqueous solution containing 250 g/l of CrO3 and 15 cm3 /l of H2 SO4 . Chemical analysis of the electrodeposited alloys was carried out by means of spectroscopic analysis in an inductively coupled argon plasma to determine metal content. Structural analysis was also performed by means of X-ray diffraction (XRD) using CuKa radiation in a Rigaku RINT )1500 operated at 40 kV and 200 mA. Tensile tests were conducted at room temperature and at a test speed of 1 mm per minute, which yielded a strain rate of about 3 · 10 3 s 1 . The tensile tests were conducted using an Instrontype machine (MECMESIN, UK) with a load capacity of 2000 lb. The grips have flat faces with double-cut fine teeth. To prevent breakage samples were tapped with Cu foils during gripping and testing. Strain (or elongation) was measured from the crosshead displacement, assuming an effective gage length of 6 mm. Microstructural observations were carried out with samples from the grip section and area near the immediate vicinity of the fractured surface using high-resolution electron microscopy (HREM). The sample was initially produced at a current density of 0.1 A/cm2 . Semi-circular discs with a diameter of 3 mm were punched from fractured samples for TEM specimens. The center of the disc is carefully located 0.5 mm from the very edge of fracture surface. Discs were directly ion milled to perforation for TEM examination. HREM was performed on a JEM-4000EX, operated at 400 kV. The fractured surface was also examined using HITACHI S900 high-resolution scanning electron microscope.
3. Results 3.1. Composition and nano-structure The tungsten content of the electrodeposited Ni–W samples, as well as the current density used to produce the samples, are listed in Table 2. The tungsten content in the alloys was in a narrow range of 11.7–12.7% and increased only slightly with increasing applied current density. XRD and electron diffraction patterns revealed that the Ni–W foils were single-phased with an FCC structure, indicating a solid solution alloy. Although the equilibrium phase diagram of the Ni–W system is speculative at room temperature, the solid solubility of tungsten in Ni is expected to be up to 12.5 at% [9]. The compositions of the electrodeposited foils listed in Table 2 are noted to be very close to this limit. Grain sizes of the Ni–W foils determined from the XRD data, are almost at the same value of 8 nm, as listed in Table 2. Thus, the effects of current density on the tungsten content and the grain size appear to be insignificant in the present experimental conditions. The in-Pane microstructure of Ni–W alloy deposited at the current
Table 2 Tungsten contents and grain sizes of the electrodeposited Ni–W Current density (A/cm2 )
W content (at%)
0.05 0.07 0.085
11.7 12.3 12.5
XRD grain size (nm) 8.1 8.6 8.1
Fig. 1. Bright field image of nanocrystalline Ni–W alloy deposited at a current density of 0.10 A/cm2 .
H. Iwasaki et al. / Scripta Materialia 50 (2004) 395–399
397
Engineering stress, MPa
1200 1000 800
Current density: 0.05 A/ cm2 maximum stress: 815 MPa
0.07 A/ cm2 1032 MPa
0.10 A/ cm2 1004 MPa
0.085 A/ cm2 820 MPa
600 400 200 1% 0
Elongation, % Fig. 3. Engineering stress vs. elongation curves for electrodeposited Ni–W alloys at room temperature.
Fig. 2. A high-resolution image (the bright circular region) of the same Ni–W foil in Fig. 1.
density of 0.10 A/cm2 is shown in Fig. 1. The grain shape appears to be isotropic, and there is no obvious tendency for grains with similar orientation to cluster together. Presumably, grain boundaries are mainly of high-angle varieties. A high-resolution image of Fig. 1 revealing the detailed structure of the Ni–W foil is shown in Fig. 2. The structure consists of fine domains of lattice fringes with a spacing of 0.203 nm, which corresponds to the spacing of the {1 1 1} plane for Ni. The grain size is estimated to be about 4.5–6.2 nm, and the intercrystalline regions (i.e., grain boundaries and triple junctions) are manifested as structureless bands about 0.5–1 nm wide. It is noted that, although not shown here, there exist some nano-pores (about 1–2 nm) in the as-deposited samples, primarily along grain boundaries. The density of these nano-pores is, however, relatively low.
4. Mechanical properties Typical stress–strain curves for the as-electrodeposited Ni–W specimens at different current densities are shown in Fig. 3. It is noted that these curves were deduced from the recorded load–displacement data, thus may suffer some uncertainty, especially for the strain. The tensile strength varies in the range of 800–1050 MPa. The yield stress, however, cannot be accurately determined from these curves. The elongation to fracture (<1%) was also difficult to measure from the gage length measurement. However, there are no obvious relationship between the current density and the tensile strength or between the current density and the elon-
gation under the present experimental conditions. The variation of fracture strength is probably, in part, caused by the presence of some residual nano-pores in the materials. Low ductility observed in nanocrystalline metals has been often suggested to be caused by the presence of nm-sized porosity [10]. Furthermore, since the tensile samples are only 0.02 mm thick, surface defects from processing or machining can also contribute to the poor ductility. It is noted, however, that many nano-metals exhibit decent ductility [11–13]. The disparity in ductility has to be associated with the defective structures, such as voids, impurities, and precipitates, in nano-materials processed by different techniques, as discussed by Shaw [14]. 4.1. Fractured surface The fracture surfaces observed using SEM were shown in Fig. 4. Fig. 4(a) indicates that the sample fractured in a brittle manner. At a higher magnification (Fig. 4(b)), however, the surface shows many dimpled patterns. The dimple size varies from 20 to 200 nm, which is about 2.5–25 times the mean grain size. As revealed at a higher magnification (Fig. 4(c)), the inner surface of each dimple appears to consist of many tiny bubble-like substructures. These bubbles have a similar dimension as the grain size. 4.2. High-resolution image of the fractured samples Legros et al. [11] recently studied the microstructure of nanocrystalline Ni after deformation to find evidences of dislocation activity, but did not observe any resultant dislocation debris. This observation, together with computer simulation results, have led to the general notion that grain boundary sliding is the dominant
398
H. Iwasaki et al. / Scripta Materialia 50 (2004) 395–399
Fig. 5. High-resolution TEM microstructure of deformed nanocrystalline Ni–W. The fracture surface is located at the left of the picture and the tensile axis is horizontal.
Fig. 4. Fracture surfaces of electrodeposited Ni–W alloys revealed by SEM. Despite the low elongation to failure, it exhibits ductile dimpled features.
deformation mechanism in these materials, particularly at grain sizes less than 20 nm. In the present study, we also examined the microstructures of nanocrystalline Ni–W alloy at the immediate vicinity of fractured surface using high-resolution transmission electron microscopy, to investigate dislocation activity and grain boundary sliding mechanism. A typical nano-scale structure is presented in Fig. 5, which shows many nano-sized grains with various contrasts. One of the contrasts consists of parallel straight lines penetrating through individual nanograin (marked by A); this may indicate the presence of stacking faults. Another contrast exhibits slightly wide curved lines (marked by B), which may indicate the presence complete dislocations. Twins are also observed in the figure, as marked by C. It is of particular interest to note that a few tangled dislocations were also observed within some grains, indicating cross slips may have occurred. These
dislocation structures were not observed in nanocrystalline Ni before [15], probably because Ni–W alloys have a lower stacking fault energy or have a smaller grain size. Analysis of the above fine microstructural features is currently underway and will be reported in the near future. In Fig. 5, a long curved band of linear defects runs from the top left to the bottom right of the figure. This band lies along grain boundaries. For nanocrystalline materials, grain boundary sliding is expected to take place. In this case, high stress concentration at grain triple junctions (for instance, locations marked 1, 2, and 3) can readily occur if sliding is not properly accommodated. In fact, structure defects described above, i.e., dislocation, twin and stacking fault, may be caused by grain boundary sliding. Voids formed at triple junctions as a result of grain boundary sliding can subsequently nucleate, grow, and interlink to form the band shown in Fig. 5. The above observations lead us to propose that the dominant deformation mechanism in nano-Ni–W is grain boundary sliding. Each grain boundary undergoes different extents of sliding, depending on its boundary character. Often, a group of neighboring nanograins slides together as grain clusters. Fracture path is along
H. Iwasaki et al. / Scripta Materialia 50 (2004) 395–399
the boundary of these grain clusters. Therefore, fracture does not occur at a scale of individual nanograin. Rather, the size of dimple shown in Fig. 4 depends on the size of grain cluster suitable for sliding.
399
Acknowledgements This work was performed under the auspices of the US Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract no. W-7405-Eng-48.
5. Summary References Tensile deformation and fracture characteristics of electrodeposited nanocrystalline Ni–W alloy with a grain size of 8 nm were investigated. It was found that the elongation to failure of the material was less than 1% and the maximum strength was in the range of 800–1050 MPa. Despite the low elongation to failure, fracture surface showed ductile dimple features. The size of dimples is about 2.5–25 times larger than the size of nanograins and the dimple surface consists of many nanograins. The microstructure of deformed specimens was further examined using high-resolution TEM and showed that at the immediate vicinity of the fracture surface there existed a long deformation band primarily along grain boundaries. We propose that the fracture of the nanocrystalline Ni–W alloy occurs by interlinking voids formed at grain triple junctions, as a result of grain boundary sliding. Instead of sliding individually, grains slide in cluster. This results in a dimpled fracture surface with the dimple size approximately 2.5–25 times as large as the grain size.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
Schuh C, Nieh TG, Yamasaki T. Scripta Mater 2002;46:735. Schuh CA, Nieh TG, Iwasaki H. Acta Mater 2003;51:431. Swygenhoven VH, Derlet PM. Phys Rev B 2001;64:224105. Torre FD, Swygenhoven HV, Vitoria M. Acta Mater 2002;50:3957. Kumar KS, Suresh S, Chisholm MF, Horton JA, Wang P. Acta Mater 2003;51:387. Iwasaki H, Higashi K, Nieh TG. Mater Res Soc Symp Proc 2003;740:119. Yamasaki T, Tomohira R, Ogino Y, Schlossmacher P, Ehrlich K. Plat Surf Finish 1999;87:148. Yamasaki T. Scripta Mater 2001;44:1497. ASM Handbook, Alloy Phase Diagrams, 3, 1992, Metals Park, OH: ASM. Koch CC, Morris DG, Lu K, Inoue A. MRS Bull 1999;24(2):54. Legros M, Elliott BR, Rittner MN, Weertman JR, Hemker KJ. Phil Mag A 2000;80:1017. Sanders PG, Youngdahi CJ, Weertman JR. Mater Sci Eng A 1997;77:234. Ebrahimi F, Bourne GR, Kelly MS, Matthews TE. Nanostruct Mater 1999;11:343. Shaw LL. JOM 2000;52(12):41. Dalla Torre F, Van Swygenhoven H, Victoria M. Acta Mater 2002;50:3957.
Tensile deformation and microstructure of a nanocrystalline Ni–W alloy produced by electrodeposition Hajime Iwasaki a, Kenji Higashi b, T.G. Nieh
c,*
a
Department of Materials Science and Engineering, Himeji Institute of Technology, 2167 Shosha, Himeji, Hyogo 671-2201, Japan b Department of Metallurgy and Materials Science, Osaka Prefecture University, Sakai 599-8531, Japan c Lawrence Livermore National Laboratory, Chemistry and Materials Science, L-350, P.O. Box 808, Livermore, CA 94551, USA Received 15 August 2003; received in revised form 29 September 2003; accepted 30 September 2003
Abstract Deformation and fracture characteristics of the electrodeposited nanocrystalline Ni–W alloy with a grain size of 8.1 nm were investigated. Tensile tests were carried out at room temperature with specimen of 25–30 lm in thickness. The fractured surface was examined using SEM and high-resolution TEM was used to study the microstructure of deformed specimens. Based on these observations we propose a deformation mechanism and fracture process for nanocrystalline Ni–W during tensile deformation are initiated by grain boundary sliding. 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Nanocrystalline material; Electrodeposition; Mechanical property; High resolution TEM; Fracture surface
1. Introduction The technological development of nanocrystalline alloys has been driven by the promise of having exceptional properties. Scientifically, the study of nanocrystalline materials is also of great interest because the potential breakdown of classical scaling laws and the accompanying need for new materials physics in the nanostructured state. It has been shown that the hardness of nanocrystalline Ni increased with decreasing grain size down to at least 14 nm before the Hall–Petch breakdown occurred [1]. For electrodeposited Ni–W alloys, a similar phenomenon was observed below grain size of 8 nm [2]. Computational simulations also indicated that in the grain size of about 10–15 nm grain boundary sliding and dislocation glide were predicted, whereas at a grain size of about 5 nm no dislocation activity is observed [3]. The reasons for this behavior are still under debate as dislocation sources within grains are not expected to operate at these grain sizes. In fact, there is no documented evidence of dislocation pile-ups in deformed specimens, and any dislocation activity is primarily believed to originate and terminate at grain
boundaries. Furthermore, it has been reported that the nanocrystalline metals are less ductile than their microcrystalline counterparts [4]. Recently, the mechanisms of deformation and damage evolution in electrodeposited nanocrystalline Ni with an average grain size of 30 nm have been investigated by conducting tensile tests performed in situ in a transmission electron microscope and microscopic observations made at high-resolution following ex situ deformation [5]. It was shown that dislocation-mediated plasticity played a dominant role during the deformation of nanocrystalline Ni and fracture surface exhibited dimpled feature with the scale of the dimples being several times larger than the grain size. Previously [6], we studied the grain and grain boundary structures of nanocrystalline Ni–W alloys with grain sizes below 10 nm produced by direct current (DC) electrodeposition. The present paper is devoted to the investigation of the microstructural characteristics of deformed nanocrystalline Ni–W alloy using high-resolution electron microscopy.
2. Material and experimental procedure *
Corresponding author. Tel.: +1-925-4239802; fax: +1-9254238034. E-mail address: [email protected] (T.G. Nieh).
Electrodeposition of Ni–W alloys has been discussed described elsewhere [7,8], and a similar method was used
1359-6462/$ - see front matter 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2003.09.056
396
H. Iwasaki et al. / Scripta Materialia 50 (2004) 395–399
Table 1 Composition of the plating bath for Ni–W electrodeposition Nickel sulfate (NiSO4 Æ 6H2 O) Citric acid (Na3 C6 H5 O7 Æ 2H2 O) Sodium tungstate (Na2 WO4 Æ 2H2 O) Ammonium chloride (NH4 Cl) Sodium bromide (NaBr) Saccharin (C7 H5 NO3 S) Sodium dodecyl sulfate (CH3 (CH2 )10 CH2 OSO3 Na) pH
0.06 0.50 0.14 0.50 0.15 0.60 0.60
mol/l mol/l mol/l mol/l mol/l g/l g/l
7.6
to produce foils about 30 lm thick; the composition of the plating bath is given in Table 1. A new plating bath was made of analytical reagent-grade chemicals and pure water for each experiment. The cathode was an electro-polished copper sheet of 0.2 mm thick having the shape of a tensile specimen with 10 mm in gage length and 5 mm in gage width. The anode was high purity platinum. During deposition citric acid and ammonium chloride were introduced to form complexes with Ni and W, while sodium bromide was used to improve conductivity. A 500 ml bath was maintained at 72 C, and the applied DC current density was kept constant at 0.05, 0.07, 0.085, and 0.10 A/cm2 . The plating time was 60 min. After plating, all electrodeposited foils were degassed in vacuum at 80 C for 24 h. The Cu substrate was dissolved in an aqueous solution containing 250 g/l of CrO3 and 15 cm3 /l of H2 SO4 . Chemical analysis of the electrodeposited alloys was carried out by means of spectroscopic analysis in an inductively coupled argon plasma to determine metal content. Structural analysis was also performed by means of X-ray diffraction (XRD) using CuKa radiation in a Rigaku RINT )1500 operated at 40 kV and 200 mA. Tensile tests were conducted at room temperature and at a test speed of 1 mm per minute, which yielded a strain rate of about 3 · 10 3 s 1 . The tensile tests were conducted using an Instrontype machine (MECMESIN, UK) with a load capacity of 2000 lb. The grips have flat faces with double-cut fine teeth. To prevent breakage samples were tapped with Cu foils during gripping and testing. Strain (or elongation) was measured from the crosshead displacement, assuming an effective gage length of 6 mm. Microstructural observations were carried out with samples from the grip section and area near the immediate vicinity of the fractured surface using high-resolution electron microscopy (HREM). The sample was initially produced at a current density of 0.1 A/cm2 . Semi-circular discs with a diameter of 3 mm were punched from fractured samples for TEM specimens. The center of the disc is carefully located 0.5 mm from the very edge of fracture surface. Discs were directly ion milled to perforation for TEM examination. HREM was performed on a JEM-4000EX, operated at 400 kV. The fractured surface was also examined using HITACHI S900 high-resolution scanning electron microscope.
3. Results 3.1. Composition and nano-structure The tungsten content of the electrodeposited Ni–W samples, as well as the current density used to produce the samples, are listed in Table 2. The tungsten content in the alloys was in a narrow range of 11.7–12.7% and increased only slightly with increasing applied current density. XRD and electron diffraction patterns revealed that the Ni–W foils were single-phased with an FCC structure, indicating a solid solution alloy. Although the equilibrium phase diagram of the Ni–W system is speculative at room temperature, the solid solubility of tungsten in Ni is expected to be up to 12.5 at% [9]. The compositions of the electrodeposited foils listed in Table 2 are noted to be very close to this limit. Grain sizes of the Ni–W foils determined from the XRD data, are almost at the same value of 8 nm, as listed in Table 2. Thus, the effects of current density on the tungsten content and the grain size appear to be insignificant in the present experimental conditions. The in-Pane microstructure of Ni–W alloy deposited at the current
Table 2 Tungsten contents and grain sizes of the electrodeposited Ni–W Current density (A/cm2 )
W content (at%)
0.05 0.07 0.085
11.7 12.3 12.5
XRD grain size (nm) 8.1 8.6 8.1
Fig. 1. Bright field image of nanocrystalline Ni–W alloy deposited at a current density of 0.10 A/cm2 .
H. Iwasaki et al. / Scripta Materialia 50 (2004) 395–399
397
Engineering stress, MPa
1200 1000 800
Current density: 0.05 A/ cm2 maximum stress: 815 MPa
0.07 A/ cm2 1032 MPa
0.10 A/ cm2 1004 MPa
0.085 A/ cm2 820 MPa
600 400 200 1% 0
Elongation, % Fig. 3. Engineering stress vs. elongation curves for electrodeposited Ni–W alloys at room temperature.
Fig. 2. A high-resolution image (the bright circular region) of the same Ni–W foil in Fig. 1.
density of 0.10 A/cm2 is shown in Fig. 1. The grain shape appears to be isotropic, and there is no obvious tendency for grains with similar orientation to cluster together. Presumably, grain boundaries are mainly of high-angle varieties. A high-resolution image of Fig. 1 revealing the detailed structure of the Ni–W foil is shown in Fig. 2. The structure consists of fine domains of lattice fringes with a spacing of 0.203 nm, which corresponds to the spacing of the {1 1 1} plane for Ni. The grain size is estimated to be about 4.5–6.2 nm, and the intercrystalline regions (i.e., grain boundaries and triple junctions) are manifested as structureless bands about 0.5–1 nm wide. It is noted that, although not shown here, there exist some nano-pores (about 1–2 nm) in the as-deposited samples, primarily along grain boundaries. The density of these nano-pores is, however, relatively low.
4. Mechanical properties Typical stress–strain curves for the as-electrodeposited Ni–W specimens at different current densities are shown in Fig. 3. It is noted that these curves were deduced from the recorded load–displacement data, thus may suffer some uncertainty, especially for the strain. The tensile strength varies in the range of 800–1050 MPa. The yield stress, however, cannot be accurately determined from these curves. The elongation to fracture (<1%) was also difficult to measure from the gage length measurement. However, there are no obvious relationship between the current density and the tensile strength or between the current density and the elon-
gation under the present experimental conditions. The variation of fracture strength is probably, in part, caused by the presence of some residual nano-pores in the materials. Low ductility observed in nanocrystalline metals has been often suggested to be caused by the presence of nm-sized porosity [10]. Furthermore, since the tensile samples are only 0.02 mm thick, surface defects from processing or machining can also contribute to the poor ductility. It is noted, however, that many nano-metals exhibit decent ductility [11–13]. The disparity in ductility has to be associated with the defective structures, such as voids, impurities, and precipitates, in nano-materials processed by different techniques, as discussed by Shaw [14]. 4.1. Fractured surface The fracture surfaces observed using SEM were shown in Fig. 4. Fig. 4(a) indicates that the sample fractured in a brittle manner. At a higher magnification (Fig. 4(b)), however, the surface shows many dimpled patterns. The dimple size varies from 20 to 200 nm, which is about 2.5–25 times the mean grain size. As revealed at a higher magnification (Fig. 4(c)), the inner surface of each dimple appears to consist of many tiny bubble-like substructures. These bubbles have a similar dimension as the grain size. 4.2. High-resolution image of the fractured samples Legros et al. [11] recently studied the microstructure of nanocrystalline Ni after deformation to find evidences of dislocation activity, but did not observe any resultant dislocation debris. This observation, together with computer simulation results, have led to the general notion that grain boundary sliding is the dominant
398
H. Iwasaki et al. / Scripta Materialia 50 (2004) 395–399
Fig. 5. High-resolution TEM microstructure of deformed nanocrystalline Ni–W. The fracture surface is located at the left of the picture and the tensile axis is horizontal.
Fig. 4. Fracture surfaces of electrodeposited Ni–W alloys revealed by SEM. Despite the low elongation to failure, it exhibits ductile dimpled features.
deformation mechanism in these materials, particularly at grain sizes less than 20 nm. In the present study, we also examined the microstructures of nanocrystalline Ni–W alloy at the immediate vicinity of fractured surface using high-resolution transmission electron microscopy, to investigate dislocation activity and grain boundary sliding mechanism. A typical nano-scale structure is presented in Fig. 5, which shows many nano-sized grains with various contrasts. One of the contrasts consists of parallel straight lines penetrating through individual nanograin (marked by A); this may indicate the presence of stacking faults. Another contrast exhibits slightly wide curved lines (marked by B), which may indicate the presence complete dislocations. Twins are also observed in the figure, as marked by C. It is of particular interest to note that a few tangled dislocations were also observed within some grains, indicating cross slips may have occurred. These
dislocation structures were not observed in nanocrystalline Ni before [15], probably because Ni–W alloys have a lower stacking fault energy or have a smaller grain size. Analysis of the above fine microstructural features is currently underway and will be reported in the near future. In Fig. 5, a long curved band of linear defects runs from the top left to the bottom right of the figure. This band lies along grain boundaries. For nanocrystalline materials, grain boundary sliding is expected to take place. In this case, high stress concentration at grain triple junctions (for instance, locations marked 1, 2, and 3) can readily occur if sliding is not properly accommodated. In fact, structure defects described above, i.e., dislocation, twin and stacking fault, may be caused by grain boundary sliding. Voids formed at triple junctions as a result of grain boundary sliding can subsequently nucleate, grow, and interlink to form the band shown in Fig. 5. The above observations lead us to propose that the dominant deformation mechanism in nano-Ni–W is grain boundary sliding. Each grain boundary undergoes different extents of sliding, depending on its boundary character. Often, a group of neighboring nanograins slides together as grain clusters. Fracture path is along
H. Iwasaki et al. / Scripta Materialia 50 (2004) 395–399
the boundary of these grain clusters. Therefore, fracture does not occur at a scale of individual nanograin. Rather, the size of dimple shown in Fig. 4 depends on the size of grain cluster suitable for sliding.
399
Acknowledgements This work was performed under the auspices of the US Department of Energy by the University of California, Lawrence Livermore National Laboratory under Contract no. W-7405-Eng-48.
5. Summary References Tensile deformation and fracture characteristics of electrodeposited nanocrystalline Ni–W alloy with a grain size of 8 nm were investigated. It was found that the elongation to failure of the material was less than 1% and the maximum strength was in the range of 800–1050 MPa. Despite the low elongation to failure, fracture surface showed ductile dimple features. The size of dimples is about 2.5–25 times larger than the size of nanograins and the dimple surface consists of many nanograins. The microstructure of deformed specimens was further examined using high-resolution TEM and showed that at the immediate vicinity of the fracture surface there existed a long deformation band primarily along grain boundaries. We propose that the fracture of the nanocrystalline Ni–W alloy occurs by interlinking voids formed at grain triple junctions, as a result of grain boundary sliding. Instead of sliding individually, grains slide in cluster. This results in a dimpled fracture surface with the dimple size approximately 2.5–25 times as large as the grain size.
[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15]
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