- Email: [email protected]
In situ joining of dissimilar nanocrystalline materials by spark plasma sintering
Scripta Materialia 48 (2003) 1225–1230 www.actamat-journals.com
In situ joining of dissimilar nanocrystalline materials by spark plasma sintering Weiping Liu
a,b,*
, Masaaki Naka
a
a
b
Joining and Welding Research Institute, Osaka University, Osaka 567-0047, Japan Department of Materials Science and Engineering, Lehigh University, 5 E. Packer Avenue, Bethlehem, PA 18015, USA Received 30 October 2002; received in revised form 16 January 2003; accepted 4 February 2003
Abstract Joining of nanocrystalline materials will be a great challenge. In this paper, a reaction synthesis-based in situ joining technique was developed for joining dissimilar nanocrystalline materials by use of the spark plasma sintering (SPS) process. The joining technique combines the nanocrystalline material processing and joining operations into a single process. Ó 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Nanomaterials; Reaction synthesis; Mechanical activation; Joining; Spark plasma sintering; Composites
1. Introduction Nanocrystalline materials, which are defined as materials with grain sizes below 100 nm, have attracted great attention and interest due to their improved properties [1,2]. As a class of potential engineering materials, joining of nanocrystalline materials will be a great challenge. Obviously, most of the conventional joining methods are not suitable for this new class of materials in order to preserve their ultra-fine grain structures. Therefore, developing new joining processes will be necessary.
* Corresponding author. Address: Department of Materials Science and Engineering, Lehigh University, 5 E. Packer Avenue, Bethlehem, PA 18015, USA. Tel.: +1-610-7584270; fax: +1-610-7586407. E-mail address: [email protected] (W. Liu).
Recently, a number of researchers [3–7] have successfully utilized the pressurized reaction synthesis or self-propagating high-temperature synthesis (SHS) process for joining of the difficultto-weld materials. SHS is an advanced material processing method that uses highly exothermic reactions between powdered constituents [8,9]. As a combination of SHS processing and joining technology, the reactive synthesis joining process provides innovative joining capabilities for advanced materials [5,7]. In particular, the process can be used for near-net-shape fabrication of joints with simultaneous bulk materials synthesis and in situ joining. The near-net-shape fabrication capabilities of the reactive synthesis joining process are attractive since they not only imply savings on operations and materials, but also imply more design freedom and improved properties. Spark plasma sintering (SPS) or pulsed electric current sintering (PECS) is a recent innovation in activated
1359-6462/03/$ - see front matter Ó 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. doi:10.1016/S1359-6462(03)00074-5
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sintering and densification by charging a high pulsed electric current directly through the powders in a graphite die under externally applied pressure [10,11]. It was reported to be an effective method for sintering of powder products at lower temperatures and for shorter times than conventional processes. This process can also be used to achieve the materials synthesis and densification simultaneously and rapidly. In this study, an in situ joining technique has been developed to fabricate dissimilar nanocrystalline material joints based on reaction synthesis of mechanically activated powders by use of the SPS process.
2. Experimental procedure The starting powders used in this study were elemental Ni (99.9% pure), Al (99.9% pure), Ti (99.9% pure) and graphite C (99.7% pure) with average particle sizes of 3, 3, 10 and 5 lm respectively. Two sets of powder mixtures were prepared in this study. One consisted of Ni and Al powders for the fabrication of Ni3 Al. B powder with particle sizes less than 40 lm was added to this powder mixture as a dopant for the desired Ni3 Al product. The Ni, Al and B powders were mixed to a nominal composition of Ni76 Al23:6 B0:4 (at.%). The other set of powder mixture consisted of Ni, Al, Ti and C powders for the fabrication of TiC/Ni3 Al composites. The Ni, Al, Ti and C powders were mixed to a nominal composition of Ni3 Al–40vol%TiC. Mechanical activation (MA) of the powder mixture was performed in a vibrational ball mill with a water-cooled chamber for 10–30 h prior to reactive synthesis by SPS. Typically, 20 g of the powder mixture were put into a cylindrical stainless steel vial together with stainless steel balls (11 mm in diameter). The ball-to-powder weight ratio was 10:1. Methanol with a ratio of 2–3 wt.% of the powder mixture was used as a process control agent (PCA) to avoid the sticking of powders to the balls and the vial. Ball milling was performed under an Ar gas atmosphere. After ball milling, typically about 5 g of the powder mixture was poured into a 10-mm-diameter (inner diameter) graphite die that had previously been lined with a BN coating for facilitating
the removal of the sample after SPS was completed. The powder mixture in the die was coldpressed under an uniaxial pressure of 40 MPa. For fabrication of in situ joints, the reactant powder mixture for one material was pre-compacted first, and the powder mixture for the other material was then stacked and cold-pressed together. The particle sizes of the ball-milled powders used for the in situ joining were less than 10 lm for the Ni–Al powder and less than 15 lm for the Ni–Al–Ti–C powder respectively. A carbon sheet was placed between the graphite punch and the powder mixture for prevention of adhesion. The sample-die assembly was then put into the chamber of the SPS machine. In this work, SPS was carried out in a DR. SINTER type SPS-1050 apparatus (Sumitomo Coal Mining Co. Ltd.). The sample was heated, in a vacuum of 5–10 Pa, to a processing temperature at a heating rate of 150–200 °C/min for a hold time of 5 min at the processing temperature. During the SPS processing, an uniaxial pressure of 65–80 MPa was applied upon the sample through the graphite punches. A cooling rate of 40 °C/min was used. The processing or joining temperature was measured using a photoelectric pyrometer focused at a hole in the graphite die approximately 2–3 mm apart from the loaded powders. X-ray diffraction (XRD) analyses using Cu–Ka radiation were conducted on the mechanically activated powders and the SPS synthesized samples with a M03XHF X-ray diffractometer (MAC Science Co. Ltd.). The average grain sizes of the milled powders and synthesized samples were estimated based on XRD peak broadening by the Scherrer equation [12]. The correction for instrumental broadening was taken into account in the measurement of peak broadening according to the Warren method [12] using the Si powder as the standard for calibration. Microstructures of the SPS processed samples were investigated using light optical microscopy (LOM), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) in a JXA-8600 Superprobe (JEOL, Japan), and transmission electron microscopy (TEM). TEM observations were performed in a JEOL JEM-2000FX microscope operated at 200 kV. The densities of the sintered samples were
W. Liu, M. Naka / Scripta Materialia 48 (2003) 1225–1230
measured by the Archimedes method. Microhardness measurements were conducted on the bonded joints using a Vickers indenter under a 200 g load. Shear tests were performed in a tensile test machine at room temperature on the selected joints to examine the bonding strength. A specially designed jig was used in the shear tests to prevent the generation of a moment at the joint interface during shear testing. The specimens were of the size 4 mm 3 mm (cross section) 10 mm (length). The crosshead speed used was 0.5 mm/ min in the shear tests.
3. Results and discussion 3.1. Fabrication of nanocrystalline Ni3 Al and TiC/ Ni3 Al Fig. 1 shows the X-ray diffraction patterns of Ni–Al powders before and after MA by ball milling for 10 and 20 h respectively. The Al diffractive peaks could hardly be seen after 10 h of milling, and the Ni peaks shifted slightly toward the lower AB
A− Ni B− Al A
C− Ni(Al) D− Ni3 Al
(a) before MA
Relative Intensity (a. u.)
A B
B
A
A B
B
(b) 10 h of MA C
(c) 20 h of MA
C
C
D
(d) SPS 900 C 5 min
D D
D
C o
D D
D
D D o
(e) SPS 1100 C 5 min
20
40
60
80
100
2 Theta (degree) Fig. 1. XRD patterns of MA Ni–Al powders after different milling times in the as-milled conditions and after SPS processing at different temperatures for 5 min.
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angle direction. These changes were caused by the formation of a fcc Ni(Al) solid solution due to the diffusion of Al into the Ni lattice during milling. In the MA process, the Ni and Al powders are deformed, cold-welded into microsandwiches, and broken repeatedly. The lamella spacing in the microsandwiches decreases with increased milling time, leading to the formation of the Ni(Al) solid solution. After MA up to 20 h, no additional peak was detected by XRD except the observed decrease of intensity and broadening of the diffraction peaks from the Ni(Al) solid solution. Significant broadening of the peak widths in the XRD patterns suggested a large decrease in the grain sizes of the MA powders. The mean grain size was estimated to be 28.7 and 14.5 nm for the Ni solid solution respectively after 10 and 20 h of MA. The MA powders were then subjected to SPS processing. As indicated by the XRD patterns shown at the bottom of Fig. 1, single-phase Ni3 Al was synthesized by SPS processing at 900 and 1100 °C for 5 min respectively. The presence of (1 0 0), (1 1 0), (2 1 0) and (2 1 1) superlattice diffraction peaks in the XRD patterns is the characteristic of an ordered Ni3 Al intermetallic compound. The average grain sizes were calculated to be 60 and 96 nm from the XRD data for the Ni3 Al synthesized at 900 and 1100 °C for 5 min, respectively. Nearly fully dense Ni3 Al bulk samples were obtained by SPS processing at a temperature of 900 °C or higher. The Ni3 Al samples synthesized at 900 and 1100 °C for 5 min exhibited a relative density of 98.8% and 99.5% respectively, as measured by the ArchimedeÕs method. Most previous investigations on nanocrystalline materials reported relative densities ranging from 60% to 97% [1,2]. The very high relative densities achieved in this study demonstrate that the SPS process is an effective method for fabricating dense nanocrystalline bulk materials. Fig. 2 shows the XRD patterns of the Ni–Al– Ti–C powder mixture after various times of MA. The results show that, at the early stage of milling, the intensities of C and Al diffraction peaks decreased much faster with the increasing milling time than those of Ni and Ti peaks. The C and Al peaks disappeared after 4 and 10 h of MA,
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W. Liu, M. Naka / Scripta Materialia 48 (2003) 1225–1230 Ni
C
Ti
(a) 1 h of MA Ni
Ti Ti Al
Ti
TiAl Ti
Ni Ti Al
Ni
Ni
Relative Intensity (a. u.)
(b) 2 h of MA
(c) 4 h of MA (d) 10 h of MA (e) 20 h of MA (f) 30 h of MA A− Ni3 Al B− TiC B B A A
20
40
A
(g) SPS 1100 oC 5 min
A A
B
60
A
A B B
80
A
A
100
2 Theta (degree) Fig. 2. XRD patterns of MA Ni–Al–Ti–C powders after different milling times in the as-milled conditions and after SPS processing at 1100 °C for 5 min.
respectively. At the same time the Ni and Ti peaks shifted slightly towards lower angles, indicating that their lattice parameters were increased due to the formation of both Ni and Ti solid solutions at this stage of the milling process. Increasing the MA time led to remarkable decreases in the intensities as well as broadening of the Ni and Ti peaks. After 10 h of MA, the mean crystallite sizes, determined by the Scherrer equation, were 20.8 and 20.4 nm for Ni and Ti solid solutions. Pro-
longation of the MA to 20 and 30 h resulted in an increased background level in the vicinity of TiC diffraction lines (especially at 2h ¼ 35–43°), suggesting a partial formation of nanocrystalline TiC particles from the Ti solid solution. The Ti peaks were still visible in the XRD pattern, and the formation of TiC was thus incomplete. Therefore, after 30 h of milling, the Ni–Al–Ti–C MA powder consisted of nanostructured Ni and Ti solid solutions plus finely dispersed TiC particles. The XRD pattern of a sample SPS processed at 1100 °C for 5 min from the Ni–Al–Ti–C MA powder of the composition Ni3 Al–40vol%TiC is shown at the bottom in Fig. 2. As can be seen, Ni3 Al and TiC were the two phases present in the synthesized bulk samples. All the superlattice reflections of Ni3 Al appeared, indicating the formation of an ordered Ni3 Al intermetallic compound. The obtained TiC was of the NaCl type crystal structure. The mean grain sizes of Ni3 Al and TiC in the sample were estimated by the Scherrer equation to be 66 nm for Ni3 Al and 56 nm for TiC. The density of the SPS processed sample was evaluated to be 6.353 g/cm3 by the ArchimedeÕs method, which is 99% of the theoretical density. Fig. 3 shows a TEM bright-field image (BFI) along with a corresponding selected area diffraction pattern (SADP) of the SPS processed Ni3 Al–40vol%TiC sample. The observed grains in the composite sample are essentially equiaxed. From the TEM micrographs, the average grain size of the sample was measured to be 60.2 nm (17.6 nm), which is in agreement with the estimated values from the XRD data.
Fig. 3. TEM (a) bright field image and (b) corresponding SADP of a Ni3 Al–40vol% TiC composite sample obtained by SPS processing at 1100 °C for 5 min from the MA Ni–Al–Ti–C powder.
W. Liu, M. Naka / Scripta Materialia 48 (2003) 1225–1230
The in situ joining process uses the reaction synthesis technique to fabricate the two materials to be joined and to achieve simultaneous bonding at the joint interface between the two. If necessary, interlayers can also be used for materials compatibility and mechanical considerations. This joining process combines the materials preparation and joining operations into a single process. As a result, it can fully utilize the advantages of the material fabrication processes for production of high-performance dissimilar material joints. During actual in situ joining processing, the powders for fabrication of the materials to be joined together with interlayers, if any, are stacked in the graphite die for pressurized reactive sintering. The most important point for in situ joining is to ensure the concurrent synthesis and densification of the two dissimilar materials under the same processing conditions. This technique has the potential for near-net-shape fabrication of dissimilar material joints through the use of special graphite dies designed for specific products. Fig. 4(a) shows a SEM micrograph of the joint between the nanocrystalline Ni3 Al and the Ni3 Al– 40vol% TiC composite. The joint was fabricated by SPS at a processing temperature of 1100 °C for a hold time of 5 min. Good bonding without detectable defects (cracks, voids) was observed at the joint interface. Fully dense materials consisting of the desired phases at either side of the joint were obtained. XRD analysis conducted on the in situ
joints indicated the presence of only Ni3 Al and TiC phases. A back-scattered electron image together with EPMA line scans of the elements Ni, Al, Ti and C across the joint interface is shown in Fig. 4(b). The light phase in the back-scattered electron image is the Ni3 Al, and the gray area is a mixture of Ni3 Al and TiC at the composite side of the joint. The line scan results suggest that the nanocrystalline TiC phase is uniformly distributed in the composite material. Fig. 5 shows microhardness distributions across the in situ joints made at different temperatures (1000 and 1100 °C) for 5 min. As expected, the nanocrystalline Ni3 Al exhibited a much higher hardness than the conventionally reaction-synthesized Ni3 Al products of micron-sized grains, which have hardness in the range of 300–380 kg/mm2
1200 1000
Microhardness,HV0.2
3.2. In situ joining technique and characterization of joints
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800 600
Monolithic Ni 3 Al
400
Ni3 Al-TiC composite o
1000 C x 5 min
200
o
1100 C x 5 min 0 0
2
4
6
8
10
Distance along the joint, mm
Fig. 5. Microhardness distributions across the in situ joints made at different SPS temperatures.
Fig. 4. (a) SEM micrograph and (b) line scans of the elements across the joint interface between nanocrystalline Ni3 Al and Ni3 Al– 40vol% TiC made at 1100 °C for 5 min.
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[6,7]. The hardness in the Ni3 Al side was higher for the joint processed at 1000 °C than that processed at 1100 °C, due to smaller grain sizes obtained at the lower processing temperature. On the contrary, the hardness in the composite side was slightly increased by processing at the higher temperature. This increase probably resulted from a higher density achieved in the composite with a higher processing temperature (relative density being 99% for 1100 °C versus 96.1% for 1000 °C processing temperature), where the grain growth was less obvious in the composite (grain sizes being 60.2 17.6 nm for 1100 °C versus 52.7 21.5 nm for 1000 °C processing temperature) than in the monolithic Ni3 Al. The average microhardness in the TiC/Ni3 Al composite side was higher than that in the monolithic Ni3 Al side due to the reinforcement of TiC phase. A relatively lower microhardness value in the central part of the composite side was caused by a lower density observed in this location. Preliminary shear tests were performed on five specimens of the in situ joints made at 1100 °C for 5 min. The shear strengths of the joints averaged 765.2 MPa (37.1 MPa). Fracture occurred mainly in the composite side, sometimes across the joint interface. Future work is needed on characterization of the mechanical properties of the in situ joints.
4. Conclusions A reaction synthesis-based in situ joining technique was developed for joining dissimilar nanocrystalline materials by use of the SPS process. This technique combines the nanocrystalline material fabrication and joining operations into a single process. After up to 20 h of MA, the Ni–Al powder consisted of a nanostructured Ni(Al) solid solution, while the MA Ni–Al–Ti–C powder was composed of nanostructured Ni and Ti solid solutions plus partially formed TiC after 30 h of mechanical activation. Nearly fully dense, nanocrystalline Ni3 Al and Ni3 Al–40vol%TiC were
fabricated respectively from the mechanically activated Ni–Al and Ni–Al–Ti–C powder mixtures by SPS processing. The SPS process is therefore an effective method for fabricating dense nanocrystalline bulk materials. In situ joints between nanocrystalline Ni3 Al and TiC/Ni3 Al were successfully produced at processing temperatures of 1000 and 1100 °C for a hold time of 5 min. The joints exhibited an average shear strength of 765 MPa.
Acknowledgements This work was sponsored by the Japanese Ministry of Science and Education. The authors would like to thank Prof. J.N. DuPont at Lehigh University for reviewing the manuscript, and Mr. M. Maeda in JWRI at Osaka University for assistance in the experiments.
References [1] Chow GM, Noskova NI, editors. Nanostructured materials: science and technology. Dordrecht, The Netherlands: Kluwer Academic Publishers; 1998. [2] Choudry MS, Dollar M, Eastman JA. Mater Sci Eng A 1998;256:25–33. [3] Rabin BH. J Am Ceram Soc 1992;75:131–5. [4] Messler RW, Orling TT. J Mater Synth Proc 1994;2:315– 28. [5] Orling TT, Messler RW. Weld J 1996;75:93s–100s. [6] Liu W, Zhai F, Ding C. Acta Metall Sinica 2000;13: 217–22. [7] Liu W, Zhai F, Liu S. In: Vianco PT, Singh M, editors. Advanced Brazing and Soldering Technologies. ASM International; 2000. p. 73–80. [8] Merzhanov AG. In: Munir ZA, Holt JB, editors. Combustion and plasma synthesis of high temperature materials. New York: VCH Publishers; 1990. p. 1–18. [9] Moore JJ, Feng HJ. Prog Mater Sci 1995;39:243–316. [10] Omori M. Mater Sci Eng A 2000;287:183–8. [11] Zhan G, Kuntz J, Wan J, Garay J, Mukherjee AK. Scripta Mater 2002;47:737–41. [12] Cullity BD. Elements of X-ray diffraction. 2nd ed. MA, USA: Addison-Wesley Publishing Co; 1978.
In situ joining of dissimilar nanocrystalline materials by spark plasma sintering Weiping Liu
a,b,*
, Masaaki Naka
a
a
b
Joining and Welding Research Institute, Osaka University, Osaka 567-0047, Japan Department of Materials Science and Engineering, Lehigh University, 5 E. Packer Avenue, Bethlehem, PA 18015, USA Received 30 October 2002; received in revised form 16 January 2003; accepted 4 February 2003
Abstract Joining of nanocrystalline materials will be a great challenge. In this paper, a reaction synthesis-based in situ joining technique was developed for joining dissimilar nanocrystalline materials by use of the spark plasma sintering (SPS) process. The joining technique combines the nanocrystalline material processing and joining operations into a single process. Ó 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Nanomaterials; Reaction synthesis; Mechanical activation; Joining; Spark plasma sintering; Composites
1. Introduction Nanocrystalline materials, which are defined as materials with grain sizes below 100 nm, have attracted great attention and interest due to their improved properties [1,2]. As a class of potential engineering materials, joining of nanocrystalline materials will be a great challenge. Obviously, most of the conventional joining methods are not suitable for this new class of materials in order to preserve their ultra-fine grain structures. Therefore, developing new joining processes will be necessary.
* Corresponding author. Address: Department of Materials Science and Engineering, Lehigh University, 5 E. Packer Avenue, Bethlehem, PA 18015, USA. Tel.: +1-610-7584270; fax: +1-610-7586407. E-mail address: [email protected] (W. Liu).
Recently, a number of researchers [3–7] have successfully utilized the pressurized reaction synthesis or self-propagating high-temperature synthesis (SHS) process for joining of the difficultto-weld materials. SHS is an advanced material processing method that uses highly exothermic reactions between powdered constituents [8,9]. As a combination of SHS processing and joining technology, the reactive synthesis joining process provides innovative joining capabilities for advanced materials [5,7]. In particular, the process can be used for near-net-shape fabrication of joints with simultaneous bulk materials synthesis and in situ joining. The near-net-shape fabrication capabilities of the reactive synthesis joining process are attractive since they not only imply savings on operations and materials, but also imply more design freedom and improved properties. Spark plasma sintering (SPS) or pulsed electric current sintering (PECS) is a recent innovation in activated
1359-6462/03/$ - see front matter Ó 2003 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. doi:10.1016/S1359-6462(03)00074-5
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W. Liu, M. Naka / Scripta Materialia 48 (2003) 1225–1230
sintering and densification by charging a high pulsed electric current directly through the powders in a graphite die under externally applied pressure [10,11]. It was reported to be an effective method for sintering of powder products at lower temperatures and for shorter times than conventional processes. This process can also be used to achieve the materials synthesis and densification simultaneously and rapidly. In this study, an in situ joining technique has been developed to fabricate dissimilar nanocrystalline material joints based on reaction synthesis of mechanically activated powders by use of the SPS process.
2. Experimental procedure The starting powders used in this study were elemental Ni (99.9% pure), Al (99.9% pure), Ti (99.9% pure) and graphite C (99.7% pure) with average particle sizes of 3, 3, 10 and 5 lm respectively. Two sets of powder mixtures were prepared in this study. One consisted of Ni and Al powders for the fabrication of Ni3 Al. B powder with particle sizes less than 40 lm was added to this powder mixture as a dopant for the desired Ni3 Al product. The Ni, Al and B powders were mixed to a nominal composition of Ni76 Al23:6 B0:4 (at.%). The other set of powder mixture consisted of Ni, Al, Ti and C powders for the fabrication of TiC/Ni3 Al composites. The Ni, Al, Ti and C powders were mixed to a nominal composition of Ni3 Al–40vol%TiC. Mechanical activation (MA) of the powder mixture was performed in a vibrational ball mill with a water-cooled chamber for 10–30 h prior to reactive synthesis by SPS. Typically, 20 g of the powder mixture were put into a cylindrical stainless steel vial together with stainless steel balls (11 mm in diameter). The ball-to-powder weight ratio was 10:1. Methanol with a ratio of 2–3 wt.% of the powder mixture was used as a process control agent (PCA) to avoid the sticking of powders to the balls and the vial. Ball milling was performed under an Ar gas atmosphere. After ball milling, typically about 5 g of the powder mixture was poured into a 10-mm-diameter (inner diameter) graphite die that had previously been lined with a BN coating for facilitating
the removal of the sample after SPS was completed. The powder mixture in the die was coldpressed under an uniaxial pressure of 40 MPa. For fabrication of in situ joints, the reactant powder mixture for one material was pre-compacted first, and the powder mixture for the other material was then stacked and cold-pressed together. The particle sizes of the ball-milled powders used for the in situ joining were less than 10 lm for the Ni–Al powder and less than 15 lm for the Ni–Al–Ti–C powder respectively. A carbon sheet was placed between the graphite punch and the powder mixture for prevention of adhesion. The sample-die assembly was then put into the chamber of the SPS machine. In this work, SPS was carried out in a DR. SINTER type SPS-1050 apparatus (Sumitomo Coal Mining Co. Ltd.). The sample was heated, in a vacuum of 5–10 Pa, to a processing temperature at a heating rate of 150–200 °C/min for a hold time of 5 min at the processing temperature. During the SPS processing, an uniaxial pressure of 65–80 MPa was applied upon the sample through the graphite punches. A cooling rate of 40 °C/min was used. The processing or joining temperature was measured using a photoelectric pyrometer focused at a hole in the graphite die approximately 2–3 mm apart from the loaded powders. X-ray diffraction (XRD) analyses using Cu–Ka radiation were conducted on the mechanically activated powders and the SPS synthesized samples with a M03XHF X-ray diffractometer (MAC Science Co. Ltd.). The average grain sizes of the milled powders and synthesized samples were estimated based on XRD peak broadening by the Scherrer equation [12]. The correction for instrumental broadening was taken into account in the measurement of peak broadening according to the Warren method [12] using the Si powder as the standard for calibration. Microstructures of the SPS processed samples were investigated using light optical microscopy (LOM), scanning electron microscopy (SEM) and electron probe microanalysis (EPMA) in a JXA-8600 Superprobe (JEOL, Japan), and transmission electron microscopy (TEM). TEM observations were performed in a JEOL JEM-2000FX microscope operated at 200 kV. The densities of the sintered samples were
W. Liu, M. Naka / Scripta Materialia 48 (2003) 1225–1230
measured by the Archimedes method. Microhardness measurements were conducted on the bonded joints using a Vickers indenter under a 200 g load. Shear tests were performed in a tensile test machine at room temperature on the selected joints to examine the bonding strength. A specially designed jig was used in the shear tests to prevent the generation of a moment at the joint interface during shear testing. The specimens were of the size 4 mm 3 mm (cross section) 10 mm (length). The crosshead speed used was 0.5 mm/ min in the shear tests.
3. Results and discussion 3.1. Fabrication of nanocrystalline Ni3 Al and TiC/ Ni3 Al Fig. 1 shows the X-ray diffraction patterns of Ni–Al powders before and after MA by ball milling for 10 and 20 h respectively. The Al diffractive peaks could hardly be seen after 10 h of milling, and the Ni peaks shifted slightly toward the lower AB
A− Ni B− Al A
C− Ni(Al) D− Ni3 Al
(a) before MA
Relative Intensity (a. u.)
A B
B
A
A B
B
(b) 10 h of MA C
(c) 20 h of MA
C
C
D
(d) SPS 900 C 5 min
D D
D
C o
D D
D
D D o
(e) SPS 1100 C 5 min
20
40
60
80
100
2 Theta (degree) Fig. 1. XRD patterns of MA Ni–Al powders after different milling times in the as-milled conditions and after SPS processing at different temperatures for 5 min.
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angle direction. These changes were caused by the formation of a fcc Ni(Al) solid solution due to the diffusion of Al into the Ni lattice during milling. In the MA process, the Ni and Al powders are deformed, cold-welded into microsandwiches, and broken repeatedly. The lamella spacing in the microsandwiches decreases with increased milling time, leading to the formation of the Ni(Al) solid solution. After MA up to 20 h, no additional peak was detected by XRD except the observed decrease of intensity and broadening of the diffraction peaks from the Ni(Al) solid solution. Significant broadening of the peak widths in the XRD patterns suggested a large decrease in the grain sizes of the MA powders. The mean grain size was estimated to be 28.7 and 14.5 nm for the Ni solid solution respectively after 10 and 20 h of MA. The MA powders were then subjected to SPS processing. As indicated by the XRD patterns shown at the bottom of Fig. 1, single-phase Ni3 Al was synthesized by SPS processing at 900 and 1100 °C for 5 min respectively. The presence of (1 0 0), (1 1 0), (2 1 0) and (2 1 1) superlattice diffraction peaks in the XRD patterns is the characteristic of an ordered Ni3 Al intermetallic compound. The average grain sizes were calculated to be 60 and 96 nm from the XRD data for the Ni3 Al synthesized at 900 and 1100 °C for 5 min, respectively. Nearly fully dense Ni3 Al bulk samples were obtained by SPS processing at a temperature of 900 °C or higher. The Ni3 Al samples synthesized at 900 and 1100 °C for 5 min exhibited a relative density of 98.8% and 99.5% respectively, as measured by the ArchimedeÕs method. Most previous investigations on nanocrystalline materials reported relative densities ranging from 60% to 97% [1,2]. The very high relative densities achieved in this study demonstrate that the SPS process is an effective method for fabricating dense nanocrystalline bulk materials. Fig. 2 shows the XRD patterns of the Ni–Al– Ti–C powder mixture after various times of MA. The results show that, at the early stage of milling, the intensities of C and Al diffraction peaks decreased much faster with the increasing milling time than those of Ni and Ti peaks. The C and Al peaks disappeared after 4 and 10 h of MA,
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W. Liu, M. Naka / Scripta Materialia 48 (2003) 1225–1230 Ni
C
Ti
(a) 1 h of MA Ni
Ti Ti Al
Ti
TiAl Ti
Ni Ti Al
Ni
Ni
Relative Intensity (a. u.)
(b) 2 h of MA
(c) 4 h of MA (d) 10 h of MA (e) 20 h of MA (f) 30 h of MA A− Ni3 Al B− TiC B B A A
20
40
A
(g) SPS 1100 oC 5 min
A A
B
60
A
A B B
80
A
A
100
2 Theta (degree) Fig. 2. XRD patterns of MA Ni–Al–Ti–C powders after different milling times in the as-milled conditions and after SPS processing at 1100 °C for 5 min.
respectively. At the same time the Ni and Ti peaks shifted slightly towards lower angles, indicating that their lattice parameters were increased due to the formation of both Ni and Ti solid solutions at this stage of the milling process. Increasing the MA time led to remarkable decreases in the intensities as well as broadening of the Ni and Ti peaks. After 10 h of MA, the mean crystallite sizes, determined by the Scherrer equation, were 20.8 and 20.4 nm for Ni and Ti solid solutions. Pro-
longation of the MA to 20 and 30 h resulted in an increased background level in the vicinity of TiC diffraction lines (especially at 2h ¼ 35–43°), suggesting a partial formation of nanocrystalline TiC particles from the Ti solid solution. The Ti peaks were still visible in the XRD pattern, and the formation of TiC was thus incomplete. Therefore, after 30 h of milling, the Ni–Al–Ti–C MA powder consisted of nanostructured Ni and Ti solid solutions plus finely dispersed TiC particles. The XRD pattern of a sample SPS processed at 1100 °C for 5 min from the Ni–Al–Ti–C MA powder of the composition Ni3 Al–40vol%TiC is shown at the bottom in Fig. 2. As can be seen, Ni3 Al and TiC were the two phases present in the synthesized bulk samples. All the superlattice reflections of Ni3 Al appeared, indicating the formation of an ordered Ni3 Al intermetallic compound. The obtained TiC was of the NaCl type crystal structure. The mean grain sizes of Ni3 Al and TiC in the sample were estimated by the Scherrer equation to be 66 nm for Ni3 Al and 56 nm for TiC. The density of the SPS processed sample was evaluated to be 6.353 g/cm3 by the ArchimedeÕs method, which is 99% of the theoretical density. Fig. 3 shows a TEM bright-field image (BFI) along with a corresponding selected area diffraction pattern (SADP) of the SPS processed Ni3 Al–40vol%TiC sample. The observed grains in the composite sample are essentially equiaxed. From the TEM micrographs, the average grain size of the sample was measured to be 60.2 nm (17.6 nm), which is in agreement with the estimated values from the XRD data.
Fig. 3. TEM (a) bright field image and (b) corresponding SADP of a Ni3 Al–40vol% TiC composite sample obtained by SPS processing at 1100 °C for 5 min from the MA Ni–Al–Ti–C powder.
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The in situ joining process uses the reaction synthesis technique to fabricate the two materials to be joined and to achieve simultaneous bonding at the joint interface between the two. If necessary, interlayers can also be used for materials compatibility and mechanical considerations. This joining process combines the materials preparation and joining operations into a single process. As a result, it can fully utilize the advantages of the material fabrication processes for production of high-performance dissimilar material joints. During actual in situ joining processing, the powders for fabrication of the materials to be joined together with interlayers, if any, are stacked in the graphite die for pressurized reactive sintering. The most important point for in situ joining is to ensure the concurrent synthesis and densification of the two dissimilar materials under the same processing conditions. This technique has the potential for near-net-shape fabrication of dissimilar material joints through the use of special graphite dies designed for specific products. Fig. 4(a) shows a SEM micrograph of the joint between the nanocrystalline Ni3 Al and the Ni3 Al– 40vol% TiC composite. The joint was fabricated by SPS at a processing temperature of 1100 °C for a hold time of 5 min. Good bonding without detectable defects (cracks, voids) was observed at the joint interface. Fully dense materials consisting of the desired phases at either side of the joint were obtained. XRD analysis conducted on the in situ
joints indicated the presence of only Ni3 Al and TiC phases. A back-scattered electron image together with EPMA line scans of the elements Ni, Al, Ti and C across the joint interface is shown in Fig. 4(b). The light phase in the back-scattered electron image is the Ni3 Al, and the gray area is a mixture of Ni3 Al and TiC at the composite side of the joint. The line scan results suggest that the nanocrystalline TiC phase is uniformly distributed in the composite material. Fig. 5 shows microhardness distributions across the in situ joints made at different temperatures (1000 and 1100 °C) for 5 min. As expected, the nanocrystalline Ni3 Al exhibited a much higher hardness than the conventionally reaction-synthesized Ni3 Al products of micron-sized grains, which have hardness in the range of 300–380 kg/mm2
1200 1000
Microhardness,HV0.2
3.2. In situ joining technique and characterization of joints
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800 600
Monolithic Ni 3 Al
400
Ni3 Al-TiC composite o
1000 C x 5 min
200
o
1100 C x 5 min 0 0
2
4
6
8
10
Distance along the joint, mm
Fig. 5. Microhardness distributions across the in situ joints made at different SPS temperatures.
Fig. 4. (a) SEM micrograph and (b) line scans of the elements across the joint interface between nanocrystalline Ni3 Al and Ni3 Al– 40vol% TiC made at 1100 °C for 5 min.
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[6,7]. The hardness in the Ni3 Al side was higher for the joint processed at 1000 °C than that processed at 1100 °C, due to smaller grain sizes obtained at the lower processing temperature. On the contrary, the hardness in the composite side was slightly increased by processing at the higher temperature. This increase probably resulted from a higher density achieved in the composite with a higher processing temperature (relative density being 99% for 1100 °C versus 96.1% for 1000 °C processing temperature), where the grain growth was less obvious in the composite (grain sizes being 60.2 17.6 nm for 1100 °C versus 52.7 21.5 nm for 1000 °C processing temperature) than in the monolithic Ni3 Al. The average microhardness in the TiC/Ni3 Al composite side was higher than that in the monolithic Ni3 Al side due to the reinforcement of TiC phase. A relatively lower microhardness value in the central part of the composite side was caused by a lower density observed in this location. Preliminary shear tests were performed on five specimens of the in situ joints made at 1100 °C for 5 min. The shear strengths of the joints averaged 765.2 MPa (37.1 MPa). Fracture occurred mainly in the composite side, sometimes across the joint interface. Future work is needed on characterization of the mechanical properties of the in situ joints.
4. Conclusions A reaction synthesis-based in situ joining technique was developed for joining dissimilar nanocrystalline materials by use of the SPS process. This technique combines the nanocrystalline material fabrication and joining operations into a single process. After up to 20 h of MA, the Ni–Al powder consisted of a nanostructured Ni(Al) solid solution, while the MA Ni–Al–Ti–C powder was composed of nanostructured Ni and Ti solid solutions plus partially formed TiC after 30 h of mechanical activation. Nearly fully dense, nanocrystalline Ni3 Al and Ni3 Al–40vol%TiC were
fabricated respectively from the mechanically activated Ni–Al and Ni–Al–Ti–C powder mixtures by SPS processing. The SPS process is therefore an effective method for fabricating dense nanocrystalline bulk materials. In situ joints between nanocrystalline Ni3 Al and TiC/Ni3 Al were successfully produced at processing temperatures of 1000 and 1100 °C for a hold time of 5 min. The joints exhibited an average shear strength of 765 MPa.
Acknowledgements This work was sponsored by the Japanese Ministry of Science and Education. The authors would like to thank Prof. J.N. DuPont at Lehigh University for reviewing the manuscript, and Mr. M. Maeda in JWRI at Osaka University for assistance in the experiments.
References [1] Chow GM, Noskova NI, editors. Nanostructured materials: science and technology. Dordrecht, The Netherlands: Kluwer Academic Publishers; 1998. [2] Choudry MS, Dollar M, Eastman JA. Mater Sci Eng A 1998;256:25–33. [3] Rabin BH. J Am Ceram Soc 1992;75:131–5. [4] Messler RW, Orling TT. J Mater Synth Proc 1994;2:315– 28. [5] Orling TT, Messler RW. Weld J 1996;75:93s–100s. [6] Liu W, Zhai F, Ding C. Acta Metall Sinica 2000;13: 217–22. [7] Liu W, Zhai F, Liu S. In: Vianco PT, Singh M, editors. Advanced Brazing and Soldering Technologies. ASM International; 2000. p. 73–80. [8] Merzhanov AG. In: Munir ZA, Holt JB, editors. Combustion and plasma synthesis of high temperature materials. New York: VCH Publishers; 1990. p. 1–18. [9] Moore JJ, Feng HJ. Prog Mater Sci 1995;39:243–316. [10] Omori M. Mater Sci Eng A 2000;287:183–8. [11] Zhan G, Kuntz J, Wan J, Garay J, Mukherjee AK. Scripta Mater 2002;47:737–41. [12] Cullity BD. Elements of X-ray diffraction. 2nd ed. MA, USA: Addison-Wesley Publishing Co; 1978.