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Nickel particle embedded aluminium matrix composite with high ductility
Materials Letters 64 (2010) 664–667
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m a t l e t
Nickel particle embedded aluminium matrix composite with high ductility Devinder Yadav, Ranjit Bauri ⁎ Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600 036, India
a r t i c l e
i n f o
Article history: Received 4 December 2009 Accepted 12 December 2009 Available online 23 December 2009 Keywords: Composite materials Mechanical properties Ductility Transmission electron microscopy Grain refining Friction stir processing
a b s t r a c t Nickel particulate reinforced aluminium matrix composite was processed without formation of deleterious Al3Ni intermetallic by friction stir processing (FSP). FSP resulted in uniform dispersion of nickel particles in the aluminium matrix with excellent interfacial bonding and also lead to grain refinement of the matrix. The composite exhibited a threefold increase in the yield stress (0.2% proof stress). The most novel feature of the composite is that an appreciable amount of ductility is retained while the strength increases significantly. The microstructure evolution was studied by transmission electron microscopy and electron backscattered diffraction analysis. EBSD analysis showed a dynamically recrystallized equiaxed microstructure having a considerable fraction of low-angle boundaries. TEM observations revealed that these low-angle boundaries are essentially subgrain boundaries formed by dislocation rearrangement and absorption during friction stir processing. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Al and its alloys are an important class of materials because of their versatile properties which render them suitable for use in a variety of applications. There has been a constant effort to improve the mechanical properties of Al alloys by various means. Metal matrix composite (MMC) technology is one such method which has become very popular in the past three decades. The Al matrix is reinforced with ceramic reinforcements like SiC, Al2O3, AlN, B4C, and TiC to obtain higher strength. However, these MMCs suffer from the disadvantage of low ductility. An alternative approach is to design composites having hard metallic phases as the reinforcement. The metallic reinforcement can offer the beneficial effect on both strength and ductility [1,2]. Nickel is an attractive choice as reinforcement because of its high strength, high stiffness and good high temperature properties. However, processing of metallic particle reinforced MMCs can pose serious challenges. For example, addition of Ni to Al melt produces brittle Al–Ni intermetallics [3] due to low solubility of Ni in Al. Although reinforcing Al with Ti particles is reported by disintegrating the melt [1], the possibility of formation of brittle intermetallics can never be excluded. Wong et al. [4] reported processing Ni particle reinforced Al composite by the same method of disintegrated melt deposition (DMD). However, in their study also formation of Al–Ni intermetallics and consequent reduction in ductility could not be avoided. Powder metallurgy processing from Al and Ni powders also leads to formation of Al–Ni intermetallics [5,6].
⁎ Corresponding author. Tel.: + 91 44 22574778; fax: + 91 44 22574752. E-mail address: [email protected] (R. Bauri). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.12.030
Friction stir processing (FSP), which originated from friction stir welding (FSW), uses a cylindrical rotating tool which is traversed horizontally. The applied downward force and localized heating due to friction softens and deforms the material [7]. The technique can be used for a variety of processing purposes like grain refinement and microstructure homogenization [7,8]. There are a few studies [7,8] on processing composites by FSP. However, as far as authors' knowledge are concerned there is no report on processing metal particle reinforced composites by FSP. In the present investigation, FSP has been used to embed Ni particles in to an Al matrix to produce metal particle reinforced composites without harmful intermetallics. The novel feature of the processed composite is that it retains most of the ductility while the strength is improved significantly. 2. Materials and methods A groove (50 × 1 × 2 mm) was made on a commercially pure (98.2%) 1050 aluminium plate. The groove was filled with nickel powder (200 mesh). A tool with shoulder diameter of 12 mm, pin diameter of 4 mm and pin length of 3.5 mm was used to carry out FSP at a rotation speed of 1000 rpm and traverse speed of 60 mm/min with a downward force of 5 kN. Pure Al was also processed (with somewhat different FSP parameters though, as the same parameters were not optimum for Ni incorporation) for comparison. The microstructure was characterized by scanning and transmission electron microscopy (SEM and TEM) and electron backscattered diffraction (EBSD). X-ray diffraction (XRD) was performed for phase analysis. Samples were metallographically polished for SEM and EBSD studies. TEM observations were made using twin-jet polished thin samples in a Philips CM-20 TEM operating at 200 kV. For EBSD studies,
D. Yadav, R. Bauri / Materials Letters 64 (2010) 664–667
samples were electropolished using a mixture of perchloric acid and methanol at −12 °C and observed in a FEI Quanta FEG SEM equipped with TSL-OIM software operating at 30 kV. XRD was carried out in a Brukers D8 diffractometer using Cu Kα radiation. 1 mm thick tensile samples were sliced from the stir zone parallel to the tool traverse direction by electro discharge machining (EDM). Tests were carried out on an Instron (Model 3367) machine at a strain rate of 10− 3 s− 1 as per ASTM standard. 3. Results and discussion 3.1. Microstructure FSP resulted in uniform distribution of nickel particles into the Al matrix with good bonding. Fig. 1(a) shows the SEM image of surface of stir zone. The volume fraction of nickel particles was found to be 7% using an image analyzer (Image J). The BSE image in Fig 1(b) shows a sharp particle–matrix interface with excellent bonding. It can be seen from the collage of BSE images in Fig. 1(c) that the nickel particles delineate the shape of the nugget and are uniformly dispersed into the volume of the stir zone up to a depth of 2 mm. The microstructure was further analyzed by EBSD and TEM. It is generally considered that the fine grains are formed during FSW/FSP by dynamic recrystallization (DRX) [7,9,10]. The EBSD image in Fig. 2(a) shows dynamically recrystallized equiaxed grains in the stir zone. A nickel particle is shown at the center of the image. The average grain size of the composite was found to be 7 µm compared to initial grain size of 74 µm. FSP serves the dual purpose of grain refinement and incorporation of nickel into the Al matrix. The misorientation angle distribution (from 2°) in Fig. 2(b) shows a high fraction of low-angle boundaries. The corresponding images of the friction stir processed (FSPed) Al are shown in Fig. 2(c) and (d) respectively. It shows similar features. The grain size is however
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somewhat finer due to less grain growth as the heat input was lower due to lower rpm used. TEM observations revealed grains in different stages of recovery (Fig. 3a). Most of the larger grains were seen to be divided into many subgrains by subgrain boundaries formed by dislocation rearrangement as shown in Fig. 3(b). The diffraction contrast between the subgrains (Fig. 3c) suggests that the subgrain boundaries are turning in to high angle boundaries. High density of such subgrain boundaries (<5°) is also supported by EBSD results. Dynamic recovery (DRV) readily occurs in aluminium and its alloys during thermo-mechanical processing due to their high stacking fault energy. This leads to arrangement of dislocations into subgrain boundaries and incorporation of additional dislocations, generated during the deformation process, into the subgrain boundaries increases their misorientation. The presence of Ni particles may also generate additional dislocations. Dislocation glide assisted rotation may turn these boundaries into high angle grain boundaries [9]. Thus it appears that a continuous type dynamic recrystallization mechanism leads to the final equiaxed grain structure of the composite. The FSPed Al shows similar microstructural features (Fig. 3d). 3.2. X-ray diffraction analysis Because of extremely low solid solubility of nickel in aluminium, Al3Ni intermetallics readily form when Ni is added to Al. However, in the present study there is no sign of any intermetallic formation. This is confirmed by XRD analysis of the composite. The XRD plot in Fig. 4 shows no peaks corresponding to any intermetallic phase and nickel was found to be present in elemental form. The temperature of the stir zone during FSP depends on the ratio of tool rotation speed to the traverse speed. Higher ratio leads to higher heat input into the material. The temperature of the stir zone has been found to be in the range of 400–500 °C during FSP of aluminium [11,12]. However, since no intermetallic was observed in our study, it is believed that the ratio
Fig. 1. SEM micrographs of (a) surface of stir zone, (b) particle matrix interface and (c) collage of BSE images showing the cross section of stir zone.
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D. Yadav, R. Bauri / Materials Letters 64 (2010) 664–667
Fig. 2. (a) EBSD image of stir zone and (b) misorientation angle distribution. (c) and (d) Corresponding pictures for the FSPed Al.
of tool rotation speed to traverse speed selected here is not high enough to raise the temperature of the stir zone to the level required to initiate aluminium nickel reaction and hence is suitable for making a composite free of deleterious intermetallics. 3.3. Mechanical properties Table 1 shows the tensile properties of the unreinforced Al, FSPed Al and Al–Ni composite. 0.2% proof stress of the Al–Ni composite improved by a factor of 3 (200%) compared to the unreinforced metal. The novel feature of the composite is that the strength is improved significantly with appreciable amount of ductility being retained unlike conventional particulate reinforced composites which invariably exhibit significant reduction in ductility [13,14]. For example, an Al–40 vol.% Al2O3 composite showed 0.2% proof stress of 89 MPa and only 2.7% ductility [14]. The corresponding values in the Al–Ni composite in the present
study are 104 MPa and 25% respectively with only 7% Ni (Table 1). Al–Ni composite also shows higher strength than the FSPed Al. As the interfacial bonding between the Al matrix and Ni particles is good (Fig. 1b), the load is effectively transferred from the Al matrix to the stronger Ni particles via the interface and hence leads to enhancement in the strength. The hindrance to dislocation motion by the Ni particles is also responsible for the increased strength. The grain refinement also contributes to strength enhancement. The ductile nature of the FCC Ni particles and the absence of any brittle intermetallic retain the overall ductility in the composite. 4. Conclusions 1. Nickel particle reinforced aluminium matrix composite is successfully prepared using FSP with uniform dispersion of nickel particles and without formation of any harmful intermetallic. FSP also resulted in significant grain refinement of the matrix.
D. Yadav, R. Bauri / Materials Letters 64 (2010) 664–667
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Fig. 3. TEM micrograph of Al–Ni composite showing (a) recovery structure in grains, (b) dislocations arranged into subgrain boundary and (c) diffraction contrast between subgrains divided by subgrain boundaries. (d) TEM micrograph of FSPed Al. Table 1 Tensile properties of the composite. Material
0.2% proof stress (MPa)
UTS (MPa)
% elongation
Base aluminium FSPed Al Al–7% Ni composite Al–40.4% Al2O3 [14]
35 82 104 89
72 90 127 249
39 35 25 2.7
References
Fig. 4. XRD plot of the composite.
2. Reinforcement of Ni particles leads to a threefold increase in the yield strength while appreciable amount of ductility is retained. 3. EBSD and TEM analyses revealed a dynamic recrystallization process driven by dynamic recovery to be operating during FSP. Acknowledgement The authors would like to thank Prof. K. Prasad Rao, IIT Madras and Prof. Indradev Samajdar, IIT Bombay, for providing access to the FSP and EBSD facility respectively.
[1] Thakur SK, Gupta M. Composites Part A 2007;38:1010–8. [2] Hassan SF, Gupta M. J Alloys Comp 2002;345:246–51. [3] Massalski T, Murray JL, Bennett LH, Baker H. Binary alloy phase diagrams, vol. 1. Metals Park, OH: American Society for Metals; 1986. [4] Wong WLE, Gupta M, Lim CYH. Sol Stat Pheno 2006;111:39–42. [5] Morsi K. Mater Sci Eng A 2001;299:1–15. [6] Marcelo T, Carvalho MH, Lopes EB, Bartout JD, Bienvenu Y. Key Eng Mater 2002;230:189–92. [7] Mishra RS, Ma ZY. Mater Sci Eng R 2005;50:1–78. [8] Hsu CJ, Chang CY, Kao PW, Ho NJ, Chang CP. Acta Mater 2006;54:5241–9. [9] Jata KV, Semiatin SL. Scripta Mater 2000;43:743–9. [10] Su J-Q, Nelson WT, Sterling CJ. Mater Sci Eng A 2005;405:277–86. [11] Rhodes CG, Mahoney MW, Bingel WH, Spurling RA, Bampton CC. Scripta Mater 1997;36:69–75. [12] Tang W, Guo X, McClure JC, Murr LE. J Mater Process Manufac Sci 1998;7:163–72. [13] McKimpson MG, Scott TE. Mater Sci Eng A 1989;107:93–106. [14] Kouzeli M, Weber L, San Marchi C, Mortensen A. Acta Mater 2001;49:3699-09.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m a t l e t
Nickel particle embedded aluminium matrix composite with high ductility Devinder Yadav, Ranjit Bauri ⁎ Department of Metallurgical and Materials Engineering, Indian Institute of Technology Madras, Chennai 600 036, India
a r t i c l e
i n f o
Article history: Received 4 December 2009 Accepted 12 December 2009 Available online 23 December 2009 Keywords: Composite materials Mechanical properties Ductility Transmission electron microscopy Grain refining Friction stir processing
a b s t r a c t Nickel particulate reinforced aluminium matrix composite was processed without formation of deleterious Al3Ni intermetallic by friction stir processing (FSP). FSP resulted in uniform dispersion of nickel particles in the aluminium matrix with excellent interfacial bonding and also lead to grain refinement of the matrix. The composite exhibited a threefold increase in the yield stress (0.2% proof stress). The most novel feature of the composite is that an appreciable amount of ductility is retained while the strength increases significantly. The microstructure evolution was studied by transmission electron microscopy and electron backscattered diffraction analysis. EBSD analysis showed a dynamically recrystallized equiaxed microstructure having a considerable fraction of low-angle boundaries. TEM observations revealed that these low-angle boundaries are essentially subgrain boundaries formed by dislocation rearrangement and absorption during friction stir processing. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Al and its alloys are an important class of materials because of their versatile properties which render them suitable for use in a variety of applications. There has been a constant effort to improve the mechanical properties of Al alloys by various means. Metal matrix composite (MMC) technology is one such method which has become very popular in the past three decades. The Al matrix is reinforced with ceramic reinforcements like SiC, Al2O3, AlN, B4C, and TiC to obtain higher strength. However, these MMCs suffer from the disadvantage of low ductility. An alternative approach is to design composites having hard metallic phases as the reinforcement. The metallic reinforcement can offer the beneficial effect on both strength and ductility [1,2]. Nickel is an attractive choice as reinforcement because of its high strength, high stiffness and good high temperature properties. However, processing of metallic particle reinforced MMCs can pose serious challenges. For example, addition of Ni to Al melt produces brittle Al–Ni intermetallics [3] due to low solubility of Ni in Al. Although reinforcing Al with Ti particles is reported by disintegrating the melt [1], the possibility of formation of brittle intermetallics can never be excluded. Wong et al. [4] reported processing Ni particle reinforced Al composite by the same method of disintegrated melt deposition (DMD). However, in their study also formation of Al–Ni intermetallics and consequent reduction in ductility could not be avoided. Powder metallurgy processing from Al and Ni powders also leads to formation of Al–Ni intermetallics [5,6].
⁎ Corresponding author. Tel.: + 91 44 22574778; fax: + 91 44 22574752. E-mail address: [email protected] (R. Bauri). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.12.030
Friction stir processing (FSP), which originated from friction stir welding (FSW), uses a cylindrical rotating tool which is traversed horizontally. The applied downward force and localized heating due to friction softens and deforms the material [7]. The technique can be used for a variety of processing purposes like grain refinement and microstructure homogenization [7,8]. There are a few studies [7,8] on processing composites by FSP. However, as far as authors' knowledge are concerned there is no report on processing metal particle reinforced composites by FSP. In the present investigation, FSP has been used to embed Ni particles in to an Al matrix to produce metal particle reinforced composites without harmful intermetallics. The novel feature of the processed composite is that it retains most of the ductility while the strength is improved significantly. 2. Materials and methods A groove (50 × 1 × 2 mm) was made on a commercially pure (98.2%) 1050 aluminium plate. The groove was filled with nickel powder (200 mesh). A tool with shoulder diameter of 12 mm, pin diameter of 4 mm and pin length of 3.5 mm was used to carry out FSP at a rotation speed of 1000 rpm and traverse speed of 60 mm/min with a downward force of 5 kN. Pure Al was also processed (with somewhat different FSP parameters though, as the same parameters were not optimum for Ni incorporation) for comparison. The microstructure was characterized by scanning and transmission electron microscopy (SEM and TEM) and electron backscattered diffraction (EBSD). X-ray diffraction (XRD) was performed for phase analysis. Samples were metallographically polished for SEM and EBSD studies. TEM observations were made using twin-jet polished thin samples in a Philips CM-20 TEM operating at 200 kV. For EBSD studies,
D. Yadav, R. Bauri / Materials Letters 64 (2010) 664–667
samples were electropolished using a mixture of perchloric acid and methanol at −12 °C and observed in a FEI Quanta FEG SEM equipped with TSL-OIM software operating at 30 kV. XRD was carried out in a Brukers D8 diffractometer using Cu Kα radiation. 1 mm thick tensile samples were sliced from the stir zone parallel to the tool traverse direction by electro discharge machining (EDM). Tests were carried out on an Instron (Model 3367) machine at a strain rate of 10− 3 s− 1 as per ASTM standard. 3. Results and discussion 3.1. Microstructure FSP resulted in uniform distribution of nickel particles into the Al matrix with good bonding. Fig. 1(a) shows the SEM image of surface of stir zone. The volume fraction of nickel particles was found to be 7% using an image analyzer (Image J). The BSE image in Fig 1(b) shows a sharp particle–matrix interface with excellent bonding. It can be seen from the collage of BSE images in Fig. 1(c) that the nickel particles delineate the shape of the nugget and are uniformly dispersed into the volume of the stir zone up to a depth of 2 mm. The microstructure was further analyzed by EBSD and TEM. It is generally considered that the fine grains are formed during FSW/FSP by dynamic recrystallization (DRX) [7,9,10]. The EBSD image in Fig. 2(a) shows dynamically recrystallized equiaxed grains in the stir zone. A nickel particle is shown at the center of the image. The average grain size of the composite was found to be 7 µm compared to initial grain size of 74 µm. FSP serves the dual purpose of grain refinement and incorporation of nickel into the Al matrix. The misorientation angle distribution (from 2°) in Fig. 2(b) shows a high fraction of low-angle boundaries. The corresponding images of the friction stir processed (FSPed) Al are shown in Fig. 2(c) and (d) respectively. It shows similar features. The grain size is however
665
somewhat finer due to less grain growth as the heat input was lower due to lower rpm used. TEM observations revealed grains in different stages of recovery (Fig. 3a). Most of the larger grains were seen to be divided into many subgrains by subgrain boundaries formed by dislocation rearrangement as shown in Fig. 3(b). The diffraction contrast between the subgrains (Fig. 3c) suggests that the subgrain boundaries are turning in to high angle boundaries. High density of such subgrain boundaries (<5°) is also supported by EBSD results. Dynamic recovery (DRV) readily occurs in aluminium and its alloys during thermo-mechanical processing due to their high stacking fault energy. This leads to arrangement of dislocations into subgrain boundaries and incorporation of additional dislocations, generated during the deformation process, into the subgrain boundaries increases their misorientation. The presence of Ni particles may also generate additional dislocations. Dislocation glide assisted rotation may turn these boundaries into high angle grain boundaries [9]. Thus it appears that a continuous type dynamic recrystallization mechanism leads to the final equiaxed grain structure of the composite. The FSPed Al shows similar microstructural features (Fig. 3d). 3.2. X-ray diffraction analysis Because of extremely low solid solubility of nickel in aluminium, Al3Ni intermetallics readily form when Ni is added to Al. However, in the present study there is no sign of any intermetallic formation. This is confirmed by XRD analysis of the composite. The XRD plot in Fig. 4 shows no peaks corresponding to any intermetallic phase and nickel was found to be present in elemental form. The temperature of the stir zone during FSP depends on the ratio of tool rotation speed to the traverse speed. Higher ratio leads to higher heat input into the material. The temperature of the stir zone has been found to be in the range of 400–500 °C during FSP of aluminium [11,12]. However, since no intermetallic was observed in our study, it is believed that the ratio
Fig. 1. SEM micrographs of (a) surface of stir zone, (b) particle matrix interface and (c) collage of BSE images showing the cross section of stir zone.
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D. Yadav, R. Bauri / Materials Letters 64 (2010) 664–667
Fig. 2. (a) EBSD image of stir zone and (b) misorientation angle distribution. (c) and (d) Corresponding pictures for the FSPed Al.
of tool rotation speed to traverse speed selected here is not high enough to raise the temperature of the stir zone to the level required to initiate aluminium nickel reaction and hence is suitable for making a composite free of deleterious intermetallics. 3.3. Mechanical properties Table 1 shows the tensile properties of the unreinforced Al, FSPed Al and Al–Ni composite. 0.2% proof stress of the Al–Ni composite improved by a factor of 3 (200%) compared to the unreinforced metal. The novel feature of the composite is that the strength is improved significantly with appreciable amount of ductility being retained unlike conventional particulate reinforced composites which invariably exhibit significant reduction in ductility [13,14]. For example, an Al–40 vol.% Al2O3 composite showed 0.2% proof stress of 89 MPa and only 2.7% ductility [14]. The corresponding values in the Al–Ni composite in the present
study are 104 MPa and 25% respectively with only 7% Ni (Table 1). Al–Ni composite also shows higher strength than the FSPed Al. As the interfacial bonding between the Al matrix and Ni particles is good (Fig. 1b), the load is effectively transferred from the Al matrix to the stronger Ni particles via the interface and hence leads to enhancement in the strength. The hindrance to dislocation motion by the Ni particles is also responsible for the increased strength. The grain refinement also contributes to strength enhancement. The ductile nature of the FCC Ni particles and the absence of any brittle intermetallic retain the overall ductility in the composite. 4. Conclusions 1. Nickel particle reinforced aluminium matrix composite is successfully prepared using FSP with uniform dispersion of nickel particles and without formation of any harmful intermetallic. FSP also resulted in significant grain refinement of the matrix.
D. Yadav, R. Bauri / Materials Letters 64 (2010) 664–667
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Fig. 3. TEM micrograph of Al–Ni composite showing (a) recovery structure in grains, (b) dislocations arranged into subgrain boundary and (c) diffraction contrast between subgrains divided by subgrain boundaries. (d) TEM micrograph of FSPed Al. Table 1 Tensile properties of the composite. Material
0.2% proof stress (MPa)
UTS (MPa)
% elongation
Base aluminium FSPed Al Al–7% Ni composite Al–40.4% Al2O3 [14]
35 82 104 89
72 90 127 249
39 35 25 2.7
References
Fig. 4. XRD plot of the composite.
2. Reinforcement of Ni particles leads to a threefold increase in the yield strength while appreciable amount of ductility is retained. 3. EBSD and TEM analyses revealed a dynamic recrystallization process driven by dynamic recovery to be operating during FSP. Acknowledgement The authors would like to thank Prof. K. Prasad Rao, IIT Madras and Prof. Indradev Samajdar, IIT Bombay, for providing access to the FSP and EBSD facility respectively.
[1] Thakur SK, Gupta M. Composites Part A 2007;38:1010–8. [2] Hassan SF, Gupta M. J Alloys Comp 2002;345:246–51. [3] Massalski T, Murray JL, Bennett LH, Baker H. Binary alloy phase diagrams, vol. 1. Metals Park, OH: American Society for Metals; 1986. [4] Wong WLE, Gupta M, Lim CYH. Sol Stat Pheno 2006;111:39–42. [5] Morsi K. Mater Sci Eng A 2001;299:1–15. [6] Marcelo T, Carvalho MH, Lopes EB, Bartout JD, Bienvenu Y. Key Eng Mater 2002;230:189–92. [7] Mishra RS, Ma ZY. Mater Sci Eng R 2005;50:1–78. [8] Hsu CJ, Chang CY, Kao PW, Ho NJ, Chang CP. Acta Mater 2006;54:5241–9. [9] Jata KV, Semiatin SL. Scripta Mater 2000;43:743–9. [10] Su J-Q, Nelson WT, Sterling CJ. Mater Sci Eng A 2005;405:277–86. [11] Rhodes CG, Mahoney MW, Bingel WH, Spurling RA, Bampton CC. Scripta Mater 1997;36:69–75. [12] Tang W, Guo X, McClure JC, Murr LE. J Mater Process Manufac Sci 1998;7:163–72. [13] McKimpson MG, Scott TE. Mater Sci Eng A 1989;107:93–106. [14] Kouzeli M, Weber L, San Marchi C, Mortensen A. Acta Mater 2001;49:3699-09.