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Pore structures of high-porosity NiTi alloys made from elemental powders with NaCl temporary space-holders
Materials Letters 63 (2009) 2402–2404
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 ev i e r. c o m / l o c a t e / m a t l e t
Pore structures of high-porosity NiTi alloys made from elemental powders with NaCl temporary space-holders Xingke Zhao a,⁎, Hongbo Sun a, Lan Lan a, Jihua Huang a, Hua Zhang a, Yu Wang b a b
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing, 100083, China Naval Academy of Armament, Beijing 100073, China
a r t i c l e
i n f o
Article history: Received 24 June 2009 Accepted 28 July 2009 Available online 8 August 2009 Keywords: Powder metallurgy Shape memory materials Scanning electron microscopy Microstructure Porosity Temporary space-holder
a b s t r a c t The unusual pseudo-elasticity and shape memory effect make NiTi alloys promising energy absorption materials. In the present study, powders of Ti, Ni and NaCl particles were mixed and cold-pressed into green ingots and green ingots were then desalted and sintered in vacuum to form high-porosity NiTi alloy specimens with porosity up to 90%. Microstructure observation shows that two kinds of pores with sizes of 200–400 μm and 10–50 μm respectively are well-distributed in these high-porosity NiTi alloys. Characteristics of pores were studied and formation mechanism was discussed. © 2009 Elsevier B.V. All rights reserved.
1. Introduction High-porosity materials generally exhibit a higher damping capacity than their corresponding compact materials. So the highporosity materials, such as aluminum foams, have been developed and used to fabricate damping components [1,2]. However, the pore structures of this kind of metallic foam will be destroyed as a plastic deformation occurs during its energy absorbing process, then the damping component is out of use and must be replaced by a new one before its next energy absorbing process. It is inconvenience to use these damping components in long-distance vehicles. One key technique toward this goal is the development and implementation of a smart material, which can recover itself from distorted state automatically. NiTi alloy shape memory alloys (SMA) are famous functional structural materials that have found more and more applications in many areas [3]. The shape memory effect (SME) and pseudo-elasticity, two major properties of SMA associated with the thermal–elastic martensitic phase transformation will enable it to recover from distorted state conveniently. Since the melting point of NiTi alloys is much higher than that of aluminum or magnesium, preparing techniques of a high-porosity NiTi alloy are more difficult than those of aluminum foams or magnesium foams, which can be prepared by adding a gas-release foaming agent to the molten metal. Powder metallurgy is widely used for the preparation of porous NiTi alloys, and porous NiTi alloys with ⁎ Corresponding author. Tel.: +86 10 62334859. E-mail address: [email protected] (X. Zhao). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.07.069
porosity up to 60%–65% have been obtained [4,5]. In recent years temporary space-holders (NH4HCO3, or CO(NH2)2 ) have been used to adjust porosity and pore structures [6,7]. However, the rapid decomposition of NH4HCO3 at a lower temperature led to an imprecise control of porosity and pore structures, as well as an unsatisfactory repeatability of mechanical properties of the porous alloys. In addition, the presence of sharp corners or notches of the pores in the alloys, which are due to the needle-like or flake NH4HCO3 space-holder particles, can obviously result in stress concentration and thus deteriorate mechanical properties of the porous NiTi SMAs [8]. In the present paper, we have prepared high-porosity TiNi alloys by using NaCl particles as temporary space-holders. 2. Experimental Equal molar fraction titanium and nickel powders with particle sizes of 50–75 μm (purity N 99.5%) and 4–7 μm (purity N 99.9%) respectively and NaCl particles (with size of 500–800 μm and purity N 99.5%) were weighed and mixed together. The powder mixtures were pressed at room temperature with pressure of 200– 600 MPa into cylindrical ingots of 24 mm in diameter and 15 mm in height. The ingots were then put into water to dissolve their NaCl particles. After desalinization was completed, sintering process was carried out in a vacuum furnace by heating to 950 °C for 2 h, furnacecooled to 200 °C, and water quenched to the room temperature. Porosity was calculated by the formula: Porosity = (1 − ρ / ρ0) × 100%, where ρ is the density of porous NiTi and ρ0 is the density of corresponding bulk NiTi alloys (approximately 6.45 g/cm3 ). The pore structures were analyzed with scanning-electron microscopy (SEM,
X. Zhao et al. / Materials Letters 63 (2009) 2402–2404
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JEOL JSM-6335F) and optical microscope (Leica DMIRM). The phase constituent of the product was determined by X-ray diffraction analysis (XRD, Philips PW3710) using Cu Kα radiation. 3. Results and discussions 3.1. Porosity Porosity of high-porosity NiTi alloy specimens as the function of NaCl additive amount and ingot forming pressure is shown in Fig. 1. Porosity increases linearly with NaCl additive amount under the same forming pressure. And at the same NaCl additive amount, the porosity decreases slightly with increasing the ingots forming pressure. It indicates that porosity depends mainly on NaCl additive amount. A porous NiTi alloy specimen with a high-porosity up to 90% can be prepared by this NaCl temporary space-holder process. 3.2. Pore structures The macroscopic appearances of green (before sintering) cylindrical ingot specimens are shown in Fig. 2. Fig. 2 (a) and (b) represents green ingot specimens before and after desalinization, respectively. Before desalinization the green ingot shows a compact smooth surface, see Fig. 2 (a). After desalinization, the ingot specimen shows a rough surface and a porous section surface (Fig. 2 (b)). The cavities in surface of a desalinized ingot were formed as NaCl particles dissolved during desalinization. As NaCl particles have mixed uniformly with metal powder mixture, so the pores distribute uniformly in the green ingot specimen. During sintering, a metallurgical reaction occurred between powders of titanium and nickel, resulting in the separate metal powders joining together to form a continuous NiTi alloy metal wall. As the ingots were sintered at a temperature below the melting point of NiTi alloys, it is a solid phase metallurgical process. And the pores in the ingot can maintain their original structures during sintering process. Most of pores are three-dimensionally interconnected, and the high-porosity NiTi alloys show a sponge-like structure, see Fig. 2 (c). SEM photos of the section of a high-porosity NiTi alloy specimen are shown in Fig. 3. Fig. 3 a) is a low amplification image. It shows many black pores with size of 200–400 μm islanded by white metal walls. Many of these island pores have clear edges and corners, the same as NaCl space-holder particles. It would prove that the black pores in the high-porosity NiTi alloy specimen are produced by NaCl space-holding function. Therefore, pore characteristics of a porous NiTi alloy are dominated by the shape and amount of NaCl particle additive. Fig. 3 (a) also shows that the white metal walls are not perfect, but small flaws can be seen in the white metal walls. To identify those Fig. 2. Optical macroscopic photos of green ingots (a, b) and high-porosity NiTi alloy (c). a) Before desalinization, b) after desalinization and before sintering, and c) after sintering.
Fig. 1. Porosity as a function of NaCl amount and ingot forming pressure.
flaws higher amplification pictures have been taken and shown in Fig. 3 b) and c). From the higher amplification photos one can see that the flaws on white metal walls are micro-pores with the sizes of a few microns. These micro-pores are thought to be formed by the Kirkendall effect which exists universally in powder metallurgical products. As mentioned above, there are two kinds of pores in the present high-porosity NiTi alloy: the macroscopic island pores produced by NaCl particles space-holding function and the microscopic ones distributing in metal walls produced by the Kirkendall effect. The NaCl space-holding function is the main porosity forming mechanism in this process. The present research may be greatly helpful for fabricating high-porosity NiTi alloys, new damping materials.
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X. Zhao et al. / Materials Letters 63 (2009) 2402–2404
Fig. 4. XRD spectrum of high-porosity NiTi alloys.
fully removed during desalinization process, and 2) the elemental Ni and Ti have synthesized completely under the present experimental condition. 4. Conclusions In present paper, high-porosity NiTi alloys were fabricated by powder metallurgical process with NaCl particles as a temporary spaceholder. The pore structures as well as their formation mechanism have been studied. The main conclusions are as follows. 1) There are two kinds of pores formed in porous NiTi alloys, sized of few hundreds of microns and few microns, respectively. The bigger pores are islanded by NiTi metal walls and the smaller pores distribute in NiTi metal walls. The porosity of a porous NiTi alloy is dominated by the bigger pores. 2) Porosity increases linearly with NaCl additive amount under the same forming pressure. And at the same NaCl additive amount, the porosity decreases slightly with increasing the forming pressure. NiTi alloys with a high-porosity up to 90% can be prepared by this NaCl temporary space-holder process. 3) NaCl additive has been removed totally during desalinization process, and the elemental Ni and Ti have synthesized completely after sintering in a vacuum furnace at 950 °C for 2 h. References [1] [2] [3] [4] [5]
Banhart J, Baurneister J, Weber M. Mat Sci Een A-STruct 1996;205:221–8. Xie ZK, Tane M, Hyun SK, Okuda Y, Nakajima H. Mat Sci Een A-STruct 2006;417:129–33. Songa G, Ma N, Li HN. Eng Struct 2006;28:1266–74. Li BY, Rong LJ, Li YY, Gjunter VE. Acta mater 2000;48:3895–904. Tay BY, Goh CW, Gu YW, Lim CS, Yong MS, Ho MK, Myint MH. J Mater Process Tech 2008;202:359–64. [6] Patil KC, Aruna ST, Mimani T. Curr Opin Solid State Mater Sci 2002;6:507–12. [7] Barrabés M, Sevilla P, Planell JA, Gil FJ. Sci Een C-Bios 2008;28:23–7. [8] Li DS, Zhang YP, Ma X, Zhang XP. J Alloy Compd 2009;474:l1–15.
Fig. 3. SEM photos of the section of high-porosity NiTi alloys. a) Low amplification pictures, b) higher amplification pictures of (a), and c) higher amplification pictures of (b).
3.3. Phase constitution X-ray analysis result shows that the high-porosity NiTi alloy consists of a dominant NiTi phase and few Ni3Ti, NiTi2, and no NaCl phase, see Fig. 4. It confirms the two facts: 1) NaCl additive has been
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 ev i e r. c o m / l o c a t e / m a t l e t
Pore structures of high-porosity NiTi alloys made from elemental powders with NaCl temporary space-holders Xingke Zhao a,⁎, Hongbo Sun a, Lan Lan a, Jihua Huang a, Hua Zhang a, Yu Wang b a b
School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing, 100083, China Naval Academy of Armament, Beijing 100073, China
a r t i c l e
i n f o
Article history: Received 24 June 2009 Accepted 28 July 2009 Available online 8 August 2009 Keywords: Powder metallurgy Shape memory materials Scanning electron microscopy Microstructure Porosity Temporary space-holder
a b s t r a c t The unusual pseudo-elasticity and shape memory effect make NiTi alloys promising energy absorption materials. In the present study, powders of Ti, Ni and NaCl particles were mixed and cold-pressed into green ingots and green ingots were then desalted and sintered in vacuum to form high-porosity NiTi alloy specimens with porosity up to 90%. Microstructure observation shows that two kinds of pores with sizes of 200–400 μm and 10–50 μm respectively are well-distributed in these high-porosity NiTi alloys. Characteristics of pores were studied and formation mechanism was discussed. © 2009 Elsevier B.V. All rights reserved.
1. Introduction High-porosity materials generally exhibit a higher damping capacity than their corresponding compact materials. So the highporosity materials, such as aluminum foams, have been developed and used to fabricate damping components [1,2]. However, the pore structures of this kind of metallic foam will be destroyed as a plastic deformation occurs during its energy absorbing process, then the damping component is out of use and must be replaced by a new one before its next energy absorbing process. It is inconvenience to use these damping components in long-distance vehicles. One key technique toward this goal is the development and implementation of a smart material, which can recover itself from distorted state automatically. NiTi alloy shape memory alloys (SMA) are famous functional structural materials that have found more and more applications in many areas [3]. The shape memory effect (SME) and pseudo-elasticity, two major properties of SMA associated with the thermal–elastic martensitic phase transformation will enable it to recover from distorted state conveniently. Since the melting point of NiTi alloys is much higher than that of aluminum or magnesium, preparing techniques of a high-porosity NiTi alloy are more difficult than those of aluminum foams or magnesium foams, which can be prepared by adding a gas-release foaming agent to the molten metal. Powder metallurgy is widely used for the preparation of porous NiTi alloys, and porous NiTi alloys with ⁎ Corresponding author. Tel.: +86 10 62334859. E-mail address: [email protected] (X. Zhao). 0167-577X/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2009.07.069
porosity up to 60%–65% have been obtained [4,5]. In recent years temporary space-holders (NH4HCO3, or CO(NH2)2 ) have been used to adjust porosity and pore structures [6,7]. However, the rapid decomposition of NH4HCO3 at a lower temperature led to an imprecise control of porosity and pore structures, as well as an unsatisfactory repeatability of mechanical properties of the porous alloys. In addition, the presence of sharp corners or notches of the pores in the alloys, which are due to the needle-like or flake NH4HCO3 space-holder particles, can obviously result in stress concentration and thus deteriorate mechanical properties of the porous NiTi SMAs [8]. In the present paper, we have prepared high-porosity TiNi alloys by using NaCl particles as temporary space-holders. 2. Experimental Equal molar fraction titanium and nickel powders with particle sizes of 50–75 μm (purity N 99.5%) and 4–7 μm (purity N 99.9%) respectively and NaCl particles (with size of 500–800 μm and purity N 99.5%) were weighed and mixed together. The powder mixtures were pressed at room temperature with pressure of 200– 600 MPa into cylindrical ingots of 24 mm in diameter and 15 mm in height. The ingots were then put into water to dissolve their NaCl particles. After desalinization was completed, sintering process was carried out in a vacuum furnace by heating to 950 °C for 2 h, furnacecooled to 200 °C, and water quenched to the room temperature. Porosity was calculated by the formula: Porosity = (1 − ρ / ρ0) × 100%, where ρ is the density of porous NiTi and ρ0 is the density of corresponding bulk NiTi alloys (approximately 6.45 g/cm3 ). The pore structures were analyzed with scanning-electron microscopy (SEM,
X. Zhao et al. / Materials Letters 63 (2009) 2402–2404
2403
JEOL JSM-6335F) and optical microscope (Leica DMIRM). The phase constituent of the product was determined by X-ray diffraction analysis (XRD, Philips PW3710) using Cu Kα radiation. 3. Results and discussions 3.1. Porosity Porosity of high-porosity NiTi alloy specimens as the function of NaCl additive amount and ingot forming pressure is shown in Fig. 1. Porosity increases linearly with NaCl additive amount under the same forming pressure. And at the same NaCl additive amount, the porosity decreases slightly with increasing the ingots forming pressure. It indicates that porosity depends mainly on NaCl additive amount. A porous NiTi alloy specimen with a high-porosity up to 90% can be prepared by this NaCl temporary space-holder process. 3.2. Pore structures The macroscopic appearances of green (before sintering) cylindrical ingot specimens are shown in Fig. 2. Fig. 2 (a) and (b) represents green ingot specimens before and after desalinization, respectively. Before desalinization the green ingot shows a compact smooth surface, see Fig. 2 (a). After desalinization, the ingot specimen shows a rough surface and a porous section surface (Fig. 2 (b)). The cavities in surface of a desalinized ingot were formed as NaCl particles dissolved during desalinization. As NaCl particles have mixed uniformly with metal powder mixture, so the pores distribute uniformly in the green ingot specimen. During sintering, a metallurgical reaction occurred between powders of titanium and nickel, resulting in the separate metal powders joining together to form a continuous NiTi alloy metal wall. As the ingots were sintered at a temperature below the melting point of NiTi alloys, it is a solid phase metallurgical process. And the pores in the ingot can maintain their original structures during sintering process. Most of pores are three-dimensionally interconnected, and the high-porosity NiTi alloys show a sponge-like structure, see Fig. 2 (c). SEM photos of the section of a high-porosity NiTi alloy specimen are shown in Fig. 3. Fig. 3 a) is a low amplification image. It shows many black pores with size of 200–400 μm islanded by white metal walls. Many of these island pores have clear edges and corners, the same as NaCl space-holder particles. It would prove that the black pores in the high-porosity NiTi alloy specimen are produced by NaCl space-holding function. Therefore, pore characteristics of a porous NiTi alloy are dominated by the shape and amount of NaCl particle additive. Fig. 3 (a) also shows that the white metal walls are not perfect, but small flaws can be seen in the white metal walls. To identify those Fig. 2. Optical macroscopic photos of green ingots (a, b) and high-porosity NiTi alloy (c). a) Before desalinization, b) after desalinization and before sintering, and c) after sintering.
Fig. 1. Porosity as a function of NaCl amount and ingot forming pressure.
flaws higher amplification pictures have been taken and shown in Fig. 3 b) and c). From the higher amplification photos one can see that the flaws on white metal walls are micro-pores with the sizes of a few microns. These micro-pores are thought to be formed by the Kirkendall effect which exists universally in powder metallurgical products. As mentioned above, there are two kinds of pores in the present high-porosity NiTi alloy: the macroscopic island pores produced by NaCl particles space-holding function and the microscopic ones distributing in metal walls produced by the Kirkendall effect. The NaCl space-holding function is the main porosity forming mechanism in this process. The present research may be greatly helpful for fabricating high-porosity NiTi alloys, new damping materials.
2404
X. Zhao et al. / Materials Letters 63 (2009) 2402–2404
Fig. 4. XRD spectrum of high-porosity NiTi alloys.
fully removed during desalinization process, and 2) the elemental Ni and Ti have synthesized completely under the present experimental condition. 4. Conclusions In present paper, high-porosity NiTi alloys were fabricated by powder metallurgical process with NaCl particles as a temporary spaceholder. The pore structures as well as their formation mechanism have been studied. The main conclusions are as follows. 1) There are two kinds of pores formed in porous NiTi alloys, sized of few hundreds of microns and few microns, respectively. The bigger pores are islanded by NiTi metal walls and the smaller pores distribute in NiTi metal walls. The porosity of a porous NiTi alloy is dominated by the bigger pores. 2) Porosity increases linearly with NaCl additive amount under the same forming pressure. And at the same NaCl additive amount, the porosity decreases slightly with increasing the forming pressure. NiTi alloys with a high-porosity up to 90% can be prepared by this NaCl temporary space-holder process. 3) NaCl additive has been removed totally during desalinization process, and the elemental Ni and Ti have synthesized completely after sintering in a vacuum furnace at 950 °C for 2 h. References [1] [2] [3] [4] [5]
Banhart J, Baurneister J, Weber M. Mat Sci Een A-STruct 1996;205:221–8. Xie ZK, Tane M, Hyun SK, Okuda Y, Nakajima H. Mat Sci Een A-STruct 2006;417:129–33. Songa G, Ma N, Li HN. Eng Struct 2006;28:1266–74. Li BY, Rong LJ, Li YY, Gjunter VE. Acta mater 2000;48:3895–904. Tay BY, Goh CW, Gu YW, Lim CS, Yong MS, Ho MK, Myint MH. J Mater Process Tech 2008;202:359–64. [6] Patil KC, Aruna ST, Mimani T. Curr Opin Solid State Mater Sci 2002;6:507–12. [7] Barrabés M, Sevilla P, Planell JA, Gil FJ. Sci Een C-Bios 2008;28:23–7. [8] Li DS, Zhang YP, Ma X, Zhang XP. J Alloy Compd 2009;474:l1–15.
Fig. 3. SEM photos of the section of high-porosity NiTi alloys. a) Low amplification pictures, b) higher amplification pictures of (a), and c) higher amplification pictures of (b).
3.3. Phase constitution X-ray analysis result shows that the high-porosity NiTi alloy consists of a dominant NiTi phase and few Ni3Ti, NiTi2, and no NaCl phase, see Fig. 4. It confirms the two facts: 1) NaCl additive has been