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Fabrication of nano-porous γ-Al2O3 layers on porous Ti-48Al-6Nb alloys
Materials and Design 109 (2016) 700–708
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Fabrication of nano-porous γ-Al2O3 layers on porous Ti-48Al-6Nb alloys Fan Wang a, Jun-Pin Lin a, Yong-Feng Liang a,⁎, Shun-Li Shang b, Zi-Kui Liu b a b
State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, USA
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• Nano-porous γ-Al2O3 layers were fabricated by soaking in 2.0 mol/L NaOH solution for 120 h. • Nano-pore-diameter was 60 ± 10 nm and the thickness of layers was 200 nm. • Total pore area increased nearly to 0.25m2/g due to nano-porous layer. • The porous alloys transformed to gradient porous materials with nano and micron range pores.
a r t i c l e
i n f o
Article history: Received 9 January 2016 Received in revised form 15 July 2016 Accepted 20 July 2016 Available online 21 July 2016 Keywords: Porous TiAl alloys Nano-porous γ-Al2O3 Total pore area Polarization
a b s t r a c t Nano-porous materials are spreading widely in various fields, such as electrochemistry, biochemistry, adsorption, and catalysis. In the present work, nano-porous γ-Al2O3 layers with 200 nm thickness have been fabricated on porous Ti-48Al-6Nb alloys by soaking in NaOH solution. Corrosion products of γ-Al2O3 with 200 nm thickness form after 48 h and begin to transform to nano-porous structures at 72 h in the areas with a smaller radius of curvature. Then the nano-pores shrink and reach the final size of 60 ± 10 nm within 120 h. Increasing the concentration of NaOH solution or soaking time accelerates the formation of nano-porous layers and the best situation is soaking for 120 h in a 2.0 mol/L NaOH solution. Meanwhile, the nano-porous γ-Al2O3 layers improve the electrochemical corrosion resistance of the porous alloys due to the more positive self-corrosion potential/current and the right-moved passive region. The NaOH corrosion method to fabricate nano-porous γ-Al2O3 layers is much easier than other methods and the porous alloys possess the better corrosion resistance and absorbability thanks to the nano-porous γ-Al2O3 layers and the micron-porous Ti-48Al-6Nb matrix. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Porous intermetallics attract a worldwide attention in the last decades due to their good performances, such as the high specific stiffness, ⁎ Corresponding author. E-mail addresses: [email protected] (F. Wang), [email protected] (Y.-F. Liang).
http://dx.doi.org/10.1016/j.matdes.2016.07.105 0264-1275/© 2016 Elsevier Ltd. All rights reserved.
high energy absorption capacity, and good oxidation resistance and corrosion resistance among foam metals. In our group, more attention has been paid for the high Nb-containing TiAl alloys because of the attractive properties of high melting point, good oxidation resistance, high specific strength and modulus due to the high Nb contents [1–6]. Porous Ti-48Al-6Nb (at.%) alloys were synthesized by powder metallurgy (PM) method using elemental powders [7]. The Kirkendall effect and phase transformation contribute to the pore-formation and the
F. Wang et al. / Materials and Design 109 (2016) 700–708
pores have mainly micron scale [8]. Although the porous Ti-48Al-6Nb coatings were fabricated by cold gas spraying and reactive sintering by Yang et al. [9], the pore size is still in the micron range of 1.8 μm due to the difficulty to control the pore size in nanometer range. However, nano-porous or nano-coatings have been fabricated by various methods for other materials. Nano Cu coatings were prepared on porous Si by acid etching the Al-Si alloy powder by Li et al. [10] and nano-porous bioactive glass film was prepared by the sol-gel method by Ma et al. [11]. The nano-porous structures have implications for bioactivity [12], electrochemical properties [13] and batteries efficiency [14]. In addition, the nano-porous γ-Al2O3 has special properties. The γ-Al2O3 nano-structured hollow microspheres, used to treat dye wastewater, were synthesized by Fang et al. [15] in terms of methylene blue as structure directing agent. The nano-Al2O3 with porous structure, used to separate iridium ions from aqueous solutions due to its better adsorption properties, was synthesized by Zhang et al. [16]. Also the nano-Al2O3 can be used as the excellent catalyst carrier. The electrochemical performance of LiNi0.6Co0.2Mn0.2O2 powder has been improved greatly by coating with nano-Al2O3 particles via ultrasonic coating [17]. However, little is known about the nano-porous structure of TiAl based alloys. In the present work, the nano-porous γ-Al2O3 layers are fabricated on porous Ti48Al-6Nb alloys by soaking in NaOH solution. According to the aforementioned nano-Al2O3, the present porous Ti-48Al-6Nb alloys with nano-porous γ-Al2O3 layers have significant contributions in many fields. 2. Experimental procedures Porous Ti-48Al-6Nb (at.%) alloys were fabricated by the powder metallurgy (PM) under a pressure of 200 MPa [18]. Purity and particle sizes of elemental powders are shown in Table 1. The schematic procedure of heat-treatment is shown in Fig. 1. Air and vapor escaped from compacts were at the temperature of 120 °C and the initial temperature of Ti-Al reaction was 600 °C [19]. Samples were heat-treated at 900 and 1350 °C for 3 h, where Ti-Al and Nb-Al phase transformations occurred with much pores formed [20]. Finally, the porous Ti-48Al-6Nb alloys were achieved when the temperature decreased to room temperature in vacuum. The nano-porous γ-Al2O3 layers were synthesized on the skeleton surface of porous Ti-48Al-6Nb alloys by soaking in NaOH solution for different times. The NaOH solutions used in our research were 0.5, 2.0 and 3.0 mol/L and the soaking times were 24, 48, 72, 96, and 120 h. Xray diffraction (XRD, Multipurpose X-ray Diffractometer TTR III) was used to analyze the phases on the skeleton surface using an incident angle of 1.5°. The pore structure was examined by scanning electron microscopy (SEM, ZEISS SUPRA 55) and the pore parameters were measured by mercury intrusion porosimetry (MIP, Quantachrome AUTOSCAN-33) [21]. Polarization curves were achieved by the three electrodes method [22,23].
Fig. 1. Schematic procedure of heat treatment.
some areas are relatively flat and some are bent. After soaking in NaOH solution, the nano-porous layers emerged on the surface of these skeletons. The surface of porous skeletons present rough and stripy in the 0.5 mol/L NaOH solution for 120 h (see Fig. 3a). A representative area magnified 50,000 times is shown in Fig. 3b. The nano-porous structures are obvious wherever the porous skeletons are flat or bent. Some nanopores are deeper and some shallower, which will be analyzed in next Section. The morphology of porous Ti-48Al-6Nb alloys in the 3.0 mol/L NaOH solution is shown in Fig. 3c and d. Compared to the 0.5 mol/L solution case, lots of nano-threads emerge on the nano-porous layers with different orientations. Some nano-threads are grown from one root and some are grown out of order. In order to observe the nano-porous structure more clearly, the higher magnification and fracture photos are presented in Fig. 4. As is shown in Fig. 4a, the nano-pores are covered and connected by each other and most of them are open pores. The pore sizes distributed at 60 ± 10 nm are measured roughly by SEM and the pores present an irregular shape unlike the micron-sized ones with smooth edges as shown in Fig. 3a. Due to the long soaking time (N120 h) in a higher concentration of NaOH solution (e.g. 3.0 mol/L), more nano-threads emerge on nano-porous layers and they are measured roughly the same as the nano-pores size indicating that the nano-threads are grown from inside of the nano-pores (see Fig. 4b). The accurate pore size will be measured by the MIP method later. From the fracture photos shown in Fig. 4c and d, the growth orientation of the nano-porous layers is nearly perpendicular to the Ti-Al matrix with fully lamellar phases and there exist obvious boundaries between them. The nano-porous layers have an uniform thickness of 200 nm,
3. Results and discussion 3.1. Morphology of nano-porous γ-Al2O3 layers on porous Ti-48Al-6Nb alloys The morphology of porous Ti-48Al-6Nb alloys after sintering is shown in Fig. 2. There exist a lot of pores with different pore diameters among the Ti-Al skeletons. The inner skeletons present a sleek polygon, Table 1 Purity and particle size of the elemental powders. Element
Purity (%)
Particle size (μm)
Nb Ti Al
N99.9 N99.9 N99.9
48–75 48–75 48–75
701
Fig. 2. SEM-SE image of porous Ti-48Al-6Nb alloys.
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Fig. 3. SEM-SE images of nano-porous layers on porous Ti-48Al-6Nb alloys in (a, b) 0.5 mol/L and (c, d) 3.0 mol/L.
which will be proved in the next section. In addition, the nano-porous layers are also indicated by γ-Al2O3 from the surface XRD patterns (see Fig. 5). 3.2. Pore-diameter and total pore area distribution of nano-porous γ-Al2O3 layers on porous Ti-48Al-6Nb alloys From the above analyses of morphology and XRD, the pores size is 60 ± 10 nm and the γ-Al2O3 layers thickness is about 200 nm by SEM. However, in order to get accurate size, the MIP method is used. The pore-diameter distributions in different concentration of NaOH solutions for 24, 48, 72, 96, 120 h are shown in Fig. 6. The main pore sizes
distribute at 8–10 μm, where the cumulative pore volume curves increase seriously. These micron-pores are caused by Kirkendall effect and phase transformations [24] and they change scarcely in different densities of NaOH solutions for 120 h, indicating that the NaOH solution does not influence the micron-pore structures, but there emerge cumulative pore volume curves rising at the range of nanometer. In the 0.5 mol/L NaOH solution for 24, 48, and 72 h, the curves show straight lines indicating no nano-pores forming. They are not appeared until for 96 and 120 h, and the nano-pore sizes distribute from 100 to 150 nm for 96 h and from 60 to 120 nm for 120 h. It is indicated that the nano-pores shrink gradually and reach the final size with time going on. Compared to the 0.5 mol/L case, the nano-pores appear 24 h
Fig. 4. SEM-SE images of nano-porous structure and fracture photos: a) nano-pores; b) nano-threads; c) and d) fracture structures.
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Fig. 5. XRD pattern of nano-porous layers.
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earlier in the 2.0 mol/L NaOH solution. Meanwhile, the nano-pore size distribute from 80 to 90 nm for the 72 h case and reach to 60 nm after 96 h according to the results from morphology analysis. The same results are achieved in the 3.0 mol/L NaOH solution for different hours that the nano-pores emerge after 72 h and reach the final size of 60 nm after 96 h. It indicates that the increase of the concentration of NaOH solutions accelerates the formation of nano-pores but an excessive increase is useless. The 2.0 mol/L is relatively enough for the nano-pores formation. In order to identify the above results and find the influence of nanoporous layers on the total pore area, the distributions of pore area are presented in Fig. 7. Due to the formation of 8–10 μm pores, total pore area increases about 0.04 m2/g and is not affected by concentration of NaOH solutions or soaking time. At the range of nanometer, nanopores do not appear until soaking in the 0.5 mol/L NaOH solution for 96 h. The nano-pores size shrinks from 140 to 70 nm and the total pore area finally reaches to 0.084 m2/g. In this concentration of NaOH solution, the increase of total pore area caused by nano-pores is almost equal to that caused by micro-pores, that is also 0.04 m2/g. However,
Fig. 6. Pore-diameter distribution of nano-porous layers in 0.5, 2.0 and 3.0 mol/L NaOH solutions for different times.
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Fig. 7. Total pore area distribution of nano-porous layers in 0.5, 2.0 and 3.0 mol/L NaOH solutions for different times.
nano-pores appear 24 h earlier in the 2.0 mol/L NaOH solution. The pores reached the final size 60 nm after 96 h according to the porediameter distribution results. Differing from the 0.5 mol/L case, total pore area finally reaches to 0.25 m2/g due to the formation of nanopores. The increase caused by nano-pores is 5 times larger than that caused by micron-pores, one is 0.2 m2/g and the other is 0.04 m2/g. The same phenomena emerged in the 3.0 mol/L NaOH solution indicate that nano-pores appear after 72 h. Total pore area reaches the maximum 0.23 m2/g for 96 h but decreases to 0.154 m2/g for 120 h due to the growth of nano-threads jammed nano-pores shown in Fig. 3c and Fig. 4b. In summary, increasing the concentration of NaOH solution accelerates the formation of nano-pores and shorts the time to reach the final pore size. However, excessively increasing the concentration or soaking time is not beneficial for the nano-porous structures, leading to the decrease of total pore area instead. So the best concentration of NaOH solution and the soaking time are 2.0 mol/L and 120 h, respectively.
3.3. Formation mechanism of nano-porous γ-Al2O3 layers on porous Ti48Al-6Nb alloys Before exploring the mechanism, it should be make sure no other composition layers exist on the surface of the porous Ti-48Al-6Nb skeletons. The fracture and surface photos before and after soaking in the NaOH solution for 120 h are shown in Fig. 8. It is indicated that there was no other layers forming on the skeleton surface. EDS analysis indicates that Point A is Ti-Al phases (see Table 2), while Point B is Ti-Al-O phases, indicating that γ-Al2O3 layers had not formed until soaking in the NaOH solution. From Fig. 8c and d, the skeleton surface turns to be rough after soaking in the NaOH solution and at the same time the phase interfaces turn to be more obvious. Fracture photos after soaking in the 2.0 mol/L NaOH solution for 12 h are shown in Fig. 9 and there exist many areas containing both nanopores and flat layers. It is similar to the process where the flat layers are transforming to nano-porous layers. Combined with the total pore
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Fig. 8. SEM-SE images of the fracture (a, b) and surface (c, d) before and after soaking in NaOH solution.
Table 2 EDS analysis of Points A and B. Point
A B
Element Ti (at.%)
Al (at.%)
Nb (at.%)
O (at.%)
57.77 31.69
41.94 30.37
0.30 6.61
0 31.32
area distribution shown in Fig. 7, the phenomena indicate that there was no nano-pores just the flat γ-Al2O3 layers in previous hours and then nano-pores with bigger size begins to appear. With time going on, nano-pores shrink and reach the final size and then extend to all flat γ-Al2O3 layers. There is another interesting phenomenon (see red
arrows) that the nano-pores emerge primarily in the strips, concave and convexity. The common character is that they have a smaller radius of curvature. In order to identify the formation mechanism, Ti-48Al-6Nb ingot with scratches “WF” on its surface is soaking in the 2.0 mol/L NaOH solution for 120 h. The ingot surface becomes yellowish-brown (see Fig. 10). Fig. 11b is the magnification of white cube in Fig. 11a and there emerge some scratches at the area of “WF” caused by NaOH corrosion. Nano-pores emerge obviously on the bottom of scratch (see Fig. 11c) and no pores emerge on the flat layers just presenting dim outlines. It is because the bottom of scratches had a smaller radius of curvature than the flat areas, indicating that the analysis about mechanism is accurate.
Fig. 9. SEM-SE images of fracture after soaking in 2.0 mol/L NaOH solution for 12 h. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
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In conclusion, the flat γ-Al2O3 layers firstly form on the porous skeletons in 2.0 mol/L NaOH solution during 12-48 h and then begin to transform to bigger nano-pores primarily from the area with smaller radius of curvature, such as strips, concave and convexity. Then nanopores shrink and reach the final size 60 ± 10 nm. The simulative schematic is shown in Fig. 12.
3.4. Polarization performance of porous Ti-48Al-6Nb alloys with nano-porous layers
Fig. 10. Appearance of Ti-48Al-6Nb ingot soaking in 2.0 mol/L NaOH solution for 120 h.
Due to nano-porous γ-Al2O3 layers, the polarization performance of porous Ti-48Al-6Nb alloys should be improved. It is predicted that the layers protect the porous Ti-48Al-6Nb alloys from electrochemical corrosion. Polarization curves of the porous Ti-48Al-6Nb alloys with and
Fig. 11. SEM-SE images of Ti-48Al-6Nb ingot surface soaking in 2.0 mol/L NaOH solution for 120 h: (c) is the magnification of Area A and (d) is Area B.
Fig. 12. Formation schematic of nano-porous layers on porous Ti-48Al-6Nb alloys.
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the edge of pits that the fully lamellar phases are covered by nano-porous γ-Al2O3 layers and just the top is corroded out. That phenomena indicate that the nano-porous γ-Al2O3 layers protect the Ti-Al matrix from electrochemical corrosion in some certain areas. The protection phenomena are also observed at the narrow skeletons that the nano-porous γ-Al2O3 layers protects some skeletons from corrosion and are separated from the Ti-Al matrix (see Fig. 14d). In this photo, the thickness of nano-porous layers is observed obviously as about 200 nm, which is agreed by Fig. 4d. In summary, the 200 nm thick nano-porous γ-Al2O3 layers with 60 ± 10 nm pores improve electrochemical corrosion resistance of the porous Ti-48Al-6Nb alloys. 3.5. Discussion of advantages and applications
Fig. 13. Polarization curves of porous Ti-48Al-6Nb alloys with and without nano-porous layers.
without nano-porous γ-Al2O3 layers are shown in Fig. 13. For the original porous alloys, the obvious passive region exists at the range of −1.0 V to −0.5 V. After the passive region, current increases with increasing potential and the original porous alloys step into an obvious electrochemical corrosion stage. However, polarization curve of porous alloys with nano-porous layers have an obvious passive region from −0.25 V to 0.25 V. The current decreases more obviously with increasing potential. Due to nano-porous γ-Al2O3 layers, the porous Ti-48Al6Nb alloys could not be electrochemical corroded easily at such high potential. So the nano-porous γ-Al2O3 layers improve substantially their electrochemical corrosion resistance, which was predicted to be well used in electrochemical field as well as larger total pore area [13,25]. Polarization morphology of porous Ti-48Al-6Nb alloys with nanoporous γ-Al2O3 layers is shown in Fig. 14. For such a high potential, the porous alloys are destroyed by high current with many electrochemistry corrosion pits appearing on the surface (see Fig. 14a). Fully lamellar phases of Ti-Al alloys are exposed at the edge of pits (see Fig. 14b). Some lamellas are separated by each other and some happened to collapse. Morphology shown in Fig. 14c is observed at
Nanotechnology is spreading vastly in various demanding fields of engineering and medicines such as aerospace, defense, automobiles, electronics, materials chemistry, energy, environment, information and communication, consumer goods and biotechnology [26]. The processing methods of nano-Al2O3 and its composites are various, such as powder metallurgy, mechanical milling, casting, pressure infiltration, wet chemical method, friction stir process and so on. The disadvantages and troubles about the process focus on the longer processing time, non-uniform diameter, high energy consumption, sophisticated equipment and harm to environment. However, in the present research, the uniform nano-porous γ-Al2O3 layers are fabricated on the porous Ti48Al-6Nb alloys by just soaking in the NaOH solution for 120 h. Our method is easier to operate just using a container and the finally gained saline solution is not harmful to environment. During the process, neither electric nor heating power is needed. Combining the advantages of nano-porous γ-Al2O3 layers and porous Ti-48Al-6Nb alloys, the materials can be used as adsorption, filter and so on. 4. Conclusion Nano-porous γ-Al2O3 layers with 200 nm thickness are fabricated on porous Ti-48Al-6Nb alloys by soaking in NaOH solution. Increasing the concentration of NaOH solution can accelerate the fabrication of nanoporous γ-Al2O3 layers. Appropriately increasing the soaking time makes the nano-pores forming entirely but excessively long soaking time leads to the growth of nano-threads from pores inside decreasing
Fig. 14. SEM-SE images of porous Ti-48Al-6Nb alloys with nano-porous γ-Al2O3 layers after polarization.
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total pore area. So the most appropriate concentration and time are 2.0 mol/L and 120 h, respectively. The mechanism schematic is that the flat γ-Al2O3 layers with 200 nm thickness formed after 48 h and begin to transform to nano-porous structures at 72 h from the areas with a smaller radius of curvature and then extend to flat areas making the nano-porous γ-Al2O3 layers forming entirely within 120 h. Nanoporous γ-Al2O3 layers improve electrochemical corrosion resistance of porous Ti-48Al-6Nb alloys due to the right-moved passive region. Acknowledgments This research was supported by the National Natural Science Foundation of China (No. 51271016), and the National Basic Research Program of China (973 Program, No. 2011CB605500). References [1] F. Appel, M. Oehring, R. Wagner, Novel design concepts for gamma-base titanium aluminide alloys, Intermetallics 8 (2008) 1283–1312. [2] J.D.H. Paul, U. Lorenz, M. Oehring, F. Appel, Up-scaling the size of TiAl component made via ingot metallurgy, Intermetallics 32 (2013) 318–328. [3] Z.W. Shi, H. Wei, H.Y. Zhang, D.L. Wu, T. Jin, X.F. Sun, Q. Zheng, Investigation of microstructure in hot-pressed Nb-23Ti-15Al alloy, J. Alloys Compd. 636 (2015) 61–66. [4] D. Holec, D. Legut, L. Isaeva, P. Souvatzis, H. Clemens, S. Mayer, Interplay between effect of Mo and chemical disorder on the stability of β/β0-TiAl phase, Intermetallics 61 (2015) 85–90. [5] H. Clemens, S. Mayer, Design, processing, microstructure, properties, and applications of advanced intermetallic TiAl alloys, Adv. Eng. Mater. 15 (2013) 191–215. [6] S. Bystrzanowski, A. Bartels, H. Clemens, R. Gerling, Characteristics of the tensile flow behavior of Ti-46Al-9Nb sheet material — analysis of thermally activated processes of plastic deformation, Intermetallics 16 (2008) 717–726. [7] Y.F. Liang, F. Yang, L.Q. Zhang, J.P. Lin, S.L. Shang, Z.K. Liu, Reaction behavior and pore formation mechanism of TiAl-Nb porous alloys prepared by element powder metallurgy, Intermetallics 44 (2014) 1–7. [8] Y. Jiang, Y.H. He, N.P. Xu, J. Zou, B.Y. Huang, C.T. Liu, Effects of the Al content on pore structures of porous Ti-Al alloys, Intermetallics 16 (2008) 327–332. [9] F. Yang, J.P. Lin, Y.H. He, H. Du, G.L. Chen, Innovative fabrication of Ti-48Al-6Nb porous coating by cold gas spraying and reactive sintering, Mater. Lett. 76 (2012) 190–193. [10] C.L. Li, P. Zhang, Z.Y. Jiang, Effect of nano Cu coating on porous Si prepared by acid etching Al-Si alloy powder, Electrochim. Acta 161 (2015) 408–412.
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Fabrication of nano-porous γ-Al2O3 layers on porous Ti-48Al-6Nb alloys Fan Wang a, Jun-Pin Lin a, Yong-Feng Liang a,⁎, Shun-Li Shang b, Zi-Kui Liu b a b
State Key Laboratory for Advanced Metals and Materials, University of Science and Technology Beijing, Beijing 100083, China Department of Materials Science and Engineering, The Pennsylvania State University, University Park, PA 16802, USA
H I G H L I G H T S
G R A P H I C A L
A B S T R A C T
• Nano-porous γ-Al2O3 layers were fabricated by soaking in 2.0 mol/L NaOH solution for 120 h. • Nano-pore-diameter was 60 ± 10 nm and the thickness of layers was 200 nm. • Total pore area increased nearly to 0.25m2/g due to nano-porous layer. • The porous alloys transformed to gradient porous materials with nano and micron range pores.
a r t i c l e
i n f o
Article history: Received 9 January 2016 Received in revised form 15 July 2016 Accepted 20 July 2016 Available online 21 July 2016 Keywords: Porous TiAl alloys Nano-porous γ-Al2O3 Total pore area Polarization
a b s t r a c t Nano-porous materials are spreading widely in various fields, such as electrochemistry, biochemistry, adsorption, and catalysis. In the present work, nano-porous γ-Al2O3 layers with 200 nm thickness have been fabricated on porous Ti-48Al-6Nb alloys by soaking in NaOH solution. Corrosion products of γ-Al2O3 with 200 nm thickness form after 48 h and begin to transform to nano-porous structures at 72 h in the areas with a smaller radius of curvature. Then the nano-pores shrink and reach the final size of 60 ± 10 nm within 120 h. Increasing the concentration of NaOH solution or soaking time accelerates the formation of nano-porous layers and the best situation is soaking for 120 h in a 2.0 mol/L NaOH solution. Meanwhile, the nano-porous γ-Al2O3 layers improve the electrochemical corrosion resistance of the porous alloys due to the more positive self-corrosion potential/current and the right-moved passive region. The NaOH corrosion method to fabricate nano-porous γ-Al2O3 layers is much easier than other methods and the porous alloys possess the better corrosion resistance and absorbability thanks to the nano-porous γ-Al2O3 layers and the micron-porous Ti-48Al-6Nb matrix. © 2016 Elsevier Ltd. All rights reserved.
1. Introduction Porous intermetallics attract a worldwide attention in the last decades due to their good performances, such as the high specific stiffness, ⁎ Corresponding author. E-mail addresses: [email protected] (F. Wang), [email protected] (Y.-F. Liang).
http://dx.doi.org/10.1016/j.matdes.2016.07.105 0264-1275/© 2016 Elsevier Ltd. All rights reserved.
high energy absorption capacity, and good oxidation resistance and corrosion resistance among foam metals. In our group, more attention has been paid for the high Nb-containing TiAl alloys because of the attractive properties of high melting point, good oxidation resistance, high specific strength and modulus due to the high Nb contents [1–6]. Porous Ti-48Al-6Nb (at.%) alloys were synthesized by powder metallurgy (PM) method using elemental powders [7]. The Kirkendall effect and phase transformation contribute to the pore-formation and the
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pores have mainly micron scale [8]. Although the porous Ti-48Al-6Nb coatings were fabricated by cold gas spraying and reactive sintering by Yang et al. [9], the pore size is still in the micron range of 1.8 μm due to the difficulty to control the pore size in nanometer range. However, nano-porous or nano-coatings have been fabricated by various methods for other materials. Nano Cu coatings were prepared on porous Si by acid etching the Al-Si alloy powder by Li et al. [10] and nano-porous bioactive glass film was prepared by the sol-gel method by Ma et al. [11]. The nano-porous structures have implications for bioactivity [12], electrochemical properties [13] and batteries efficiency [14]. In addition, the nano-porous γ-Al2O3 has special properties. The γ-Al2O3 nano-structured hollow microspheres, used to treat dye wastewater, were synthesized by Fang et al. [15] in terms of methylene blue as structure directing agent. The nano-Al2O3 with porous structure, used to separate iridium ions from aqueous solutions due to its better adsorption properties, was synthesized by Zhang et al. [16]. Also the nano-Al2O3 can be used as the excellent catalyst carrier. The electrochemical performance of LiNi0.6Co0.2Mn0.2O2 powder has been improved greatly by coating with nano-Al2O3 particles via ultrasonic coating [17]. However, little is known about the nano-porous structure of TiAl based alloys. In the present work, the nano-porous γ-Al2O3 layers are fabricated on porous Ti48Al-6Nb alloys by soaking in NaOH solution. According to the aforementioned nano-Al2O3, the present porous Ti-48Al-6Nb alloys with nano-porous γ-Al2O3 layers have significant contributions in many fields. 2. Experimental procedures Porous Ti-48Al-6Nb (at.%) alloys were fabricated by the powder metallurgy (PM) under a pressure of 200 MPa [18]. Purity and particle sizes of elemental powders are shown in Table 1. The schematic procedure of heat-treatment is shown in Fig. 1. Air and vapor escaped from compacts were at the temperature of 120 °C and the initial temperature of Ti-Al reaction was 600 °C [19]. Samples were heat-treated at 900 and 1350 °C for 3 h, where Ti-Al and Nb-Al phase transformations occurred with much pores formed [20]. Finally, the porous Ti-48Al-6Nb alloys were achieved when the temperature decreased to room temperature in vacuum. The nano-porous γ-Al2O3 layers were synthesized on the skeleton surface of porous Ti-48Al-6Nb alloys by soaking in NaOH solution for different times. The NaOH solutions used in our research were 0.5, 2.0 and 3.0 mol/L and the soaking times were 24, 48, 72, 96, and 120 h. Xray diffraction (XRD, Multipurpose X-ray Diffractometer TTR III) was used to analyze the phases on the skeleton surface using an incident angle of 1.5°. The pore structure was examined by scanning electron microscopy (SEM, ZEISS SUPRA 55) and the pore parameters were measured by mercury intrusion porosimetry (MIP, Quantachrome AUTOSCAN-33) [21]. Polarization curves were achieved by the three electrodes method [22,23].
Fig. 1. Schematic procedure of heat treatment.
some areas are relatively flat and some are bent. After soaking in NaOH solution, the nano-porous layers emerged on the surface of these skeletons. The surface of porous skeletons present rough and stripy in the 0.5 mol/L NaOH solution for 120 h (see Fig. 3a). A representative area magnified 50,000 times is shown in Fig. 3b. The nano-porous structures are obvious wherever the porous skeletons are flat or bent. Some nanopores are deeper and some shallower, which will be analyzed in next Section. The morphology of porous Ti-48Al-6Nb alloys in the 3.0 mol/L NaOH solution is shown in Fig. 3c and d. Compared to the 0.5 mol/L solution case, lots of nano-threads emerge on the nano-porous layers with different orientations. Some nano-threads are grown from one root and some are grown out of order. In order to observe the nano-porous structure more clearly, the higher magnification and fracture photos are presented in Fig. 4. As is shown in Fig. 4a, the nano-pores are covered and connected by each other and most of them are open pores. The pore sizes distributed at 60 ± 10 nm are measured roughly by SEM and the pores present an irregular shape unlike the micron-sized ones with smooth edges as shown in Fig. 3a. Due to the long soaking time (N120 h) in a higher concentration of NaOH solution (e.g. 3.0 mol/L), more nano-threads emerge on nano-porous layers and they are measured roughly the same as the nano-pores size indicating that the nano-threads are grown from inside of the nano-pores (see Fig. 4b). The accurate pore size will be measured by the MIP method later. From the fracture photos shown in Fig. 4c and d, the growth orientation of the nano-porous layers is nearly perpendicular to the Ti-Al matrix with fully lamellar phases and there exist obvious boundaries between them. The nano-porous layers have an uniform thickness of 200 nm,
3. Results and discussion 3.1. Morphology of nano-porous γ-Al2O3 layers on porous Ti-48Al-6Nb alloys The morphology of porous Ti-48Al-6Nb alloys after sintering is shown in Fig. 2. There exist a lot of pores with different pore diameters among the Ti-Al skeletons. The inner skeletons present a sleek polygon, Table 1 Purity and particle size of the elemental powders. Element
Purity (%)
Particle size (μm)
Nb Ti Al
N99.9 N99.9 N99.9
48–75 48–75 48–75
701
Fig. 2. SEM-SE image of porous Ti-48Al-6Nb alloys.
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Fig. 3. SEM-SE images of nano-porous layers on porous Ti-48Al-6Nb alloys in (a, b) 0.5 mol/L and (c, d) 3.0 mol/L.
which will be proved in the next section. In addition, the nano-porous layers are also indicated by γ-Al2O3 from the surface XRD patterns (see Fig. 5). 3.2. Pore-diameter and total pore area distribution of nano-porous γ-Al2O3 layers on porous Ti-48Al-6Nb alloys From the above analyses of morphology and XRD, the pores size is 60 ± 10 nm and the γ-Al2O3 layers thickness is about 200 nm by SEM. However, in order to get accurate size, the MIP method is used. The pore-diameter distributions in different concentration of NaOH solutions for 24, 48, 72, 96, 120 h are shown in Fig. 6. The main pore sizes
distribute at 8–10 μm, where the cumulative pore volume curves increase seriously. These micron-pores are caused by Kirkendall effect and phase transformations [24] and they change scarcely in different densities of NaOH solutions for 120 h, indicating that the NaOH solution does not influence the micron-pore structures, but there emerge cumulative pore volume curves rising at the range of nanometer. In the 0.5 mol/L NaOH solution for 24, 48, and 72 h, the curves show straight lines indicating no nano-pores forming. They are not appeared until for 96 and 120 h, and the nano-pore sizes distribute from 100 to 150 nm for 96 h and from 60 to 120 nm for 120 h. It is indicated that the nano-pores shrink gradually and reach the final size with time going on. Compared to the 0.5 mol/L case, the nano-pores appear 24 h
Fig. 4. SEM-SE images of nano-porous structure and fracture photos: a) nano-pores; b) nano-threads; c) and d) fracture structures.
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Fig. 5. XRD pattern of nano-porous layers.
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earlier in the 2.0 mol/L NaOH solution. Meanwhile, the nano-pore size distribute from 80 to 90 nm for the 72 h case and reach to 60 nm after 96 h according to the results from morphology analysis. The same results are achieved in the 3.0 mol/L NaOH solution for different hours that the nano-pores emerge after 72 h and reach the final size of 60 nm after 96 h. It indicates that the increase of the concentration of NaOH solutions accelerates the formation of nano-pores but an excessive increase is useless. The 2.0 mol/L is relatively enough for the nano-pores formation. In order to identify the above results and find the influence of nanoporous layers on the total pore area, the distributions of pore area are presented in Fig. 7. Due to the formation of 8–10 μm pores, total pore area increases about 0.04 m2/g and is not affected by concentration of NaOH solutions or soaking time. At the range of nanometer, nanopores do not appear until soaking in the 0.5 mol/L NaOH solution for 96 h. The nano-pores size shrinks from 140 to 70 nm and the total pore area finally reaches to 0.084 m2/g. In this concentration of NaOH solution, the increase of total pore area caused by nano-pores is almost equal to that caused by micro-pores, that is also 0.04 m2/g. However,
Fig. 6. Pore-diameter distribution of nano-porous layers in 0.5, 2.0 and 3.0 mol/L NaOH solutions for different times.
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Fig. 7. Total pore area distribution of nano-porous layers in 0.5, 2.0 and 3.0 mol/L NaOH solutions for different times.
nano-pores appear 24 h earlier in the 2.0 mol/L NaOH solution. The pores reached the final size 60 nm after 96 h according to the porediameter distribution results. Differing from the 0.5 mol/L case, total pore area finally reaches to 0.25 m2/g due to the formation of nanopores. The increase caused by nano-pores is 5 times larger than that caused by micron-pores, one is 0.2 m2/g and the other is 0.04 m2/g. The same phenomena emerged in the 3.0 mol/L NaOH solution indicate that nano-pores appear after 72 h. Total pore area reaches the maximum 0.23 m2/g for 96 h but decreases to 0.154 m2/g for 120 h due to the growth of nano-threads jammed nano-pores shown in Fig. 3c and Fig. 4b. In summary, increasing the concentration of NaOH solution accelerates the formation of nano-pores and shorts the time to reach the final pore size. However, excessively increasing the concentration or soaking time is not beneficial for the nano-porous structures, leading to the decrease of total pore area instead. So the best concentration of NaOH solution and the soaking time are 2.0 mol/L and 120 h, respectively.
3.3. Formation mechanism of nano-porous γ-Al2O3 layers on porous Ti48Al-6Nb alloys Before exploring the mechanism, it should be make sure no other composition layers exist on the surface of the porous Ti-48Al-6Nb skeletons. The fracture and surface photos before and after soaking in the NaOH solution for 120 h are shown in Fig. 8. It is indicated that there was no other layers forming on the skeleton surface. EDS analysis indicates that Point A is Ti-Al phases (see Table 2), while Point B is Ti-Al-O phases, indicating that γ-Al2O3 layers had not formed until soaking in the NaOH solution. From Fig. 8c and d, the skeleton surface turns to be rough after soaking in the NaOH solution and at the same time the phase interfaces turn to be more obvious. Fracture photos after soaking in the 2.0 mol/L NaOH solution for 12 h are shown in Fig. 9 and there exist many areas containing both nanopores and flat layers. It is similar to the process where the flat layers are transforming to nano-porous layers. Combined with the total pore
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Fig. 8. SEM-SE images of the fracture (a, b) and surface (c, d) before and after soaking in NaOH solution.
Table 2 EDS analysis of Points A and B. Point
A B
Element Ti (at.%)
Al (at.%)
Nb (at.%)
O (at.%)
57.77 31.69
41.94 30.37
0.30 6.61
0 31.32
area distribution shown in Fig. 7, the phenomena indicate that there was no nano-pores just the flat γ-Al2O3 layers in previous hours and then nano-pores with bigger size begins to appear. With time going on, nano-pores shrink and reach the final size and then extend to all flat γ-Al2O3 layers. There is another interesting phenomenon (see red
arrows) that the nano-pores emerge primarily in the strips, concave and convexity. The common character is that they have a smaller radius of curvature. In order to identify the formation mechanism, Ti-48Al-6Nb ingot with scratches “WF” on its surface is soaking in the 2.0 mol/L NaOH solution for 120 h. The ingot surface becomes yellowish-brown (see Fig. 10). Fig. 11b is the magnification of white cube in Fig. 11a and there emerge some scratches at the area of “WF” caused by NaOH corrosion. Nano-pores emerge obviously on the bottom of scratch (see Fig. 11c) and no pores emerge on the flat layers just presenting dim outlines. It is because the bottom of scratches had a smaller radius of curvature than the flat areas, indicating that the analysis about mechanism is accurate.
Fig. 9. SEM-SE images of fracture after soaking in 2.0 mol/L NaOH solution for 12 h. (For interpretation of the references to color in this figure, the reader is referred to the web version of this article.)
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In conclusion, the flat γ-Al2O3 layers firstly form on the porous skeletons in 2.0 mol/L NaOH solution during 12-48 h and then begin to transform to bigger nano-pores primarily from the area with smaller radius of curvature, such as strips, concave and convexity. Then nanopores shrink and reach the final size 60 ± 10 nm. The simulative schematic is shown in Fig. 12.
3.4. Polarization performance of porous Ti-48Al-6Nb alloys with nano-porous layers
Fig. 10. Appearance of Ti-48Al-6Nb ingot soaking in 2.0 mol/L NaOH solution for 120 h.
Due to nano-porous γ-Al2O3 layers, the polarization performance of porous Ti-48Al-6Nb alloys should be improved. It is predicted that the layers protect the porous Ti-48Al-6Nb alloys from electrochemical corrosion. Polarization curves of the porous Ti-48Al-6Nb alloys with and
Fig. 11. SEM-SE images of Ti-48Al-6Nb ingot surface soaking in 2.0 mol/L NaOH solution for 120 h: (c) is the magnification of Area A and (d) is Area B.
Fig. 12. Formation schematic of nano-porous layers on porous Ti-48Al-6Nb alloys.
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the edge of pits that the fully lamellar phases are covered by nano-porous γ-Al2O3 layers and just the top is corroded out. That phenomena indicate that the nano-porous γ-Al2O3 layers protect the Ti-Al matrix from electrochemical corrosion in some certain areas. The protection phenomena are also observed at the narrow skeletons that the nano-porous γ-Al2O3 layers protects some skeletons from corrosion and are separated from the Ti-Al matrix (see Fig. 14d). In this photo, the thickness of nano-porous layers is observed obviously as about 200 nm, which is agreed by Fig. 4d. In summary, the 200 nm thick nano-porous γ-Al2O3 layers with 60 ± 10 nm pores improve electrochemical corrosion resistance of the porous Ti-48Al-6Nb alloys. 3.5. Discussion of advantages and applications
Fig. 13. Polarization curves of porous Ti-48Al-6Nb alloys with and without nano-porous layers.
without nano-porous γ-Al2O3 layers are shown in Fig. 13. For the original porous alloys, the obvious passive region exists at the range of −1.0 V to −0.5 V. After the passive region, current increases with increasing potential and the original porous alloys step into an obvious electrochemical corrosion stage. However, polarization curve of porous alloys with nano-porous layers have an obvious passive region from −0.25 V to 0.25 V. The current decreases more obviously with increasing potential. Due to nano-porous γ-Al2O3 layers, the porous Ti-48Al6Nb alloys could not be electrochemical corroded easily at such high potential. So the nano-porous γ-Al2O3 layers improve substantially their electrochemical corrosion resistance, which was predicted to be well used in electrochemical field as well as larger total pore area [13,25]. Polarization morphology of porous Ti-48Al-6Nb alloys with nanoporous γ-Al2O3 layers is shown in Fig. 14. For such a high potential, the porous alloys are destroyed by high current with many electrochemistry corrosion pits appearing on the surface (see Fig. 14a). Fully lamellar phases of Ti-Al alloys are exposed at the edge of pits (see Fig. 14b). Some lamellas are separated by each other and some happened to collapse. Morphology shown in Fig. 14c is observed at
Nanotechnology is spreading vastly in various demanding fields of engineering and medicines such as aerospace, defense, automobiles, electronics, materials chemistry, energy, environment, information and communication, consumer goods and biotechnology [26]. The processing methods of nano-Al2O3 and its composites are various, such as powder metallurgy, mechanical milling, casting, pressure infiltration, wet chemical method, friction stir process and so on. The disadvantages and troubles about the process focus on the longer processing time, non-uniform diameter, high energy consumption, sophisticated equipment and harm to environment. However, in the present research, the uniform nano-porous γ-Al2O3 layers are fabricated on the porous Ti48Al-6Nb alloys by just soaking in the NaOH solution for 120 h. Our method is easier to operate just using a container and the finally gained saline solution is not harmful to environment. During the process, neither electric nor heating power is needed. Combining the advantages of nano-porous γ-Al2O3 layers and porous Ti-48Al-6Nb alloys, the materials can be used as adsorption, filter and so on. 4. Conclusion Nano-porous γ-Al2O3 layers with 200 nm thickness are fabricated on porous Ti-48Al-6Nb alloys by soaking in NaOH solution. Increasing the concentration of NaOH solution can accelerate the fabrication of nanoporous γ-Al2O3 layers. Appropriately increasing the soaking time makes the nano-pores forming entirely but excessively long soaking time leads to the growth of nano-threads from pores inside decreasing
Fig. 14. SEM-SE images of porous Ti-48Al-6Nb alloys with nano-porous γ-Al2O3 layers after polarization.
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total pore area. So the most appropriate concentration and time are 2.0 mol/L and 120 h, respectively. The mechanism schematic is that the flat γ-Al2O3 layers with 200 nm thickness formed after 48 h and begin to transform to nano-porous structures at 72 h from the areas with a smaller radius of curvature and then extend to flat areas making the nano-porous γ-Al2O3 layers forming entirely within 120 h. Nanoporous γ-Al2O3 layers improve electrochemical corrosion resistance of porous Ti-48Al-6Nb alloys due to the right-moved passive region. Acknowledgments This research was supported by the National Natural Science Foundation of China (No. 51271016), and the National Basic Research Program of China (973 Program, No. 2011CB605500). References [1] F. Appel, M. Oehring, R. Wagner, Novel design concepts for gamma-base titanium aluminide alloys, Intermetallics 8 (2008) 1283–1312. [2] J.D.H. Paul, U. Lorenz, M. Oehring, F. Appel, Up-scaling the size of TiAl component made via ingot metallurgy, Intermetallics 32 (2013) 318–328. [3] Z.W. Shi, H. Wei, H.Y. Zhang, D.L. Wu, T. Jin, X.F. Sun, Q. Zheng, Investigation of microstructure in hot-pressed Nb-23Ti-15Al alloy, J. Alloys Compd. 636 (2015) 61–66. [4] D. Holec, D. Legut, L. Isaeva, P. Souvatzis, H. Clemens, S. Mayer, Interplay between effect of Mo and chemical disorder on the stability of β/β0-TiAl phase, Intermetallics 61 (2015) 85–90. [5] H. Clemens, S. Mayer, Design, processing, microstructure, properties, and applications of advanced intermetallic TiAl alloys, Adv. Eng. Mater. 15 (2013) 191–215. [6] S. Bystrzanowski, A. Bartels, H. Clemens, R. Gerling, Characteristics of the tensile flow behavior of Ti-46Al-9Nb sheet material — analysis of thermally activated processes of plastic deformation, Intermetallics 16 (2008) 717–726. [7] Y.F. Liang, F. Yang, L.Q. Zhang, J.P. Lin, S.L. Shang, Z.K. Liu, Reaction behavior and pore formation mechanism of TiAl-Nb porous alloys prepared by element powder metallurgy, Intermetallics 44 (2014) 1–7. [8] Y. Jiang, Y.H. He, N.P. Xu, J. Zou, B.Y. Huang, C.T. Liu, Effects of the Al content on pore structures of porous Ti-Al alloys, Intermetallics 16 (2008) 327–332. [9] F. Yang, J.P. Lin, Y.H. He, H. Du, G.L. Chen, Innovative fabrication of Ti-48Al-6Nb porous coating by cold gas spraying and reactive sintering, Mater. Lett. 76 (2012) 190–193. [10] C.L. Li, P. Zhang, Z.Y. Jiang, Effect of nano Cu coating on porous Si prepared by acid etching Al-Si alloy powder, Electrochim. Acta 161 (2015) 408–412.
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