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Evaluation of thermal stability of ultrafine grained aluminium matrix composites reinforced with carbon nanotubes
Composites Science and Technology 71 (2011) 1881–1885
Contents lists available at SciVerse ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Evaluation of thermal stability of ultrafine grained aluminium matrix composites reinforced with carbon nanotubes Joanna Lipecka a, Mariusz Andrzejczuk a, Małgorzata Lewandowska a,⇑, Jolanta Janczak-Rusch b, Krzysztof J. Kurzydłowski a a b
Warsaw University of Technology, Faculty of Materials Science and Engineering, Wołoska 141, 02-507 Warsaw, Poland Laboratory for Joining and Interface Technology, Empa, Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland
a r t i c l e
i n f o
Article history: Received 12 May 2011 Received in revised form 1 September 2011 Accepted 4 September 2011 Available online 12 September 2011 Keywords: Ultrafine grained aluminium alloys A. Metal matrix composites (MMCs) A. Carbon nanotubes B. Thermal stability
a b s t r a c t The effect of carbon nanotubes on the thermal stability of ultrafine grained aluminium alloy processed by the consolidation of nano-powders obtained by mechanical alloying was evaluated via measurements of grain size and mechanical property changes upon annealing at various temperatures. It was found that the grain size of the samples containing carbon nanotubes is stable up to high temperatures and even after annealing at 450 °C (0.7Tm) no evident grain growth was observed. The limited grain boundary migration was attributed to the presence of entangled networks of carbon nanotubes located at grain boundaries and to the formation of nanoscale particles of aluminium carbide Al4C3. It was also revealed that carbon nanotubes decompose at a relatively low temperature of 450 °C and form fine Al4C3 precipitates. This transformation does not significantly affect the mechanical properties due to the nanoscale size of the carbides. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Ultrafine grained (UFG) aluminium alloys possess appealing properties such as ultra-high yield and fracture strengths, toughness and superior wear resistance combined with the light weight, which make them very attractive for a number of engineering applications. On the other hand, the UFG metals intrinsically feature high values of stored energy per unit volume which may lead to high thermal instability, among others to the enhanced tendency to grain growth [1]. The tendency to grain growth, which severely reduces the temperature range of Al-alloy applications, can be reduced by the addition of an inert nano-phase in the form of particles, wires or tubes. Implementation of this fabrication route is logically related to the concept of nano-metallic composites. In recent years, there has been a steadily growing interest in the development of aluminium metal matrix composites (MMCs). It has been reported in literature [2–4] that aluminium matrix composites (AMCs) can be reinforced with various particles (e.g. SiC, Al2O3) and fibres (e.g. carbon, alumina, silicon carbide). In the past years exceptional attention has also been given to carbon nanotubes (CNTs) as a reinforcement for aluminium matrix composites [5–12,16,17]. CNT composites can be produced by a number of techniques, such as plasma spraying of spray dried powders, spark plasma sin⇑ Corresponding author. Tel.: +48 22 234 83 99; fax: +48 22 234 85 14. E-mail address: [email protected] (M. Lewandowska). 0266-3538/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2011.09.001
tering (SPS) and hot or cold isostatic pressing followed by hot extrusion [5–9,17]. The addition of CNTs can result in a significant improvement in mechanical properties of Al/CNT composites as reported in [6–10,16,17]. However, there is a lack of data on thermal stability of such composite structures. The UFG structure combined with the reinforcement by CNTs may provide new, enhanced properties of aluminium alloys also in terms of their thermal stability, which in turn may broaden the temperature range of their application. Therefore, an investigation of their thermal stability is of prime importance. In the present study, special attention was paid to the analysis of grain size changes and concurrent changes in the mechanical properties of ultrafine grained S790 aluminium alloy reinforced with CNTs. 2. Experimental 2.1. Materials The materials used in this study were two 7XXX aluminium alloys provided by Rio Tinto Alcan Company: AlZn11Mg2Cu, designated as S790, and AlZn11Mg2Cu containing 3% of CNTs, designated as S790/CNT. Aluminium S790 powder and CNTs (Baytubes from Bayer, multiwall carbon nanotubes) were first high energy milled, axial cold compacted to form a billet and finally extruded into a rod-shape at 375 °C. In order to evaluate their thermal stability and mechanical properties, the samples were annealed at various temperatures ranging from 100 to 550 °C for 10 min.
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2.2. Characterization The microstructures of the samples were analysed using a Scanning Transmission Electron Microscope (STEM) Hitachi-HD2700, equipped with a Cs corrector and operated at 200 kV. Specimens for STEM observations were prepared by slicing into thin foils oriented perpendicular to the extrusion direction. The thin foils were mechanically grinded using a Gatan Dimple Grinder and polished by argon ion milling with the use of a Gatan Precision Ion Polishing System (PIPS). The grain size changes that occurred during annealing were quantified via computer aided image analyses of the representative
STEM images. The microstructures were evaluated in terms of the average grain size and the grain size distribution. The average grain size was computed as the mean value of the equivalent diameter d2, which is defined as the diameter of a circle of equal area to the section area of a given grain. The width of the grain size distribution was expressed as the variation coefficient CV(d2) defined as the ratio of the standard deviation SD(d2) to the mean value. Sizes of more than 100 randomly selected grains were analysed for each annealing temperature. The stability of the mechanical properties was determined by plotting the microhardness measurements against the annealing temperature. Vickers microhardness was measured on
Fig. 1. STEM images of (a) non-heat treated state of S790, (b) Z-contrast image of non-heat treated state of S790, (c) non-heat treated state of S790/CNT, (d) Z-contrast image of non-heat treated state of S790/CNT showing agglomerates of CNTs around the Al grains, (e) CNT at the grain boundary zone, and (f) HR-STEM image of entangled networks of CNTs.
J. Lipecka et al. / Composites Science and Technology 71 (2011) 1881–1885
the cross-section of extruded samples using a diamond indenter and 50 g of load with a force duration of 20 s on a computer-controlled measuring system Fischerscope HM2000 Hardness Tester. 3. Results The microstructures of the non-heat treated samples of S790 and S790/CNT alloys shown in Fig. 1a–d consist of ultrafine grains with average equivalent diameters of 713 nm and 507 nm, respectively. The grains contain a significant density of dislocations and are fairly equiaxial. Small precipitates of the MgZn2 phase are also visible. HR-STEM analysis of S790/CNT samples reveals that CNTs form tangles located preferentially at the grain boundaries, as illustrated in Fig. 1e–f. Such inhomogeneous distributions of CNTs do not significantly affect the mechanical properties, but may have a substantial effect on the grain size stability, since CNTs act as barriers for the migration of the grain boundaries. A plot of the mean equivalent diameter E(d2) against the annealing temperature is shown in Fig. 2a. The annealing of S790 samples at temperatures lower than 240 °C only results in a slight increase in the grain size. At this stage, the major microstructural evolution of the alloy consists of the annihilation of the dislocations in the grain interiors. A significant grain growth occurs at higher annealing temperatures (above 240 °C) and for the annealing at 450 °C the mean equivalent diameter reaches a value of almost 1 lm. The grain growth during annealing of CNT modified S790 is very much reduced. The mean equivalent diameter increases only slightly from 500 to of 566 nm after annealing at 450 °C. The data describing the width of the grain size distributions in the ultrafine grained Al with and without CNTs are shown in
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Fig. 2b. One can notice that the size distributions for both alloys are fairly uniform. The values of CV(d2) are relatively low and do not change significantly with the annealing temperature. These results suggest that during the annealing uniform grain growth occurs in both alloys. However, HR-STEM images of the S790/ CNT annealed at 450 °C presented in Fig. 3, indicate that there is a significant morphological transformation of CNTs. At this annealing temperature the CNTs partially transform into nano-particles of aluminium carbide Al4C3 as illustrated in Fig. 3c. The average size of these particles is about 5 nm in width and 20 nm in length, which is consistent with the data presented in [11]. However in [11], Al4C3 particles were found to form at 500 °C or above, whereas in the present study they formed at 450 °C. Some of these nano-carbides are located at the opened ends of the nanotubes, as shown in Fig. 3e. Another possible locations of nano-carbides are amorphous layers along the CNTs as reported in [10,11]. Fig. 4 shows the changes in the microhardness of the investigated alloys plotted against the annealing temperature. The microhardness of both alloys prior to the annealing is similar and is controlled by the microhardness of the metal matrix. The annealing of S790 at a temperature above 100 °C causes a gradual decrease in microhardness by nearly 30% after annealing at 240 °C. Above this temperature, the alloy preserves its properties up to 500 °C (0.8Tm) and at a temperature of 550 °C the material starts to melt. On the other hand, for the S790/CNT alloy, annealing at high temperatures only leads to a slight decrease in the microhardness. The values of the microhardness are reduced only by 15% in comparison to the values measured prior to the annealing.
4. Discussion
Fig. 2. Plot of (a) the mean equivalent diameter d2 and (b) the coefficient of variation of grain size CV(d2) against the annealing temperature.
The experiments reported in this study clearly show that the addition of 3% CNTs significantly enhances the thermal stability of ultrafine grained S790 aluminium alloy by suppressing the grain growth. The results obtained also give an insight on the mechanism of the grain growth retardation brought about by the addition of CNTs. Inam et al. [12] suggested that CNTs located at the grain boundaries entangle the grains prohibiting their growth. This hypothesis is in good agreement with the observations reported here. High resolution TEM images revealed highly entangled agglomerates of CNTs at grain boundaries or in their vicinity (Fig. 1c–f). Such a location directly indicates the possible effect on the kinetics of the grain growth due to constraining migration of the grain boundaries. However, the present study also reveals another possible mechanism of the enhanced thermal stability of S790/CNT at high temperatures, the one related to the presence of nano-Al4C3 precipitates. It has been reported in literature [13] that an extensive reaction between an aluminium matrix and carbon may lead to the development of the brittle Al4C3 phase. Basically, the formation of this phase is undesirable due to the fact that Al4C3 can easily decompose in a hygroscopic atmosphere and its particles may act as corrosion sites and crack initiators. On the other hand, below a critical thickness, the Al4C3 phase may cause a stronger interfacial bond and lead to an improvement of the composite properties [14]. Nano-sized aluminium carbides, Al4C3, observed in the alloy investigated with a thickness of 5 nm are harmless and may additionally act as a nano-dispersed phase, in the way already utilised to stabilise the thermal properties of oxide dispersion-strengthened (ODS) steels. Fine precipitates located at grain boundaries may apply a pinning pressure thus impeding the mobility of grain boundaries in addition to the effect of CNTs [11]. These two structural elements may jointly contribute to the relatively high thermal stability (up to 0.8Tm) of the S790/CNT. However, it should be noted that the
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Fig. 3. STEM images of S790/CNT annealed at 450 °C (a) the microstructure of the composite, (b) Z-contrast image of the microstructure of the composite, (c) CNT networks at the grain boundary zone and the Al4C3 phase formed between the Al matrix and the CNT, (d) Z-contrast image of the Al4C3 phase in the Al matrix, (e) aluminium carbide formed on the tip of a CNT at a grain boundary zone, and (f) aluminium carbide with lattice fringes of the (0 1 2) plane with a spacing of 0.28 nm and of the (0 0 15) plane with a spacing of 0.17 nm. The insert shows a diffraction pattern indicating the main phase of these particles to be aluminium carbide Al4C3.
mechanisms related are not independent, as nano-particles form at the expense of CNTs. The investigation of the mechanical properties carried out in this study showed that the addition of 3% CNT leads to a maintenance of the already high value of the microhardness of the S790 Al alloy. Moreover, this high value is much less reduced than in the unreinforced alloy, by only 15%, as a result of the subsequent annealing operations. Although, the strengthening mechanism for CNT-reinforced aluminium matrix composites is still not fully clarified, the results reported here suggest that at higher temperatures the strengthening effect may be controlled by the formation of aluminium nano-carbides, Al4C3. These carbides are formed at the interface
between the aluminium matrix and the reinforcing CNTs on the tip of nanotubes and/or on the amorphous surface layers [10,11]. As reported by Landry et al. [15], aluminium and aluminium alloys have poor wettability with carbon with a contact angle of 130–140°. However, when the nano-Al4C3 particles are formed the contact angle decreases and the aluminium matrix can be well bonded transferring the stress [10,11,14,16,17]. 5. Conclusions The results presented in this study indicate that a significant improvement of the thermal stability of ultrafine grained S790
J. Lipecka et al. / Composites Science and Technology 71 (2011) 1881–1885
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References
Fig. 4. Plot of the microhardness HV0.05 depending on the annealing temperature.
aluminium alloy can be obtained by the addition of 3% CNTs. The grain size analysis showed that the mean diameter was stable up to an annealing temperature of 550 °C and no significant grain growth was observed. In addition, the CNTs strongly affected the mechanical properties of the aluminium alloy. The microhardness was reduced by only 15% compared to the alloy without CNTs. On the basis of the above results, one can conclude that the addition of CNTs can be a successful method of improving the thermal stability of ultrafine grained aluminium alloys. An alloy modification with CNTs enables its use up to high temperatures without significant microstructural changes and without losing a substantial part of its excellent mechanical properties resulting from grain refinement. This is very important from the application point of view and makes possible the use of high temperature methods of formation and joining of aluminium components with an ultrafine grained structure. Acknowledgment This work was carried out within a NANOMET Project financed under the European Funds for Regional Development (Contract No. POIG.01.03.01-00-015/08).
[1] Lewandowska M, Kurzydłowski KJ. Thermal stability of a nanostructured aluminium alloy. Mater Charact 2005;55(4-5):395–401. [2] Prabhu B, Suryanarayana C, An L, Vaidyanathan R. Synthesis and characterization of high volume fraction Al–Al2O3 nanocomposite powders by high-energy milling. Mater Sci Eng A 2006;425(1–2):192–200. [3] Woo KD, Bom Lee H. Fabrication of Al alloy matrix composite reinforced with subsive-sized Al2O3 particles by the in situ displacement reaction using highenergy ball-milled powder. Mater Sci Eng A 2007;449–451(1–2):829–32. [4] Rosso M. Ceramic and metal matrix composites: routes and properties. J Mater Process Technol 2006;175(1–2):364–75. [5] Kuzumaki T, Miyazawa K, Ichinose H, Ito K. Processing of carbon nanotube reinforced aluminum composite. J Mater Res 1998;13(9):2445–9. [6] Deng CF, Wang DZ, Zhang XX, Li AB. Processing and properties of carbon nanotubes reinforced aluminum composites. Mater Sci Eng A 2007;444(1– 2):138–45. [7] Esawi AMK, Morsi K, Sayed A, Abdel Gawad A, Borah P. Fabrication and properties of dispersed carbon nanotube–aluminum composites. Mater Sci Eng A 2009;508(1–2):167–73. [8] Bakshi SR, Singh V, Seal S, Agarwal A. Aluminum composite reinforced with multiwalled carbon nanotubes from plasma spraying of spray dried powders. Surf Coat Technol 2009;203(10–11):1544–54. [9] Morsi K, Esawi AMK, Lanka S, Sayed A, Taher M. Spark plasma extrusion (SPE) of ball-milled aluminum and carbon nanotube reinforced aluminum composite powders. Composites: Part A 2010;41(2):322–6. [10] Kwon H, Park DH, Silvain JF, Kawasaki A. Investigation of carbon nanotube reinforced aluminum matrix composite materials. Compos Sci Technol 2010;70(3):546–50. [11] Ci L, Ryu Z, Jin-Phillipp NY, Ruhle M. Investigation of the interfacial reaction between multi-walled carbon nanotubes and aluminum. Acta Mater 2006;54(20):5367–75. [12] Inam F, Yan X, Peijs T, Reece MJ. The sintering and grain growth behaviour of ceramic–carbon nanotube nanocomposites. Compos Sci Technol 2010;70(6):947–52. [13] Tham LM, Gupta M, Cheng L. Effect of limited matrix–reinforcement interfacial reaction on enhancing the mechanical properties of aluminium–silicon carbide composites. Acta Mater 2001;49(16):3243–53. [14] Seong HG, Lopez HF, Robertson DP, Rohatgi PK. Interface structure in carbon and graphite fiber reinforced 2014 aluminum alloy processed with active fiber cooling. Mater Sci Eng A 2008;487(1–2):201–9. [15] Landry K, Kalogeropoulou S, Eustathopoulos N. Wettability of carbon by aluminum and aluminum alloys. Mater Sci Eng A 1998;254(1–2):99–111. [16] Esawi AMK, Morsi K, Sayed A, Taher M, Lanka S. Effect of carbon nanotube (CNT) content on the mechanical properties of CNT–reinforced aluminium composites. Compos Sci Technol 2010;70:2237–41. [17] Kwon H, Estili M, Takagi K, Miyazaki T, Kawasaki A. Combination of hot extrusion and spark plasma sintering for producing carbon nanotube reinforced aluminum matrix composites. Carbon 2009;47(3):570–7.
Contents lists available at SciVerse ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Evaluation of thermal stability of ultrafine grained aluminium matrix composites reinforced with carbon nanotubes Joanna Lipecka a, Mariusz Andrzejczuk a, Małgorzata Lewandowska a,⇑, Jolanta Janczak-Rusch b, Krzysztof J. Kurzydłowski a a b
Warsaw University of Technology, Faculty of Materials Science and Engineering, Wołoska 141, 02-507 Warsaw, Poland Laboratory for Joining and Interface Technology, Empa, Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland
a r t i c l e
i n f o
Article history: Received 12 May 2011 Received in revised form 1 September 2011 Accepted 4 September 2011 Available online 12 September 2011 Keywords: Ultrafine grained aluminium alloys A. Metal matrix composites (MMCs) A. Carbon nanotubes B. Thermal stability
a b s t r a c t The effect of carbon nanotubes on the thermal stability of ultrafine grained aluminium alloy processed by the consolidation of nano-powders obtained by mechanical alloying was evaluated via measurements of grain size and mechanical property changes upon annealing at various temperatures. It was found that the grain size of the samples containing carbon nanotubes is stable up to high temperatures and even after annealing at 450 °C (0.7Tm) no evident grain growth was observed. The limited grain boundary migration was attributed to the presence of entangled networks of carbon nanotubes located at grain boundaries and to the formation of nanoscale particles of aluminium carbide Al4C3. It was also revealed that carbon nanotubes decompose at a relatively low temperature of 450 °C and form fine Al4C3 precipitates. This transformation does not significantly affect the mechanical properties due to the nanoscale size of the carbides. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction Ultrafine grained (UFG) aluminium alloys possess appealing properties such as ultra-high yield and fracture strengths, toughness and superior wear resistance combined with the light weight, which make them very attractive for a number of engineering applications. On the other hand, the UFG metals intrinsically feature high values of stored energy per unit volume which may lead to high thermal instability, among others to the enhanced tendency to grain growth [1]. The tendency to grain growth, which severely reduces the temperature range of Al-alloy applications, can be reduced by the addition of an inert nano-phase in the form of particles, wires or tubes. Implementation of this fabrication route is logically related to the concept of nano-metallic composites. In recent years, there has been a steadily growing interest in the development of aluminium metal matrix composites (MMCs). It has been reported in literature [2–4] that aluminium matrix composites (AMCs) can be reinforced with various particles (e.g. SiC, Al2O3) and fibres (e.g. carbon, alumina, silicon carbide). In the past years exceptional attention has also been given to carbon nanotubes (CNTs) as a reinforcement for aluminium matrix composites [5–12,16,17]. CNT composites can be produced by a number of techniques, such as plasma spraying of spray dried powders, spark plasma sin⇑ Corresponding author. Tel.: +48 22 234 83 99; fax: +48 22 234 85 14. E-mail address: [email protected] (M. Lewandowska). 0266-3538/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2011.09.001
tering (SPS) and hot or cold isostatic pressing followed by hot extrusion [5–9,17]. The addition of CNTs can result in a significant improvement in mechanical properties of Al/CNT composites as reported in [6–10,16,17]. However, there is a lack of data on thermal stability of such composite structures. The UFG structure combined with the reinforcement by CNTs may provide new, enhanced properties of aluminium alloys also in terms of their thermal stability, which in turn may broaden the temperature range of their application. Therefore, an investigation of their thermal stability is of prime importance. In the present study, special attention was paid to the analysis of grain size changes and concurrent changes in the mechanical properties of ultrafine grained S790 aluminium alloy reinforced with CNTs. 2. Experimental 2.1. Materials The materials used in this study were two 7XXX aluminium alloys provided by Rio Tinto Alcan Company: AlZn11Mg2Cu, designated as S790, and AlZn11Mg2Cu containing 3% of CNTs, designated as S790/CNT. Aluminium S790 powder and CNTs (Baytubes from Bayer, multiwall carbon nanotubes) were first high energy milled, axial cold compacted to form a billet and finally extruded into a rod-shape at 375 °C. In order to evaluate their thermal stability and mechanical properties, the samples were annealed at various temperatures ranging from 100 to 550 °C for 10 min.
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2.2. Characterization The microstructures of the samples were analysed using a Scanning Transmission Electron Microscope (STEM) Hitachi-HD2700, equipped with a Cs corrector and operated at 200 kV. Specimens for STEM observations were prepared by slicing into thin foils oriented perpendicular to the extrusion direction. The thin foils were mechanically grinded using a Gatan Dimple Grinder and polished by argon ion milling with the use of a Gatan Precision Ion Polishing System (PIPS). The grain size changes that occurred during annealing were quantified via computer aided image analyses of the representative
STEM images. The microstructures were evaluated in terms of the average grain size and the grain size distribution. The average grain size was computed as the mean value of the equivalent diameter d2, which is defined as the diameter of a circle of equal area to the section area of a given grain. The width of the grain size distribution was expressed as the variation coefficient CV(d2) defined as the ratio of the standard deviation SD(d2) to the mean value. Sizes of more than 100 randomly selected grains were analysed for each annealing temperature. The stability of the mechanical properties was determined by plotting the microhardness measurements against the annealing temperature. Vickers microhardness was measured on
Fig. 1. STEM images of (a) non-heat treated state of S790, (b) Z-contrast image of non-heat treated state of S790, (c) non-heat treated state of S790/CNT, (d) Z-contrast image of non-heat treated state of S790/CNT showing agglomerates of CNTs around the Al grains, (e) CNT at the grain boundary zone, and (f) HR-STEM image of entangled networks of CNTs.
J. Lipecka et al. / Composites Science and Technology 71 (2011) 1881–1885
the cross-section of extruded samples using a diamond indenter and 50 g of load with a force duration of 20 s on a computer-controlled measuring system Fischerscope HM2000 Hardness Tester. 3. Results The microstructures of the non-heat treated samples of S790 and S790/CNT alloys shown in Fig. 1a–d consist of ultrafine grains with average equivalent diameters of 713 nm and 507 nm, respectively. The grains contain a significant density of dislocations and are fairly equiaxial. Small precipitates of the MgZn2 phase are also visible. HR-STEM analysis of S790/CNT samples reveals that CNTs form tangles located preferentially at the grain boundaries, as illustrated in Fig. 1e–f. Such inhomogeneous distributions of CNTs do not significantly affect the mechanical properties, but may have a substantial effect on the grain size stability, since CNTs act as barriers for the migration of the grain boundaries. A plot of the mean equivalent diameter E(d2) against the annealing temperature is shown in Fig. 2a. The annealing of S790 samples at temperatures lower than 240 °C only results in a slight increase in the grain size. At this stage, the major microstructural evolution of the alloy consists of the annihilation of the dislocations in the grain interiors. A significant grain growth occurs at higher annealing temperatures (above 240 °C) and for the annealing at 450 °C the mean equivalent diameter reaches a value of almost 1 lm. The grain growth during annealing of CNT modified S790 is very much reduced. The mean equivalent diameter increases only slightly from 500 to of 566 nm after annealing at 450 °C. The data describing the width of the grain size distributions in the ultrafine grained Al with and without CNTs are shown in
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Fig. 2b. One can notice that the size distributions for both alloys are fairly uniform. The values of CV(d2) are relatively low and do not change significantly with the annealing temperature. These results suggest that during the annealing uniform grain growth occurs in both alloys. However, HR-STEM images of the S790/ CNT annealed at 450 °C presented in Fig. 3, indicate that there is a significant morphological transformation of CNTs. At this annealing temperature the CNTs partially transform into nano-particles of aluminium carbide Al4C3 as illustrated in Fig. 3c. The average size of these particles is about 5 nm in width and 20 nm in length, which is consistent with the data presented in [11]. However in [11], Al4C3 particles were found to form at 500 °C or above, whereas in the present study they formed at 450 °C. Some of these nano-carbides are located at the opened ends of the nanotubes, as shown in Fig. 3e. Another possible locations of nano-carbides are amorphous layers along the CNTs as reported in [10,11]. Fig. 4 shows the changes in the microhardness of the investigated alloys plotted against the annealing temperature. The microhardness of both alloys prior to the annealing is similar and is controlled by the microhardness of the metal matrix. The annealing of S790 at a temperature above 100 °C causes a gradual decrease in microhardness by nearly 30% after annealing at 240 °C. Above this temperature, the alloy preserves its properties up to 500 °C (0.8Tm) and at a temperature of 550 °C the material starts to melt. On the other hand, for the S790/CNT alloy, annealing at high temperatures only leads to a slight decrease in the microhardness. The values of the microhardness are reduced only by 15% in comparison to the values measured prior to the annealing.
4. Discussion
Fig. 2. Plot of (a) the mean equivalent diameter d2 and (b) the coefficient of variation of grain size CV(d2) against the annealing temperature.
The experiments reported in this study clearly show that the addition of 3% CNTs significantly enhances the thermal stability of ultrafine grained S790 aluminium alloy by suppressing the grain growth. The results obtained also give an insight on the mechanism of the grain growth retardation brought about by the addition of CNTs. Inam et al. [12] suggested that CNTs located at the grain boundaries entangle the grains prohibiting their growth. This hypothesis is in good agreement with the observations reported here. High resolution TEM images revealed highly entangled agglomerates of CNTs at grain boundaries or in their vicinity (Fig. 1c–f). Such a location directly indicates the possible effect on the kinetics of the grain growth due to constraining migration of the grain boundaries. However, the present study also reveals another possible mechanism of the enhanced thermal stability of S790/CNT at high temperatures, the one related to the presence of nano-Al4C3 precipitates. It has been reported in literature [13] that an extensive reaction between an aluminium matrix and carbon may lead to the development of the brittle Al4C3 phase. Basically, the formation of this phase is undesirable due to the fact that Al4C3 can easily decompose in a hygroscopic atmosphere and its particles may act as corrosion sites and crack initiators. On the other hand, below a critical thickness, the Al4C3 phase may cause a stronger interfacial bond and lead to an improvement of the composite properties [14]. Nano-sized aluminium carbides, Al4C3, observed in the alloy investigated with a thickness of 5 nm are harmless and may additionally act as a nano-dispersed phase, in the way already utilised to stabilise the thermal properties of oxide dispersion-strengthened (ODS) steels. Fine precipitates located at grain boundaries may apply a pinning pressure thus impeding the mobility of grain boundaries in addition to the effect of CNTs [11]. These two structural elements may jointly contribute to the relatively high thermal stability (up to 0.8Tm) of the S790/CNT. However, it should be noted that the
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Fig. 3. STEM images of S790/CNT annealed at 450 °C (a) the microstructure of the composite, (b) Z-contrast image of the microstructure of the composite, (c) CNT networks at the grain boundary zone and the Al4C3 phase formed between the Al matrix and the CNT, (d) Z-contrast image of the Al4C3 phase in the Al matrix, (e) aluminium carbide formed on the tip of a CNT at a grain boundary zone, and (f) aluminium carbide with lattice fringes of the (0 1 2) plane with a spacing of 0.28 nm and of the (0 0 15) plane with a spacing of 0.17 nm. The insert shows a diffraction pattern indicating the main phase of these particles to be aluminium carbide Al4C3.
mechanisms related are not independent, as nano-particles form at the expense of CNTs. The investigation of the mechanical properties carried out in this study showed that the addition of 3% CNT leads to a maintenance of the already high value of the microhardness of the S790 Al alloy. Moreover, this high value is much less reduced than in the unreinforced alloy, by only 15%, as a result of the subsequent annealing operations. Although, the strengthening mechanism for CNT-reinforced aluminium matrix composites is still not fully clarified, the results reported here suggest that at higher temperatures the strengthening effect may be controlled by the formation of aluminium nano-carbides, Al4C3. These carbides are formed at the interface
between the aluminium matrix and the reinforcing CNTs on the tip of nanotubes and/or on the amorphous surface layers [10,11]. As reported by Landry et al. [15], aluminium and aluminium alloys have poor wettability with carbon with a contact angle of 130–140°. However, when the nano-Al4C3 particles are formed the contact angle decreases and the aluminium matrix can be well bonded transferring the stress [10,11,14,16,17]. 5. Conclusions The results presented in this study indicate that a significant improvement of the thermal stability of ultrafine grained S790
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References
Fig. 4. Plot of the microhardness HV0.05 depending on the annealing temperature.
aluminium alloy can be obtained by the addition of 3% CNTs. The grain size analysis showed that the mean diameter was stable up to an annealing temperature of 550 °C and no significant grain growth was observed. In addition, the CNTs strongly affected the mechanical properties of the aluminium alloy. The microhardness was reduced by only 15% compared to the alloy without CNTs. On the basis of the above results, one can conclude that the addition of CNTs can be a successful method of improving the thermal stability of ultrafine grained aluminium alloys. An alloy modification with CNTs enables its use up to high temperatures without significant microstructural changes and without losing a substantial part of its excellent mechanical properties resulting from grain refinement. This is very important from the application point of view and makes possible the use of high temperature methods of formation and joining of aluminium components with an ultrafine grained structure. Acknowledgment This work was carried out within a NANOMET Project financed under the European Funds for Regional Development (Contract No. POIG.01.03.01-00-015/08).
[1] Lewandowska M, Kurzydłowski KJ. Thermal stability of a nanostructured aluminium alloy. Mater Charact 2005;55(4-5):395–401. [2] Prabhu B, Suryanarayana C, An L, Vaidyanathan R. Synthesis and characterization of high volume fraction Al–Al2O3 nanocomposite powders by high-energy milling. Mater Sci Eng A 2006;425(1–2):192–200. [3] Woo KD, Bom Lee H. Fabrication of Al alloy matrix composite reinforced with subsive-sized Al2O3 particles by the in situ displacement reaction using highenergy ball-milled powder. Mater Sci Eng A 2007;449–451(1–2):829–32. [4] Rosso M. Ceramic and metal matrix composites: routes and properties. J Mater Process Technol 2006;175(1–2):364–75. [5] Kuzumaki T, Miyazawa K, Ichinose H, Ito K. Processing of carbon nanotube reinforced aluminum composite. J Mater Res 1998;13(9):2445–9. [6] Deng CF, Wang DZ, Zhang XX, Li AB. Processing and properties of carbon nanotubes reinforced aluminum composites. Mater Sci Eng A 2007;444(1– 2):138–45. [7] Esawi AMK, Morsi K, Sayed A, Abdel Gawad A, Borah P. Fabrication and properties of dispersed carbon nanotube–aluminum composites. Mater Sci Eng A 2009;508(1–2):167–73. [8] Bakshi SR, Singh V, Seal S, Agarwal A. Aluminum composite reinforced with multiwalled carbon nanotubes from plasma spraying of spray dried powders. Surf Coat Technol 2009;203(10–11):1544–54. [9] Morsi K, Esawi AMK, Lanka S, Sayed A, Taher M. Spark plasma extrusion (SPE) of ball-milled aluminum and carbon nanotube reinforced aluminum composite powders. Composites: Part A 2010;41(2):322–6. [10] Kwon H, Park DH, Silvain JF, Kawasaki A. Investigation of carbon nanotube reinforced aluminum matrix composite materials. Compos Sci Technol 2010;70(3):546–50. [11] Ci L, Ryu Z, Jin-Phillipp NY, Ruhle M. Investigation of the interfacial reaction between multi-walled carbon nanotubes and aluminum. Acta Mater 2006;54(20):5367–75. [12] Inam F, Yan X, Peijs T, Reece MJ. The sintering and grain growth behaviour of ceramic–carbon nanotube nanocomposites. Compos Sci Technol 2010;70(6):947–52. [13] Tham LM, Gupta M, Cheng L. Effect of limited matrix–reinforcement interfacial reaction on enhancing the mechanical properties of aluminium–silicon carbide composites. Acta Mater 2001;49(16):3243–53. [14] Seong HG, Lopez HF, Robertson DP, Rohatgi PK. Interface structure in carbon and graphite fiber reinforced 2014 aluminum alloy processed with active fiber cooling. Mater Sci Eng A 2008;487(1–2):201–9. [15] Landry K, Kalogeropoulou S, Eustathopoulos N. Wettability of carbon by aluminum and aluminum alloys. Mater Sci Eng A 1998;254(1–2):99–111. [16] Esawi AMK, Morsi K, Sayed A, Taher M, Lanka S. Effect of carbon nanotube (CNT) content on the mechanical properties of CNT–reinforced aluminium composites. Compos Sci Technol 2010;70:2237–41. [17] Kwon H, Estili M, Takagi K, Miyazaki T, Kawasaki A. Combination of hot extrusion and spark plasma sintering for producing carbon nanotube reinforced aluminum matrix composites. Carbon 2009;47(3):570–7.