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Ni structures by ball milling
Intermetallics 18 (2010) 2219e2223
Contents lists available at ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
Synthesis of reactive Al/Ni structures by ball milling Anastasia Hadjiafxenti, Ibrahim Emre Gunduz*, Chrysostomos Tsotsos, Theodora Kyratsi, Charalabos C. Doumanidis, Claus Rebholz Mechanical and Manufacturing Engineering Department, University of Cyprus, 1678 Nicosia, Cyprus
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 July 2009 Accepted 8 July 2010 Available online 14 August 2010
Material systems that exhibit self-propagating exothermic reactions (SPER) can be potentially used for nano/microscale heater purposes in thermal nanomanufacturing applications. We propose that ball milling (BM) is a simple and cost effective technique that can be used to produce such materials. For this purpose, an interrupted BM run of aluminum/nickel powders with a molar ratio of 1:3 was performed, followed by the cold pressing of the powders into pellets that were ignited using an external heat source. Samples that exhibit SPER were characterized before and after ignition using X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) with a Back Scattered Electron Detector (BSE), Energy Dispersive X-Ray analysis (EDX) and Differential Thermal Analysis (DTA). The results show that after 7 h of BM an aluminum/nickel lamellar structure forms at the particle boundaries. The characteristic lamella dimension reduces with increasing milling time down to w200 nm after 10 h of BM. The ignition conditions for compacts of milled powders indicate that samples with a higher volume fraction of the lamellae possess the shortest ignition time at 4.25 s. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: A. Nickel aluminide C. Mechanical alloying and milling D. Microstructure B. Thermal stability G. Energy systems
1. Introduction Nanoscale materials that exhibit exothermic reactions are candidates for use as heat sources in nano/micro-heater systems (NMHS), which can be graded to control temperature distribution and duration of heat generation [1]. Manageable sources employed in such systems have higher accuracy, compared to the available macro-scale sources that require large multi-element systems. NMHS can offer innovative and creative directions in thermal technology applications with emphasis on autonomous power sources. For example, aluminum/nickel bilayer sources in NMHS can operate as miniature devices in many technological fields involving thermal manufacturing (e.g. joining and material removal). Thin films [2e7] studied as nanoheater sources appear very promising in thermal nanotechnology, since they are stable, reliable and can be ignited relatively easily. However, they are mainly produced through sputtering, which is a relatively expensive and time consuming method for large scale manufacturing. Therefore, their use is limited to specialty applications which warrant their cost. Ball milling (BM) on the other hand is a simple and relatively inexpensive processing technique that is used for applying extensive plastic deformation to metallic powders through multiple * Corresponding author. E-mail address: [email protected] (I.E. Gunduz). 0966-9795/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2010.07.009
impact events in a rotating vial containing hardened steel or ceramic balls [8e29]. BM can produce extremely fine microstructures and form non-equilibrium phases (i.e. amorphous) or intermetallic compounds, when powder mixtures of different metals are used [15,20]. Parameters that affect milling are particle size, compositional ratio of elements, ball-to-material ratio, rotation speed and milling time. BM has potential as a viable method for generating ignitable powder mixtures through microstructural refinement, which reduces diffusion distances. There is still effort to identify the exact parameters of BM that affect ignitability of materials which can exhibit self-propagating exothermic reactions (SPER), i.e. certain carbides, nitrides, carbonitrides, borides, silicates, chalcogenides, hydrides, aluminides and composites [8e10]. BM has been studied on various aluminum/nickel powder mixtures to synthesize selected compounds, including AlNi [11e14], Al9Ni2 [6], Al3Ni [14] and AlNi3 [14e16], and to deliver unique mechanical properties through increased lattice strain [17,18] which increases microhardness [18,19]. The interest in this system is due to its low material cost, the superior high-temperature mechanical properties of AlNi and AlNi3 intermetallic compounds and the highly exothermic formation enthalpies, which are particularly attractive for generating heat. Although BM has been carried out to mainly produce materials with superior high-temperature mechanical properties, we report for the first time the use of BM to synthesize materials that exhibit SPER. Specifically, BM is used to mix aluminum/nickel powders
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with a molar ratio of 1:3, to refine the composite particle microstructures up to the formation of nickel aluminides, and to determine the critical milling time in order to produce ignitable compacts. The nickel-rich overall composition was selected to delay the onset of reaction during BM as experienced in mixtures with AlNi overall composition [15,20], allowing longer milling times that result in finer structures. 2. Material and methods The low-energy Ball Miller used for the experiment was a Fritsch Planetary Mono Mill “Pulverisette 6” with a stainless steel vial, 80 ml in volume. BM was performed in nitrogen atmosphere with aluminum and nickel powders of 100 þ 325 (99.5%) and 10 þ 325 (99.8%) mesh size, respectively, at a molar ratio of 1:3, corresponding to the AlNi3 intermetallic compound. The total weight of the powders introduced in the vial was 28 g, along with five 20 mm diameter stainless steel balls. The ball-to-material weight ratio was 5.8:1. The rotational speed was set at 300 rpm. Milling was interrupted at consecutive intervals of 5, 7, 10 and 20 h, and approximately 2 g of powder was removed for analysis and ignition experiments. The vial was opened and resealed inside a glove box filled with nitrogen in order to prevent oxidation. 0.5 g of powder from each stage was compacted into 10 mm diameter pellets under approximately 17 MPa pressure. Pellets were analyzed before and after ignition using Scanning Electron Microscopy (SEM, Tescan Vega LSU), X-Ray Diffraction Analysis (XRD, Shimadzu 6000 Series), Energy Dispersive X-Ray Spectroscopy (EDX, Zeiss EVO 50 type SEM), and Differential Thermal Analysis (DTA, Linseis TG-DTA/SC). DTA measurements were carried out in alumina crucibles for temperatures up to 1773 K with a heating rate of 10 K/min, and then cooled to room temperature at the same rate. The pellets were ignited in air using a propane torch with the nozzle 300 mm away, ensuring flame coverage only at the edge of the pellet until ignition was established. The ignition tests were observed using the FLIR System A40 Thermovision Camera operated at 50 frames/s. Fig. 1 demonstrates the ignition induced by heating and propagation of the reaction in a pellet. 3. Results The evolution of the powder microstructures with milling time is shown in Fig. 2. After 5 h of milling, the aluminum particles accommodate most of the plastic deformation compared to the much harder nickel powders. Larger nickel particles are coated with the aluminum matrix that incorporated some of the smaller nickel particles. At this stage, the structure of the mixture is similar to that of the initial state. The major fragmentation of nickel particles occurs after 7 h, accompanied by the formation of a partially lamellar structure with some large nickel particles remaining. As the aluminum matrix hardens with nickel enrichment, the deformation
Fig. 1. Infrared images of the pellet and the propane torch along with the observed SPER characterized by the uniform motion of the flame front.
efficiency during milling increases substantially, leading to severe plastic deformation of nickel as well, as seen after 10 h. The lamellae at this stage have submicron characteristic dimensions with relatively thicker layers of nickel (Fig. 2c), which can facilitate interdiffusion and formation of nickel aluminides. On the other hand, the 20 h milled samples have a completely different microstructure, which shows heavily deformed fine lamellae with slight compositional variations that appear as different tints of grey in the back scattered electron (BSE) image in Fig. 2d, where the lighter shades correspond to nickel-rich compositions. Furthermore, most bulk powders at this stage are observed to metallurgically bond to the bottom of the vial and to the surface of the steel balls, as well as forming large clusters, similar to those observed in other studies with overall composition close to AlNi [20]. Ignition tests indicate that increasing milling time reduces the time required to start the SPER for samples milled under 10 h. For example, the samples milled for 5, 7 and 10 h exhibit SPER after heating for >20, 17 and 4.25 s, consecutively. The SPER in these samples is characterized by a flame front which propagates until the end of the pellet within 0.7 s, followed by a uniform heating and subsequent cooling stage (Fig. 1). The reactivity of the samples increases rapidly with the refinement of the powder microstructure. The characteristic dimensions of the lamellae after 10 h of milling are similar to nanoscale multilayer foils obtained by sputtering [5,6]. However, the ignition sensitivity of the samples is still much less than those of the sputtered foils, which can ignite with a small electrical spark [2,5,6]. Further milling for 20 h is observed to suppress SPER, and only the part of the sample in contact with the flame reaches high temperatures, which are confined within that region. XRD analysis of the samples milled up to 10 h shows only aluminum and nickel peaks (Fig. 3), which are increasingly broadened due to the formation of the fine lamellar structure (Fig. 2) and the internal stresses. The overall powder diameter is on the order of 50 mm. Further milling to 20 h results in a major change in the spectrum, which shows asymmetric peaks denoted by *, centered between those of AlNi3 and pure nickel (Fig. 3). The overall free powder diameter is also reduced to 10e20 mm due to the formation of more brittle reaction products, which tend to fragment in impact events during milling, although most of the powders are found to form large agglomerates that are bonded to the vial bottom, walls and ball surfaces. XRD spectra for the ignited pellets in Fig. 3 point out to the formation of increasing amounts of AlNi3 and AlNi (with corresponding increase in peak intensities) and decreasing amounts of nickel for the 5 h, 7 h and 10 h milling times, along with some minor peaks of the intermetallic phases Al3Ni and Al3Ni5, but there are no remaining aluminum peaks. The 20 h sample spectra shows no change after ignition. EDX analysis and elemental mapping performed on the reacted 10 h sample shows four distinct regions within the particles (Fig. 4), marked by A, B, C and D, with increasing aluminum concentration, where the black areas are voids, pointing out to the porous nature of the reacted pellet. (A) is pure nickel which remains mostly unaffected during SPER. (B) shows significant aluminum enrichment in the nickel particles, suggesting liquid diffusion. (C) denotes an area with the Al3Ni5 intermetallic composition (65 at.% nickel) and (D) has a composition close to that of AlNi (48 at.% nickel). The phase boundaries are delineated, indicating formation of different crystallographic phases as opposed to extended solid solutions. Initial aluminum appears to have been completely consumed during the reaction, which is also evident by the absence of corresponding peaks in the XRD results in Fig. 3. Only the surfaces of the powders are covered with a thin layer of Al2O3, which forms when the powder compacts are ignited in air. DTA traces show that increasing milling time results in the formation of additional peaks and peak shifts towards lower
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Fig. 2. As milled powder microstructures for BM duration of (a) 5 h, (b) 7 h, (c) 10 h and (d) 20 h, along with higher magnification images.
temperatures, related to further mixing and lamellae refinement (Fig. 5), similar to those observed in continuously rolled aluminum and nickel multilayer foils [30]. The 5 h sample shows two peaks centered at (1) 533 K and (2) 841 K. The first peak is most probably associated with the formation of the metastable phase Al9Ni2, which has only been observed in rapidly solidified alloys and sputtered nanoscale multilayer foil reactions [6]. The second peak is due to the formation of AlNi3 [13]. The 7 h milled sample exhibits three peaks centered at (1) 520 K, (2) 670 K and (3) 813 K, pointing to the formation of Al9Ni2 (1), Al3Ni (2) and AlNi3 (3) [6]. The 10 h milled sample shows four major peaks centered at (1) 516 K, (2) 557 K, (3) 645 K and (4) 732 K. The peaks are most probably associated with the formation of Al9Ni2 (1), Al3Ni (2)(3) and AlNi3 (4), similar to those observed in nanoscale multilayer foils with a bilayer thickness of 200 nm, indicating that the lamellar structure formed during milling has a similar characteristic thickness [6]. The double peak for the Al3Ni arises from kinetic reasons [6,30]. The 20 h milled sample shows only one small peak centered around w510 K, which might correspond to the formation of Al9Ni2 or a relaxation process such as stress relief or recrystallization. All the samples show the large endothermic peak associated with the melting of AlNi3, starting at around w1630 K. 4. Discussion Fig. 3. XRD spectra of the samples before and after ignition after 5, 7, 10 and 20 h ball milling. * denotes the peaks that either correspond to a disordered AlNi3 phase [13], or to a very fine mixture of AlNi, AlNi3 and nickel-rich solid solution.
The observed microstructural refinement with increasing milling time is expected as the number of collision events and deformation efficiency increases. However, the 20 h milled samples point out to
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Fig. 4. EDX analysis performed on 10 h ignited samples shows the formation of different phases along the particle boundaries. The measured compositions in (a) are (A) 100 at.% Ni, (C) 65 at.% Ni and (D) 48 at.% Ni. The EDX elemental mapping indicates extensive aluminum-rich liquid diffusion within nickel particles marked by (B). Oxide formation is limited to the surfaces of the powders.
a critical milling time between 10 h and 20 h, at which the powders undergo a transformation. The welding of the powders to form large agglomerates at the bottom of the vial and the surface of the steel balls suggests exposure to very high temperatures, where at least parts of the powders should be molten. This is also supported by the mostly pore-free microstructure in Fig. 2d, as opposed to the ignited 10 h sample in Fig. 4. A shock assisted transformation occurring during ball impacts would not propagate to most of the powders, which are accumulated at the bottom of the vial. Furthermore, gradual transformations would not result in such clumps, but rather form loose powders. It is more likely that the formation and refinement of the lamellar structure (yielding decreased diffusion distances) permit the rapid initiation of SPER during the collision events, which can sufficiently increase the local temperatures [12]. At this stage, the reaction products covering the surface of the balls, which are molten due to high formation enthalpy, would be ejected to the rest of the powder sitting at the bottom of the vial and propagate the reaction. It has been speculated that ball milled powders equivalent to the 20 h samples in this study show the formation of a single phase AlNi3 with the absence of superlattice peaks, which is then attributed to this phase being disordered [13].
However, closer examination of the XRD results (Fig. 3) reveal that this final product might instead be a very fine mixture with compositions ranging from extended nickel-rich solid solutions and AlNi3 to AlNi with broad overlapping peaks. The BSE image in Fig. 2d clearly shows some compositional variation within the particles, in accordance with this view. In fact, it is most probable that the powders rapidly react and melt partially at the interfaces to form AlNi, which is followed by excessive aluminum diffusion into the nickel layers to form AlNi3 and extended solid solutions. This is also suggested by the microstructure of the ignited 10 h milled sample in Fig. 4, which exhibits planar reaction products at the particle interfaces and large aluminum diffusion distances into the nickel particles. The formation of AlNi is facilitated by its high formation enthalpy implying a high driving force, the ease of nucleation due to the relatively simple crystal structure, and due to the interface compositions inherently being close to that of AlNi in the absence of other intermetallic compounds. Further milling after the critical milling time would only induce additional mixing that might form the structures observed in Fig. 2d. The lack of post-milling ignition and any significant change in the XRD spectrum of the 20 h sample is due to the stability of the reaction products, namely AlNi, AlNi3 and nickel-rich solid solutions, which have very high melting points and low excess enthalpy. The relatively small peak observed in DTA points out to an additional reaction, but it is not clear whether it corresponds to the formation of Al9Ni2 or a relaxation process. The low ignition sensitivity of the samples milled for up to 10 h, compared to sputtered nanoscale multilayer foils, is probably due to the remaining nickel particles which might act as heat sinks, as well as persisting voids, lack of metallurgical bonding, and good thermal contact during cold compaction of the powders into pellets. 5. Conclusions
Fig. 5. DTA results for the as milled 5, 7, 10 and 20 h samples.
Ball milling (BM) of pure aluminum and nickel particles are shown to yield powders with fine nanoscale lamellar microstructures with minimal oxidation. Furthermore, pellets produced by compacted powders can be successfully ignited to produce self-propagating exothermic reactions (SPER). Therefore, BM is shown to be a viable method for generating heat sources for nanoscale heating systems for thermal applications. The critical threshold for the reactions during milling is determined to be between 10 h and 20 h under the studied
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process parameters. It is also shown that SPER during BM starts with impact-induced local temperature rise after the powders form nanoscale lamellae, followed by the ejection of the molten reaction products from the ball surfaces that spread the reaction to the rest of the powders. The milling parameters can be optimized further to yield highly reactive compacts with nanoscale microstructures. Acknowledgements The authors would like to thank the European Commission for research funding of this project under Marie Curie Excellence Team e NanoHeaters (EXT-0023899), EC/FP6 IP Programme “ManuDirect” and EX/FP7 ICT Programme “NanoMA”, Paul Mayrhofer’s research group at the University of Leoben for the EDX analysis, the Department of Chemistry at the University of Cyprus for access to XRD instrumentation and Mr. K. Fadenberger for constructive discussions on DTA analysis and reactive nanofoils. References [1] Jogdand H, Gulsoy G, Ando T, Chen J, Doumanidis CC, Gu Z, et al. Fabrication and characterization of nanoscale heating sources (nanoheaters) for nanomanufacturing. NSTI-Nanotech 2008;1:280e3. [2] Gunduz IE, Fadenberger K, Kokonou M, Rebholz C, Doumanidis CC. Investigations on the self-propagating reactions of nickel and aluminum multilayered foils. Appl Phys Lett 2008;93:134101. [3] Gunduz IE, Fadenberger K, Kokonou M, Rebholz C, Doumanidis CC, Ando T. Modeling of the self-propagating reactions of nickel and aluminium multilayered foils. J Appl Phys 2009;105:074903. [4] Dyer TS, Munir ZA, Ruth V. The combustion synthesis of multilayer NiAl systems. Scripta Metall Mater 1994;30(10):1281e6. [5] Mann AB, Gavens AJ, Reiss ME, Heerden DV, Bao G, Weihs TP. Modeling and characterizing the propagation velocity of exothermic reactions in multilayer foils. J Appl Phys 1997;82(3):1178e88. [6] Blobaum KJ, Heerden DV, Gavens AJ, Weihs TP. Al/Ni formation reactions: characterization of the metastable Al9Ni2 phase and analysis of its formation. Acta Mater 2003;51:3871e84. [7] Trenkle JC, Koerner LJ, Tate MW, Gruner SM, Weihs TP, Hufnagel TC. Phase transformations during rapid heating of Al/Ni multilayer foils. Appl Phys Lett 2008;93:081903. [8] Takacs L. Self-sustaining reactions induced by ball milling. Prog Mater Sci 2002;47:355e414. [9] Yi HC, Moore JJ. Self-propagating high-temperature (combustion) synthesis (SHS) of powder-compacted materials. J Mater Sci 1990;25:1159e68.
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[10] Moore JJ, Feng HJ. Combustion synthesis of advanced materials: part I reaction parameters. Prog Mater Sci 1995;39:243e73. [11] Wagner CNJ, Yang E, Boldrick MS. The structure of nanocrystalline NiAl powders prepared by high energy ball milling. J Non-Cryst Solids 1995;192-193:574e7. [12] Atzmon M. Characterization of AlNi formed by a self-sustaining reaction during mechanical alloying. Mater Sci Eng A 1991;134:1326e9. [13] Joardar J, Pabi SK, Murty BS. Milling criteria for the synthesis of nanocrystalline NiAl by mechanical alloying. J Alloys Compd 2007;429:204e10. [14] Cardellini F, Mazzone G, Monotne A, Antisari MV. Solid state reactions between Ni and Al powders induced by plastic deformation. Acta Mater Mat 1994;42(7):2445e51. [15] Cardellini F, Mazzone G, Antisari MV. Solid state reactions and microstructural evolution of AleNi powders during high-energy ball milling. Acta Mater 1996;44(4):1511e7. [16] Enayati MH, Sadeghian Z, Salehi M, Saisi A. The effect of milling parameters on the synthesis of Ni3Al intermetallic compound by mechanical alloying. Mater Sci Eng A 2004;375-377:809e11. [17] Moshksar MM, Mirzae M. Formation of NiAl intermetallic by gradual and explosive exothermic reaction mechanism during ball milling. Intermetallics 2004;12:1361e6. [18] Eckert J, Börner I. Nanostructure formation and properties of ball-milled NiAl intermetallic compound. Mater Sci Eng A 1997;239:619e24. [19] Lu L, Lai MO, Zhang S. Evolution and characterization of a Ni3Al intermetallic compound during mechanical alloying. Mater Design 1994;15 (2):79e86. [20] Atzmon M. In-situ thermal observation of explosive compound-formation reaction during mechanical alloying. Phys Rev Lett 1990;64(4):487e91. [21] Moore JJ, Feng HJ. Combustion synthesis of advanced materials: part II classification, applications and modeling. Prog Mater Sci 1995;39:275e316. [22] Zhu P, Li JCM, Liu CT. Combustion reaction in multilayered nickel and aluminum foils. Mater Sci Eng A 1997;239:532e9. [23] Zhu P, Li JCM, Liu CT. Reaction mechanism of combustion synthesis of NiAl. Mater Sci Eng A 2002;329:57e68. [24] Van Dyck S, Delaey L, Froyen L, Buekenhout L. The influence of mechanical alloying on the synthesis of Ni3(Si, Ti) intermetallics. Acta Mater 1998;46 (8):2831e40. [25] Enayati MH, Karimzadeh F, Anvari SZ. Synthesis of nanocrystalline NiAl by mechanical alloying. J Mater Proc Tech 2008;200:312e5. [26] Atzmon M. Parameters affecting phase formation kinetics in mechanically alloyed Al-Ni powders. Solid State Powder Process 1990;90:15560. [27] Maric R, Ishihara KN, Shingu PH. Structural changes during low energy ball milling in the Al-Ni system. J Mater Sci Lett 1996;15:1180e3. [28] Subrahmanyan J, Vijayakumar M. Self-propagating high-temperature synthesis. J Mater Sci 1992;27:6249e73. [29] Bremer FJ, Beyss M, Wenzl H. The order-disorder transition of the intermetallic phase Ni3Al. Phys Status Solidi A 1988;110:77e82. [30] Sieber H, Park JS, Weissmuller J, Perepezko JH. Structural evolution and phase formation in cold-rolled aluminum-nickel multilayers. Acta Mater 2001;49:1139e51.
Contents lists available at ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
Synthesis of reactive Al/Ni structures by ball milling Anastasia Hadjiafxenti, Ibrahim Emre Gunduz*, Chrysostomos Tsotsos, Theodora Kyratsi, Charalabos C. Doumanidis, Claus Rebholz Mechanical and Manufacturing Engineering Department, University of Cyprus, 1678 Nicosia, Cyprus
a r t i c l e i n f o
a b s t r a c t
Article history: Received 11 July 2009 Accepted 8 July 2010 Available online 14 August 2010
Material systems that exhibit self-propagating exothermic reactions (SPER) can be potentially used for nano/microscale heater purposes in thermal nanomanufacturing applications. We propose that ball milling (BM) is a simple and cost effective technique that can be used to produce such materials. For this purpose, an interrupted BM run of aluminum/nickel powders with a molar ratio of 1:3 was performed, followed by the cold pressing of the powders into pellets that were ignited using an external heat source. Samples that exhibit SPER were characterized before and after ignition using X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM) with a Back Scattered Electron Detector (BSE), Energy Dispersive X-Ray analysis (EDX) and Differential Thermal Analysis (DTA). The results show that after 7 h of BM an aluminum/nickel lamellar structure forms at the particle boundaries. The characteristic lamella dimension reduces with increasing milling time down to w200 nm after 10 h of BM. The ignition conditions for compacts of milled powders indicate that samples with a higher volume fraction of the lamellae possess the shortest ignition time at 4.25 s. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: A. Nickel aluminide C. Mechanical alloying and milling D. Microstructure B. Thermal stability G. Energy systems
1. Introduction Nanoscale materials that exhibit exothermic reactions are candidates for use as heat sources in nano/micro-heater systems (NMHS), which can be graded to control temperature distribution and duration of heat generation [1]. Manageable sources employed in such systems have higher accuracy, compared to the available macro-scale sources that require large multi-element systems. NMHS can offer innovative and creative directions in thermal technology applications with emphasis on autonomous power sources. For example, aluminum/nickel bilayer sources in NMHS can operate as miniature devices in many technological fields involving thermal manufacturing (e.g. joining and material removal). Thin films [2e7] studied as nanoheater sources appear very promising in thermal nanotechnology, since they are stable, reliable and can be ignited relatively easily. However, they are mainly produced through sputtering, which is a relatively expensive and time consuming method for large scale manufacturing. Therefore, their use is limited to specialty applications which warrant their cost. Ball milling (BM) on the other hand is a simple and relatively inexpensive processing technique that is used for applying extensive plastic deformation to metallic powders through multiple * Corresponding author. E-mail address: [email protected] (I.E. Gunduz). 0966-9795/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2010.07.009
impact events in a rotating vial containing hardened steel or ceramic balls [8e29]. BM can produce extremely fine microstructures and form non-equilibrium phases (i.e. amorphous) or intermetallic compounds, when powder mixtures of different metals are used [15,20]. Parameters that affect milling are particle size, compositional ratio of elements, ball-to-material ratio, rotation speed and milling time. BM has potential as a viable method for generating ignitable powder mixtures through microstructural refinement, which reduces diffusion distances. There is still effort to identify the exact parameters of BM that affect ignitability of materials which can exhibit self-propagating exothermic reactions (SPER), i.e. certain carbides, nitrides, carbonitrides, borides, silicates, chalcogenides, hydrides, aluminides and composites [8e10]. BM has been studied on various aluminum/nickel powder mixtures to synthesize selected compounds, including AlNi [11e14], Al9Ni2 [6], Al3Ni [14] and AlNi3 [14e16], and to deliver unique mechanical properties through increased lattice strain [17,18] which increases microhardness [18,19]. The interest in this system is due to its low material cost, the superior high-temperature mechanical properties of AlNi and AlNi3 intermetallic compounds and the highly exothermic formation enthalpies, which are particularly attractive for generating heat. Although BM has been carried out to mainly produce materials with superior high-temperature mechanical properties, we report for the first time the use of BM to synthesize materials that exhibit SPER. Specifically, BM is used to mix aluminum/nickel powders
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with a molar ratio of 1:3, to refine the composite particle microstructures up to the formation of nickel aluminides, and to determine the critical milling time in order to produce ignitable compacts. The nickel-rich overall composition was selected to delay the onset of reaction during BM as experienced in mixtures with AlNi overall composition [15,20], allowing longer milling times that result in finer structures. 2. Material and methods The low-energy Ball Miller used for the experiment was a Fritsch Planetary Mono Mill “Pulverisette 6” with a stainless steel vial, 80 ml in volume. BM was performed in nitrogen atmosphere with aluminum and nickel powders of 100 þ 325 (99.5%) and 10 þ 325 (99.8%) mesh size, respectively, at a molar ratio of 1:3, corresponding to the AlNi3 intermetallic compound. The total weight of the powders introduced in the vial was 28 g, along with five 20 mm diameter stainless steel balls. The ball-to-material weight ratio was 5.8:1. The rotational speed was set at 300 rpm. Milling was interrupted at consecutive intervals of 5, 7, 10 and 20 h, and approximately 2 g of powder was removed for analysis and ignition experiments. The vial was opened and resealed inside a glove box filled with nitrogen in order to prevent oxidation. 0.5 g of powder from each stage was compacted into 10 mm diameter pellets under approximately 17 MPa pressure. Pellets were analyzed before and after ignition using Scanning Electron Microscopy (SEM, Tescan Vega LSU), X-Ray Diffraction Analysis (XRD, Shimadzu 6000 Series), Energy Dispersive X-Ray Spectroscopy (EDX, Zeiss EVO 50 type SEM), and Differential Thermal Analysis (DTA, Linseis TG-DTA/SC). DTA measurements were carried out in alumina crucibles for temperatures up to 1773 K with a heating rate of 10 K/min, and then cooled to room temperature at the same rate. The pellets were ignited in air using a propane torch with the nozzle 300 mm away, ensuring flame coverage only at the edge of the pellet until ignition was established. The ignition tests were observed using the FLIR System A40 Thermovision Camera operated at 50 frames/s. Fig. 1 demonstrates the ignition induced by heating and propagation of the reaction in a pellet. 3. Results The evolution of the powder microstructures with milling time is shown in Fig. 2. After 5 h of milling, the aluminum particles accommodate most of the plastic deformation compared to the much harder nickel powders. Larger nickel particles are coated with the aluminum matrix that incorporated some of the smaller nickel particles. At this stage, the structure of the mixture is similar to that of the initial state. The major fragmentation of nickel particles occurs after 7 h, accompanied by the formation of a partially lamellar structure with some large nickel particles remaining. As the aluminum matrix hardens with nickel enrichment, the deformation
Fig. 1. Infrared images of the pellet and the propane torch along with the observed SPER characterized by the uniform motion of the flame front.
efficiency during milling increases substantially, leading to severe plastic deformation of nickel as well, as seen after 10 h. The lamellae at this stage have submicron characteristic dimensions with relatively thicker layers of nickel (Fig. 2c), which can facilitate interdiffusion and formation of nickel aluminides. On the other hand, the 20 h milled samples have a completely different microstructure, which shows heavily deformed fine lamellae with slight compositional variations that appear as different tints of grey in the back scattered electron (BSE) image in Fig. 2d, where the lighter shades correspond to nickel-rich compositions. Furthermore, most bulk powders at this stage are observed to metallurgically bond to the bottom of the vial and to the surface of the steel balls, as well as forming large clusters, similar to those observed in other studies with overall composition close to AlNi [20]. Ignition tests indicate that increasing milling time reduces the time required to start the SPER for samples milled under 10 h. For example, the samples milled for 5, 7 and 10 h exhibit SPER after heating for >20, 17 and 4.25 s, consecutively. The SPER in these samples is characterized by a flame front which propagates until the end of the pellet within 0.7 s, followed by a uniform heating and subsequent cooling stage (Fig. 1). The reactivity of the samples increases rapidly with the refinement of the powder microstructure. The characteristic dimensions of the lamellae after 10 h of milling are similar to nanoscale multilayer foils obtained by sputtering [5,6]. However, the ignition sensitivity of the samples is still much less than those of the sputtered foils, which can ignite with a small electrical spark [2,5,6]. Further milling for 20 h is observed to suppress SPER, and only the part of the sample in contact with the flame reaches high temperatures, which are confined within that region. XRD analysis of the samples milled up to 10 h shows only aluminum and nickel peaks (Fig. 3), which are increasingly broadened due to the formation of the fine lamellar structure (Fig. 2) and the internal stresses. The overall powder diameter is on the order of 50 mm. Further milling to 20 h results in a major change in the spectrum, which shows asymmetric peaks denoted by *, centered between those of AlNi3 and pure nickel (Fig. 3). The overall free powder diameter is also reduced to 10e20 mm due to the formation of more brittle reaction products, which tend to fragment in impact events during milling, although most of the powders are found to form large agglomerates that are bonded to the vial bottom, walls and ball surfaces. XRD spectra for the ignited pellets in Fig. 3 point out to the formation of increasing amounts of AlNi3 and AlNi (with corresponding increase in peak intensities) and decreasing amounts of nickel for the 5 h, 7 h and 10 h milling times, along with some minor peaks of the intermetallic phases Al3Ni and Al3Ni5, but there are no remaining aluminum peaks. The 20 h sample spectra shows no change after ignition. EDX analysis and elemental mapping performed on the reacted 10 h sample shows four distinct regions within the particles (Fig. 4), marked by A, B, C and D, with increasing aluminum concentration, where the black areas are voids, pointing out to the porous nature of the reacted pellet. (A) is pure nickel which remains mostly unaffected during SPER. (B) shows significant aluminum enrichment in the nickel particles, suggesting liquid diffusion. (C) denotes an area with the Al3Ni5 intermetallic composition (65 at.% nickel) and (D) has a composition close to that of AlNi (48 at.% nickel). The phase boundaries are delineated, indicating formation of different crystallographic phases as opposed to extended solid solutions. Initial aluminum appears to have been completely consumed during the reaction, which is also evident by the absence of corresponding peaks in the XRD results in Fig. 3. Only the surfaces of the powders are covered with a thin layer of Al2O3, which forms when the powder compacts are ignited in air. DTA traces show that increasing milling time results in the formation of additional peaks and peak shifts towards lower
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Fig. 2. As milled powder microstructures for BM duration of (a) 5 h, (b) 7 h, (c) 10 h and (d) 20 h, along with higher magnification images.
temperatures, related to further mixing and lamellae refinement (Fig. 5), similar to those observed in continuously rolled aluminum and nickel multilayer foils [30]. The 5 h sample shows two peaks centered at (1) 533 K and (2) 841 K. The first peak is most probably associated with the formation of the metastable phase Al9Ni2, which has only been observed in rapidly solidified alloys and sputtered nanoscale multilayer foil reactions [6]. The second peak is due to the formation of AlNi3 [13]. The 7 h milled sample exhibits three peaks centered at (1) 520 K, (2) 670 K and (3) 813 K, pointing to the formation of Al9Ni2 (1), Al3Ni (2) and AlNi3 (3) [6]. The 10 h milled sample shows four major peaks centered at (1) 516 K, (2) 557 K, (3) 645 K and (4) 732 K. The peaks are most probably associated with the formation of Al9Ni2 (1), Al3Ni (2)(3) and AlNi3 (4), similar to those observed in nanoscale multilayer foils with a bilayer thickness of 200 nm, indicating that the lamellar structure formed during milling has a similar characteristic thickness [6]. The double peak for the Al3Ni arises from kinetic reasons [6,30]. The 20 h milled sample shows only one small peak centered around w510 K, which might correspond to the formation of Al9Ni2 or a relaxation process such as stress relief or recrystallization. All the samples show the large endothermic peak associated with the melting of AlNi3, starting at around w1630 K. 4. Discussion Fig. 3. XRD spectra of the samples before and after ignition after 5, 7, 10 and 20 h ball milling. * denotes the peaks that either correspond to a disordered AlNi3 phase [13], or to a very fine mixture of AlNi, AlNi3 and nickel-rich solid solution.
The observed microstructural refinement with increasing milling time is expected as the number of collision events and deformation efficiency increases. However, the 20 h milled samples point out to
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Fig. 4. EDX analysis performed on 10 h ignited samples shows the formation of different phases along the particle boundaries. The measured compositions in (a) are (A) 100 at.% Ni, (C) 65 at.% Ni and (D) 48 at.% Ni. The EDX elemental mapping indicates extensive aluminum-rich liquid diffusion within nickel particles marked by (B). Oxide formation is limited to the surfaces of the powders.
a critical milling time between 10 h and 20 h, at which the powders undergo a transformation. The welding of the powders to form large agglomerates at the bottom of the vial and the surface of the steel balls suggests exposure to very high temperatures, where at least parts of the powders should be molten. This is also supported by the mostly pore-free microstructure in Fig. 2d, as opposed to the ignited 10 h sample in Fig. 4. A shock assisted transformation occurring during ball impacts would not propagate to most of the powders, which are accumulated at the bottom of the vial. Furthermore, gradual transformations would not result in such clumps, but rather form loose powders. It is more likely that the formation and refinement of the lamellar structure (yielding decreased diffusion distances) permit the rapid initiation of SPER during the collision events, which can sufficiently increase the local temperatures [12]. At this stage, the reaction products covering the surface of the balls, which are molten due to high formation enthalpy, would be ejected to the rest of the powder sitting at the bottom of the vial and propagate the reaction. It has been speculated that ball milled powders equivalent to the 20 h samples in this study show the formation of a single phase AlNi3 with the absence of superlattice peaks, which is then attributed to this phase being disordered [13].
However, closer examination of the XRD results (Fig. 3) reveal that this final product might instead be a very fine mixture with compositions ranging from extended nickel-rich solid solutions and AlNi3 to AlNi with broad overlapping peaks. The BSE image in Fig. 2d clearly shows some compositional variation within the particles, in accordance with this view. In fact, it is most probable that the powders rapidly react and melt partially at the interfaces to form AlNi, which is followed by excessive aluminum diffusion into the nickel layers to form AlNi3 and extended solid solutions. This is also suggested by the microstructure of the ignited 10 h milled sample in Fig. 4, which exhibits planar reaction products at the particle interfaces and large aluminum diffusion distances into the nickel particles. The formation of AlNi is facilitated by its high formation enthalpy implying a high driving force, the ease of nucleation due to the relatively simple crystal structure, and due to the interface compositions inherently being close to that of AlNi in the absence of other intermetallic compounds. Further milling after the critical milling time would only induce additional mixing that might form the structures observed in Fig. 2d. The lack of post-milling ignition and any significant change in the XRD spectrum of the 20 h sample is due to the stability of the reaction products, namely AlNi, AlNi3 and nickel-rich solid solutions, which have very high melting points and low excess enthalpy. The relatively small peak observed in DTA points out to an additional reaction, but it is not clear whether it corresponds to the formation of Al9Ni2 or a relaxation process. The low ignition sensitivity of the samples milled for up to 10 h, compared to sputtered nanoscale multilayer foils, is probably due to the remaining nickel particles which might act as heat sinks, as well as persisting voids, lack of metallurgical bonding, and good thermal contact during cold compaction of the powders into pellets. 5. Conclusions
Fig. 5. DTA results for the as milled 5, 7, 10 and 20 h samples.
Ball milling (BM) of pure aluminum and nickel particles are shown to yield powders with fine nanoscale lamellar microstructures with minimal oxidation. Furthermore, pellets produced by compacted powders can be successfully ignited to produce self-propagating exothermic reactions (SPER). Therefore, BM is shown to be a viable method for generating heat sources for nanoscale heating systems for thermal applications. The critical threshold for the reactions during milling is determined to be between 10 h and 20 h under the studied
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process parameters. It is also shown that SPER during BM starts with impact-induced local temperature rise after the powders form nanoscale lamellae, followed by the ejection of the molten reaction products from the ball surfaces that spread the reaction to the rest of the powders. The milling parameters can be optimized further to yield highly reactive compacts with nanoscale microstructures. Acknowledgements The authors would like to thank the European Commission for research funding of this project under Marie Curie Excellence Team e NanoHeaters (EXT-0023899), EC/FP6 IP Programme “ManuDirect” and EX/FP7 ICT Programme “NanoMA”, Paul Mayrhofer’s research group at the University of Leoben for the EDX analysis, the Department of Chemistry at the University of Cyprus for access to XRD instrumentation and Mr. K. Fadenberger for constructive discussions on DTA analysis and reactive nanofoils. References [1] Jogdand H, Gulsoy G, Ando T, Chen J, Doumanidis CC, Gu Z, et al. Fabrication and characterization of nanoscale heating sources (nanoheaters) for nanomanufacturing. NSTI-Nanotech 2008;1:280e3. [2] Gunduz IE, Fadenberger K, Kokonou M, Rebholz C, Doumanidis CC. Investigations on the self-propagating reactions of nickel and aluminum multilayered foils. Appl Phys Lett 2008;93:134101. [3] Gunduz IE, Fadenberger K, Kokonou M, Rebholz C, Doumanidis CC, Ando T. Modeling of the self-propagating reactions of nickel and aluminium multilayered foils. J Appl Phys 2009;105:074903. [4] Dyer TS, Munir ZA, Ruth V. The combustion synthesis of multilayer NiAl systems. Scripta Metall Mater 1994;30(10):1281e6. [5] Mann AB, Gavens AJ, Reiss ME, Heerden DV, Bao G, Weihs TP. Modeling and characterizing the propagation velocity of exothermic reactions in multilayer foils. J Appl Phys 1997;82(3):1178e88. [6] Blobaum KJ, Heerden DV, Gavens AJ, Weihs TP. Al/Ni formation reactions: characterization of the metastable Al9Ni2 phase and analysis of its formation. Acta Mater 2003;51:3871e84. [7] Trenkle JC, Koerner LJ, Tate MW, Gruner SM, Weihs TP, Hufnagel TC. Phase transformations during rapid heating of Al/Ni multilayer foils. Appl Phys Lett 2008;93:081903. [8] Takacs L. Self-sustaining reactions induced by ball milling. Prog Mater Sci 2002;47:355e414. [9] Yi HC, Moore JJ. Self-propagating high-temperature (combustion) synthesis (SHS) of powder-compacted materials. J Mater Sci 1990;25:1159e68.
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