A flake powder metallurgy approach to Al 2 O 3 /Al biomimetic nanolaminated composites with enhanced ductility Lin Jiang, Zhiqiang Li, ⇑ Genlian Fan and Di Zhang State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China Received 4 March 2011; revised 13 May 2011; accepted 17 May 2011 Available online 23 May 2011 A simple and scalable methodology called “flake powder metallurgy” (flake PM) has been developed to fabricate biomimetic Al 2 O 3 /Al composites. Nanoflake Al powders with native Al 2 O 3 skins were used as building blocks to rapidly assemble into biomi- metic nanolaminated structures via compacting and extrusion, giving rise to strong and ductile composites with tensile strength of 262 MPa and plasticity of 22.9%. Thus, it is evidenced that well-balanced strength and ductility can be achieved in biomimetic metal–matrix composites by flake PM. Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Metal–matrix composites; Nanocomposite; Layered structures; Ductility; Flake powder metallurgy To improve the energy efficiency of structures, the applications of metal–matrix composites (MMCs) have been greatly increased in the automobile, aerospace and electronic industries due to their lighter weight and greater strength and stiffness over conventional metals and alloys [1]. However, due to the incorporation of par- ticulate and fibrous reinforcements, MMCs usually exhi- bit lower ductility and toughness, and thus have a lower damage tolerance in comparison with monolithic metals and alloys [2–4], which is an obvious drawback to their practical application. Learning from natural biological design is becoming a prevailing idea in developing new generations of synthetic materials. In the quest for strong and ductile solutions for struc- tural composites, nacre has gained tremendous interest as a model system. In nacre, the laminated assembly of alternating protein collagen layers (10–50 nm thick) and aragonite tablets (200–900 nm thick) contributes to remarkable mechanical properties far beyond that which can be achieved by man-made composites [5,6]. Ji and Gao [7] concluded from studying biomechanics that the nanoscale is the key factor for the excellent properties of such biological materials as nacre and bone, at which material failure occurs at the theoretical strength irregardless of the presence of any flaws. For this reason, there are increasing efforts to artificially cre- ate or biomimick nacre’s nanolaminated structures in MMCs. However, this task is hampered by the fact that there is no effective method of processing to implement the nanolaminated design. A recent attempt was made by Ritchie and co-workers [8,9], who used ice-templated Al 2 O 3 preforms and subsequent metal infiltration to fab- ricate Al 2 O 3 /Al–Si laminated composites with lamellae thicknesses down to 10 lm, and they achieved a tensile strength of approximately 300 MPa. Accumulative roll bonding [10] and nanostructured multilayer techniques such as physical or chemical depo- sition [11,12] are also feasible methods to obtain submi- cron lamellar thicknesses in MMCs, but these methods are inherently laborious and time-consuming, thus are restricted to the fabrication of miniature sized sheets and films [8,13]. Therefore, conceptually new methodol- ogies are needed to mass produce bulk MMCs with a nacre-like nanolaminated structure. Herein, we report a simple, quick, and mass-produc- ing approach called “flake powder metallurgy” (flake PM), in which Al flake powders with native Al 2 O 3 skins and a two-dimensional (2-D) planar morphology are used as building blocks for assembly into Al 2 O 3 /Al nan- olaminated composites. Normally, in the widely used powder metallurgy routes, spherical metal powders serve as the building blocks to make up dense materials by a process of compacting and secondary deformation. Little attention is paid to the control of the shape and the stacking mode of the metal powders, thus the MMC’s structure is left to evolve randomly during the subsequent consolidation process. The proposed flake PM essentially exploits the fact that well-defined, flake 1359-6462/$ - see front matter Ó 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2011.05.022 ⇑ Corresponding author. Tel.: +86 21 3420 2584; fax: +86 21 3420 3913; e-mail: [email protected]Available online at www.sciencedirect.com Scripta Materialia 65 (2011) 412–415 www.elsevier.com/locate/scriptamat
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Available online at www.sciencedirect.com
Scripta Materialia 65 (2011) 412–415
www.elsevier.com/locate/scriptamat
A flake powder metallurgy approach to Al2O3/Al biomimeticnanolaminated composites with enhanced ductility
Lin Jiang, Zhiqiang Li,⇑ Genlian Fan and Di Zhang
State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China
Received 4 March 2011; revised 13 May 2011; accepted 17 May 2011Available online 23 May 2011
A simple and scalable methodology called “flake powder metallurgy” (flake PM) has been developed to fabricate biomimeticAl2O3/Al composites. Nanoflake Al powders with native Al2O3 skins were used as building blocks to rapidly assemble into biomi-metic nanolaminated structures via compacting and extrusion, giving rise to strong and ductile composites with tensile strength of262 MPa and plasticity of 22.9%. Thus, it is evidenced that well-balanced strength and ductility can be achieved in biomimeticmetal–matrix composites by flake PM.� 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
To improve the energy efficiency of structures,the applications of metal–matrix composites (MMCs)have been greatly increased in the automobile, aerospaceand electronic industries due to their lighter weight andgreater strength and stiffness over conventional metalsand alloys [1]. However, due to the incorporation of par-ticulate and fibrous reinforcements, MMCs usually exhi-bit lower ductility and toughness, and thus have a lowerdamage tolerance in comparison with monolithic metalsand alloys [2–4], which is an obvious drawback to theirpractical application. Learning from natural biologicaldesign is becoming a prevailing idea in developing newgenerations of synthetic materials.
In the quest for strong and ductile solutions for struc-tural composites, nacre has gained tremendous interestas a model system. In nacre, the laminated assemblyof alternating protein collagen layers (10–50 nm thick)and aragonite tablets (200–900 nm thick) contributesto remarkable mechanical properties far beyond thatwhich can be achieved by man-made composites [5,6].Ji and Gao [7] concluded from studying biomechanicsthat the nanoscale is the key factor for the excellentproperties of such biological materials as nacre andbone, at which material failure occurs at the theoreticalstrength irregardless of the presence of any flaws. Forthis reason, there are increasing efforts to artificially cre-ate or biomimick nacre’s nanolaminated structures in
1359-6462/$ - see front matter � 2011 Acta Materialia Inc. Published by Eldoi:10.1016/j.scriptamat.2011.05.022
MMCs. However, this task is hampered by the fact thatthere is no effective method of processing to implementthe nanolaminated design. A recent attempt was madeby Ritchie and co-workers [8,9], who used ice-templatedAl2O3 preforms and subsequent metal infiltration to fab-ricate Al2O3/Al–Si laminated composites with lamellaethicknesses down to 10 lm, and they achieved a tensilestrength of approximately 300 MPa.
Accumulative roll bonding [10] and nanostructuredmultilayer techniques such as physical or chemical depo-sition [11,12] are also feasible methods to obtain submi-cron lamellar thicknesses in MMCs, but these methodsare inherently laborious and time-consuming, thus arerestricted to the fabrication of miniature sized sheetsand films [8,13]. Therefore, conceptually new methodol-ogies are needed to mass produce bulk MMCs with anacre-like nanolaminated structure.
Herein, we report a simple, quick, and mass-produc-ing approach called “flake powder metallurgy” (flakePM), in which Al flake powders with native Al2O3 skinsand a two-dimensional (2-D) planar morphology areused as building blocks for assembly into Al2O3/Al nan-olaminated composites. Normally, in the widely usedpowder metallurgy routes, spherical metal powdersserve as the building blocks to make up dense materialsby a process of compacting and secondary deformation.Little attention is paid to the control of the shape andthe stacking mode of the metal powders, thus theMMC’s structure is left to evolve randomly during thesubsequent consolidation process. The proposed flakePM essentially exploits the fact that well-defined, flake
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(platelet)-shaped building blocks can be ordered intoastonishingly well-aligned macroscopic assemblies evenunder common and time-efficient processes, such ashot pressing and extrusion. The primary result showsthat, using the flake PM route, Al2O3/Al nanolaminatedcomposites with an enhanced tensile plasticity of 22.9%at a strength of 262 MPa can be achieved.
In a typical flake PM process, three steps are in-volved: (i) preparation of flake powders. The flake Alpowders can be easily obtained by microrolling (ballmilling) the spherical powder, as illustrated in Figure1a. In our experiment, a near-spherical powder (10 lmin diameter and 99.5 wt.% in purity) with 1 wt.% stearicacid can quickly reach a flake thickness of several hun-dred nanometers by ball milling in an attritor at423 rpm at room temperature for only 1–2 h. (ii) Thein situ introduction of Al2O3. The as-prepared flakepowder is heated in a flowing Ar atmosphere at 400 �Cfor 1 h to remove all stearic acid from the powder andthen kept in air at room temperature for several daysto grow a native Al2O3 membrane, as illustrated in Fig-ure 1b. (iii) Alignment and consolidation of the flakepowder. Compacting was used to align the flake pow-ders into a column (U40 � 30 mm) under 500 MPa pres-sure. As illustrated in Figure 1c, the flake powders witha native Al2O3 membrane tend to lie flat on each otherin an irregular manner, so that the potential energy isminimized under self-gravitational force and/or the ap-plied force fields, in the same way that the leaves falland spread flatly on the ground due to the gravity. Aftercompacting, a laminated structure with a relatively lowdensity (70–85%) can be achieved and the direction ofthe compacted powder layer is perpendicular to the axisof the column. Sintering in a flowing Ar atmosphere at630 �C for 2 h and hot extrusion at 400 �C with an extru-sion ratio of 20:1 at a ram speed of 0.5 mm�min�1 con-solidates the flake powders.
After extrusion, the density of the rods can be over99% and the alignment of the layers is enhanced. Theextrusion direction is parallel to the axis of the column,so the layer direction of the final product is changed toparallel to the axis of the rods. The final diameter of theextruded rods is 8 mm and there is no cracking on thesurface of the rods. Moreover, there is no obvious differ-ence in the periodicity of the layers between the centerand the edge of the rods.
Having produced the nanolaminated structure, it wasthen characterized by field emission scanning electronmicroscopy (FESEM) using a LEO Supra 55 FESEMand by high-resolution transmission electron
Figure 1. Self-assembly strategy for the preparation of Al2O3/Al biomimetic(b) in situ introduction of Al2O3; (c) consolidation.
microscopy (HRTEM) in a Philips CM200 microscopeoperated at 200 kV. The Al2O3 phase in the compositewas analyzed by X-ray diffraction (XRD; Rigaku andCN2301) with a Cu Ka radiation source. The chemicalcompositions of the flake powders and extrusion rodswere analyzed using inductively coupled plasma–opticalemission spectroscopy to determine the amount of oxidelayers. To evaluate the tensile strength, specimens weremachined from the extruded rods with the tensile axisparallel to the extrusion direction. The gauge length ofthe specimens was 25 mm and the diameter was 5 mm.The tensile strength was measured with a universal test-ing machine at an initial strain rate of 5 � 10�4 s�1 atroom temperature (AUTOGRAPH AG-I 50 KN, Shi-madzu Co. Ltd., Japan).
As seen from Figure 2a, the as-prepared flake Alpowder has a 2-D planar morphology with an averagediameter of 70 lm, thus giving a large aspect ratio of140 (diameter to thickness). Additionally, due to theuse of stearic acid during ball milling, the flake powdersso produced tend to be individual platelets, as seen inFigure 2a, which is helpful for the alignment and consol-idation of the flake powder. Observing the flake powderunder high magnification (Fig. 2a inset), we can see thatthere exists a very thin outer Al2O3 layer, which shows aquite different contrast to the Al core. As seen in Figure2b, the compacting column of flake powders exhibits astructure with a strikingly strong alignment of flakepowders, and the high-resolution SEM image (Fig. 2c)reveals well-defined and highly aligned self-assemblieswith a periodicity of 500–600 nm.
Typical optical microscopy (OM) of the extrusioncomposite shows the overall microstructures in boththe vertical (extrusion) and transverse directions. Ascan be seen in Figure 3a and b, the transverse view(Fig. 3a) presents polygonal platelets with differentdimensions and the vertical section (Fig. 3b) shows thatall the platelets are organized with their faces parallel tothe extrusion direction. So, the highly ordered nanola-minated structure was well maintained after consolida-tion. The Al2O3 layers were so thin that it was difficultto observe them in both the SEM and OM images. How-ever, from the TEM image (Fig. 3c), the extruded mul-tilayer structures with alternating Al (300–500 nm) andAl2O3 (�10 nm) layers can be readily observed fromthe vertical section of the extrusion samples. TheAl2O3 layers originate from the native oxide skin andtheir crystal structures can be controlled by the sinteringtemperature. It has been reported that the air-formedoxide layer, which is mostly viewed as amorphous, expe-
nanolaminated composites by flake PM: (a) flake powder preparation;
Figure 2. FESEM of: (a) flake Al powder (the inset shows the native Al2O3 skin on the Al surface); (b) self-assembled nanolaminated structure; (c)magnified image of (b).
Figure 3. OM images of the Al2O3/Al nanolaminated composites in (a)the transverse direction and (b) the vertical direction; (c) TEM imagesshowing the laminate structure; (d) HRTEM showing the crystallinestructure of the Al2O3 layer (the inset shows the XRD pattern of thenanolaminated composites).
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riences a gradual transition from amorphous to crystal-line Al2O3 with increasing sintering temperature [14]. Inthis study, the sintering temperature of the compositereached 630 �C, so the amorphous oxide layers predom-inantly transformed to crystalline c-Al2O3 phases(Fig. 3d), as revealed by HRTEM examination and fur-ther confirmed by XRD (inset of Fig. 3d). The transfor-mation of the Al2O3 oxide from the amorphous to thecrystalline phase has been proven to enhance the ductil-ity [15].
The tensile stress–strain curve of the Al2O3/Al nan-olaminated composites is shown in Fig. 4a, which
Figure 4. (a) Tensile properties and (b) fracture sur
displays a plasticity of 22.9% at a tensile strength of262 MPa. Even more important, the Al2O3/Al nanola-minated composites have a uniform elongation of16.5%, which is much higher than the critical ductility(5%) that is required for many structural applications.It is supposed that such a remarkable uniform elonga-tion comes from the nanoscaled Al2O3 layers that char-acterize the laminated structure. The nanoscaled Al2O3
layers may act to hinder the recovery and recrystalliza-tion processes of Al matrix, thus significantly increasingthe strain hardening capacity. The ductile failure of theAl2O3/Al nanolaminated composites can also be seenfrom the fracture surface shown in Figure 4b: the elon-gated dimples exhibited a size about 300–500 nm, whichis similar to the thickness of the laminated Al layers.
A better appreciation of the unique mechanical prop-erties of the Al2O3/Al biomimetic nanolaminated com-posites can be gained by comparing them with thosefabricated by severe plastic deformation (SPD) methods,such as equal channel angular pressing (ECAP) [16],high-pressure torsion (HPT) straining [17] and frictionstir process (FSP) [18]. The room-temperature tensileproperties of Al2O3/Al composites produced by the dif-ferent processes are summarized in Table 1. It is interest-ing to observe that ultrafine-grained Al fabricated byECAP [16] has a very low uniform elongation (strain be-fore necking), of less than 1%. It has been argued thatthe low uniform elongation of ultrafine-grained andnanocrystalline metals is a result of diminishing strainhardening capacity and insufficient strain rate hardeningdue to the intrinsic difficulty in keeping dislocations in-side the tiny grains [18]. Strengthening metallic materialswithout a substantial degradation of strain hardeningcapacity can also be achieved by FSP or HPT. Forexample, Hu et al. [18] inserted 15 wt.% a-Al2O3 disper-soids (�30 nm) homogeneously into pure Al by FSP to
face of Al/Al2O3 nanolaminated composites.
Table 1. Room-temperature tensile properties of Al2O3/Al composites.
Ref. Al2O3 (wt%) Grain size(nm)
Strength (MPa) Uniform elongation (%) Total elongation (%) Method Testing direction
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significantly increase the work hardening, and thus thetensile elongation was enhanced.
The lamellar structure, which enables the full poten-tial of Al2O3 to hinder recovery and keep dislocationsin the small grains, is the main reason for the increaseduniform elongation in the flake PM materials. TheAl2O3 in the flake PM materials has a 2-D planar mor-phology with a large aspect ratio, and the efficiency ofAl2O3 in hindering recovery in the lamellar structure isrelatively higher than that of Al2O3 in the FSP materials.Thus, increased uniform elongation appears in the flakePM materials. Moreover, the improved total elongationof the flake PM materials may be also attributed to thetexture forming in the Al layers, the crack deflection atthe interface between the layers and the crack tip blunt-ing due to nanoplasticity at the interface [19]. Contin-uing work will focus on fully decoding the underlyingmechanisms behind these unique properties.
Finally, it worth mentioning here that, althoughremarkable success has been achieved in enhancing ten-sile elongation in laboratory-scale models by HPT orFSP, it remains difficult to foresee the potential indus-trial applications of such time-consuming and energy-intensive processes. Compared with the reported SPDmethods, the novel approach of the flake PM describedin this research is simple, feasible and applicable formass production.
In summary, utilizing nature’s solution for strong andductile design, we developed an upgraded flake PM tech-nique to fabricate bulk metal–matrix composites with arefined nanolaminated structure over large-scale dimen-sions. The resultant Al2O3/Al biomimetic compositesexhibited excellent combinations of strength and ductil-ity, which indicates that the proposed flake powder met-allurgy is a very effective way to fabricate a biomimeticnanolaminated metal–matrix composite with well-bal-anced mechanical properties. Furthermore, the mechan-ical properties should be further improved if thelamellae thicknesses could be reduced down to tens ofnanometers from the present 300–500 nm. Thus, our fu-ture studies will focus on the manipulation of the mor-phology of the flake powders, the refinement of the
lamellae thicknesses and extending the flake PM tech-nique to other alloy composites.
We acknowledge the financial support from theNational Natural Science Foundation (Nos. 50890174and 51071100) and the International S&T CooperationProgram (No. 2010DFA52550) of China.
