Dynamic mechanical response of a 20 vol% silicon carbide particles (SiCp) reinforced 2024 Al composite prepared by powder metallurgy techniques were studied with a split Hopkinson bar. The fracture mechanisms and the deformation microstructure were examined with Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The present results indicate that the composite has a strong SiC-Al interfacial bonding; failure of the material is mainly caused by fracture of SiC particles and tearing failure of the SiC-Al interface. This failure by interface tearing with adhesion of an aluminium layer on SiC particles on the fracture surfaces has not been reported in SiC particle-reinforced aluminium composites. High-resolution transmission electron microscopy studies showed that many of the SiC-Al interfaces have coincident site lattice structures, which are considered to make a significant contribution to the strong interfacial bonding

Lee CS

Li RKY

Lu YX

Lu, YX (reprint author), City Univ Hong Kong, Dept Phys & Mat Sci, Tat Chee Ave, Hong Kong, Hong Kong.

Meng XM

黄晨光

Institute Of Mechanics,Chinese Academy of Sciences

phys. stat. sol. (a) 169, 49 (1998)Subject classification: 62.20.Fe; 62.20.Mk; 68.35.Gy; S4; S6Failure Mechanisms of a SiC Particles/2024Al Compositeunder Dynamic LoadingY.X. Lu (a, b), X.M. Meng (a, b), C.S. Lee (a), R.K.Y. Li (a),and C.G. Huang (a, c)(a) Department of Physics and Materials Science, City University of Hong Kong,Tat Chee Avenue, Hong Kong(b) Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110015,People's Republic of China(c) Institute of Mechanics, Chinese Academy of Sciences, Beijing,People's Republic of China(Received April 28, 1998; in revised form July 14, 1998)Dynamic mechanical response of a 20 vol% silicon carbide particles (SiCp) reinforced 2024 Alcomposite prepared by powder metallurgy techniques were studied with a split Hopkinson bar.The fracture mechanisms and the deformation microstructure were examined with Scanning Elec-tron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The present results indi-cate that the composite has a strong SiC±Al interfacial bonding; failure of the material is mainlycaused by fracture of SiC particles and tearing failure of the SiC±Al interface. This failure byinterface tearing with adhesion of an aluminium layer on SiC particles on the fracture surfaces hasnot been reported in SiC particle±reinforced aluminium composites. High-resolution transmissionelectron microscopy studies showed that many of the SiC±Al interfaces have coincident site latticestructures, which are considered to make a significant contribution to the strong interfacial bond-ing.1. IntroductionSiC reinforced aluminium composites have various superior properties including highelastic moduli, high yield strength and wear resistance. These composites can also beprocessed by conventional metal processing techniques at reasonable cost. Thus, SiC/Alcomposites have good potential for large-scale applications in various structural compo-nents. In order to improve their fracture resistance, failure mechanisms of these compo-sites have been extensively studied in the past decade [1 to 6]. These works suggestedthat the failure mechanisms are related to various factors including size, volume frac-tion and distribution of the SiC phase, chemical composition and heat treatment of thematrix as well as processing history of the composites. Based on TEM observations,Nutt and Duva [7] suggested that failure of SiC whisker-reinforced aluminium compo-sites is mainly initiated by nucleation and growth of voids and cracks at the corners ofthe whiskers where the stress concentration is high. These cracks would then propagatealong the SiC/Al interface and lead to the final fracture. While this failure mechanismfor SiC whisker-reinforced aluminium composites is widely accepted, there is no singlegenerally agreed failure mechanism for SiC particle-reinforced aluminium composites.Most commonly reported failure mechanisms are ductile failure of the matrix [3, 8],Y.X. Lu et al.: Failure Mechanisms of a SiC Particles/2024Al Composite 49fracture of the reinforcement particles [9 to 11] and interfacial debonding [10, 11].Lloyd et al. [12] attributed the three types of failure to the relative strength of thematrix, reinforcement phase and their interfacial bonding. When the SiC±Al interfacialbonding is weak, the composite will fail by interfacial debonding. In composites withgood interfacial bonding but low matrix strength, failure of the composites will be in-itiated by ductile yielding of the matrix. When strengths of both the matrix and theinterfacial bonding are high, the composite will fail by brittle fracture of the reinforce-ment phase.While substantial works have been done on the fracture mechanisms of SiCp/Al com-posites under static loading, there are relative few works on the fracture mechanismunder dynamic loading [14, 15]. In particular, the present understanding on the relation-ships between the mechanical properties, fracture mechanisms and microstructure arestill far from adequate. These understandings are obviously needed for large-scale in-dustrial applications of the materials. In the present work, the deformation microstruc-ture and fracture mechanisms of a 20 vol% 3.5 mm SiCp/2024Al composite deformedunder dynamic loading conditions were studied.2. Materials and ExperimentalThe SiCp/2024Al composite used in the present paper was fabricated by a powder me-tallurgy process using 2024Al powder of 280 grits and SiC particles with an averagegrain size of 3.5 mm. The mixed powder with a SiC volume fraction of 20% was com-pacted by hot compression under vacuum. The compacted powder was then extrudedat 450 C to rods of 1.2 cm diameter with an extrusion ratio of 40 : 1. The compositewas then solution treated at 500 C for 1 h followed by water quenching and subse-quently ageing at 170 C for a duration of 6 h (T6 treatment). The microstructure ofthe as-prepared composite is shown in Fig. 1, which indicates that SiC particles areevenly distributed and there is no observable defect. Tensile samples of 0.32 cm diam-eter  1 cm gauge length and compression samples of 0.8 cm diameter  0.6 cm height50 Y.X. Lu, X.M. Meng, C.S. Lee, R.K.Y. Li, and C.G. HuangFig. 1. Optical micrograph of the as-prepared 20 vol% 3.5 mm SiCp/2024Al compositewere machined from the rod along the extrusion direction. Dynamic tensile and com-pression tests were carried out with a split Hopkinson bar at a strain rate of 103 s± 1.The working principle and set-up of a Hopkinson bar can be found in the literature[16]. TEM specimens were made from thin slices of about 150 mm cut parallel to theextrusion direction. The slices were mechanically ground to 70 mm, dimpled to 25 mmand finally ion thinned at 4 kV with an incident angle of 11. TEM specimens wereexamined with either a Philips CM20 or a JEOL 2000EX operated at 200 kV. Fracturesurfaces of the tensile samples were observed with a SEM. The microstructure and thedistribution of SiC particles were observed with an optical microscope.3. Results and DiscussionDynamic stress±strain curves of the composite tested in tension and in compression ata strain rate of 103 s± 1 are shown in Fig. 2. Work hardening can be observed in theinitial stage of the plastic deformation. The flow stresses reach their saturation valuesafter about 5% strain. The tensile flow stress is higher than the compressive flow stressat all strains. This asymmetry of the tensile and compressive strength is due to the inter-nal residual stresses, originating in the difference in thermal expansion coefficient of thematrix and SiC particles, generated during the production process of the composite[13].The tensile fracture surface shows features of brittle fracture at a relatively macro-scopic scale (Fig. 3). However, in a microscopic view, dimples showing ductile shearingcan also be seen. Dimples observed are of two different size ranges. The large and thesmall dimples have sizes similar to those of the SiC particles and precipitated phases(about 1 mm size), respectively. SiC particles found on the fracture surface are of twodifferent types of feature. The first type shows a smooth and flat fracture surface alongthe cleavage planes of SiC. As SiC has several low index cleavage planes, the fractureFailure Mechanisms of a SiC Particles/2024Al Composite under Dynamical Load 51Fig. 2. Stress±strain response of the composite at a strain rate of 103 s± 1of SiC particles can proceed with cleavage along several planes simultaneously andgives rise to secondary cracks (marker 1 in Fig. 3). Some SiC particles on the fracturesurface show a ductile tearing failure along the SiC±Al interface. This type of failure isdifferent from the commonly observed interfacial debonding which would give rise to amuch smoother fracture surface. Marker 2 in Fig. 3 shows a SiC particle adhering withridges originated from tearing of the aluminium matrix. This was also verified by EDXanalysis which showed that the aluminium content at marker 2 is much higher than that52 Y.X. Lu, X.M. Meng, C.S. Lee, R.K.Y. Li, and C.G. HuangFig. 3. Tensile fracture surface of the SiCp/2024Al compositeFig. 4. EDX spectra taken at a)marker 1 and b) marker 2 of thesample as shown in Fig. 3at marker 1 (Fig. 4). This tearing failure near the SiCp/Al interface is similar to thecorresponding observation in SiC whisker-reinforced Al composite [17]. However, tothe authors' knowledge, it has not been reported in SiC particles reinforced aluminiumcomposites. The tearing failure indicates that the composite used here has a highSiC±Al interfacial bonding strength.TEM observations of the dynamically loaded samples show that various forms ofdefects were accumulated in the SiC particles. In addition to stacking faults, locationswith high stress concentration, such as sharp corners of the SiC particles, were oftenfractured (Fig. 5). The dislocation density increased tremendously in both the interfaceand the matrix regions. Fracture of SiC was also observed with the cracks usually in-itiated from locations of high stress concentrations such as stacking faults or corners ofthe SiC particles (Fig. 6). Tearing failure found in the fracture surface (Fig. 3) and adhe-Failure Mechanisms of a SiC Particles/2024Al Composite under Dynamical Load 53Fig. 5. A TEM micrograph of the SiCp/2024Al composite after dynamic loading showing damagesin the microstructureFig. 6. A TEM micrograph show-ing cracking of a SiC particle afterdynamic loadingsion of aluminium to the SiC particle (indicated by an arrow) can also be observed inFig. 7.High-resolution TEM studies of the SiC±Al interface show that many of the SiCparticles have coincident lattice sites with the matrix along the interface. Fig. 8 showsone of these interfaces (marked by double arrows), where the orientation relationship110Al k 2110SiC; 111Al k 0006SiC was observed (there is a small angle, about 2,between the Al (111 and the SiC (0006) planes). It can be seen that the interface is54 Y.X. Lu, X.M. Meng, C.S. Lee, R.K.Y. Li, and C.G. HuangFig. 7. A TEM micrograph show-ing tearing of an Al±SiC interfaceFig. 8. A high-resolution TEM micrograph showing a coincident site lattice relationship in anAl±SiC interface. Selected area electron diffraction pattern from the interface is shown in the insetsmooth with some mismatch dislocations (marked by single arrows) due to the differentspacings of the Al 111 and the SiC (0006) planes (0.234 and 0.251 nm, respectively).The high interfacial bonding strength in the present composite can be attributed tothese interfaces with coincident site lattice. More systematic high-resolution TEM workon this issue is under way to fully characterize the interfaces in the present composite.4. ConclusionsFailure mechanisms of a 20 vol% 3.5 mm SiCp/2024Al under dynamic loading conditionwere studied by SEM and TEM analysis. The present results indicated that the compo-site fails by tearing of the SiC±Al interface, which has not been reported in SiC parti-cles reinforced aluminium matrix composites. This failure mechanism is attributed tothe high interfacial bonding strength between the aluminium matrix and the SiC parti-cles. The high interfacial strength can, in turn, be explained by the observation thatmany of the Al±SiC boundaries have coincident site lattice relationships.Acknowledgement Financial support by the City University of Hong Kong throughresearch grant number 7000531 is gratefully acknowledged.References[1] A.P. Divecha, S.G. Fishman, and S.D. Karmaker, J. Metals 33, 12 (1981).[2] D.L. McDonels, Metals Trans. 16A, 1105 (1985).[3] C.P. You, A.W. Thompson, and I.M. Bernstein, Scripta Metall. 21, 181 (1987).[4] D.J. Lloyd, Acta Metall. et Mater. 39, 59 (1991).[5] J.C. Lee and K.N. Subramanian, J. Mater. Sci. 27, 5453 (1992).[6] B. Wang, G.M. Janowski, and B.R. Patterson, Metals Trans. 16A, 2457 (1995).[7] S.R. Nutt and J.M. Duva, Scripta Metall. 20, 1055 (1986).[8] R.J. Arsenault, N. Shi, C.R. Feng, and L. Wang, Mater. Sci. Engng. A 131, 56 (1991).[9] J. Lorca, A. Martin, J. Ruiz, and M. Elices, Metals Trans. 24A, 1575 (1993).[10] J.J. Lewandowaki, C. Liu, and W. H. Hunt, Mater. Sci. Engng. A 107, 241 (1989).[11] P. Mummery and B. Derby, Mater. Sci. Engng. A 135, 221 (1991).[12] D.J. Lloyd, H.P. Lagace, and A.D. Mcleod, Proc. ICCI-III, Ed. H. Ishinda, Elsevier Publ. Co.,Amsterdam 1990 (p. 359).[13] S.F. Corbin and D.S. Wilkinson, Acta Metall. et Mater. 42, 1319 (1994).[14] R.U. Vaidya, S.G. Song, and A.K. Zurek, Phil. Mag. A70, 819 (1994).[15] R.U. Vaidya and A.K. Zurek, J. Mater. Sci. 30, 2541 (1995).[16] P.S. Follansbee, Metals Handbook, 9th ed., Vol. 8, ASM, Metals Park, Ohio 1985 (p. 190).[17] J.G. Nieh, R.A. Rainen, and D.J. Chellman, Proc. ICCM5, San Diego, Eds. W.C. Harrigan, Jr.Strife, and A.K. Dhingra, TMS-AIME 1985 (p. 843).Failure Mechanisms of a SiC Particles/2024Al Composite under Dynamical Load 55

Failure mechanisms of a SiC particles 2024Al composite under dynamic loading

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Failure mechanisms of a SiC particles 2024Al composite under dynamic loading

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Institute Of Mechanics,Chinese Academy of Sciences