Nội dung

AJSTD Vol. 23 Issue 4 pp. 307-321 (2006) METAL COMPOSITES BEHAVIOUR UNDER BIAXIAL STRESSES J. Jai∗ Faculty of Chemical Engineering, Universiti Teknologi MARA, Malaysia M.N. Berhan Faculty of Mechanical Engineering, Universiti Teknologi MARA, Malaysia Received 07 August 2005 ABSTRACT For this study, different volume fraction (vol.%) of particulate alumina (Al2O3) reinforced aluminium alloy (Al 6061) with 5 vol.%, 15 vol.% and 25 vol.% are produced by powder metallurgy method. These samples were subjected to biaxial stresses in order to investigate the behaviour of the metal matrix composites (MMCs). Microstructure analysis on the individual sample before and after loading was performed under scanning electron microscopy. The small particles of 2 µm in size have exhibited strong interfacial bonding with the matrix. The particles of 5 µm in size have shown fractures and debonding interface. Large particles of above 20 µm in size have revealed severe fractures and particles pulled out. Behavior of the MMC was explained by relating the microstructures and displacement directions of the undeformed and deformed samples. Some understandings on the behaviour of the MMCs with different vol.% of Al2O3 due to biaxial stresses have been established. Keywords: Metal matrix composites, biaxial stresses, microstructures, displacement, fracture, debonding 1. INTRODUCTION Increasing demands on materials with high strength and stiffness with properties attainable at elevated temperatures have encouraged the development of the advanced composites [1]. A composite is produced when two materials are combined to produce a blend of properties that cannot be obtained in the original materials. In designing composite materials, engineers and scientists have combined various metals, ceramics and polymers to produce a new generation of materials. MMCs are manufactured for aerospace, automotive and other structural applications. Cost saving and weight reduction are among the topics most often discussed [2]. Weight saving is the challenge of the structural designers and materials specialists, whereas cost reduction is the daily request of the manufacturing department. This leads to the development of various processing routes with various types of metal matrices and reinforcements. The available ∗ Corresponding author e-mail: junejai@salam.uitm.edu.my J. Jai and M.N. Berhan Metal composites behaviour under biaxial stresses reinforcements in the market are carbides, nitrides, oxides as well as elemental materials, which are in the form of continuous fibres, chopped fibres, whiskers, platelets or particulates. The metal matrices obtainable are aluminium, magnesium, copper or titanium. The processing routes of the MMCs can be classified into three categories known as the liquid phase, solid phase and solid-liquid phases processes. Different materials of matrix and reinforcement with dissimilar processing method will give rise to different physical and mechanical properties of the produced composites. Besides cost saving and weight reduction, knowledge on the mechanical properties of the produced MMCs is very essential for its application purposes. The mechanical properties of the MMCs are dominated by the microstructures of the reinforcing phase, matrix alloying composition and reinforcement/matrix interfacial strength. The enhancement of composite mechanical properties is not only based on the volume fraction, size, shape and spatial distribution of the reinforcement, but is also dependent upon how well the externally applied load is transferred to the reinforcing phase [3]. Strength of the MMCs is based on the load transfer from matrix to the reinforcement. There are two types of strengthening mechanisms of the MMCs, direct and indirect, depending on the characteristics of the reinforcement [4]. Direct strengthening results by the load transfer from the matrix to the reinforcement. A strong interfacial bond facilitates the load transfer. Indirect strengthening results from the influence that the reinforcement may have on the matrix microstructure such as particle size and volume fraction. Investigations on the mechanical properties of MMCs have focused mainly on the determination of tensile characteristics. The performance of the MMC materials under compressive loading has received minor interest, in spite of the fact that MMCs have great potential in fields such as aviation and automotive components such as engine pistons, where knowledge of the compressive characteristics is essential. This research work focuses on the influence of different volume fraction of Al2O3 particulate reinforced aluminium metal matrix on the biaxial compressive characteristics. Behaviour of the MMCs with different volume fraction of the reinforcement experiencing biaxial compressive deformation loading has been investigated. 2. EXPERIMENTAL WORK 2.1 Raw materials In this research work alumina (Al2O3) particulate reinforced aluminium alloy (Al 6061) MMCs have been investigated. Shape of the Al 6061 was irregular while the Al2O3 was a combination of irregular and angular as shown in Fig. 1. Table 1 shows the chemical composition of the Al 6061 and the Al2O3. (a) Fig. 1: Particle shape of (a) Aluminium alloy and (b) Alumina 308 (b) AJSTD Vol. 23 Issue 4 Table 1: Chemical composition of Aluminium alloy and Alumina Aluminium alloy (6061 Al) Alumina (α-Al2O3) Elements Weight percent (wt %) Oxide Weight percent (wt %) Mg 0.64 Na2O 0.02 Si 0.66 SiO 0.05 Fe 0.38 Fe2O3 0.02 Cu 0.19 TiO2 0.01 Cr 0.17 MgO 0.01 Zc 0.11 CaO 0.03 Ti 0.04 K2 O 0.01 Al 83.1 α-Al2O3 99.85 2.2 Samples preparation Al2O3/Al 6061 MMCs with 5 percentage volume (vol.%), 15 vol.%, and 25 vol.% of Al2O3 were produced by powder metallurgy (PM) method using hot pressing technique. Figure 2(a) illustrates the processing flow in the production of the MMCs samples. Figure 2(b) illustrates the heating profile during the consolidation process. The solidified MMCs were in the shape of a flat circular disk with 35 mm diameter and 5 mm thickness. Figure 3 illustrates the method involved in the investigation of behaviour of the MMCs. All the samples were prepared in exactly the same way, comparisons between the undeformed and deformed samples should not be greatly affected by such uncertainties. Zinc stearate Al2O3 5 vol.% 15 vol.% 25 vol.% Formulation Al 6061 Mixing Vacuum hot pressing 280 MPa 10 min Temperature (oC) Cold compaction 580 10 min 450 Time (min) Sample ready for analysis (a) (b) Fig. 2: Fabrication of the MMCs. (a): Processing flow in producing the MMC samples; (b): Heating profile during the vacuum hot pressing process 309 J. Jai and M.N. Berhan Metal composites behaviour under biaxial stresses MMCs Samples Polishing and Grinding Microstructures analysis by SEM Deformation Compression Tests Fig. 3: Research methods in the study of behaviour of the MMCs 2.3 Deformation test In this study the effect of biaxial stresses on the behaviour of the MMCs is investigated. The MMC sample was placed on top of the bottom ring plunger, which was set to a fixed condition, while the upper ring plunger was appointed to move downward at the ramp speed of 1.5 mm/min in room condition as illustrated in Fig. 4(a). The effect of uniaxial loading introduces biaxial stresses at the bottom of the MMC samples as illustrated in Fig. 4(b). Biaxial strain gauge was cemented at the bottom of the sample which is experiencing the maximum tensile stress. The strain, stress, modulus and displacement of the samples were recorded as a function of the applied load. For this investigation the deformation test was performed in the tensile machine (Instron 100kN-UTM 5582E16). Due to samples limitation, a maximum of two samples per each test analysis were used. σττ F R1 σrr Sample σττ R σττ = tangential stress σrr = radial stress (a) (b) Fig. 4: (a): Diagram of circular plate sample subjected to ring load F, distributed around a circular sample of radius R1; (b): The stress components at the bottom surface of the sample after compressive loading 2.4 Microstructure analysis The undeformed MMC samples were observed under SEM (Philips-XL 40) for any defect such as in the matrix, individual particles and particle clusters. Similarly, the deformed samples were 310 AJSTD Vol. 23 Issue 4 also observed under the SEM, which focuses on the bottom surface of the samples. Cracks propagation, cracks initiation, void, interface, particle clusters, individual particle and matrix cracks that exist in the microstructures were observed. 3. RESULTS AND DISCUSSION 3.1 Undeformed MMC samples The microstructures of undeformed 5 vol.% Al2O3 MMC samples revealed that there were small and large clusters of Al2O3 particles randomly distributed in the MMCs as shown in Fig. 5(a). The small number of particles within the cluster indicates small clusters while the large number of particles within the clusters shows large clusters. From the observation it has been shown that the large clusters actually consist of small particles. Most of the individual large particles were seriously fractured especially for the particles of 20 µm in size and lager as shown in Fig. 5(b). Intergranular cracks and voids at the interface of Al2O3 particles have been observed in some of the small particles of 5 µm in size as shown in Figs. 5(c) and (d). However, the voids and intergranular cracks propagations were stopped at the interface of the Al2O3 particles. There was no void or intergranular crack discovered in most of the small particle of 2 µm in size. Minor microcracks were observed in the matrix material of 5 vol.% Al2O3/Al 6061. (a) (b) (c) (d) Fig. 5: Microstructures of 5 vol.% Al2O3/Al 6061 MMCs before deformation test The microstructures of the undeformed MMC samples at 15 vol.% Al2O3 showed the presence of small and large individual Al2O3 particles, which were randomly distributed in the MMCs as shown in Fig. 6(a). On the other hand, small and large clusters were also discovered in the MMCs. Most of the large particles shown in Figs. 6(b) and (c) were seriously fractured with parts of the particle pulled out. On the contrary, minor cracks have been observed in some of the smaller particles of 5 µm in size as shown in Fig. 6(d). Microcracks were observed in the matrix, propagated along the grain boundary of the Al 6061. There was no particle pulled out observed in the large cluster. From the observation it has been shown that the large clusters consist of large and small particles. For the smaller particles of 2 µm in size the same phenomena have been observed as in 5 vol.% Al2O3 MMC samples. 311 J. Jai and M.N. Berhan Metal composites behaviour under biaxial stresses (a) (c) (b) (d) Fig. 6: Microstructures of 15 vol.% Al2O3/Al 6061 MMCs before deformation test The microstructure study of the undeformed samples of 25 vol.% Al2O3 shows a combination of individual particles and clusters, which were randomly distributed in the MMCs as shown in Fig. 7(a). From the observation it has been observed that the large clusters consist of large and small particles. Microcracks and debonding at interface were observed within the small cluster of the MMCs as shown in Fig. 7(b). Most of the large particles were heavily fractured with particle pulled out and voids at the interface of the particles as shown in Fig. 7(c). For the smaller particles of 25 vol.% Al2O3 MMC samples the same phenomena have been observed as in 5 vol.% and 15 vol.% Al2O3 MMC samples. Microcracks were propagated along the grain boundary of the Al 6061 matrix and stopped at the interface of a particle as illustrated in Fig. 7(d). (a) (b) (c) (d) Fig. 7: Microstructures of 25 vol.% Al2O3/Al 6061 MMCs before deformation test 312 AJSTD Vol. 23 Issue 4 There are similarities in the 5 vol.%, 15 vol.% and 25 vol.% Al2O3 MMC especially on the behaviour of the small particles, large particles and small clusters. However there are slight differences in the behaviour of the large cluster. Perhaps this is due to the effect of the different vol.% of reinforcement in the MMC. 3.2 Compressive deformation test analysis Displacement data of the MMC samples have been obtained from the compressive deformation test. Figure 8 illustrates the displacement verses time curves of 5 vol.%, 15 vol.% and 25 vol.% Al2O3 MMC. For 5 vol.% Al2O3 MMC samples as shown in Fig. 8(a), the displacement started at around 10 seconds after compression. The displacement increases with time in the radial and tangential directions, but the displacement was more towards the radial direction. After 94 seconds of compression loading, there was a drastic drop in the displacement. For 15 vol.% Al2O3 MMC samples as shown in Fig. 8(b), the displacement started at around 8 seconds after loading. The displacement increases with time in the radial and tangential directions. However at 35 and 47 seconds of the compression loading there was a sudden drop in the displacement of tangential and radial directions respectively before it stabilized. The stable displacement was more towards the radial direction. For 25 vol.% Al2O3 MMC sample as shown in Fig. 8(c), the displacement of the material started after 5.5 seconds of the compression loading. The displacement increases with time in the radial and tangential directions. There was a sudden change and a drop in the displacement at 30 and 32.5 seconds of radial and tangential directions respectively before they stabilized. The stable displacement was more towards the radial direction. From the same illustration the maximum displacement length of the MMC sample can be deduced. The maximum displacement length of 5 vol.%, 15 vol.% and 25 vol.% Al2O3 MMC samples were 28 mm, 19 mm and 11 mm, respectively. The displacement and compression load of the MMCs at the early stage of the compression test are used in the investigation of the fracture initiation. Figure 9 shows the behaviour of compression load and displacement of MMC samples at the fracture initiation. For 5 vol.% Al2O3 MMC as shown in Fig. 9(a), the increase in the displacement and the drop in the compression loading were observed after 7.5 seconds of the compression loading. The displacement of the sample at this point was towards the radial direction. The displacement towards the tangential direction only started 13 seconds after loading. For the MMC sample with 15 vol.% Al2O3 as shown in Fig. 9(b), the displacement started to take place with the sudden drop in the compression loading after 7 seconds. The displacement at this point was more towards the radial direction. For 25 vol.% Al2O3 MMC sample as illustrated in Fig. 9(c), at 4.5 seconds there was a drastic increase in displacement and a sudden drop in the compression load reading. At this point the displacement was more towards the tangential direction. The displacement behaviour at the fracture initiation of 5 vol.% and 15 vol.% Al2O3 MMC samples is more towards the radial direction. On the other hand, the fracture initiation of 25 vol.% Al2O3 MMC sample is more towards tangential direction. Ductility of the MMC samples can be explained from the displacement behaviour due to deformation loading. Brittleness of the materials indicated by the time taken for the MMC samples to reach the maximum displacement before fracture. The ductile material requires a long time to reach its maximum displacement before fracture. The 25 vol.% Al2O3 MMC is the most brittle material followed by 15 vol.% and 5 vol.% Al2O3 MMC. In addition, [5] stated that three primary factors affecting the ductility of the composites: reinforcement strength, interface bond strength and matrix toughness. Any void created either by particle cracking or by interface failure can lead to a decrease in ductility. Displacement length as well indicates the ductility of the MMC sample. For 5 vol.% Al2O3 MMC is the most ductile material followed by 15 vol.% and 25 vol.% Al2O3 MMC samples in descending order. This is agreed by [6] whereby ductility of the MMCs decreases with the increase in the volume fraction of particulate. 313 J. Jai and M.N. Berhan Metal composites behaviour under biaxial stresses 30 (7 8 .5 ,2 8 ) 25 (9 4 ,2 7 ) 20 Displacement (mm) (9 4 ,2 1 ) 15 10 5 0 0 20 40 60 T im e (s ) 80 100 120 -5 (a) 25 Displacement (mm) 20 (4 7 ,1 9 ) 15 (3 5 ,1 2 ) 10 5 0 0 .5 5 .5 1 0 .5 1 5 .5 2 0 .5 2 5 .5 3 0 .5 3 5 .5 4 0 .5 4 5 .5 5 0 .5 5 5 .5 6 0 .5 6 5 .5 7 0 .5 7 5 .5 T im e (s ) (b) 12 (3 2 .5 ,1 1 ) 10 Displacement (mm) (3 0 ,8 .7 ) 8 6 4 2 0 0 .5 5 .5 1 0 .5 1 5 .5 2 0 .5 2 5 .5 3 0 .5 3 5 .5 4 0 .5 4 5 .5 5 0 .5 5 5 .5 T im e (s ) r a d ia l d ir e c tio n ta n g e n tia l d ir e c tio n (c) Fig. 8: Displacement verses time of the MMC with, (a): 5 vol.% of Al2O3; (b): 15 vol.% Al2O3 and (c): 25 vol.% Al2O3 314 AJSTD Vol. 23 Issue 4 650 500 550 (3,124) Displacement (um) 350 -500 250 150 -1000 50 (7.5) (13) -50 Compression load (N) 0 450 -1500 -150 0 2 4 6 8 10 12 14 16 Time (s) -250 -2000 (a) (7,131.26) 300 200 (7,131.26) 100 250 Displacement (um) -100 150 -200 100 -300 50 Compression load (N) 0 200 -400 0 0 1 2 3 4 5 6 7 8 9 10 -50 -500 -600 Time (s) (b) 100 200 (4.5,130.7) Displacement (um) 0 60 -100 40 -200 20 -300 0 Compression load (N) 100 80 -400 -20 -500 0 1 2 3 4 5 6 7 8 time (s) radial direction tangential direction compression load (c) Fig. 9: Displacement and deformation load verses time at the initial stage of compression loading of the MMC with, (a): 5 vol.% Al2O3; (b): 15 vol.% Al2O3 and (c): 25 vol.% Al2O3 3.3 Microstructures analysis A study on the deformed MMC revealed that the small particles of 2 µm in size were rarely fractured and less debonding at interface observed. However, for the particles of 5 µm in size, some debonding at interfaces besides fractured particles have been observed. These findings were clearly observed in the samples of 5 vol.% Al2O3 where some of the particles were fractured with serious debonding interface as shown in Fig. 10(a). On the other hand, for 15 and 25 vol.% Al2O3 MMC sample, less fractured particles and fine debonding at interface were observed as illustrated in Figs. 10(b) and (c), minor fractured particles and debonding at interface have been observed. 315 J. Jai and M.N. Berhan Metal composites behaviour under biaxial stresses (a) (b) (c) Fig. 10: Individual small particles of the MMC after deformation test with; (a): 5 vol.% Al2O3; (b): 15 vol.% Al2O3 and (c): 25 vol.% Al2O3 A study on the individual large particle of all the deformed MMC samples shows that the particles of 20 µm in size and above are badly fractured and some parts of the particles are missing. Figure 11(a) shows the behaviour of the large particles in 5 vol.% Al2O3 MMC samples. Cracks at the edge of the particles, particles pulled out and debonding interface of the large particles have been observed. Short microcracks were propagated from the interface of the particles to the matrix. Figure 11(b) illustrates the large particles of 15 vol.% Al2O3 MMC samples, serious cracks with particles pulled out were observed on the large particles. Short and long microcracks were branching out from the interface of the particles to the matrix. For 25 vol.% Al2O3 MMC samples as illustrated in Fig. 11(c), large particles were seriously fractured and particle pulled out were observed. There were minor microcracks observed around the interface of the particles. (a) (b) (c) Fig. 11: Individual large particles of the MMC after deformation test with; (a): 5 vol.% Al2O3; (b): 15 vol.% Al2O3 and (c): 25 vol.% Al2O3 (a) (b) (c) Fig. 12: Small cluster in the MMC after deformation test with; (a): 5 vol.% Al2O3; (b): 15 vol.% Al2O3 and (c): 25 vol.% Al2O3 316