Fabrication and characterisation of pure magnesium-30 vol.% SiC P particle composite

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1 Materials Science and Engineering A276 (2000) Fabrication and characterisation of pure magnesium-30 vol.% SiC P particle composite R.A. Saravanan, M.K. Surappa * Department of Metallurgy, Indian Institute of Science, Bangalore , India Received 7 January 1999; received in revised form 5 July 1999 Abstract Pure magnesium-30 Vol.% SiC particle composite are fabricated by melt stir technique without the use of a flux or protective inert gas atmosphere. After hot extrusion with an extrusion ratio of 13, Mg-30 vol.% SiC P composites have been evaluated for their tensile properties at room and elevated temperatures (up to 400 C). Composites in the as-cast conditions do not show any change in dendrite arm spacing/cell size compared to unreinforced pure magnesium. However, in the extruded conditions average grain size of the composites is 20 m compared to 50 m in the pure magnesium. Microstructure shows no evidence of reaction product at particle/matrix interface. At room temperature, stiffness and UTS of the extruded composites are 40 and 30% higher compared to unreinforced pure magnesium, signifying significant strengthening due to the presence of the SiC particles. Further, up to temperatures of 400 C, composites exhibit higher UTS compared to pure magnesium. Mg composites show a wear rate lower by two orders of magnitude compared to pure Mg, when tested against steel disc using pin-on disc machine Elsevier Science S.A. All rights reserved. Keywords: Pure magnesium; Melt stir technique; Tensile properties 1. Introduction Metal matrix composite (MMC) systems are currently under various stages of development. Composites based on aluminium and magnesium matrices reinforced with SiC particles are of great interest to the automotive and aerospace industries. In the last two decades much of the research has enveloped around cast aluminium based MMCs and also has been the topic of discussions at many international forums. In contrast, research efforts on the processing and properties of magnesium based MMCs have been rather limited [1 10]. Magnesium and its alloys with low density and high stiffness to weight ratio is an excellent candidature matrix material for compositing. It is anticipated that reinforcing Mg with SiC P reinforcement can lead to significant improvement in stiffness and strength, * Corresponding author. Tel.: ; fax: address: mirle@metalrg.iisc.ernet.in (M.K. Surappa) both at room and elevated temperatures besides improvement in wear resistance and damping capacity. In general, MMCs based on magnesium and its alloys reinforced with SiC have been produced by stir casting [1 4], powder metallurgy [5,6], squeeze casting [7 9] and spray forming [10]. Luo and his co-workers [1,2] report liquid mixing and casting process for the fabrication of Mg/Mg alloy SiC P composites. However, in their process, mixing has been done under SF 6 /CO 2 protective atmosphere and this makes the process tedious and hence cannot be easily adaptable in conventional magnesium foundries. Hence, an investigation was undertaken to develop an affordable and easily adaptable casting process for the synthesis of particulate reinforced Mg based composites. The present paper reports on the use of conventional stir casting process for the fabrication of Mg-30 vol.% SiC P composite without the use of flux or calcium or SF 6 /CO 2 protective gas atmosphere. Cast composites have been subsequently hot extruded at temperature of 500 C and characterized for their microstructure, mechanical properties and sliding wear resistance /00/$ - see front matter 2000 Elsevier Science S.A. All rights reserved. PII: S (99)

2 R.A. Sara anan, M.K. Surappa / Materials Science and Engineering A276 (2000) Experimental details The magnesium 30 vol.% SiC P composite was processed by melt stir technique. The matrix material was 99% pure magnesium and SiC particles with 40 m average size. Steps involved and the procedure employed for the fabrication of composites are as follows; the melting was carried out using a resistance heating furnace of 5 KW. Stainless steel crucible with a capacity to hold 5 kg of Mg was used for melting Mg and subsequent processing of composites. Magnesium ingot weighing about 2.5 Kg was fully wrapped with aluminium foil and charged into the stainless steel crucible. During melting the furnace was covered with lid. This was done to minimize contact between the atmospheric oxygen and Mg metal. In addition aluminium foil (used for wrapping Mg metal) helped to prevent oxidation and burning of magnesium metal. Temperature was monitored and controlled closely and care was taken to maintain the furnace temperature around 700 C. The metal was melted with very little oxidation. The lid was removed and oxide layers present on the melt were removed. This was quickly followed by the additions of pre heated SiC particles in to the vortex of the melt created by the rotating impeller. Complete additions of SiC particles was done in less than a minute. During this period very little burning was observed on the surface of the melt. The composite melt was poured into cast iron cylindrical mould of 65 mm diameter and 200 mm height. The material loss incurred by this method was less than 15 wt.%. The cast billets were subsequently hot extruded using CBJ-250 Ton hydraulic press with an extrusion ratio of 13:1. (Prior to hot extrusion, the billets were wrapped in aluminium foil and homogenized at 500 C for 2 h.) For the purpose of comparison unreinforced Mg was cast and extruded in a similar way. Both unreinforced Mg and Mg-30 vol.% SiC P composite specimens in the as-cast and extruded conditions were metallographically polished and examined using optical microscope and SEM. Vickers macro and microhardness (Shimadzu 2000 HMV, at 25 g load) of the materials were measured. Tensile properties of the unreinforced Mg and Mg-30 vol.% SiC P composite specimens in the as-cast and materials in the extruded condition were evaluated using Hounsefield Tensometer with furnace attachment. Tests were done at room temperature, 250, 300, 350 and 450 C at a strain rate of s 1. In the case of pure magnesium tests could be done only up to 350 C, since beyond 350 C specimens were too soft for gripping. Elastic modulus of both unreinforced Mg and composite were measured using elastosonic method. Fracture surfaces were observed under SEM (JEOl-JSM-840A). Polished sections parallel to tensile axis were examined under optical microscope (Olympus B061) to obtain information on fracture mechanisms. Wear resistance of both unreinforced Mg and its composite were evaluated using pin-on disc machine at a sliding speed of 0.5 ms 1 and at loads of 5, 10 and 50 N. All the tests were carried out at a constant sliding distance of 1.5 km. The wear rate of the worn out specimens was calculated from the weight loss measurements. Wear rate is defined as weight loss per unit distance (g/m). 3. Results and discussions 3.1. Microstructures Visual inspection of machined surface of the composite billets showed reasonably uniform distribution of SiC particle throughout the casting. Casting defects were not noticed on the casting surface. Fig. 1a and b shows the optical microstructure of the pure Mg and its composite in the as-cast condition. Pure Mg exhibits typical dendritic structure where as, Mg-30 vol.% SiC P composite show cellular dendritic structure. It is possible that the presence of reinforcement particles change the morphology of the growing interface from dendrite in pure Mg into cellular-dendrite interface in Mg-30 vol.% SiC P composite. During solidification of Al MMCs, similar changes in morphology of interfaces has been reported by Trivedi et al. [11] and Dutta et al. [12] and this has been attributed to changes in temperature gradient and/or convection in the melt during solidification. Our microstructural observations are in agreement with the prediction of Trivedi et al. [11]. There is no marked difference in the dendrite arm spacing/cell size of composite compared to pure magnesium. Distribution of SiC particles is not completely uniform in the as-cast composite. Size of the SiC particles are either comparable or more than the cell size of the matrix in the composite. Occasionally fine size SiC particles are present within the cell (Fig. 1c). Average DAS/cell size is around 20 m in both unreinforced pure Mg and in the composite. Occurrence of SiC P clusters were observed at some locations. Fig. 2a c shows microstructure of the extruded materials. Both shows recrystallized grain structure. Microstructure of extruded composite shown in Figs. 2b and 3c is characterized by excellent distribution of SiC particles. Particle clusters and pores were rarely observed. In the extruded condition Mg and its composites had a density of 1.71 and 2.19 g/cc, respectively. Average grain size is about 50 and 20 m for the unreinforced magnesium and composites, respectively. Fine grain size observed in the case of the extruded composites indicate that SiC P particle could have played the role of nuclei during recrystallization during hot working. SEM photomicrograph of the composite clearly shows good and clean interface between SiC particle and Mg matrix

3 110 R.A. Sara anan, M.K. Surappa / Materials Science and Engineering A276 (2000) (Fig. 3). There are no reaction products such as MgO or Mg 2 Si at the interface which are generally reported to form in the SiC w / magnesium matrix in the case of magnesium alloys [1,6]. All these indicate good wetting between Mg melt and SiC particle under processing conditions employed in the present studies Hardness The hardness of the as extruded unreinforced Mg and its composites were listed in Table 1. Both macro and micro hardness (vickers) of the extruded composite are higher compared to the unreinforced magnesium as expected. Fig. 2. Optical micrograph of (a) as extruded pure magnesium; (b) as extruded Mg-30 vol.% SiC P composite; and (c) high magnification of photograph Fig. 2b Tensile properties Fig. 1. Optical micrograph of (a) as-cast pure magnesium; (b and c) as-cast Mg-30 vol.% SiC P composites (arrow mark show finer size SiC particles inside the cell). Stress strain behavior of the unreinforced magnesium and Mg-30 vol.% SiC P composite at room temperature as obtained from the tensile test is shown in the Fig. 4. Values of stiffness (elastic modulus), 0.2% proof stress, UTS and % elongation are given in the Table 2. Presence of 30 vol.% SiC P increased the modulus of Mg by more than 40% indicating effective stress transfer from matrix to SiC P. In the case of pure magnesium yielding occurred at 135 MPa. However, in the case of

4 R.A. Sara anan, M.K. Surappa / Materials Science and Engineering A276 (2000) Fig. 3. SEM micrograph of Mg SiC P composite showing a clean particle/matrix interface. Fig. 5. Stress strain curves for unreinforced magnesium and its composite tested at elevated temperatures. tensile property data is listed in Table 3. At all temperatures UTS of the composites are higher and percentage elongation are lower compared to the pure magnesium. From this, it is evident that strengthening due to SiC P is retained even at elevated temperatures. UTS and 0.2% proof stress of Mg and its composites are plotted as a function of temperature in Fig. 6. It is clear that the magnitude of difference in strength between composite and pure magnesium decrease with increase in temperature. On the other hand magnitude of difference in ductility increases with increase in temperature. Fig. 4. Stress strain curves for unreinforced magnesium and its composite tested at room temperatures. composite, there is no distinct yield point though the composite exhibited 2% elongation, suggesting that the plastic deformation commenced from the onset of testing. 0.2% proof stress and UTS of the composite are 229 and 258 MPa, respectively compared to 135 and 196 MPa for the pure magnesium. From these it is clear that there is significant strengthening of Mg matrix due to the presence of SiC particle. It is also important to note that even with 30 vol.% SiC particles, the extruded composite exhibits ductility of 2%. Stress strain curves of pure Mg and composite at 250, 300 and 350 C are shown in Fig. 5. Corresponding 3.4. Fracture beha ior SEM fractrograph of pure magnesium tested at room and elevated temperatures are shown in Fig. 7a c. In the case of pure magnesium fracture surface exhibits elongated dimples of nearly uniform size indicating ductile fracture. With increase in temperature dimple Table 1 Hardness of the extruded pure Mg and Mg-30 vol.% SiC P composites Material Macro hardness (VHN) Pure Mg Mg-30 vol.% SiC P Micro hardness (VHN) Table 2 Room temperature tensile properties of pure Mg and its 30 vol.% SiC P composite (all are in the extruded condition) Material Young s modulus (Gpa) 0.2% proof stress (MPa) UTS (MPa) % elongation Pure Mg Mg-30 vol.% SiC P

5 112 R.A. Sara anan, M.K. Surappa / Materials Science and Engineering A276 (2000) Table 3 Tensile properties of pure Mg and Mg-30 vol.% SiC P composite at elevated temperatures Temperature ( C) 0.2% proof stress (MPa) UTS (MPa) % elongation Mg Mg-30 vol.% SiC P Mg Mg-30 Vol.% SiC P Mg Mg-30 Vol.% SiC P morphology changes from elongated to nearly circular and this evident from the Fig. 7b which corresponds to the specimen fractured at 250 C. At 350 C fracture surface of Mg shows deep circular cavities (Fig. 7c). In contrast, fracture behavior of Mg-30 vol.% SiC P composite is different. Extensive SiC particle fracture was observed where as matrix exhibits quasi-brittle fracture (Fig. 8a). Occasionally matrix adherence on SiC particle was seen indicating good bonding. However, at 250 C fracture surface of composite begins to show fine size dimples (Fig. 8b). In addition, signs of shear of deformation especially at ridges were observed at higher temperatures (Fig. 8c). All these indicate that at elevated temperatures more than one mode of deformation is operative in both unreinforced Mg and its composite. This explains increased ductility observed in both the materials at higher temperatures. Microstructure of the mid sections of the Mg specimen (along the gauge length) cut parallel to the tensile axis are shown in the Fig. 9a f. At room temperature the grains are equiaxed and the fracture seems to have occurred along the grain boundaries (Fig. 9a). In some grains of the gauge section, slip bands or deformation twins are seen (Fig. 9b). With increase in temperature, the microstructure reveals grains severely deformed parallel to the tensile axis. At 250 C unreinforced Mg specimen show elongated grains parallel to the tensile axis both in the gauge section and fracture tip region (Fig. 9c, d). The morphology of the large grains and fine grains nucleating on the large grain boundaries clearly indicate that the structure is dynamically recovered. In the gauge section, the slip bands and deformation twins are seen more clearly compared to the specimen deformed at room temperature. These are seen both in large and in small grains (recrystallized grains). At 350 C the microstructures reveal equiaxed fine grains near fracture tip as well as in the gauge section (Fig. 9e, f). Further, the structure show deep cavities or pipes near the fracture tip running parallel to the tensile axis. At higher magnification the optical microstructures shows extensive grain coarsening. In the case of Mg-30 vol.% SiC P composite, the specimens deformed at room temperature shows no change in the microstructure in the vicinity of tip as well as in the gauge section (Fig. 10a b) similar to that of the pure Mg. At higher temperatures the microstructures of the composites do not show any elongated grains as in the case of unreinforced pure magnesium. In addition, there is no change in the matrix grain size of the composites indicating particles strongly inhibit the grain growth. At 250 C in the gauge section of the specimen, the microstructure reveals micro bands in the grains in the direction of tensile deformation (Fig. 10c). At high temperatures these bands become more predominant in both near the fractured tip as well as in the gauge section (Fig. 10d, e). Further, at high temperature fracture has occurred by void coalescence and crack bridging between the reinforcement particle Wear results Fig. 6. UTS and % elongation of pure Mg and Mg SiC P composite as a function of temperature. The sliding wear rate of the unreinforced Mg and its composite tested at loads in the range 5 50 N and at a sliding speed of 0.5 m/s is listed in the Table 4. The wear rate of the composite is generally less by two order of magnitude compared to unreinforced Mg at all loads. Improved wear resistance could be attributed in part due to the presence of SiC particles and to the improved strength of the composite. Though there is substantial improvement in wear resistance, a detailed study has to be carried out to understand the wear mechanisms in the magnesium composite.

6 R.A. Sara anan, M.K. Surappa / Materials Science and Engineering A276 (2000) Fig. 7. SEM fractographs of unreinforced Mg tested at (a) room temperature; (b) 250 C; and (c) 350 C 4. Conclusions 1. It is possible to fabricate Mg-30 vol.% SiC P composites using conventional melt stir technique without using protective atmosphere or flux. 2. Grain size of extruded Mg-30 vol.% SiC P composites are finer compared to unreinforced pure magnesium. 3. At room temperature elastic modulus and UTS of the extruded Mg-30 vol.% SiC P composite are higher compared to that of the unreinforced pure Fig. 8. SEM fractographs of Mg SiC P composite tested at (a) room temperature; (b) 250 C; and (c) 400 C Table 4 Wear rate of unreinforced Mg and Mg-30 vol.% SiC P composite tested at a sliding speed 0.5 m/s Load (N) Wear rate (g/m) Mg Mg-30 vol.% SiC P

7 114 R.A. Sara anan, M.K. Surappa / Materials Science and Engineering A276 (2000) Fig. 9. Optical micrographs of longitudinal sections of tensile specimens (pure Mg) failed at: (a) room temperature showing fracture path; (b) room temperature in the gauge section; (c) 250 C showing fracture path; (d) 250 C in the gauge section; (e) 350 C showing fracture path; and (f) 350 C in the gauge section.

8 R.A. Sara anan, M.K. Surappa / Materials Science and Engineering A276 (2000) Fig. 10. Optical micrographs of longitudinal sections of tensile specimens (Mg SiC P ) composite failed at: (a) room temperature showing fracture path; (b) room temperature in the gauge section; (c) 250 C in the gauge section; (d) 400 C showing fracture path; and (e) 400 C in the gauge section.

9 116 R.A. Sara anan, M.K. Surappa / Materials Science and Engineering A276 (2000) Mg. Up to a temperature of 400 C composite exhibit superior strength compared to pure Mg. 4. Fracture behavior of pure magnesium reveals elongated dimples at room temperature and circular dimples at high temperatures. 5. Sliding wear rate of the Mg-30 vol.% SiC P composite is two order of magnitude less compared to unreinforced magnesium. References [1] A. Luo, M.O. Pekguleryuz, in: Proceedings of the 51st International Magnesium Conference, International Magnesium Association (IMA), Berlin, 1994, pp. 74. [2] A. Luo, Metall. Trans. 26A (1995) [3] R.A. Saravanan, S. Seshan, M.K. Surappa, in: Y. Miyano, M. Yamabe (Eds.), Proceedings of the Fifth Japanese International SAMPE Symposium, Tokyo, Japan, 28th 31st October, 1997, pp [4] A. Martin, J. Llorca, Mater. Sci. Eng. 201A (1995) 77. [5] K.U. Kainer, J. Schroder, B.L. Mordike, in: T. Chandra, A.K. Dhingra (Eds.), Proceedings of the International Conference on Advanced Composite Materials, TMS, 1993, pp [6] D.M. Lee, B.K. Suh, B.G. Kim, J.S. Lee, C.H. Lee, Mater. Sci. Technol. 13 (1997) 590. [7] S.-Y. Chang, H. Tezuka, A. Kamio, Mater. Trans. JIM. 38 (1997) 18. [8] K. Purazrang, K.U. Kainer, B.L. Mordike, Composites 22 (1991) 456. [9] D.J. Towbe, C.M. Friend, Mater. Sci. Eng. 188A (1994) 153. [10] T. Ebert, F. Moll, K.U. Kainer, Powder Metall. 40 (1997) 126. [11] J. Sehkar, R. Tridevi:, Mater. Sci. Eng. 147A (1991) 9. [12] B. Dutta, M.K. Surappa, Metal. Trans. 29A (1998)