- Email: [email protected]
SiC particulate composites
Journal of Alloys and Compounds 466 (2008) 268–272
Precipitation hardening characteristics of Al–Si–Fe/SiC particulate composites S.B. Hassan, O. Aponbiede, V.S. Aigbodion ∗ Department of Metallurgical Engineering, Ahmadu Bello University, Zaria, Nigeria Received 6 June 2007; received in revised form 7 November 2007; accepted 11 November 2007 Available online 17 November 2007
Abstract The effect of age-hardening on the microstructure and mechanical properties of SiCp/Al–Si–Fe particulate composites have been studied. 5 and 15% SiC additions were used for the production of two grades of composites. The composite samples were solution heat-treated at 500 ◦ C for 3 h and quenched in warm water at 65 ◦ C. The samples were then aged at 100, 200 and 300 ◦ C for various ageing times between 1 and 11 h. The microstructure obtained reveals a dark ceramic and white metal phases, which resulted into increase in the dislocation density at the particles–matrix interfaces. The mechanical properties measured included: tensile strength, yield strength, hardness values and impact energy. The composites produced exhibited maximum mechanical properties at peak ageing at various ageing temperatures. These increases in mechanical properties during ageing are attributed to the formation of a coherent and uniform precipitation of the second phase in the matrix of the metal. This result shows that a substantial improvement in mechanical properties has been achieved in the reinforced metal matrices produced after age-hardening. © 2007 Elsevier B.V. All rights reserved. Keywords: Al–Si–Fe alloy; Age-hardening; Composites; Microstructure; Silicon carbide and particulate
1. Introduction The age-hardening characteristics of an alloy are generally modified by the introduction of reinforcement. These modifications are due to the manufacturing process, the reactivity between the reinforcement and the matrix, the size, the morphology and volume fraction of the reinforcement [1,2]. A great deal of research has been carried out in the past to develop and improved aluminium-based composites with the aim of achieving better mechanical properties [3]. Donne et al. [4] studied the age-hardening behavior of a 30 wt% (SiCp/Al–Si) composite and found that the ageing response was very rapid, in that order only 1 h ageing at 120 ◦ C, 45% of the peak tensile strength and 69% of the peak hardness were attained, compared to the unreinforced alloy. Cottu et al. [5] showed that the hardening kinetics of A1–Cu–Mg alloy/10 wt% SiC fiber composite was enhanced by ∗
Corresponding author. Tel.: +234 8028433576. E-mail addresses: [email protected] (S.B. Hassan), [email protected] (V.S. Aigbodion). 0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.11.023
the presence of the reinforcement during heat-treatment. They explained this by the plastic deformation induced by the heattreatment due to the coefficient of thermal expansion (CTE) mismatch between matrix and reinforcement, as well as the highest dislocation density in the composite that supplied precipitation sites. Srivatson and Mattingly [6] also attributed the enhanced properties of their heat-treated 2124 Al alloy/SiC fiber composites to CTE mismatch and higher dislocation density factor. On the other hand, Skibo et al. [7] reported no difference in the ageing response of as-cast unreinforced A6061 and 10 and 20 wt% of SiC reinforced A6061 material in their studies of the strength and elongation as a function of ageing time in these materials at 175 ◦ C. With extensive data available on the heat-treatment of Al–Si–Fe alloys with conventional alloying elements [8], no much is documented on the age-hardening characteristics of this alloy particularly with silicon carbide reinforcement. Hence, the need for research in this area is justified. The objective of this present research therefore is to study the age-hardening characteristics of SiCp/Al–Si–Fe particulate composites.
S.B. Hassan et al. / Journal of Alloys and Compounds 466 (2008) 268–272
269
2. Materials and methods 2.1. Materials The materials used in this study are: high purity aluminium electrical wire obtained from Northern Cable Company NOCACO (Kaduna), ferrosilicon, silicon carbide with average particle size of 10 m, moulding box, silica sand, bentonite were obtained from the Foundry Shop of National Metallurgical Development Centre, Jos, Nigeria.
2.2. Equipment The equipment used in this study included: Pyrometer, mechanical stirrer, crucible, electrical resistance furnace, Rockwell hardness tester, Avery Denison impact machine, Tinus–Olsen tensile machine and Metallurgical microscope.
2.3. Methods The synthesis of the metal matrix composite that was use in this study was produced using double stir-casting method at the Foundry Shop of the National Metallurgical Development Center, Jos, Nigeria. The samples were produced by keeping the percentage of iron and silicon constant and varying the reinforcing material (silicon carbide) particles in the range 5–15% SiC. High purity aluminium electrical wires free from dust and contamination was charged in a graphite crucible kept in an electric resistance furnace and 0.01% NaCl–KCl powder was used as a cover for melting the alloy. The NaCl–KCl powder aids in minimizing oxidation of aluminium by excluding oxygen and creating a protective atmosphere inside the furnace. Upon initiation of melting of pure aluminium the temperature of the furnace was raised to 720 ◦ C. The required quantity of Silicon (2.8 wt%) and iron (0.8 wt%) was added using ferrosilicon, and the resulting melt thoroughly stirred. With progressive melting the furnace temperature was raised to 780 ◦ C and the melt was held at this temperature for 10 min. It was then skimmed to remove the oxides and impurities. The molten metal was continuously stirred in order to ensure a near-uniform distribution of alloying elements and prevent the elements from settling at the bottom on account of their higher density. For each melting 400 g of charge materials was used to produce the alloy. The SiC particles were preheated to 1000 ◦ C to oxidize their surface [9]. The alloy was then cooled down to just below the liquidus to keep the slurry in a semi-solid state. At this stage the preheated SiC particles were added and mixed manually. Manual mixing was used because it was very difficult to achieve mixing using automatic device when the alloy was in a semi-solid state. After proper manual mixing was done, the composite slurry was re-heated to full liquid state and then automatic mechanical mixing was carried out for about 20 min at an average stirring rate of 150 rpm. In the final mixing processes, the furnace temperature was controlled between 730 and 740 ◦ C and, the pouring temperature were controlled to be about 720 ◦ C. A preheated sand mould with
Fig. 1. Microstructure of the reinforced alloy (5% SiC). Dissolution of the eutectic silicon phase and uniform distribution of SiC (black) (×200).
Fig. 2. Microstructure of the reinforced alloy (15% SiC). Dissolution of the eutectic silicon phase and uniform distribution of SiC (black) (×200).
diameter 18 mm and 300 mm length was used to prepare cast bars [9]. After casting, the test samples were machined into standard test samples and solution heat-treated at temperature of 500 ◦ C in an electrically heated furnace, soaked for 3 h at this temperature and then rapidly quenched in warm water at 65 ◦ C. Ageing of the test samples were carried out at temperatures of 100, 200 and 300 ◦ C, for various ageing times of 60–660 min, and then cooled in air [9]. The ageing characteristic of these grades of composites was evaluated using hardness values obtained from solution heat-treated samples of the investigated composites subject to the aforementioned temperature conditions. The tensile and impact test were conducted at peak ageing time of various ageing temperatures.
Fig. 3. Microstructure of the age-hardened reinforced alloy (5% SiC). Dissolution of the eutectic silicon phase and uniform distribution of SiC (black) with precipitation (×200).
Fig. 4. Microstructure of the age-hardened reinforced alloy (15% SiC). Dissolution of the eutectic silicon phase and uniform distribution of SiC (black) with precipitation (×200).
270
S.B. Hassan et al. / Journal of Alloys and Compounds 466 (2008) 268–272
Fig. 5. Effect of age-hardening time on the hardness of 5SiCp/Al–Si–Fe composite.
2.3.1. Determination of hardness values Hardness was determined using the Rockwell hardness test machine. The indenter used was a 1.56 mm steel ball and minor load of 10 kg and major load of 100 kg was applied. Rockwell B scale and Hardness of 101.2HRB standard block were used as recommended by American Society for testing and Materials (ASTM) [9,10]. 2.3.2. Determination of impact strength Impact tests were conducted using a fully instrumented Avery Denison test machine. With impact energies ranging from 0 to 300 J. The mass of the hammer was 22.7 kg and the striking velocity 3.5 m/s. Charpy impact tests were conducted on notched specimens. Standard square impact test specimen measured 50 mm × 10 mm × 10 mm with notch depth of 2 mm and a notch tip radius of 0.02 mm at angle of 45◦ was used in this research as recommended by American Society for Testing and Materials (ASTM) [9,10]. 2.3.3. Determination of tensile properties The tensile properties were conducted on Tinus–Olsen test machine. The test pieces were machined to the standard shape and dimensions as specified by the American Society for Testing and Materials (ASTM) [9,10]. The yield strength, ultimate tensile strength, percentage elongation and reduction in cross-sectional area of the samples were determined.
Fig. 7. Effect of age-hardening temperature of 100 ◦ C on the hardness of Al–Si–Fe, 5SiCp/Al–Si–Fe and 15SiCp/Al–Si–Fe.
3. Results and discussion 3.1. Results The various microstructures developed of both grades of composites show distribution of the SiCp in the metal matrix, but the 15SiCp/Al–Si–Fe grade has more SiC uniformly distributed in the metal matrix than 5SiCp/Al–Si–Fe. There was precipitation of the second phase in age-hardened conditions (Figs. 1–4). Figs. 5 and 6 show the effect of ageing time at 100, 200 and 300 ◦ C on the hardness values of age-hardened SiCp/Al–Si–Fe composites. The age-hardening curves for 5SiCp/Al–Si–Fe and 15SiCp/Al–Si–Fe at 100, 200 and 300 ◦ C are shown in Figs. 7–9, respectively. The results of the tensile properties, impact energy and hardness values of Al–Si–Fe alloy, 5SiCp/Al–Si–Fe and 15SiCp/Al–Si–Fe for both as-cast and age-hardened conditions are shown in Table 1. 3.2. Discussion
2.3.4. Microstructural examination Metallographic specimens were cut from the as-cast and age-hardened samples of the SiCp/Al–Si–Fe particulate composite. The cut samples were mounted in Bakelite, and mechanically ground progressively on grades of SiC impregnated emery paper (80–600 grits) sizes using water as the coolant. The ground samples were polished using 1 m size alumina polishing powder suspended in distilled water. Final polish was achieved using 0.5-m alumina polishing powder suspended in distilled water. Following the polishing operation, etching of the polished sample was done using Keller’s reagent. The structure obtained was photographically recorded using an optical microscope with built in camera [9].
The 5SiCp/Al–Si–Fe exhibited a maximum hardness value of 72.0HRB at 100 ◦ C for 9 h, 73.0HRB at 200 ◦ C for 6 h and 74.0HRB at 300 ◦ C for 5 h (see Fig. 5). While 15SiCp/Al–Si–Fe have maximum hardness value of 82.0HRB at 100 ◦ C for 9 h, 84.0HRB at 200 ◦ C for 6 h and 85.0HRB at 300 ◦ C for 5 h (see Fig. 6). The tensile properties, hardness values and impact energy of the samples increases in the age-hardened than the ascast condition (see Table 1) for example, at peak ageing of 200 ◦ C, the yield strength of 5SiCp/Al–Si–Fe increased from
Fig. 6. Effect of age-hardening time on the hardness of 15SiCp/Al–Si–Fe composite.
Fig. 8. Effect of age-hardening temperature of 200 ◦ C on the hardness Al–Si–Fe, 5SiCp/Al–Si–Fe and 15SiCp/Al–Si–Fe.
S.B. Hassan et al. / Journal of Alloys and Compounds 466 (2008) 268–272
271
Table 1 Mechanical properties of the various grades of Al–Si–Fe/SiCp composites Mechanical properties
Ageing temperatures (◦ C)
Al–Si–Fe
5SiCp/Al–Si–Fe
15SiCp/Al–Si–Fe As-cast
As-cast
Peak aged
As-cast
Peak aged
100 200 300
45.60
59.90 56.40 52.40
57.57
65.20 64.80 63.00
72.38
78.0 78.91 74.00
Tensile strength (N/mm2 )
100 200 300
58.69
62.50 67.18 64.20
73.35
80.75 79.50 76.80
105.20
115.67 118.96 117.20
% elongation
100 200 300
6.00
13.00 12.00 10.80
5.50
10.40 9.00 9.80
3.00
4.40 4.44 4.40
% reduction in area
100 200 300
10.34
15.34 14.00 14.00
10.34
15.34 12.09 13.05
7.80
9.80 9.80 9.40
Hardness values (HRB)
100 200 300
31.10
60.10 61.50 62.00
54.50
72.00 73.00 74.00
64.30
82.00 84.00 85.00
Intact energy (J)
100 200 300
17.00
28.00 24.00 22.00
17.00
26.00 20.00 20.00
13.00
17.50 18.00 18.00
Yield strength
(N/mm2 )
57.57 to 64.80 N/mm2 , tensile strength increased from 73.35 to 79.50 N/mm2 in the as-cast and peak ageing, respectively. The yield and tensile strength of 15SiCp/Al–Si–Fe also increased from 72.38 to 78.91 N/mm2 and 105.20 to 118.96 N/mm2 in the as-cast and age-hardened condition at the same temperature (see Table 1). These results show that 5SiCp/Al–Si–Fe has 8.38, 12.56 and 17.65% increases in yield strength, tensile strength and impact energy, respectively, as compared to the as-cast samples at peak ageing temperature of 200 ◦ C, while 13.08, 7.76 and 38.46% increments in yield strength, tensile strength and impact energy were obtained for the 15SiCp/Al–Si–Fe, respectively. Both the age-hardened grade of composites exhibited higher ductility and impact energy than the as-cast condition (see Table 1). The ductility and impact energy of 5SiCp/Al–Si–Fe are higher than that of 15SiCp/Al–Si–Fe this is as a result of the high hardness values of the 15% SiCp. From the Figs. 5–9, it can be seen that there is steep rise in hardness values of each grade of the composite at initial stages for all ageing temperatures and then fell after reaching the various peak ageing time. Thereafter the hardness values decreases, corresponding to over-ageing. However, at higher ageing temperature the materials develop peak hardness at shorter ageing time, because the rate of precipitation of the second phase is faster and hence the rate of increases in hardness is also faster [11,12]. The age-hardening behavior of the SiCp/Al–Si–Fe composites are similar to 5SiCp/Al–Si–Mg as reported by Cottu et al. [5], i.e. hardness continuously increases to maximum value during ageing and them decreases later due to overageing. It is interesting to note that in the reinforced aluminium alloy metal–matrix, as the volume fraction of SiCp in the aluminium alloy metal–matrix increases there is a mono-
Peak aged
tonic reduction in the time required to reach peak hardness (see Figs. 5 and 6). The addition of SiCp reinforcement to the aluminium alloy metal–matrix (Al–Si–Fe) raises the dislocation density at the particle–matrix interfaces [5,6]. This is due to the differences in coefficient of thermal expansion (CTE) between the hard and brittle reinforcing particles and the soft and ductile metal–matrix. The presence of high dislocation density at interfaces facilitates precipitation at these locations. The formation and presence of precipitates at the particles–matrix interfaces may be appreciated by comparing micrographs of the composite in the as-cast (see Figs. 1 and 2) and in the peak age-hardened condition (see Figs. 3 and 4). The micrograph in the peak age-hardened condition (Figs. 3 and 4) reveals precipitates covering the surface at the particles–matrix interfaces. Cottu and co-workers reported similar behavior of increased precipitation at interfaces [5,11] for aluminium alloy 6061 reinforced with whiskers of silicon carbide. They attributed this to increase in dislocation density at interfaces. Increase in dislocation density strain
Fig. 9. Effect of age-hardening temperature of 300 ◦ C on the hardness Al–Si–Fe, 5SiCp/Al–Si–Fe and 15SiCp/Al–Si–Fe.
272
S.B. Hassan et al. / Journal of Alloys and Compounds 466 (2008) 268–272
hardens the metal–matrix locally and provides heterogeneous nucleation sites for precipitation, thereby accelerating the ageing response [4,6,12]. The higher values of both ultimate tensile strength and yield strength obtained at age-hardened condition as compared with the as-cast condition are attributed to uniform distribution and precipitation of the SiC particles in the microstructure of the agehardened condition (see Figs. 3 and 4) than the microstructure of as-cast condition (see Figs. 1 and 2), it can be seen that the inter-atomic spacing in later is larger than in the former. The contribution of inter-atomic spacing to strain hardening arises from the fact that the space permitted a dislocation to manoeuvre round obstacles limit the yield stress given by the formula:
(3) This study therefore, has established that these grade of composites response very well to precipitation hardening heat-treatment.
Gb λ where τ is the yield stress, λ the inter-particle spacing, G the shear energy of dislocation and b is the Burger vector. The smaller the values of λ the greater the yield stress as a result of strain hardening. It therefore supports the fact that (Figs. 3 and 4) with smaller λ have greater strengths than (Figs. 1 and 2) with larger λ [13]. These modulated structure formed during ageing in both grade of composites enhanced this mechanical properties and is in agreement with the earlier work of 5SiCp/Al–Si–Mg [5,11].
References
τ=
4. Conclusions From the results obtained from this study, the following conclusions can be made: (1) The mechanical properties of the two grades of composites produced are higher in the age-hardened samples than that of the as-cast samples, due to the precipitates of the second phase during ageing which cover the surface at the particles–matrix interfaces. (2) It was found that hardness increases with increasing weight fraction of silicon carbide in the alloy and decreases with increasing ageing time after the peak ageing time have been exceeded.
Acknowledgments The Authors acknowledge with thanks the management of the National Metallurgical Development Centre, Jos, Nigeria for allowing us use their equipment. We also acknowledge the assistance given by Dr. S.A. Yaro, M.U. Suleiman and A.W. Usman for their assistance during the production of the research material.
[1] T.W. Clyne (Ed.), Comprehensive Composite Materials, Metal Matrix Composites (ser. eds), Elsevier, vol. 3, 2000, 1–8. [2] A. Sagail, G. Leisk, Scripta Metall. Mater. (1992) 871–876. [3] T.W. Clyne, in: A. Mortensen (Ed.), Encyclopedia of Material Science and Technology, Elsevier, 2001, pp. 26–38. [4] S. Donne, M.R. Krishnadev, R. Bouchard, Minerals Met. Mater. Soc. (1990) 243–251. [5] J.P. Cottu, J.J. Coudere, B. Viguier, L. Bernard, J. Mater. Sci. 27 (1992) 3068–3074. [6] T.S. Srivatsan, J. Mattingly, J. Mater. Sci. 289 (1993) 611–620. [7] M. Skibo, P.L. Morris, D.J. Lloyd, in: S.G. Fishman, A.K. Dhirngra (Eds.), Cast Reinforced Composites, ASM International, 1988, pp. 257–261. [8] S.A. Yaro, V.S. Aigbodion, O.Y. Mohammed, Int. Res. J. Eng. Technol. (2006) 32–39. [9] V.S. Aigbodion, Development of Al–Si–Fe/SiC particulate composite as advanced materials for engineering applications, Unpublished M.Sc Thesis, Department of Metallurgical Engineering, Ahmadu Bello University, Zaria, Nigeria, 2007, pp. 3–10. [10] Annual Books of ASTM Standards, Section 1. Iron and Steel Products, American Society of Testing Materials, vol. 01.04, 1990, pp. 13–63. [11] N.A.O. Ikechukwuka, Characterization of aluminium alloy 2618 and its composites containing alumina particles, unpublished PhD Thesis Department of Mechanical engineering, University of Saskatchewan, Saskatoon, 1999, pp. 1–200. [12] S. Jayalakshmi, S. Seshan, S.V. Kailas, S Srivatsan, Influence Processing and Reinforcement on microstructure and impact behaviour of magnesium alloy AM100 S¯adhan¯a, 29(5, October), 2004 509–523, ©Printed in India. [13] G.E. Dieter, Mechanical Metallurgy, McGraw-Hill Company, 1988, pp. 184–223.
Precipitation hardening characteristics of Al–Si–Fe/SiC particulate composites S.B. Hassan, O. Aponbiede, V.S. Aigbodion ∗ Department of Metallurgical Engineering, Ahmadu Bello University, Zaria, Nigeria Received 6 June 2007; received in revised form 7 November 2007; accepted 11 November 2007 Available online 17 November 2007
Abstract The effect of age-hardening on the microstructure and mechanical properties of SiCp/Al–Si–Fe particulate composites have been studied. 5 and 15% SiC additions were used for the production of two grades of composites. The composite samples were solution heat-treated at 500 ◦ C for 3 h and quenched in warm water at 65 ◦ C. The samples were then aged at 100, 200 and 300 ◦ C for various ageing times between 1 and 11 h. The microstructure obtained reveals a dark ceramic and white metal phases, which resulted into increase in the dislocation density at the particles–matrix interfaces. The mechanical properties measured included: tensile strength, yield strength, hardness values and impact energy. The composites produced exhibited maximum mechanical properties at peak ageing at various ageing temperatures. These increases in mechanical properties during ageing are attributed to the formation of a coherent and uniform precipitation of the second phase in the matrix of the metal. This result shows that a substantial improvement in mechanical properties has been achieved in the reinforced metal matrices produced after age-hardening. © 2007 Elsevier B.V. All rights reserved. Keywords: Al–Si–Fe alloy; Age-hardening; Composites; Microstructure; Silicon carbide and particulate
1. Introduction The age-hardening characteristics of an alloy are generally modified by the introduction of reinforcement. These modifications are due to the manufacturing process, the reactivity between the reinforcement and the matrix, the size, the morphology and volume fraction of the reinforcement [1,2]. A great deal of research has been carried out in the past to develop and improved aluminium-based composites with the aim of achieving better mechanical properties [3]. Donne et al. [4] studied the age-hardening behavior of a 30 wt% (SiCp/Al–Si) composite and found that the ageing response was very rapid, in that order only 1 h ageing at 120 ◦ C, 45% of the peak tensile strength and 69% of the peak hardness were attained, compared to the unreinforced alloy. Cottu et al. [5] showed that the hardening kinetics of A1–Cu–Mg alloy/10 wt% SiC fiber composite was enhanced by ∗
Corresponding author. Tel.: +234 8028433576. E-mail addresses: [email protected] (S.B. Hassan), [email protected] (V.S. Aigbodion). 0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.11.023
the presence of the reinforcement during heat-treatment. They explained this by the plastic deformation induced by the heattreatment due to the coefficient of thermal expansion (CTE) mismatch between matrix and reinforcement, as well as the highest dislocation density in the composite that supplied precipitation sites. Srivatson and Mattingly [6] also attributed the enhanced properties of their heat-treated 2124 Al alloy/SiC fiber composites to CTE mismatch and higher dislocation density factor. On the other hand, Skibo et al. [7] reported no difference in the ageing response of as-cast unreinforced A6061 and 10 and 20 wt% of SiC reinforced A6061 material in their studies of the strength and elongation as a function of ageing time in these materials at 175 ◦ C. With extensive data available on the heat-treatment of Al–Si–Fe alloys with conventional alloying elements [8], no much is documented on the age-hardening characteristics of this alloy particularly with silicon carbide reinforcement. Hence, the need for research in this area is justified. The objective of this present research therefore is to study the age-hardening characteristics of SiCp/Al–Si–Fe particulate composites.
S.B. Hassan et al. / Journal of Alloys and Compounds 466 (2008) 268–272
269
2. Materials and methods 2.1. Materials The materials used in this study are: high purity aluminium electrical wire obtained from Northern Cable Company NOCACO (Kaduna), ferrosilicon, silicon carbide with average particle size of 10 m, moulding box, silica sand, bentonite were obtained from the Foundry Shop of National Metallurgical Development Centre, Jos, Nigeria.
2.2. Equipment The equipment used in this study included: Pyrometer, mechanical stirrer, crucible, electrical resistance furnace, Rockwell hardness tester, Avery Denison impact machine, Tinus–Olsen tensile machine and Metallurgical microscope.
2.3. Methods The synthesis of the metal matrix composite that was use in this study was produced using double stir-casting method at the Foundry Shop of the National Metallurgical Development Center, Jos, Nigeria. The samples were produced by keeping the percentage of iron and silicon constant and varying the reinforcing material (silicon carbide) particles in the range 5–15% SiC. High purity aluminium electrical wires free from dust and contamination was charged in a graphite crucible kept in an electric resistance furnace and 0.01% NaCl–KCl powder was used as a cover for melting the alloy. The NaCl–KCl powder aids in minimizing oxidation of aluminium by excluding oxygen and creating a protective atmosphere inside the furnace. Upon initiation of melting of pure aluminium the temperature of the furnace was raised to 720 ◦ C. The required quantity of Silicon (2.8 wt%) and iron (0.8 wt%) was added using ferrosilicon, and the resulting melt thoroughly stirred. With progressive melting the furnace temperature was raised to 780 ◦ C and the melt was held at this temperature for 10 min. It was then skimmed to remove the oxides and impurities. The molten metal was continuously stirred in order to ensure a near-uniform distribution of alloying elements and prevent the elements from settling at the bottom on account of their higher density. For each melting 400 g of charge materials was used to produce the alloy. The SiC particles were preheated to 1000 ◦ C to oxidize their surface [9]. The alloy was then cooled down to just below the liquidus to keep the slurry in a semi-solid state. At this stage the preheated SiC particles were added and mixed manually. Manual mixing was used because it was very difficult to achieve mixing using automatic device when the alloy was in a semi-solid state. After proper manual mixing was done, the composite slurry was re-heated to full liquid state and then automatic mechanical mixing was carried out for about 20 min at an average stirring rate of 150 rpm. In the final mixing processes, the furnace temperature was controlled between 730 and 740 ◦ C and, the pouring temperature were controlled to be about 720 ◦ C. A preheated sand mould with
Fig. 1. Microstructure of the reinforced alloy (5% SiC). Dissolution of the eutectic silicon phase and uniform distribution of SiC (black) (×200).
Fig. 2. Microstructure of the reinforced alloy (15% SiC). Dissolution of the eutectic silicon phase and uniform distribution of SiC (black) (×200).
diameter 18 mm and 300 mm length was used to prepare cast bars [9]. After casting, the test samples were machined into standard test samples and solution heat-treated at temperature of 500 ◦ C in an electrically heated furnace, soaked for 3 h at this temperature and then rapidly quenched in warm water at 65 ◦ C. Ageing of the test samples were carried out at temperatures of 100, 200 and 300 ◦ C, for various ageing times of 60–660 min, and then cooled in air [9]. The ageing characteristic of these grades of composites was evaluated using hardness values obtained from solution heat-treated samples of the investigated composites subject to the aforementioned temperature conditions. The tensile and impact test were conducted at peak ageing time of various ageing temperatures.
Fig. 3. Microstructure of the age-hardened reinforced alloy (5% SiC). Dissolution of the eutectic silicon phase and uniform distribution of SiC (black) with precipitation (×200).
Fig. 4. Microstructure of the age-hardened reinforced alloy (15% SiC). Dissolution of the eutectic silicon phase and uniform distribution of SiC (black) with precipitation (×200).
270
S.B. Hassan et al. / Journal of Alloys and Compounds 466 (2008) 268–272
Fig. 5. Effect of age-hardening time on the hardness of 5SiCp/Al–Si–Fe composite.
2.3.1. Determination of hardness values Hardness was determined using the Rockwell hardness test machine. The indenter used was a 1.56 mm steel ball and minor load of 10 kg and major load of 100 kg was applied. Rockwell B scale and Hardness of 101.2HRB standard block were used as recommended by American Society for testing and Materials (ASTM) [9,10]. 2.3.2. Determination of impact strength Impact tests were conducted using a fully instrumented Avery Denison test machine. With impact energies ranging from 0 to 300 J. The mass of the hammer was 22.7 kg and the striking velocity 3.5 m/s. Charpy impact tests were conducted on notched specimens. Standard square impact test specimen measured 50 mm × 10 mm × 10 mm with notch depth of 2 mm and a notch tip radius of 0.02 mm at angle of 45◦ was used in this research as recommended by American Society for Testing and Materials (ASTM) [9,10]. 2.3.3. Determination of tensile properties The tensile properties were conducted on Tinus–Olsen test machine. The test pieces were machined to the standard shape and dimensions as specified by the American Society for Testing and Materials (ASTM) [9,10]. The yield strength, ultimate tensile strength, percentage elongation and reduction in cross-sectional area of the samples were determined.
Fig. 7. Effect of age-hardening temperature of 100 ◦ C on the hardness of Al–Si–Fe, 5SiCp/Al–Si–Fe and 15SiCp/Al–Si–Fe.
3. Results and discussion 3.1. Results The various microstructures developed of both grades of composites show distribution of the SiCp in the metal matrix, but the 15SiCp/Al–Si–Fe grade has more SiC uniformly distributed in the metal matrix than 5SiCp/Al–Si–Fe. There was precipitation of the second phase in age-hardened conditions (Figs. 1–4). Figs. 5 and 6 show the effect of ageing time at 100, 200 and 300 ◦ C on the hardness values of age-hardened SiCp/Al–Si–Fe composites. The age-hardening curves for 5SiCp/Al–Si–Fe and 15SiCp/Al–Si–Fe at 100, 200 and 300 ◦ C are shown in Figs. 7–9, respectively. The results of the tensile properties, impact energy and hardness values of Al–Si–Fe alloy, 5SiCp/Al–Si–Fe and 15SiCp/Al–Si–Fe for both as-cast and age-hardened conditions are shown in Table 1. 3.2. Discussion
2.3.4. Microstructural examination Metallographic specimens were cut from the as-cast and age-hardened samples of the SiCp/Al–Si–Fe particulate composite. The cut samples were mounted in Bakelite, and mechanically ground progressively on grades of SiC impregnated emery paper (80–600 grits) sizes using water as the coolant. The ground samples were polished using 1 m size alumina polishing powder suspended in distilled water. Final polish was achieved using 0.5-m alumina polishing powder suspended in distilled water. Following the polishing operation, etching of the polished sample was done using Keller’s reagent. The structure obtained was photographically recorded using an optical microscope with built in camera [9].
The 5SiCp/Al–Si–Fe exhibited a maximum hardness value of 72.0HRB at 100 ◦ C for 9 h, 73.0HRB at 200 ◦ C for 6 h and 74.0HRB at 300 ◦ C for 5 h (see Fig. 5). While 15SiCp/Al–Si–Fe have maximum hardness value of 82.0HRB at 100 ◦ C for 9 h, 84.0HRB at 200 ◦ C for 6 h and 85.0HRB at 300 ◦ C for 5 h (see Fig. 6). The tensile properties, hardness values and impact energy of the samples increases in the age-hardened than the ascast condition (see Table 1) for example, at peak ageing of 200 ◦ C, the yield strength of 5SiCp/Al–Si–Fe increased from
Fig. 6. Effect of age-hardening time on the hardness of 15SiCp/Al–Si–Fe composite.
Fig. 8. Effect of age-hardening temperature of 200 ◦ C on the hardness Al–Si–Fe, 5SiCp/Al–Si–Fe and 15SiCp/Al–Si–Fe.
S.B. Hassan et al. / Journal of Alloys and Compounds 466 (2008) 268–272
271
Table 1 Mechanical properties of the various grades of Al–Si–Fe/SiCp composites Mechanical properties
Ageing temperatures (◦ C)
Al–Si–Fe
5SiCp/Al–Si–Fe
15SiCp/Al–Si–Fe As-cast
As-cast
Peak aged
As-cast
Peak aged
100 200 300
45.60
59.90 56.40 52.40
57.57
65.20 64.80 63.00
72.38
78.0 78.91 74.00
Tensile strength (N/mm2 )
100 200 300
58.69
62.50 67.18 64.20
73.35
80.75 79.50 76.80
105.20
115.67 118.96 117.20
% elongation
100 200 300
6.00
13.00 12.00 10.80
5.50
10.40 9.00 9.80
3.00
4.40 4.44 4.40
% reduction in area
100 200 300
10.34
15.34 14.00 14.00
10.34
15.34 12.09 13.05
7.80
9.80 9.80 9.40
Hardness values (HRB)
100 200 300
31.10
60.10 61.50 62.00
54.50
72.00 73.00 74.00
64.30
82.00 84.00 85.00
Intact energy (J)
100 200 300
17.00
28.00 24.00 22.00
17.00
26.00 20.00 20.00
13.00
17.50 18.00 18.00
Yield strength
(N/mm2 )
57.57 to 64.80 N/mm2 , tensile strength increased from 73.35 to 79.50 N/mm2 in the as-cast and peak ageing, respectively. The yield and tensile strength of 15SiCp/Al–Si–Fe also increased from 72.38 to 78.91 N/mm2 and 105.20 to 118.96 N/mm2 in the as-cast and age-hardened condition at the same temperature (see Table 1). These results show that 5SiCp/Al–Si–Fe has 8.38, 12.56 and 17.65% increases in yield strength, tensile strength and impact energy, respectively, as compared to the as-cast samples at peak ageing temperature of 200 ◦ C, while 13.08, 7.76 and 38.46% increments in yield strength, tensile strength and impact energy were obtained for the 15SiCp/Al–Si–Fe, respectively. Both the age-hardened grade of composites exhibited higher ductility and impact energy than the as-cast condition (see Table 1). The ductility and impact energy of 5SiCp/Al–Si–Fe are higher than that of 15SiCp/Al–Si–Fe this is as a result of the high hardness values of the 15% SiCp. From the Figs. 5–9, it can be seen that there is steep rise in hardness values of each grade of the composite at initial stages for all ageing temperatures and then fell after reaching the various peak ageing time. Thereafter the hardness values decreases, corresponding to over-ageing. However, at higher ageing temperature the materials develop peak hardness at shorter ageing time, because the rate of precipitation of the second phase is faster and hence the rate of increases in hardness is also faster [11,12]. The age-hardening behavior of the SiCp/Al–Si–Fe composites are similar to 5SiCp/Al–Si–Mg as reported by Cottu et al. [5], i.e. hardness continuously increases to maximum value during ageing and them decreases later due to overageing. It is interesting to note that in the reinforced aluminium alloy metal–matrix, as the volume fraction of SiCp in the aluminium alloy metal–matrix increases there is a mono-
Peak aged
tonic reduction in the time required to reach peak hardness (see Figs. 5 and 6). The addition of SiCp reinforcement to the aluminium alloy metal–matrix (Al–Si–Fe) raises the dislocation density at the particle–matrix interfaces [5,6]. This is due to the differences in coefficient of thermal expansion (CTE) between the hard and brittle reinforcing particles and the soft and ductile metal–matrix. The presence of high dislocation density at interfaces facilitates precipitation at these locations. The formation and presence of precipitates at the particles–matrix interfaces may be appreciated by comparing micrographs of the composite in the as-cast (see Figs. 1 and 2) and in the peak age-hardened condition (see Figs. 3 and 4). The micrograph in the peak age-hardened condition (Figs. 3 and 4) reveals precipitates covering the surface at the particles–matrix interfaces. Cottu and co-workers reported similar behavior of increased precipitation at interfaces [5,11] for aluminium alloy 6061 reinforced with whiskers of silicon carbide. They attributed this to increase in dislocation density at interfaces. Increase in dislocation density strain
Fig. 9. Effect of age-hardening temperature of 300 ◦ C on the hardness Al–Si–Fe, 5SiCp/Al–Si–Fe and 15SiCp/Al–Si–Fe.
272
S.B. Hassan et al. / Journal of Alloys and Compounds 466 (2008) 268–272
hardens the metal–matrix locally and provides heterogeneous nucleation sites for precipitation, thereby accelerating the ageing response [4,6,12]. The higher values of both ultimate tensile strength and yield strength obtained at age-hardened condition as compared with the as-cast condition are attributed to uniform distribution and precipitation of the SiC particles in the microstructure of the agehardened condition (see Figs. 3 and 4) than the microstructure of as-cast condition (see Figs. 1 and 2), it can be seen that the inter-atomic spacing in later is larger than in the former. The contribution of inter-atomic spacing to strain hardening arises from the fact that the space permitted a dislocation to manoeuvre round obstacles limit the yield stress given by the formula:
(3) This study therefore, has established that these grade of composites response very well to precipitation hardening heat-treatment.
Gb λ where τ is the yield stress, λ the inter-particle spacing, G the shear energy of dislocation and b is the Burger vector. The smaller the values of λ the greater the yield stress as a result of strain hardening. It therefore supports the fact that (Figs. 3 and 4) with smaller λ have greater strengths than (Figs. 1 and 2) with larger λ [13]. These modulated structure formed during ageing in both grade of composites enhanced this mechanical properties and is in agreement with the earlier work of 5SiCp/Al–Si–Mg [5,11].
References
τ=
4. Conclusions From the results obtained from this study, the following conclusions can be made: (1) The mechanical properties of the two grades of composites produced are higher in the age-hardened samples than that of the as-cast samples, due to the precipitates of the second phase during ageing which cover the surface at the particles–matrix interfaces. (2) It was found that hardness increases with increasing weight fraction of silicon carbide in the alloy and decreases with increasing ageing time after the peak ageing time have been exceeded.
Acknowledgments The Authors acknowledge with thanks the management of the National Metallurgical Development Centre, Jos, Nigeria for allowing us use their equipment. We also acknowledge the assistance given by Dr. S.A. Yaro, M.U. Suleiman and A.W. Usman for their assistance during the production of the research material.
[1] T.W. Clyne (Ed.), Comprehensive Composite Materials, Metal Matrix Composites (ser. eds), Elsevier, vol. 3, 2000, 1–8. [2] A. Sagail, G. Leisk, Scripta Metall. Mater. (1992) 871–876. [3] T.W. Clyne, in: A. Mortensen (Ed.), Encyclopedia of Material Science and Technology, Elsevier, 2001, pp. 26–38. [4] S. Donne, M.R. Krishnadev, R. Bouchard, Minerals Met. Mater. Soc. (1990) 243–251. [5] J.P. Cottu, J.J. Coudere, B. Viguier, L. Bernard, J. Mater. Sci. 27 (1992) 3068–3074. [6] T.S. Srivatsan, J. Mattingly, J. Mater. Sci. 289 (1993) 611–620. [7] M. Skibo, P.L. Morris, D.J. Lloyd, in: S.G. Fishman, A.K. Dhirngra (Eds.), Cast Reinforced Composites, ASM International, 1988, pp. 257–261. [8] S.A. Yaro, V.S. Aigbodion, O.Y. Mohammed, Int. Res. J. Eng. Technol. (2006) 32–39. [9] V.S. Aigbodion, Development of Al–Si–Fe/SiC particulate composite as advanced materials for engineering applications, Unpublished M.Sc Thesis, Department of Metallurgical Engineering, Ahmadu Bello University, Zaria, Nigeria, 2007, pp. 3–10. [10] Annual Books of ASTM Standards, Section 1. Iron and Steel Products, American Society of Testing Materials, vol. 01.04, 1990, pp. 13–63. [11] N.A.O. Ikechukwuka, Characterization of aluminium alloy 2618 and its composites containing alumina particles, unpublished PhD Thesis Department of Mechanical engineering, University of Saskatchewan, Saskatoon, 1999, pp. 1–200. [12] S. Jayalakshmi, S. Seshan, S.V. Kailas, S Srivatsan, Influence Processing and Reinforcement on microstructure and impact behaviour of magnesium alloy AM100 S¯adhan¯a, 29(5, October), 2004 509–523, ©Printed in India. [13] G.E. Dieter, Mechanical Metallurgy, McGraw-Hill Company, 1988, pp. 184–223.