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Effect of mechanical alloying process on mechanical properties of α-Si3N4 reinforced aluminum-based composite materials
Materials and Design 29 (2008) 1856–1861
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Materials and Design journal homepage: www.elsevier.com/locate/matdes
Effect of mechanical alloying process on mechanical properties of a-Si3N4 reinforced aluminum-based composite materials Halil Arik Department of Metallurgy, Faculty of Technical Education, Gazi University, Ankara 06500, Turkey
a r t i c l e
i n f o
Article history: Received 2 June 2006 Accepted 15 May 2007 Available online 28 March 2008 Keywords: Metal–matrix composites (MMCs) Mechanical alloying Al–a-Si3N4 Mechanical properties
a b s t r a c t In this study, Al–a-Si3N4 metal matrix composite (MMCs) materials were produced by using powder metallurgy technique. To prepare mixture of Al and a-Si3N4 powders two different methods were used by milling for 5 h in a ball mill with alumina balls and mechanically alloying for 5 h in a high-energy attritor mill. Then mixed powders were compacted under 1000 MPa pressure to produce standard transverse rupture block specimens with dimension of 6.35 12.70 31.70 mm. Compacted samples were then sintered at 620; 640; 660; and 680 °C for 2 h under argon atmosphere in a tube furnace. The mechanical properties of the composite specimens were determined by measuring the density, hardness and transverse rupture strength values. Using optical and scanning electron microscopy (SEM) were also observed microstructures of the composites. The results showed that, more homogenous dispersion of a-Si3N4 powders were obtained in Al matrix using the mechanical alloying technique according to classical mixing method. Especially, as a result of homogenous dispersion each other of Al and a-Si3N4 powders high hardness and transverse repute strength values were obtained from these samples. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Due to ease of formability and lightweight, aluminum is finding many applications in industrial area [1,2]. But because of poor elevated temperature properties of aluminum and its alloys, much research work had been carried out to strengthen them by producing aluminum matrix composites [3–5]. Aluminum matrix composites, produced combining aluminum matrix with ceramic reinforcement such as SiC, Al2O3, and Si3N4, have become increasingly used for critical structural applications in industrial sector because of their excellent stiffness to density and strength to density ratios [6–9]. A wide range of production techniques have been developed for aluminum matrix composites materials. This is generally done by dispersion of high temperature resistant fine ceramic reinforcement particles in the aluminum structure. Dispersion of fine particles is either carried out in the solid or liquid-state. In the liquid methods, particles are added to liquid aluminum by stirring before casting, but the resulting distribution is generally inhomogeneous. The solid-state route is powder metallurgy processing of the blended powders of each constituent. Among the others, however, the powder metallurgy (PM) method is the most attractive due to several reasons. Firstly, PM offers microstructural control of the phases. Secondly, the lower temperatures employed during the process accounts for the strict control of interface kinetics. The conventional PM route for making Al matrix composites include: (1) Al is blended
E-mail address: [email protected] 0261-3069/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2008.03.010
or mechanically alloyed with reinforcement particles; (2) compaction by die pressing, and (3) sintering [10–13]. The aim of this study was to produce a-Si3N4 reinforcement Al-based composites materials using powder metallurgy method and determination of their mechanical properties. As a starting material, Al and a-Si3N4 were prepared using two different methods namely; mechanical alloying, and conventional mixing. Finally, effects of some parameters, such as sintering temperatures, amount of Si3N4, and preparation of starting powders, on the mechanical properties of produced Al matrix composite materials were discussed. 2. Experimental procedure In this study gas atomized Al powders (maximum particle size of 100 lm) and a-Si3N4 powders (specifications in Table 1) were used as the initial materials. As a first method the powders were mixed together according to the proportions of Al– a-Si3N4 (5; 10; 15 wt%) and then ball milled for 5 h with ceramic balls in a ball miller (Fig. 1). As a second method the powders at he same proportions were mechanically alloyed for 5 h. with steel balls in a high-energy ball miller (Fig. 2). The parameters for mechanical alloying are given in Table 2. The use of argon gas was to prevent possible oxidation of the new surfaces of Al particles created on fracturing. In order to minimize the extreme tendency of aluminum to get itself welded during milling, 2 wt% of stearic acid was used as a process control agent. After the milling and alloying shape and size of mixed powders were characterized by using optic microscope and Malvern Master Seizer E version 1.2b laser scattering machine. Then mixed powders were compacted at 1000 MPa pressure to produce blocks 6.35 12.70 31.70 mm in size, according to ASTM B 312, by using a single action press [14]. Block samples were then put to graphite boats which were placed in an atmosphere controlled tube furnace and heated to test temperature in a flowing argon atmosphere for predetermined times. Sintering was performed at 620; 640; 660, and 680 °C for 2 h. The surfaces of all sintered blanks were lightly ground and polished to remove any irregularity or debris. The samples were then subjected
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H. Arik / Materials and Design 29 (2008) 1856–1861 Table 1 Specifications of a-Si3N4 powders
Table 2 Mechanical alloying condition for the powder mixture
Grade
SN-E-10
Degree of crystallinity (%)
100
Morphology
Equiaxed
Phase composition
a-Phase 95% b-Phase 5%
Particle size (lm) Specific surface area (m2/g)
0.1–0.3 10–14
Purity
N > 38.0% O < 2.0% C < 2.0% Cl < 100 ppm Fe < 100 ppm Ca < 50 ppm Al < 50 ppm
MA MA Rotor PCA Charge ratio (mass Sample Al aatmosphere speed (g) Si2N4 (%) of grinding balls: mass (rev. min1) of powder in mill) (g) 1 2 3
95 90 85
5 10 5
1.5 1.5 1.5
10:1 10:1 10:1
5 5 5
Ar Ar Ar
450 450 450
P
Materials (Al+Si3N4)
Steel balls Jar
25.4±0.3 mm
Drive rollers Fig. 3. Transverse rupture test apparatus. Fig. 1. Schematic illustration of ball milling of powders.
3. Results and discussion to transverse rupture test in a special device, designed and manufactured according to MPIF Standard 41 at Gazi P/M Lab (Fig. 3). The load was applied by a lever mechanism with balls of 0.040 kN (minimum load reading). Calculate the transverse rupture strength as follows: S ¼ 3PL=2t2 w; where S = transverse rupture strength, psi (MPa); P = force required to rupture, lbf (N); L = length of specimen span of fixture, in. (mm); W = width of specimen, in. (mm); t = thickness of specimen, in. (mm). Hardness’s of all samples were measured by the Vickers hardness method and mean of at least five readings was taken. SEM microscopy was used in order to determine the effect of mixing method and sintering temperature on the microstructure of Al–a-Si3N4 composite block materials.
3.1. Effect of a-Si3N4 amount on particle size and morphology of mechanically alloyed powders Microstructures show no significant effect of conventional mixing on the particle size and the shape of the mixture powders. For instance, the mean particle size of starting mixture (10 wt% Si3N4 balance Al) was 93.52 lm, whereas it was 91.35 lm after 5 h mixing (Fig. 4a). However, the results obtained from mechanically alloyed powders showed considerable change in particle size and shape after processing the powders by mechanical alloying
Ar
Ar
Ball and Si3N4- Al mixing
Cooling Water Rotating impeller
Cooling Water
Fig. 2. Schematic illustration of mechanically alloying set up.
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H. Arik / Materials and Design 29 (2008) 1856–1861
Fig. 4. Effect of mechanical alloying process and amount of a-Si3N4 on the mean particle size of composite materials.
(Fig. 4b and c). The mean particle size of Al powders containing 5 wt% Si3N4 decreased from 93.52 lm to 70.46 lm after mechanical alloying for 2 h. At the same time spherical morphology of Al particles turned to flaky shape by mechanical alloying process (Fig. 5a and b). Average particle size of alloyed powders containing 10 wt% Si3N4 decreased to 36.94 lm after mechanical alloying for 2 h. In these samples, deformation of Al particles was partly restricted with increasing the Si3N4 amount in the matrix. Thus, more fragmentation into smaller particles took place and consequently particle size decreased in Al matrix (Fig. 5c). In general, increasing the amount of a-Si3N4 in the matrix resulted in a decrease in the final powder size. Particle got smaller, homogenized and turned to flake like morphology with increasing the alloying time. At the same time, some of the particles cold welded to each other as shown in Fig. 5c. The mean particle size of the powders containing 15 wt% aSi3N4 increased to 50.71 lm after alloying for 2 h. This was coarser than that was observed on the powders contained 10 wt% a-Si3N4. This increase was attributed to the increasing of the a-Si3N4 in Al matrix as shown in Fig. 5d. High amount of a-Si3N4 in the matrix resulted in both easy fracturing and cold welding of Al particles. Cold welding caused of Al particles to be coarser at certain regions which consequently affecting the mean particle size of mixture powders. 3.2. Effect of sintering temperature on the density, microstructure and transverse rupture strength of the block samples As shown in Fig. 6, densities of the samples increased with increasing sintering temperatures up to 660 °C. However, samples sintered at 680 °C showed a considerable decrease in densities compared to those sintered at 660 °C. The reason for this was partly melting of blocks since sintering temperature exceeded the melting point of pure Al. But this melting did not make any
detectable change in the shape of the composite samples. As seen in Fig. 6, sintered density increase was more pronounced on mechanically alloyed powders compared to conventionally mixed powders. Inhomogeneous distribution of a-Si3N4 particles in the matrix mixed with classical method prevented the sintering and consequently reduced the density. On the other hand mechanical alloying resulted in a homogenous distribution of a-Si3N4 particles which facilitated the sintering. At the same time, MA process increased the internal energy of the particles, which positively affected the sintering behavior. All block containing high amount of a-Si3N4 processed with conventionally mixing exhibited lower density. Maximum density was measured on the MA processed sample containing 10 wt% a-Si3N4. This was attributed to the finer powder particle size together with a homogeneously distributed reinforcement elements. Finer particles and homogenous dispersion of a-Si3N4 affects sintering behavior positively and consequently increase the density of the composite material. As a general, transverse rupture strength of blocks was increased with increasing sintering temperatures. This increase was more evident on the sample prepared by MA. Mainly, there are two important reasons for this increase. Firstly, these samples have better sintering ability and higher densities comparing to those that mixed with conventional method. Secondly, and more important reason is; more homogenous dispersion of a-Si3N4 in Al matrix which could only be obtained by MA process (Fig. 7). As it is known, the most important factors affecting the fracture strength of the particle-reinforced composites are size, shape and dispersion of reinforcement materials in the matrix structure [15,16]. 3.3. Effect of sintering temperature on hardness of the block samples Hardness values of the samples, containing different amounts of a-Si3N4 and sintered at various temperatures, are given in Fig. 8.
H. Arik / Materials and Design 29 (2008) 1856–1861
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Fig. 5. Morphology of samples mechanically alloyed for 5 h. (a) Original Al powder; (b) Al–5a-Si3N4 (wt%); (c) Al–10a-Si3N4 (wt%) and (d) Al–15a-Si3N4 (wt%).
100
Density (%)
98
96
94
92
90 610
620
630
640
650
660
670
680
690
Sintering Temperatures (°C) %5 α-Si3N4 MA.
%15 α-Si3N4 MA.
%10 α-Si3N4 Mix.
%5 α-Si3N4 Mix.
%10 α-Si3N4 MA. %15 α-Si3N4 Mix.
Fig. 6. Effect of sintering temperature on the density of samples.
The results showed that, there were three different parameters affecting the hardness of the blocks. These are; a-Si3N4 amount, powders preparation method, and sintering temperature. The most important one to produce composite materials is preparation method of the mixture powders. It is clear that, all mechanically alloyed samples had higher hardness values compared to the conventionally mixed samples. The sample containing 10 wt% aSi3N4 had the highest hardness among the samples since it had the finest powder particle size together with more homogenous dispersion of a-Si3N4 in Al matrix. However, samples prepared by conventional mixing method, exhibited coarse particle size and coarse a-Si3N4 ceramic reinforcement which generally located at the Al particles interfaces instead of homogenous dispersion in the Al powder particles. Because of this, hardness did not increase with increasing sintering temperature as it was expected. All samples exhibited significant hardness increase with sintering temperature. The highest hardness among the mechanically alloyed samples was detected on the samples sintered at 660 °C. Further increase in sintering temperature resulted in a decrease in hardness in all samples. This was probably due to the partially melting of the samples. It seems that the mixing method has an important factor on the distribution of reinforcing elements in matrix. In this study, more uniform distribution of a-Si3N4 particles was obtained by mechanically alloyed samples compared to conventional mixed one. It can be concluded that mechanical alloying
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H. Arik / Materials and Design 29 (2008) 1856–1861
3000
Transverse rupture strengh (MPa)
2500
2000
1500
1000
500
0 600
620
640
660
680
700
Sintering Temperatures (°C) % 5 α-Si3N4 MA.
% 15 α-Si3N4 MA.
% 10 α-Si3N4 MA.
%5 α-Si3N4 Mix.
%10 α-Si3N4 Mix.
%15 α-Si3N4 Mix.
Fig. 7. Effect of sintering temperature on the transverse rupture strength of samples.
175
150
Fig. 9. Distribution of a-Si3N4 (10 wt%) particles in composite materials sintered at 660 °C for 2 h: (a) composite was prepared by using conventional mixing method and (b) composite was prepared by using mechanical alloying technique.
Hardness (Hv)
125
conventional milling and MA techniques were used for 5 h. The results were given below:
100
75
50
25
0 600
620
640
660
680
700
Temperatures (°C ) %5 α-Si3N4 MA.
%10 α-Si3N4 MA.
%15 α-Si3N4 MA.
%5 α-Si3N4 Mix.
%10 α-Si3N4 Mix.
%15 α-Si3N4 Mix.
Fig. 8. Effect of sintering temperature on the hardness of samples.
technique improves the mechanical properties of the composite materials considerably (see Fig. 9). 4. Conclusion Al–a-Si3N4 metal matrix composite were produced by using PM techniques. In order to prepare mixture of Al and a-Si3N4 powders
1. More homogenous distribution of a-Si3N4 in Al matrix was obtained by using MA techniques compared to a conventional mixing method. 2. As a result of homogenous dispersion of a-Si3N4 in Al matrix, a considerable improvement was obtained on density, transverse rupture strength and hardness of composite blocks. 3. MA process resulted in high deformation of Al particles which facilitated the sintering behavior of blocks due to the high internal energy of powder particle 4. The results obtained from this study showed that the ideal sintering temperature and amount of a-Si3N4 were 660 °C and 10 wt%, respectively for better mechanical properties.
References [1] Arık H. Production and characterization of in situ Al4C3 reinforced aluminumbased composite produced by mechanical alloying technique. Mater Des 2004;25:31–40. [2] Ge D, Gu M. Mechanical properties of hybrid reinforced aluminum based composites. Mater Lett 2001;49:334–9. [3] Kostka A, Lelatko J, Gigla M, Moraviec H, Janas A. TEM study of the interface in ceramic-reinforced aluminum-based composites. Mater Chem Phys 2003;81:323–5. [4] Jeong HG, Kıraga K, Mabuchi M. Effect of addition of magnesium on interface structure and high-strain-rate superplastıcity in Si3N4-reinforced Al-alloy composites. Acta Mater 1998;46:6009–20. [5] Arık H, Turker M, Saritas S. Investigation of mechanical properties of in situ Al4C3 reinforced aluminum based composites by mechanical alloying
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[6] [7] [8]
[9] [10]
technique. Powder Metallurgy World Congress and Exhibition, Kyoto, Japan, 2000. p. 543–9. Gingu O, Mangra M, Orban RL. In-situ production of Al/SiCp composite by laser deposition technology. J Mater Process Technol 1999;89–90:187–90. Cao GH, Wu SQ, Liu JM, Liu ZG. Wear-resistance mechanism of an Al–12Si alloy reinforced with aluminosilicate short fibers. Tribology 1999;32:721–4. Gür CH. Investigation of microstructure – ultrasonic velocity relationship in SiCp-reinforced aluminum metal matrix composites.. Mater Sci Eng A 2003;A361:29–35. Tjong SC, Tam KF, Wu SQ. Thermal characteristics of in-situ Al-based composites prepared by reactive hot pressing. Compos Sci Technol 2003;63:89–97. Ogel B, Gurbuz R. Microstructural characterization and tensile properties of hot pressed Al–SiC composites prepared from pure Al and Cu powders. Mater Sci Eng A 2001;A 301:213–20.
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[11] Sankar R. Synthesis of 7075 Al/SiC particulate composite powders by mechanical alloying. Mater Lett 1998;36:201–5. [12] Zhang DL. Processing of a transverse rupture strength advanced materials using high-energy mechanical milling. Prog Mater Sci 2004;49:537–60. [13] Suryanarayana C. Mechanical alloying and milling. Prog Mater Sci 2001;46:1–184. [14] ASTM B 312. Standard Test Method for Green Strength for Compacted Metal Powder Specimens, vol. 02.05, 1996. p. 80–91. [15] Peng LM, Zhu SJ, Ma ZY, Bi J. The effect of Si3 N4 whiskers on the hightemperature creep behavior of an Al–Fe–V–Si alloy matrix composite. Compos Sci Technol 1999;59:769–73. [16] Chen LQ, Kanetake N. Fabrication and mechanical behavior of powder metallurgy processed in situ Nb/Al-sheet metal–metal composites. Mater Sci Eng A 2004;367:295–300.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Effect of mechanical alloying process on mechanical properties of a-Si3N4 reinforced aluminum-based composite materials Halil Arik Department of Metallurgy, Faculty of Technical Education, Gazi University, Ankara 06500, Turkey
a r t i c l e
i n f o
Article history: Received 2 June 2006 Accepted 15 May 2007 Available online 28 March 2008 Keywords: Metal–matrix composites (MMCs) Mechanical alloying Al–a-Si3N4 Mechanical properties
a b s t r a c t In this study, Al–a-Si3N4 metal matrix composite (MMCs) materials were produced by using powder metallurgy technique. To prepare mixture of Al and a-Si3N4 powders two different methods were used by milling for 5 h in a ball mill with alumina balls and mechanically alloying for 5 h in a high-energy attritor mill. Then mixed powders were compacted under 1000 MPa pressure to produce standard transverse rupture block specimens with dimension of 6.35 12.70 31.70 mm. Compacted samples were then sintered at 620; 640; 660; and 680 °C for 2 h under argon atmosphere in a tube furnace. The mechanical properties of the composite specimens were determined by measuring the density, hardness and transverse rupture strength values. Using optical and scanning electron microscopy (SEM) were also observed microstructures of the composites. The results showed that, more homogenous dispersion of a-Si3N4 powders were obtained in Al matrix using the mechanical alloying technique according to classical mixing method. Especially, as a result of homogenous dispersion each other of Al and a-Si3N4 powders high hardness and transverse repute strength values were obtained from these samples. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Due to ease of formability and lightweight, aluminum is finding many applications in industrial area [1,2]. But because of poor elevated temperature properties of aluminum and its alloys, much research work had been carried out to strengthen them by producing aluminum matrix composites [3–5]. Aluminum matrix composites, produced combining aluminum matrix with ceramic reinforcement such as SiC, Al2O3, and Si3N4, have become increasingly used for critical structural applications in industrial sector because of their excellent stiffness to density and strength to density ratios [6–9]. A wide range of production techniques have been developed for aluminum matrix composites materials. This is generally done by dispersion of high temperature resistant fine ceramic reinforcement particles in the aluminum structure. Dispersion of fine particles is either carried out in the solid or liquid-state. In the liquid methods, particles are added to liquid aluminum by stirring before casting, but the resulting distribution is generally inhomogeneous. The solid-state route is powder metallurgy processing of the blended powders of each constituent. Among the others, however, the powder metallurgy (PM) method is the most attractive due to several reasons. Firstly, PM offers microstructural control of the phases. Secondly, the lower temperatures employed during the process accounts for the strict control of interface kinetics. The conventional PM route for making Al matrix composites include: (1) Al is blended
E-mail address: [email protected] 0261-3069/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2008.03.010
or mechanically alloyed with reinforcement particles; (2) compaction by die pressing, and (3) sintering [10–13]. The aim of this study was to produce a-Si3N4 reinforcement Al-based composites materials using powder metallurgy method and determination of their mechanical properties. As a starting material, Al and a-Si3N4 were prepared using two different methods namely; mechanical alloying, and conventional mixing. Finally, effects of some parameters, such as sintering temperatures, amount of Si3N4, and preparation of starting powders, on the mechanical properties of produced Al matrix composite materials were discussed. 2. Experimental procedure In this study gas atomized Al powders (maximum particle size of 100 lm) and a-Si3N4 powders (specifications in Table 1) were used as the initial materials. As a first method the powders were mixed together according to the proportions of Al– a-Si3N4 (5; 10; 15 wt%) and then ball milled for 5 h with ceramic balls in a ball miller (Fig. 1). As a second method the powders at he same proportions were mechanically alloyed for 5 h. with steel balls in a high-energy ball miller (Fig. 2). The parameters for mechanical alloying are given in Table 2. The use of argon gas was to prevent possible oxidation of the new surfaces of Al particles created on fracturing. In order to minimize the extreme tendency of aluminum to get itself welded during milling, 2 wt% of stearic acid was used as a process control agent. After the milling and alloying shape and size of mixed powders were characterized by using optic microscope and Malvern Master Seizer E version 1.2b laser scattering machine. Then mixed powders were compacted at 1000 MPa pressure to produce blocks 6.35 12.70 31.70 mm in size, according to ASTM B 312, by using a single action press [14]. Block samples were then put to graphite boats which were placed in an atmosphere controlled tube furnace and heated to test temperature in a flowing argon atmosphere for predetermined times. Sintering was performed at 620; 640; 660, and 680 °C for 2 h. The surfaces of all sintered blanks were lightly ground and polished to remove any irregularity or debris. The samples were then subjected
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H. Arik / Materials and Design 29 (2008) 1856–1861 Table 1 Specifications of a-Si3N4 powders
Table 2 Mechanical alloying condition for the powder mixture
Grade
SN-E-10
Degree of crystallinity (%)
100
Morphology
Equiaxed
Phase composition
a-Phase 95% b-Phase 5%
Particle size (lm) Specific surface area (m2/g)
0.1–0.3 10–14
Purity
N > 38.0% O < 2.0% C < 2.0% Cl < 100 ppm Fe < 100 ppm Ca < 50 ppm Al < 50 ppm
MA MA Rotor PCA Charge ratio (mass Sample Al aatmosphere speed (g) Si2N4 (%) of grinding balls: mass (rev. min1) of powder in mill) (g) 1 2 3
95 90 85
5 10 5
1.5 1.5 1.5
10:1 10:1 10:1
5 5 5
Ar Ar Ar
450 450 450
P
Materials (Al+Si3N4)
Steel balls Jar
25.4±0.3 mm
Drive rollers Fig. 3. Transverse rupture test apparatus. Fig. 1. Schematic illustration of ball milling of powders.
3. Results and discussion to transverse rupture test in a special device, designed and manufactured according to MPIF Standard 41 at Gazi P/M Lab (Fig. 3). The load was applied by a lever mechanism with balls of 0.040 kN (minimum load reading). Calculate the transverse rupture strength as follows: S ¼ 3PL=2t2 w; where S = transverse rupture strength, psi (MPa); P = force required to rupture, lbf (N); L = length of specimen span of fixture, in. (mm); W = width of specimen, in. (mm); t = thickness of specimen, in. (mm). Hardness’s of all samples were measured by the Vickers hardness method and mean of at least five readings was taken. SEM microscopy was used in order to determine the effect of mixing method and sintering temperature on the microstructure of Al–a-Si3N4 composite block materials.
3.1. Effect of a-Si3N4 amount on particle size and morphology of mechanically alloyed powders Microstructures show no significant effect of conventional mixing on the particle size and the shape of the mixture powders. For instance, the mean particle size of starting mixture (10 wt% Si3N4 balance Al) was 93.52 lm, whereas it was 91.35 lm after 5 h mixing (Fig. 4a). However, the results obtained from mechanically alloyed powders showed considerable change in particle size and shape after processing the powders by mechanical alloying
Ar
Ar
Ball and Si3N4- Al mixing
Cooling Water Rotating impeller
Cooling Water
Fig. 2. Schematic illustration of mechanically alloying set up.
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H. Arik / Materials and Design 29 (2008) 1856–1861
Fig. 4. Effect of mechanical alloying process and amount of a-Si3N4 on the mean particle size of composite materials.
(Fig. 4b and c). The mean particle size of Al powders containing 5 wt% Si3N4 decreased from 93.52 lm to 70.46 lm after mechanical alloying for 2 h. At the same time spherical morphology of Al particles turned to flaky shape by mechanical alloying process (Fig. 5a and b). Average particle size of alloyed powders containing 10 wt% Si3N4 decreased to 36.94 lm after mechanical alloying for 2 h. In these samples, deformation of Al particles was partly restricted with increasing the Si3N4 amount in the matrix. Thus, more fragmentation into smaller particles took place and consequently particle size decreased in Al matrix (Fig. 5c). In general, increasing the amount of a-Si3N4 in the matrix resulted in a decrease in the final powder size. Particle got smaller, homogenized and turned to flake like morphology with increasing the alloying time. At the same time, some of the particles cold welded to each other as shown in Fig. 5c. The mean particle size of the powders containing 15 wt% aSi3N4 increased to 50.71 lm after alloying for 2 h. This was coarser than that was observed on the powders contained 10 wt% a-Si3N4. This increase was attributed to the increasing of the a-Si3N4 in Al matrix as shown in Fig. 5d. High amount of a-Si3N4 in the matrix resulted in both easy fracturing and cold welding of Al particles. Cold welding caused of Al particles to be coarser at certain regions which consequently affecting the mean particle size of mixture powders. 3.2. Effect of sintering temperature on the density, microstructure and transverse rupture strength of the block samples As shown in Fig. 6, densities of the samples increased with increasing sintering temperatures up to 660 °C. However, samples sintered at 680 °C showed a considerable decrease in densities compared to those sintered at 660 °C. The reason for this was partly melting of blocks since sintering temperature exceeded the melting point of pure Al. But this melting did not make any
detectable change in the shape of the composite samples. As seen in Fig. 6, sintered density increase was more pronounced on mechanically alloyed powders compared to conventionally mixed powders. Inhomogeneous distribution of a-Si3N4 particles in the matrix mixed with classical method prevented the sintering and consequently reduced the density. On the other hand mechanical alloying resulted in a homogenous distribution of a-Si3N4 particles which facilitated the sintering. At the same time, MA process increased the internal energy of the particles, which positively affected the sintering behavior. All block containing high amount of a-Si3N4 processed with conventionally mixing exhibited lower density. Maximum density was measured on the MA processed sample containing 10 wt% a-Si3N4. This was attributed to the finer powder particle size together with a homogeneously distributed reinforcement elements. Finer particles and homogenous dispersion of a-Si3N4 affects sintering behavior positively and consequently increase the density of the composite material. As a general, transverse rupture strength of blocks was increased with increasing sintering temperatures. This increase was more evident on the sample prepared by MA. Mainly, there are two important reasons for this increase. Firstly, these samples have better sintering ability and higher densities comparing to those that mixed with conventional method. Secondly, and more important reason is; more homogenous dispersion of a-Si3N4 in Al matrix which could only be obtained by MA process (Fig. 7). As it is known, the most important factors affecting the fracture strength of the particle-reinforced composites are size, shape and dispersion of reinforcement materials in the matrix structure [15,16]. 3.3. Effect of sintering temperature on hardness of the block samples Hardness values of the samples, containing different amounts of a-Si3N4 and sintered at various temperatures, are given in Fig. 8.
H. Arik / Materials and Design 29 (2008) 1856–1861
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Fig. 5. Morphology of samples mechanically alloyed for 5 h. (a) Original Al powder; (b) Al–5a-Si3N4 (wt%); (c) Al–10a-Si3N4 (wt%) and (d) Al–15a-Si3N4 (wt%).
100
Density (%)
98
96
94
92
90 610
620
630
640
650
660
670
680
690
Sintering Temperatures (°C) %5 α-Si3N4 MA.
%15 α-Si3N4 MA.
%10 α-Si3N4 Mix.
%5 α-Si3N4 Mix.
%10 α-Si3N4 MA. %15 α-Si3N4 Mix.
Fig. 6. Effect of sintering temperature on the density of samples.
The results showed that, there were three different parameters affecting the hardness of the blocks. These are; a-Si3N4 amount, powders preparation method, and sintering temperature. The most important one to produce composite materials is preparation method of the mixture powders. It is clear that, all mechanically alloyed samples had higher hardness values compared to the conventionally mixed samples. The sample containing 10 wt% aSi3N4 had the highest hardness among the samples since it had the finest powder particle size together with more homogenous dispersion of a-Si3N4 in Al matrix. However, samples prepared by conventional mixing method, exhibited coarse particle size and coarse a-Si3N4 ceramic reinforcement which generally located at the Al particles interfaces instead of homogenous dispersion in the Al powder particles. Because of this, hardness did not increase with increasing sintering temperature as it was expected. All samples exhibited significant hardness increase with sintering temperature. The highest hardness among the mechanically alloyed samples was detected on the samples sintered at 660 °C. Further increase in sintering temperature resulted in a decrease in hardness in all samples. This was probably due to the partially melting of the samples. It seems that the mixing method has an important factor on the distribution of reinforcing elements in matrix. In this study, more uniform distribution of a-Si3N4 particles was obtained by mechanically alloyed samples compared to conventional mixed one. It can be concluded that mechanical alloying
1860
H. Arik / Materials and Design 29 (2008) 1856–1861
3000
Transverse rupture strengh (MPa)
2500
2000
1500
1000
500
0 600
620
640
660
680
700
Sintering Temperatures (°C) % 5 α-Si3N4 MA.
% 15 α-Si3N4 MA.
% 10 α-Si3N4 MA.
%5 α-Si3N4 Mix.
%10 α-Si3N4 Mix.
%15 α-Si3N4 Mix.
Fig. 7. Effect of sintering temperature on the transverse rupture strength of samples.
175
150
Fig. 9. Distribution of a-Si3N4 (10 wt%) particles in composite materials sintered at 660 °C for 2 h: (a) composite was prepared by using conventional mixing method and (b) composite was prepared by using mechanical alloying technique.
Hardness (Hv)
125
conventional milling and MA techniques were used for 5 h. The results were given below:
100
75
50
25
0 600
620
640
660
680
700
Temperatures (°C ) %5 α-Si3N4 MA.
%10 α-Si3N4 MA.
%15 α-Si3N4 MA.
%5 α-Si3N4 Mix.
%10 α-Si3N4 Mix.
%15 α-Si3N4 Mix.
Fig. 8. Effect of sintering temperature on the hardness of samples.
technique improves the mechanical properties of the composite materials considerably (see Fig. 9). 4. Conclusion Al–a-Si3N4 metal matrix composite were produced by using PM techniques. In order to prepare mixture of Al and a-Si3N4 powders
1. More homogenous distribution of a-Si3N4 in Al matrix was obtained by using MA techniques compared to a conventional mixing method. 2. As a result of homogenous dispersion of a-Si3N4 in Al matrix, a considerable improvement was obtained on density, transverse rupture strength and hardness of composite blocks. 3. MA process resulted in high deformation of Al particles which facilitated the sintering behavior of blocks due to the high internal energy of powder particle 4. The results obtained from this study showed that the ideal sintering temperature and amount of a-Si3N4 were 660 °C and 10 wt%, respectively for better mechanical properties.
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