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Effects of SiC particulates on the fatigue behaviour of an Al-alloy matrix composite
Materials & Design Materials and Design 27 (2006) 776–782 www.elsevier.com/locate/matdes
Short communication
Effects of SiC particulates on the fatigue behaviour of an Al-alloy matrix composite Cevdet Kaynak *, Suha Boylu Department of Metallurgical and Materials Engineering, Middle East Technical University, TR-06531 Ankara, Turkey Received 4 October 2004; accepted 11 January 2005 Available online 19 February 2005
Abstract In this study, the fatigue behaviour of aluminium matrix–silicon carbide (SiC) particulate reinforced composite specimens was investigated in comparison to the matrix aluminium alloy containing 12 wt% Si. Three different weight percentages of SiC particulates: 5, 10, and 15 in the size range of 40–60 lm were injected into the melt. Mg was also added to improve the wettability of Alalloy matrix over SiC particulates resulting in better interfacial bonding. Test specimens were machined from squeeze casting billets. Hardness, bending and fatigue tests indicated that reinforcing the Al-alloy matrix with SiC particulates improved the hardness, flexural strength and fatigue resistance with increasing content of SiC particulates. Stress levels yielding less than 107 cycles to fracture were applied. Cracks initiated at the debonded particulate–matrix interface and/or by the cracking of the coarse particulates. SiC particulates improved fatigue resistance mainly by acting as barriers to cracks and/or deflecting the growth plane of cracks resulting in decreased crack propagation rates. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Metal matrix composites; Fatigue
1. Introduction Since metal matrix composites have distinct advantages in aerospace, automotive, and other structural applications their fatigue behaviour is an important factor that must be considered. Therefore, the purpose of this work was to contribute to the fatigue literature through observations and S–N data on an aluminium alloy matrix containing silicon carbide (SiC) particulates. First, a brief literature survey will be given. Skolianos [1] indicated that in stress-controlled fatigue testing under fully reversed bending conditions, the fatigue life of definite type of composite specimens is improved over that of the unreinforced specimens at intermediate and lower stress levels. At higher stress levels the improvement was negligible. However, the fati*
Corresponding author. Tel.: +90 312 210 5920; fax: +90 312 210 1267. E-mail address: [email protected] (C. Kaynak). 0261-3069/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2005.01.009
gue strength at 107 cycles decreased with increasing SiC particle size. The effect of SiC volume fraction and particle size on the fatigue behaviour of 2080 Al alloy was investigated by Chawla et al. [2]. They found that increasing volume fraction (10%, 20% and 30%) and decreasing particle size (down to 5 lm) resulted in an increase in fatigue resistance. Mechanisms responsible for this behaviour were described in terms of load transfer from the matrix to the high stiffness reinforcement, increasing obstacles for dislocation motion and the decrease in strain localization with decreasing interparticle spacing as a result of reduced particle size. The effects of particle size (2, 5, 9, and 20 lm), volume fraction (10%, 20% and 35%) and matrix strength on stress-controlled axial fatigue behaviour and the probability of particle fracture were evaluated for 2124 aluminium alloy reinforced with SiC particles by Hall et al. [3]. Strength and fatigue life increased as reinforcement
C. Kaynak, S. Boylu / Materials and Design 27 (2006) 776–782
particle size decreased and volume fraction increased. The frequency of particle fracture during crack propagation was found to be dependent on matrix strength, particle size and volume fraction and on maximum crack tip stress intensity. The low-cycle fatigue behaviour of a SiC-particulatereinforced Al–Si cast alloy with two different volume fractions has been investigated under strain-controlled conditions with and without tensile mean strains by Koh et al. [4]. They found that the composites and the unreinforced matrix alloy showed cyclic hardening behaviour. The composite containing higher volume fraction of SiC particles exhibited a more pronounced strain-hardening rate leading to shorter fatigue life at a given strain amplitude. The inferior strain-life of the composite can be attributed to the low ductility and the corresponding poor resistance to cyclic plasticity caused by the brittle reinforcement. Bonnen et al. [5] researched the influence of the mean stress on the fatigue behaviour of a naturally aged powder metallurgy 2xxx series aluminium alloy and a composite made of this alloy with 15 vol% SiCp by using different stress ratios of 1, 0, 1, and 0.7. Results showed that mean stress had a significant influence on fatigue life and this influence was consistent with that normally observed in metals. At each stress ratio, the incorporation of SiC reinforcement led to an increase in fatigue life at low and intermediate stresses. Nieh et al. [6] investigated the tensile and fatigue properties of a 25 vol% SiC particulate reinforced 6090 Al composite at 300 °C and they indicated that room temperature fatigue resistance of the composites were better than at 300 °C. Fracture behaviour of a SiC particulate reinforced 7075 aluminium alloy was studied under uniaxial tensile loading in the temperature range 25–400 °C by Razaghian et al. [7]. The ductility of the composite was found to be much lower than that of the monolithic alloy at all temperatures, but both materials exhibited similar strength levels above 300 °C. Particle fracture was the main damage mechanism prior to final fracture at room temperature, while interface debonding together with interparticle voids were dominant features in fracture at high temperature. Large particles and regions of clustered particles were found to be the locations prone to damage in the composite at both room and high temperatures. At room temperature, particle fracture was observed at clusters of particles as well as in large particles, whereas at high temperature voids nucleated in the matrix closely adjacent to particles and at particle ends. This can be attributed to the high local stress in these regions and the high probability of flaws in large particles. 2. Experimental work The matrix material used was Si-rich aluminium alloy simply designated as Al–Si 12 due to the main alloying
777
Table 1 The starting composition of Al-alloy matrix material (wt%) Cu
Fe
Si
Zn
Mn
Mg
0.10
0.60
11.5–13.5
0.10
0.40
0.10
Table 2 Particle size fractions of the SiC particulates used as reinforcements Mesh size (lm)
SiCp (%)
+53 +45 +38
64 28 8
Table 3 Detailed designation of the specimens Designation
Description
Al–0% SiCp Al–5% SiCp Al–10% SiCp Al–15% SiCp
Squeeze Squeeze Squeeze Squeeze
cast cast cast cast
Al-alloy Al-alloy Al-alloy Al-alloy
matrix matrix matrix matrix
with with with with
0 wt% SiCp 5 wt% SiCp 10 wt% SiCp 15 wt% SiCp
element being 12 wt% Si. The chemical composition of this Al-alloy prior to Mg addition is given in Table 1. The SiC particulates used as reinforcement material had a size range of 40–60 lm and their sieve analysis is given in Table 2. X-ray analysis indicated that these SiC particles have an a-crystal structure. The squeeze casting method was used for the production of large composite billets after which small specimens were extracted. First, an induction furnace was used to melt and hold the Al matrix alloy at 700 °C. During melting 1.7 wt% Mg was also added to improve the wettability of the matrix. Secondly, SiC particulates were added by a vibratory particle injection apparatus. Then, the squeeze casting was used for the production of large size billets by means of a hydraulic press of 600 kN capacity and a permanent metallic mould which was heated to 300 °C prior to casting. A pressure of 80 MPa was maintained until complete solidification. After the production of composite billets, specimens were cut and machined from these billets. There were four types of specimens: Al-alloy matrix without SiCp and Al-alloy matrix with 5, 10 and 15 wt% SiC particulates, their designation is given in Table 3. For flexural and fatigue tests 6-mm thick, 10-mm width and 60-mm span length three-point bending specimens were tested under a 100-kN servohydraulic MTS universal testing machine. In fatigue tests, constant amplitude sinusoidal loading at a stress ratio of 0.1 and a frequency of 15 Hz was applied. According to the flexural strength values of the specimens, fatigue tests were conducted at three different maximum stress levels: 200, 225 and 250 MPa, so that less than 107 cycles to fracture were obtained. At each stress level at least three specimens were tested. Fatigue crack initiation and propagation
778
C. Kaynak, S. Boylu / Materials and Design 27 (2006) 776–782
behaviour was examined by using a JEOL 6400 scanning electron microscope both on the fractured surfaces (fractography) and on the upper surfaces of specimens before breakage (surface micrography).
3. Results and discussion
squeeze cast aluminium matrix composites. Before fatigue tests, the hardness and flexural behaviour of the specimens were also investigated. In order to determine the tress levels to be used in fatigue tests, first threepoint bending tests were conducted for all specimens. Flexural stress and strain behaviour of the specimens are compared in Fig. 1. Then, flexural strength, flexural
The aim of this study was to observe the effects of SiC particulate reinforcement on the fatigue behaviour of MAXIMUM STRESS (MPa)
300
FLEXURAL STRESS (MPa)
300 250 200 150 Al-0%SiC Al-5%SiC Al-10%SiC Al-15%SiC
100 50
5
10 15 FLEXURAL STRAIN (%)
1,0E+03
1,0E+05
1,0E+07
NUMBER OF CYCLES TO FAILURE
Fig. 3. Effect of SiC particulates on the S–N behaviour of the specimens.
(b) 20
300
275
250
225 -5
0
5
10
15
20
FLEXURAL STRAIN AT FAILURE (%)
FLEXURAL STRENGTH (MPa)
200
20
Fig. 1. Comparison of the flexural behaviour of all specimens.
(a)
250
150 1,0E+01
0 0
Al-0%SiC (p) Al-5%SiC (p) Al-10%SiC(p) Al-15%SiC(p)
15
10
5
0
-5
0
5
10
15
20
Wt % SiC (p)
Wt % SiC(p)
(d) 100
HARDNESS (HBN)
FLEXURAL MODULUS (GPa)
(c) 120
100
80
60
-5
0
5
10
Wt % SiC (%)
15
20
80
60
40
20
0
5
10
15
20
Wt % SiC (p)
Fig. 2. Effect of reinforcement content on the flexural behaviour and hardness of Al–SiCp composites: (a) flexural strength; (b) flexural strain at failure; (c) flexural modulus; (d) hardness.
C. Kaynak, S. Boylu / Materials and Design 27 (2006) 776–782
strain at failure, and flexural modulus values were determined. These data together with hardness test results are given in Fig. 2.
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As shown in Fig. 2(a), increasing the SiC content increases the flexural strength of the specimens. Increase in the dislocation density which is due to the coefficient of
Fig. 4. Fractographs (a, b) showing adhesion at the interface between the Al-alloy matrix and SiC particulates.
Fig. 5. Crack initiation at SiC particles especially due to debonding at the interface under high stress levels: (a, b) surface micrographs; (c, d) fractographs.
780
C. Kaynak, S. Boylu / Materials and Design 27 (2006) 776–782
thermal expansion (CTE) mismatch between the matrix and the reinforcement and a decrease in subgrain size are the major factors affecting the strength improvement of the matrix. There are other possible contributions: residual stresses, differences in texture between the composite matrix and the matrix material without reinforcement, classical composite load transfer and dispersion strengthening. Also the reinforcement particle is a barrier to slip. A decrease in the particle size can lead to a potential decrease in the slip length. However, as shown in Fig. 2(b) flexural strain at failure decreases as the weight fraction of the SiCp increases. Because, the mechanisms that increase the strength of materials decrease their flexibility and ductility. Fig. 2(c) indicates slight increases in flexural modulus of the specimens. Additionally, Fig. 2(d) shows that as the SiC content increases the hardness values also increase for the same reasons mentioned above for the strength improvement. This is also because SiC particles are ceramic materials which are much harder than the Al-alloy matrix. Three different maximum stress levels used in fatigue loading were determined according to the flexural strength values of the specimens so that less than 107 cy-
cles to failure was obtained. Then the data were used to construct the S–N behaviour of the specimens as shown in Fig. 3. The S–N curves show that as the weight fraction of SiCp increases the fatigue life increases. It has been frequently reported in the literature that under stress controlled fatigue, the fatigue lives of discontinuously reinforced metals are generally longer than those of unreinforced metals [2,3,5]. The fatigue strength increases with volume fraction of the particles because of the significant amount of the load being transferred to the stiffer particulate reinforcement and overall lower total strains for a given fatigue stress. Increase in fatigue strength can also be attributed to the decreased elastic and plastic strains that result from the modulus and rate of work hardening, both of which increase with increasing volume fraction of the reinforcements. SEM studies indicated that adequate interfacial bonding was achieved between the SiC particulates and the Al-alloy matrix as shown in Fig. 4. This is largely due to the Mg addition improving the wettability of the Al-alloy matrix over SiC particulates. This behaviour was also observed by Choh and Oki [8], and Henriksen and Johnsen [9].
Fig. 6. Main and secondary crack initiation at the fractured SiC particles: (a, b) surface micrographs; (c, d) fractographs.
C. Kaynak, S. Boylu / Materials and Design 27 (2006) 776–782
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Fig. 7. (a, b) fractographs showing fatigue striations radiating from SiC particles.
There are basically two mechanisms of crack initiation in metal matrix composites. In the first cracks may initiate at the debonded interface between the matrix and the particulates. In the second cracks may also initiate due to cracking of the particles. As shown in Fig. 5 cracks sometimes initiated at the interface between the
Al-alloy matrix and SiCp. This behaviour was especially observed at higher stress levels. The second crack initiation mechanism due to SiC particle cracking was also observed as shown in Fig. 6. As the particle size increases the propensity for particle fracture increases. Because larger particles will have a higher probability of faults
Fig. 8. Main and secondary crack growth retardation at SiC particles located at different planes: (a, b) surface micrographs; (c, d) fractographs.
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C. Kaynak, S. Boylu / Materials and Design 27 (2006) 776–782
and will thus fracture at lower stress levels than small SiC particles. Fatigue crack striations were also observed in the Alalloy matrix during early growth and propagation periods. Figs. 5(c) and (d) show the striations radiating from the debonded interface between the Al-alloy matrix and SiCp, while Figs. 6(c) and (d) show striations associated with cracks that initiated at the fractured SiCp. Finally, Fig. 7 shows striations during early growth and propagation periods. The improvement of fatigue resistance in metal matrix composites is mainly due to the increased yield strength in LCF regime (as for 5% SiCp specimens), whereas in HCF regime (as for 10% and 15% SiCp specimens), it is especially due to the decreased crack growth rate by the hindering effect of the particles or the change of the growth plane of the cracks. Fig. 8 shows that SiCp acted as crack stoppers and/or points deflecting the growth plane of the main and secondary cracks. 4. Conclusions The aim of this study was to contribute to the fatigue literature through S–N data and observations on the behaviour of an Al-alloy matrix SiC particulate reinforced composites under cyclic loading. Hardness and three point bending tests were also performed. These tests revealed the following conclusions: Hardness and flexural strength values increased as the weight fraction of SiCp increased. This is due to strain hardening in the regions surrounding the particles, due to the CTE mismatch between the matrix and the particulates resulting in a high dislocation density. Conversely, flexural strain at fracture values decreased due to the decreased ductility. Mg addition during melting of the Al-alloy matrix improved the wettability resulting in certain bonding at the particulate-matrix interface. However, under higher stress levels the interfacial bond strength was
not sufficient and cracks initiated at the debonded interface. Another crack initiation mechanism was the fracture of SiCp especially seen in the specimens with coarse particulates. Fatigue striations were observed in the Al-alloy matrix during early crack growth and propagation periods. S–N curves simply indicated that SiCp reinforcement improved the fatigue resistance of the specimens due to the increased yield strength in the LCF regime. However, in the HCF regime the particles acted as crack stoppers and/or points deflecting the growth planes of the main and secondary cracks resulting in lower propagation rates. Therefore, increasing the SiCp content increased the fatigue life of the specimens.
References [1] Skolianos S. Mechanical behavior of cast SiCp-reinforced Al– 4.5%Cu–1.5%Mg alloy. Mater Sci Eng 1996;210A:76–82. [2] Chawla N, Andres C, Jones JW, Allison JE. Effect of SiC volume fraction and particle size on the fatigue resistance of a 2080 Al/SiCp composite. Metall Mater Trans A 1998;29A: 2843–54. [3] Hall JN, Wayne Jones J, Sachdev Anil K. Particle size, volume fraction and matrix strength effects on fatigue behavior and particle fracture in 2124 aluminum–SiCp composites. Mater Sci Eng 1994;183A:69–80. [4] Koh SK, Oh SJ, Li C, Ellyin F. Low-cycle fatigue life of SiCparticulate-reinforced Al–Si cast alloy composites with tensile mean strain effects. Int J Fatigue 1999;21:1019–32. [5] Bonnen JJ, Allison JE, Jones JW. Fatigue behavior of a 2xxx series aluminum alloy reinforced with 15 vol% SiCp. Metall Mater Trans 1991;22A:1007–19. [6] Nieh TG, Lesuer DR, Syn CK. Tensile and fatigue properties of a 25 vol% SiC particulate reinforced 6090 Al composite at 300 °C. Scr Metall Mater 1995;32:707–12. [7] Razaghian A, Yu D, Chandra T. Fracture behavior of a SiCparticle-reinforced aluminum alloy at high temperature. Compos Sci Technol 1998;58:293–8. [8] Choh T, Oki T. Wettability of SiC to aluminum and aluminum alloys. Mater Sci Technol 1987;3:378–85. [9] Henriksen BR, Johnsen TE. Influence of microstructure of fiber/ matrix interface on mechanical properties of Al/SiC composites. Mater Sci Technol 1990;6:857–63.
Short communication
Effects of SiC particulates on the fatigue behaviour of an Al-alloy matrix composite Cevdet Kaynak *, Suha Boylu Department of Metallurgical and Materials Engineering, Middle East Technical University, TR-06531 Ankara, Turkey Received 4 October 2004; accepted 11 January 2005 Available online 19 February 2005
Abstract In this study, the fatigue behaviour of aluminium matrix–silicon carbide (SiC) particulate reinforced composite specimens was investigated in comparison to the matrix aluminium alloy containing 12 wt% Si. Three different weight percentages of SiC particulates: 5, 10, and 15 in the size range of 40–60 lm were injected into the melt. Mg was also added to improve the wettability of Alalloy matrix over SiC particulates resulting in better interfacial bonding. Test specimens were machined from squeeze casting billets. Hardness, bending and fatigue tests indicated that reinforcing the Al-alloy matrix with SiC particulates improved the hardness, flexural strength and fatigue resistance with increasing content of SiC particulates. Stress levels yielding less than 107 cycles to fracture were applied. Cracks initiated at the debonded particulate–matrix interface and/or by the cracking of the coarse particulates. SiC particulates improved fatigue resistance mainly by acting as barriers to cracks and/or deflecting the growth plane of cracks resulting in decreased crack propagation rates. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Metal matrix composites; Fatigue
1. Introduction Since metal matrix composites have distinct advantages in aerospace, automotive, and other structural applications their fatigue behaviour is an important factor that must be considered. Therefore, the purpose of this work was to contribute to the fatigue literature through observations and S–N data on an aluminium alloy matrix containing silicon carbide (SiC) particulates. First, a brief literature survey will be given. Skolianos [1] indicated that in stress-controlled fatigue testing under fully reversed bending conditions, the fatigue life of definite type of composite specimens is improved over that of the unreinforced specimens at intermediate and lower stress levels. At higher stress levels the improvement was negligible. However, the fati*
Corresponding author. Tel.: +90 312 210 5920; fax: +90 312 210 1267. E-mail address: [email protected] (C. Kaynak). 0261-3069/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2005.01.009
gue strength at 107 cycles decreased with increasing SiC particle size. The effect of SiC volume fraction and particle size on the fatigue behaviour of 2080 Al alloy was investigated by Chawla et al. [2]. They found that increasing volume fraction (10%, 20% and 30%) and decreasing particle size (down to 5 lm) resulted in an increase in fatigue resistance. Mechanisms responsible for this behaviour were described in terms of load transfer from the matrix to the high stiffness reinforcement, increasing obstacles for dislocation motion and the decrease in strain localization with decreasing interparticle spacing as a result of reduced particle size. The effects of particle size (2, 5, 9, and 20 lm), volume fraction (10%, 20% and 35%) and matrix strength on stress-controlled axial fatigue behaviour and the probability of particle fracture were evaluated for 2124 aluminium alloy reinforced with SiC particles by Hall et al. [3]. Strength and fatigue life increased as reinforcement
C. Kaynak, S. Boylu / Materials and Design 27 (2006) 776–782
particle size decreased and volume fraction increased. The frequency of particle fracture during crack propagation was found to be dependent on matrix strength, particle size and volume fraction and on maximum crack tip stress intensity. The low-cycle fatigue behaviour of a SiC-particulatereinforced Al–Si cast alloy with two different volume fractions has been investigated under strain-controlled conditions with and without tensile mean strains by Koh et al. [4]. They found that the composites and the unreinforced matrix alloy showed cyclic hardening behaviour. The composite containing higher volume fraction of SiC particles exhibited a more pronounced strain-hardening rate leading to shorter fatigue life at a given strain amplitude. The inferior strain-life of the composite can be attributed to the low ductility and the corresponding poor resistance to cyclic plasticity caused by the brittle reinforcement. Bonnen et al. [5] researched the influence of the mean stress on the fatigue behaviour of a naturally aged powder metallurgy 2xxx series aluminium alloy and a composite made of this alloy with 15 vol% SiCp by using different stress ratios of 1, 0, 1, and 0.7. Results showed that mean stress had a significant influence on fatigue life and this influence was consistent with that normally observed in metals. At each stress ratio, the incorporation of SiC reinforcement led to an increase in fatigue life at low and intermediate stresses. Nieh et al. [6] investigated the tensile and fatigue properties of a 25 vol% SiC particulate reinforced 6090 Al composite at 300 °C and they indicated that room temperature fatigue resistance of the composites were better than at 300 °C. Fracture behaviour of a SiC particulate reinforced 7075 aluminium alloy was studied under uniaxial tensile loading in the temperature range 25–400 °C by Razaghian et al. [7]. The ductility of the composite was found to be much lower than that of the monolithic alloy at all temperatures, but both materials exhibited similar strength levels above 300 °C. Particle fracture was the main damage mechanism prior to final fracture at room temperature, while interface debonding together with interparticle voids were dominant features in fracture at high temperature. Large particles and regions of clustered particles were found to be the locations prone to damage in the composite at both room and high temperatures. At room temperature, particle fracture was observed at clusters of particles as well as in large particles, whereas at high temperature voids nucleated in the matrix closely adjacent to particles and at particle ends. This can be attributed to the high local stress in these regions and the high probability of flaws in large particles. 2. Experimental work The matrix material used was Si-rich aluminium alloy simply designated as Al–Si 12 due to the main alloying
777
Table 1 The starting composition of Al-alloy matrix material (wt%) Cu
Fe
Si
Zn
Mn
Mg
0.10
0.60
11.5–13.5
0.10
0.40
0.10
Table 2 Particle size fractions of the SiC particulates used as reinforcements Mesh size (lm)
SiCp (%)
+53 +45 +38
64 28 8
Table 3 Detailed designation of the specimens Designation
Description
Al–0% SiCp Al–5% SiCp Al–10% SiCp Al–15% SiCp
Squeeze Squeeze Squeeze Squeeze
cast cast cast cast
Al-alloy Al-alloy Al-alloy Al-alloy
matrix matrix matrix matrix
with with with with
0 wt% SiCp 5 wt% SiCp 10 wt% SiCp 15 wt% SiCp
element being 12 wt% Si. The chemical composition of this Al-alloy prior to Mg addition is given in Table 1. The SiC particulates used as reinforcement material had a size range of 40–60 lm and their sieve analysis is given in Table 2. X-ray analysis indicated that these SiC particles have an a-crystal structure. The squeeze casting method was used for the production of large composite billets after which small specimens were extracted. First, an induction furnace was used to melt and hold the Al matrix alloy at 700 °C. During melting 1.7 wt% Mg was also added to improve the wettability of the matrix. Secondly, SiC particulates were added by a vibratory particle injection apparatus. Then, the squeeze casting was used for the production of large size billets by means of a hydraulic press of 600 kN capacity and a permanent metallic mould which was heated to 300 °C prior to casting. A pressure of 80 MPa was maintained until complete solidification. After the production of composite billets, specimens were cut and machined from these billets. There were four types of specimens: Al-alloy matrix without SiCp and Al-alloy matrix with 5, 10 and 15 wt% SiC particulates, their designation is given in Table 3. For flexural and fatigue tests 6-mm thick, 10-mm width and 60-mm span length three-point bending specimens were tested under a 100-kN servohydraulic MTS universal testing machine. In fatigue tests, constant amplitude sinusoidal loading at a stress ratio of 0.1 and a frequency of 15 Hz was applied. According to the flexural strength values of the specimens, fatigue tests were conducted at three different maximum stress levels: 200, 225 and 250 MPa, so that less than 107 cycles to fracture were obtained. At each stress level at least three specimens were tested. Fatigue crack initiation and propagation
778
C. Kaynak, S. Boylu / Materials and Design 27 (2006) 776–782
behaviour was examined by using a JEOL 6400 scanning electron microscope both on the fractured surfaces (fractography) and on the upper surfaces of specimens before breakage (surface micrography).
3. Results and discussion
squeeze cast aluminium matrix composites. Before fatigue tests, the hardness and flexural behaviour of the specimens were also investigated. In order to determine the tress levels to be used in fatigue tests, first threepoint bending tests were conducted for all specimens. Flexural stress and strain behaviour of the specimens are compared in Fig. 1. Then, flexural strength, flexural
The aim of this study was to observe the effects of SiC particulate reinforcement on the fatigue behaviour of MAXIMUM STRESS (MPa)
300
FLEXURAL STRESS (MPa)
300 250 200 150 Al-0%SiC Al-5%SiC Al-10%SiC Al-15%SiC
100 50
5
10 15 FLEXURAL STRAIN (%)
1,0E+03
1,0E+05
1,0E+07
NUMBER OF CYCLES TO FAILURE
Fig. 3. Effect of SiC particulates on the S–N behaviour of the specimens.
(b) 20
300
275
250
225 -5
0
5
10
15
20
FLEXURAL STRAIN AT FAILURE (%)
FLEXURAL STRENGTH (MPa)
200
20
Fig. 1. Comparison of the flexural behaviour of all specimens.
(a)
250
150 1,0E+01
0 0
Al-0%SiC (p) Al-5%SiC (p) Al-10%SiC(p) Al-15%SiC(p)
15
10
5
0
-5
0
5
10
15
20
Wt % SiC (p)
Wt % SiC(p)
(d) 100
HARDNESS (HBN)
FLEXURAL MODULUS (GPa)
(c) 120
100
80
60
-5
0
5
10
Wt % SiC (%)
15
20
80
60
40
20
0
5
10
15
20
Wt % SiC (p)
Fig. 2. Effect of reinforcement content on the flexural behaviour and hardness of Al–SiCp composites: (a) flexural strength; (b) flexural strain at failure; (c) flexural modulus; (d) hardness.
C. Kaynak, S. Boylu / Materials and Design 27 (2006) 776–782
strain at failure, and flexural modulus values were determined. These data together with hardness test results are given in Fig. 2.
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As shown in Fig. 2(a), increasing the SiC content increases the flexural strength of the specimens. Increase in the dislocation density which is due to the coefficient of
Fig. 4. Fractographs (a, b) showing adhesion at the interface between the Al-alloy matrix and SiC particulates.
Fig. 5. Crack initiation at SiC particles especially due to debonding at the interface under high stress levels: (a, b) surface micrographs; (c, d) fractographs.
780
C. Kaynak, S. Boylu / Materials and Design 27 (2006) 776–782
thermal expansion (CTE) mismatch between the matrix and the reinforcement and a decrease in subgrain size are the major factors affecting the strength improvement of the matrix. There are other possible contributions: residual stresses, differences in texture between the composite matrix and the matrix material without reinforcement, classical composite load transfer and dispersion strengthening. Also the reinforcement particle is a barrier to slip. A decrease in the particle size can lead to a potential decrease in the slip length. However, as shown in Fig. 2(b) flexural strain at failure decreases as the weight fraction of the SiCp increases. Because, the mechanisms that increase the strength of materials decrease their flexibility and ductility. Fig. 2(c) indicates slight increases in flexural modulus of the specimens. Additionally, Fig. 2(d) shows that as the SiC content increases the hardness values also increase for the same reasons mentioned above for the strength improvement. This is also because SiC particles are ceramic materials which are much harder than the Al-alloy matrix. Three different maximum stress levels used in fatigue loading were determined according to the flexural strength values of the specimens so that less than 107 cy-
cles to failure was obtained. Then the data were used to construct the S–N behaviour of the specimens as shown in Fig. 3. The S–N curves show that as the weight fraction of SiCp increases the fatigue life increases. It has been frequently reported in the literature that under stress controlled fatigue, the fatigue lives of discontinuously reinforced metals are generally longer than those of unreinforced metals [2,3,5]. The fatigue strength increases with volume fraction of the particles because of the significant amount of the load being transferred to the stiffer particulate reinforcement and overall lower total strains for a given fatigue stress. Increase in fatigue strength can also be attributed to the decreased elastic and plastic strains that result from the modulus and rate of work hardening, both of which increase with increasing volume fraction of the reinforcements. SEM studies indicated that adequate interfacial bonding was achieved between the SiC particulates and the Al-alloy matrix as shown in Fig. 4. This is largely due to the Mg addition improving the wettability of the Al-alloy matrix over SiC particulates. This behaviour was also observed by Choh and Oki [8], and Henriksen and Johnsen [9].
Fig. 6. Main and secondary crack initiation at the fractured SiC particles: (a, b) surface micrographs; (c, d) fractographs.
C. Kaynak, S. Boylu / Materials and Design 27 (2006) 776–782
781
Fig. 7. (a, b) fractographs showing fatigue striations radiating from SiC particles.
There are basically two mechanisms of crack initiation in metal matrix composites. In the first cracks may initiate at the debonded interface between the matrix and the particulates. In the second cracks may also initiate due to cracking of the particles. As shown in Fig. 5 cracks sometimes initiated at the interface between the
Al-alloy matrix and SiCp. This behaviour was especially observed at higher stress levels. The second crack initiation mechanism due to SiC particle cracking was also observed as shown in Fig. 6. As the particle size increases the propensity for particle fracture increases. Because larger particles will have a higher probability of faults
Fig. 8. Main and secondary crack growth retardation at SiC particles located at different planes: (a, b) surface micrographs; (c, d) fractographs.
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C. Kaynak, S. Boylu / Materials and Design 27 (2006) 776–782
and will thus fracture at lower stress levels than small SiC particles. Fatigue crack striations were also observed in the Alalloy matrix during early growth and propagation periods. Figs. 5(c) and (d) show the striations radiating from the debonded interface between the Al-alloy matrix and SiCp, while Figs. 6(c) and (d) show striations associated with cracks that initiated at the fractured SiCp. Finally, Fig. 7 shows striations during early growth and propagation periods. The improvement of fatigue resistance in metal matrix composites is mainly due to the increased yield strength in LCF regime (as for 5% SiCp specimens), whereas in HCF regime (as for 10% and 15% SiCp specimens), it is especially due to the decreased crack growth rate by the hindering effect of the particles or the change of the growth plane of the cracks. Fig. 8 shows that SiCp acted as crack stoppers and/or points deflecting the growth plane of the main and secondary cracks. 4. Conclusions The aim of this study was to contribute to the fatigue literature through S–N data and observations on the behaviour of an Al-alloy matrix SiC particulate reinforced composites under cyclic loading. Hardness and three point bending tests were also performed. These tests revealed the following conclusions: Hardness and flexural strength values increased as the weight fraction of SiCp increased. This is due to strain hardening in the regions surrounding the particles, due to the CTE mismatch between the matrix and the particulates resulting in a high dislocation density. Conversely, flexural strain at fracture values decreased due to the decreased ductility. Mg addition during melting of the Al-alloy matrix improved the wettability resulting in certain bonding at the particulate-matrix interface. However, under higher stress levels the interfacial bond strength was
not sufficient and cracks initiated at the debonded interface. Another crack initiation mechanism was the fracture of SiCp especially seen in the specimens with coarse particulates. Fatigue striations were observed in the Al-alloy matrix during early crack growth and propagation periods. S–N curves simply indicated that SiCp reinforcement improved the fatigue resistance of the specimens due to the increased yield strength in the LCF regime. However, in the HCF regime the particles acted as crack stoppers and/or points deflecting the growth planes of the main and secondary cracks resulting in lower propagation rates. Therefore, increasing the SiCp content increased the fatigue life of the specimens.
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