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Fatigue behaviour of SiC-particulate-reinforced aluminium alloy composites with different particle sizes at elevated temperatures
Composites Science and Technology 68 (2008) 2785–2791
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Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Fatigue behaviour of SiC-particulate-reinforced aluminium alloy composites with different particle sizes at elevated temperatures Y. Uematsu a,*, K. Tokaji a, M. Kawamura b a b
Department of Mechanical and Systems Engineering, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan NTN Corporation, 1-3-17 Kyomachibori, Nishi-ku, Osaka 550-0003, Japan
a r t i c l e
i n f o
Article history: Received 3 April 2008 Received in revised form 29 May 2008 Accepted 3 June 2008 Available online 11 June 2008 Keywords: A. Metal–matrix composite B. Fatigue B. High-temperature properties D. Fractography
a b s t r a c t Fully reversed axial fatigue tests have been performed at 150 °C (423 K) and 250 °C (523 K) using smooth specimens of SiC-particulate-reinforced aluminium alloy composites with different particle sizes at a constant wt.% of SiC particles. Regardless of particle size, fatigue strength decreased with increasing temperature with a remarkable reduction at 250 °C. The particle size dependence of fatigue strength was clearly seen at ambient temperature, while became small at 150 °C and almost disappeared at 250 °C. Crack initiation depended on temperature and particle size and small crack growth rates were an order faster at 250 °C than at ambient temperature and 150 °C in all materials studied. It was indicated that the softening and associated loss in strength of the matrix at elevated temperatures were the primary causes for the observed temperature and particle size dependence of fatigue behaviour. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction SiC-particulate-reinforced aluminium alloy composites (hereafter, SiCp/Al composites) posses many advantages such as high specific strength and stiffness and good wear resistance. In addition to those properties, they can be processed by conventional means such as forging, rolling, extrusion and subsequent machining. Therefore SiCp/Al composites are suitable for structural components, and they are further expected to be widely used in various industries because of weight and energy saving and high performance in machines and structures, where fatigue properties are critical. Various fatigue properties of SiCp/Al composites have been investigated extensively [1], most of which have been devoted to the characterization of the ambient temperature fatigue behaviour. Since it is believed that SiCp/Al composites possess superior hightemperature performance to the matrix alloy due to the presence of ceramic reinforcement, they are potential candidates for components exposed to elevated temperatures. However, there have been very limited studies on the high-temperature fatigue behaviour of SiCp/Al composites [2–5], in particular, crack initiation and small crack growth have not been studied in any great detail because of experimental difficulties for measurement. There are many factors influencing the fatigue properties of SiCp/Al composites, such as the matrix properties, and the size, shape, distribution and volume fraction of the reinforcement, in * Corresponding author. Tel.: +81 58 293 2501; fax: +81 58 230 1892. E-mail address: [email protected] (Y. Uematsu). 0266-3538/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2008.06.005
which particle size is one of the important microstructural variables. Therefore there are still recent works about the effect of particle size on plastic deformation [6,7], fracture toughness [8] and so on. Hence, the authors have recently studied the fatigue crack propagation and the fatigue behaviour of SiCp/2024Al composites with different particle sizes at ambient temperature [9–12]. However, it is expected that particle size would play different roles in fatigue behaviour at elevated temperatures. The purpose of the present study is to understand the high-temperature fatigue behaviour of SiCp/Al composites with different particle sizes at a constant wt.% of SiC particles. Fully reversed axial fatigue tests were performed at ambient temperature, 150 °C and 250 °C using smooth specimens of the unreinforced alloy and the SiCp/2024Al composites, and the effects of temperature and particle size on fatigue behaviour were discussed based on crack initiation, small crack growth and fractographic analysis. 2. Experimental details 2.1. Material Aluminium alloy 2024 fabricated by powder metallurgy was used in the unreinforced condition and with 10 wt.% SiCp (about 9.0 vol.%) reinforcement. The chemical composition (wt.%) of the matrix alloy is Si 0.13, Fe 0.24, Cu 4.57, Mn 0.63, Mg 1.65, Cr 0.01, Zn 0.091, Ti 0.02, balance Al. Gas atomized 2024 alloy powders were blended with SiC particles and then the mixed powders were cold compacted in an aluminium container. Subsequently, the degassing process was performed at 450 °C for 1 h in vacuum
Y. Uematsu et al. / Composites Science and Technology 68 (2008) 2785–2791
below 10 1b Torr. The same processing route was employed for the unreinforced alloy. Finally, extrusion at 430 °C was applied to produce bars with a diameter of 35 mm. The composites with the different nominal particle sizes of 5 lm, 20 lm and 60 lm were prepared, hereafter denoted in this paper as 5 lm SiCp/Al, 20 lm SiCp/Al and 60 lm SiCp/Al, respectively. It should be noted that the unreinforced alloy and the composites were prepared from different lots from those used in previous reports [9–12]. The microstructures of the unreinforced alloy and the composites can be found in previous reports [9–11], where SiC particles tended to be weakly aligned in the extrusion direction and the aspect ratio of particles was approximately 1.7. Any pores were not found in the microstructures of all materials. 2.2. Specimen preparation Fatigue specimens with a width of 8 mm, a thickness of 4 mm and a gauge length of 12 mm were machined from the as-received bars. A shallow notch with a depth of 0.4 mm was introduced in one side of the gauge section in order to facilitate the observation of crack initiation and subsequent small crack growth. The stress concentration factor of the notch is small (Kt = 1.06), thus the specimens can be regarded as smooth ones. All specimens were solution treated at 495 °C for 1 h, quenched in water and then aged at 190 °C to achieve the peak-aged condition for the matrix. The ageing time was 5 h for the unreinforced alloy and 5 lm SiCp/Al composite, 3 h for 20 lm SiCp/Al composite, and 2.5 h for 60 lm SiCp/Al composite. These ageing conditions are based on the previous work [9]. Before fatigue test, the surface of the gauge section was mechanically polished using progressively finer grades of emery paper. 2.3. Procedures Fully reversed axial fatigue tests were performed on a 19 kN capacity electro-servohydraulic fatigue testing machine operating at a sinusoidal frequency of 10 Hz. Test temperatures are ambient temperature, 150 °C and 250 °C. An induction heating system was used to control temperature of the specimen. Prior to fatigue test, temperature calibration was conducted between the centre of the specimen and the location of 20 mm away from the center. Based on this temperature calibration, the temperature at the latter position was controlled to the desired temperatures at the center of the specimen during experiment. The accuracy of temperature was within 1% of the desired temperatures. Crack initiation and subsequent crack growth were monitored with replication technique. Experiment was interrupted and the specimen was cooled down to ambient temperature and then replicas were taken before temperature rise. This procedure was repeated until crack length was reached a critical size. Prior to experiment, it was confirmed that there was no discernible difference in fatigue life between the interrupted and continuous tests. The crack length was measured by means of an optical microscope on a plastic replica and defined as the projected length vertical to the loading direction. After experiment, fracture surfaces were examined in detail using a scanning electron microscope (SEM) to identify crack initiation and growth mechanisms operated at elevated temperatures.
3. Results 3.1. Temperature dependence of tensile strength The tensile strengths, rB, of the unreinforced alloy and of the composites are shown in Fig. 1 as a function of temperature, where
600
Tensile strength σ B (MPa)
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SiC p/2024Al composite
500 400 300 Unreinforced 5μmSiC p/Al 20μmSiC p/Al 60μmSiC p/Al
200 100
0
100 200 Temperature (ºC)
300
Fig. 1. Temperature dependence of tensile strength in unreinforced alloy and SiCp/ Al composites.
tensile tests were conducted on fatigue testing machine using smooth fatigue specimens without notch. In this case, the yield strength was not measured, but it is considered that the tensile strength could be correlated with the yield strength. It can be seen that regardless of reinforcement incorporation and particle size, tensile strength decreases with increasing temperature with an abrupt decrease above 200 °C. Similar behaviour has been reported by Nair et al. in unreinforced 2024 alloy and SiCp/2024Al composite with 21 vol.% SiC [13]. In the temperature range studied, 60 lm SiCp/Al composite exhibits slightly lower tensile strengths than the other materials, but the tensile strengths of 5 lm SiCp/Al and 20 lm SiCp/Al composites are the same as that of the unreinforced alloy. The brittle fracture of large particles in 60 lm SiCp/Al might be detrimental to static strength, but the detail is unclear. Han et al. have also indicated that the tensile strengths of the composites with different particle sizes became nearly equal at elevated temperature [3]. Therefore, it is believed that the properties of the matrix itself may play a dominant role in determining high-temperature composite strength. The detailed analysis of the fracture surfaces is not performed because this paper aims at fatigue properties. Some fractographic analysis of the fracture under monotonic loading at elevated temperatures can be referred for example to [14]. 3.2. Fatigue behaviour 3.2.1. Fatigue strength The S–N diagram is represented in Fig. 2 for the unreinforced alloy and the composites at ambient and elevated temperatures. In both the unreinforced alloy and the composites, fatigue strength decreases with increasing temperature with a considerable reduction at 250 °C, which is the same trend as tensile strength. At ambient temperature, fatigue strength is the highest in 5 lm SiCp/Al composite, then 20 lm SiCp/Al composite, the unreinforced alloy, 60 lm SiCp/Al composite in decreasing order. This particle size dependence of fatigue strength is similar to that obtained in previous reports [9,11]. At 150 °C, the particle size dependence observed at ambient temperature can be still seen, but differences in fatigue strength among the unreinforced alloy, 5 lm SiCp/Al composite and 20 lm SiCp/Al composite become small. Furthermore, at 250 °C, the fatigue strengths for all materials are nearly the same, i.e. the particle size dependence almost disappears, but 60 lm SiCp/Al composite still exhibits slightly lower fatigue strength in long life regime. Fig. 3 shows the fatigue strengths characterized in terms of fatigue ratio, r/rB, i.e. stress amplitude normalized by tensile
Y. Uematsu et al. / Composites Science and Technology 68 (2008) 2785–2791
Stress amplitude σ (MPa)
250
SiC p/2024Al composite Temp.(ºC) R.T.150 250 Axial loading, R =–1 Unreinforced 5μmSiC p/Al 20μmSiC p/Al 60μmSiC p/Al
200 150 100 50 0 3 10
4
5
6
7
10 10 10 10 Number of cycles to failure Nf
10
8
Fig. 2. S–N diagram for unreinforced alloy and SiCp/Al composites at ambient and elevated temperatures.
0.8
Fatigue ratio σ/σB
0.7
SiC p/2024Al composite Axial loading, R=–1 Temp.(˚C) R.T.150 250 Unreinforced 5μmSiC p/Al 20μmSiC p/Al 60μmSiC p/Al
0.6 0.5 0.4 0.3 0.2 0.1 3 10
4
5
6
7
10 10 10 10 Number of cycles to failure Nf
10
8
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5 lm SiCp/Al. In 20 lm SiCp/Al composite, cracks are generated by cyclic slip deformation at ambient temperature, while due to particle fracture at 150 °C and at the particle/matrix interface at 250 °C. In 60 lm SiCp/Al composite, similar initiation behaviour can be seen at elevated temperatures, but the particle/matrix interface is the dominant crack initiation site at ambient temperature. Examples of matching SEM micrographs of fracture surfaces are shown in Figs. 5–8. Figs. 5 and 6 are crack initiation due to particle fracture (150 °C) and at the particle/matrix interface (250 °C), respectively, in 20 lm SiCp/Al composite. Figs. 7 and 8 are crack initiation due to due to particle fracture (150 °C) and at the particle/matrix interface (250 °C), respectively, in 60 lm SiCp/Al composite. It is evident in Figs. 5 and 7 that the crack was initiated due to particle fracture at 150 °C because a particle can be seen on a pair of fracture surfaces. In Figs. 6 and 8, a particle can be seen on one side of fracture surfaces (Figs. 6 and 8b), but the corresponding hole is observed on the opposite side of fracture surfaces (Figs. 6 and 8a), clearly indicating that the crack was generated at the particle/matrix interface at 250 °C. Based on detailed examination and analysis of the crack initiation behaviour for all fatigue-failed specimens, the predominant crack initiation mechanisms operated are summarized in Table 1 for each temperature and material. When two different mechanisms were identified, both are indicated in the table. The unreinforced alloy and 5 lm SiCp/Al composite show the same crack initiation mechanism regardless of temperature; slip or defects in the former and clustering or agglomeration of particles in the latter. On the contrary, in the composites with larger particle sizes, it should be noted that the crack initiation mechanism changes with temperature, where particle fracture and particle/matrix interface are the dominant mechanisms at 150 °C and 250 °C, respectively. Illustrated in Fig. 9 are the relationships between crack length, 2c, and number of cycles, N, at elevated temperatures in the unreinforced alloy and the composites. As indicated in previous report [9,11], at ambient temperature, the crack initiation resistance decreased with increasing particle size. In Fig. 9, similar behaviour can be seen at 150 °C, where crack initiation becomes faster with increasing particle size. At 250 °C, however, it becomes nearly the same for all materials, indicating that the particle size dependence of crack initiation observed at ambient temperature and 150 °C completely disappears at this temperature.
Fig. 3. Fatigue strength characterized in terms of fatigue ratio.
strength. Nieh et al. have indicated that the fatigue ratio at 107 cycles increased with increasing temperature in various Al–matrix composites [2]. In the present study, however, regardless of reinforcement incorporation and particle size, the relative fatigue strengths are the same between ambient temperature and 150 °C, but slightly lower at 250 °C, particularly in the composites. This suggests that the fatigue strength decreases more than the tensile strength and some localized damage relating to the presence of reinforcement would take place in crack initiation or small crack growth during fatigue testing at 250 °C. 3.2.2. Crack initiation Revealed in Fig. 4 are the typical examples of SEM micrographs showing crack initiation site of the composites at ambient and elevated temperatures. The crack initiation sites for the unreinforced alloy were found to be defects or due to cyclic slip deformation regardless of temperature and applied stress level. As can be seen in the micrographs, the crack initiation site for 5 lm SiCp/ Al composite is the clustering or agglomeration of particles at all temperatures. This clustering of particles was recognized only in
3.2.3. Small crack growth Crack growth rate, da/dN, is shown in Fig. 10 as a function of maximum stress intensity factor, Kmax, for the unreinforced alloy and the composites. Crack growth rate was calculated as a slop of a–N curve based on a polynomial approximation using five data points. Kmax was calculated using Newman–Raju formula [15], assuming a semicircular surface small crack. Regardless of reinforcement incorporation and particle size, crack growth rates become faster with increasing temperature. The difference in crack growth rate between ambient temperature and 150 °C is small, but the crack growth rates at 250 °C are an order of magnitude faster. At ambient temperature, the crack growth rates of the unreinforced alloy and 5 lm SiCp/Al composite are slightly faster than those of 20 lm SiCp/Al and 60 lm SiCp/Al composites. At 150 °C, there are no discernible differences among the unreinforced alloy, 5 lm SiCp/Al composite and 20 lm SiCp/Al composite, but 60 lm SiCp/Al composite shows slightly faster crack growth rates. At 250 °C, on the contrary, the da/dN–Kmax relationships for all materials are similar, indicating little influence of reinforcement incorporation and particle size. It is known that elastic modulus, E, is the most important material variables influencing fatigue crack propagation. Elastic
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Y. Uematsu et al. / Composites Science and Technology 68 (2008) 2785–2791
Temperature
5μmSiCp/Al
60μmSiCp/Al
20μmSiCp/Al
R.T. 20μm
10μm
20μm
50μm
20μm
20μm
10μm
20μm
20μm
150 ºC
250 ºC
Fig. 4. SEM micrographs showing fracture surfaces near crack initiation site in SiCp/Al composites at ambient and elevated temperatures.
Fig. 5. Matching SEM micrographs of crack initiation site at 150 °C in 20 lm SiCp/Al composite (r = 160 MPa, Nf = 1.3 105).
Fig. 6. Matching SEM micrographs of crack initiation site at 250 °C in 20 lm SiCp/Al composite (r = 60 MPa, Nf = 1.6 105).
modulus decreases with increasing temperature, thus the small crack growth data were characterized in terms of Kmax normalized with respect to elastic modulus, Kmax/E, and the results are shown in Fig. 11. In the present study, the elastic modulus at each temperature was estimated based on the results in the Ref. [16]. As can be seen in the figure, the crack growth rates of the unreinforced alloy,
5 lm SiCp/Al composite and 20 lm SiCp/Al composite are nearly the same between ambient temperature and 150 °C, but 60 lm SiCp/Al composite still exhibits slightly faster crack growth rates at 150 °C. At 250 °C, the crack growth rates are still approximately an order of magnitude faster than those at ambient temperature and 150 °C.
Y. Uematsu et al. / Composites Science and Technology 68 (2008) 2785–2791
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Fig. 7. Matching SEM micrographs of crack initiation site at 150 °C in 60 lm SiCp/Al composite (r = 160 MPa, Nf = 4.9 104).
Fig. 8. Matching SEM micrographs of crack initiation site at 250 °C in 60 lm SiCp/Al composite (r = 50 MPa, Nf = 2.7 105).
Table 1 Predominant crack initiation mechanisms depending on temperature and particle size Temperature (°C)
Unreinforced
5 lm SiCp/Al
20 lm SiCp/Al
60 lm SiCp/Al
R.T.
Slip or Defect
Particle cluster
Slip or particle fracture Particle fracture Interface
Slip or interface
150 250
Particle fracture Interface
4. Discussion 4.1. High-temperature fatigue strength of the composites As described in introduction, there have been limited studies on the high-temperature fatigue behaviour of SiCp/Al composites. Nieh et al. [2] and Han et al. [3] have studied the fatigue behaviour of SiCp/6090Al at 300 °C and the low cycle fatigue behaviour of SiCp/pure Al composite at 168 °C, respectively. Srivatsan and Al-Hajri have also investigated the fatigue behaviour of SiCp/ 7034Al composite at 120 °C [4]. LLorca have indicated the fatigue strength of SiCp/2080Al composites at 150 °C and 170 °C in his review for the fatigue behaviour of discontinuously-reinforced metal–matrix composites [5]. In those works, the fatigue strengths of the composites were found to be lowered at elevated temperatures independent of volume fraction and particle size of reinforcement and matrix alloy, as well as in the present study. However, the range of variables such as temperature and particle size was relatively narrow in those works. The present study clearly indicates that the improvement of the fatigue strength of the composites is not always achieved. As shown in Fig. 2, at ambient temperature, the composites with smaller particle sizes showed higher fatigue strength than the unrein-
forced alloy, but with increasing temperature, their fatigue strengths become nearly equal to those of the unreinforced alloy, i.e. the composites lose the advantage for improving fatigue strength by reinforcement incorporation at elevated temperatures. Therefore, it may be concluded that the matrix plays a dominant role in determining the high-temperature fatigue strength of the composites. Fig. 12 shows Vickers hardness change of the matrix with exposure time at elevated temperatures in the unreinforced alloy and 20 lm SiCp/Al composite, where measurements were done until 27 h corresponding to 106 fatigue cycles. Both the unreinforced alloy and the composite exhibit slightly lower hardness at 150 °C than at ambient temperature. This temperature is lower than the ageing temperature of the materials, but the matrix becomes overaged. At 250 °C, on the contrary, considerable decrease in hardness can be seen in both materials, indicating the remarkable softening and associated loss in strength of the matrix. Therefore, the matrix becomes more plastically deformable at elevated temperatures, particularly at 250 °C. The enhanced ambient temperature fatigue strength of the composites with smaller particles resulted primarily from the increased crack initiation resistance and early small crack growth resistance [11]. However, the effects of factors contributing to the improvement of the resistance would be minimized or eliminated at elevated temperatures. Crack initiation will be discussed in the following section. At ambient temperature, early small crack growth was significantly affected by particles where small cracks grew avoiding particles, i.e. hard particles acted as barriers to small crack growth [11]. With increasing temperature, it is assumed that the particle/matrix interfaces become easy to decohere, because of the large difference in the thermal-expansion coefficient between particle and matrix. The coefficients of particle and matrix are about 4.4 and 22.9 (10 6 °C 1), respectively. Therefore, particles no longer act as effective barriers to small crack growth.
Y. Uematsu et al. / Composites Science and Technology 68 (2008) 2785–2791
SiC p/2024Al composite
Unreinforced (σ=160MPa) 5μmSiCp/Al (σ=160MPa) 20μmSiCp/Al (σ=160MPa) 60μmSiCp/Al (σ=150MPa)
2.0
1.0
3.0
0
0.4 0.8 1.2 1.6 2.0 5 Number of cycles N (x10 )
2.4
SiC p/2024Al composite Temperature:250 ºC
Crack growth rate da/dN (mm/cycle)
Temperature:150 ºC
0
Surface crack length 2c (mm)
–3
10
3.0 SiC p/2024Al composite
2.0
1.0
–4
Axial loading R=–1
10
–5
10
–6
10
Temp.(ºC ) –7
10
250
R.T. 150
Unreinforced 5μmSiC p/Al 20μmSiCp/Al 60μmSiCp/Al
Unreinforced (σ=65MPa) 5μmSiCp/Al (σ=70MPa) 20μmSiCp/Al (σ=65MPa) 60μmSiCp/Al (σ=60MPa)
–8
10
0.3 1 2 Maximum stress intensity factor 1/2 Kmax (MPam )
10
Fig. 10. Relationship between crack growth rate and maximum stress intensity factor for unreinforced alloy and composites at ambient and elevated temperatures.
0
0
0.2 0.4 0.6 0.8 1.0 1.2 5 Number of cycles N (x10 )
1.4
Fig. 9. Surface crack length as a function of number of cycles for unreinforced alloy and composites: (a) 150 °C; (b) 250 °C.
4.2. Particle size dependence of high-temperature fatigue behaviour At ambient temperature, fatigue strength increased with decreasing particle size, but the particle size dependence of fatigue strength became small at 150 °C and almost disappeared at 250 °C. Such tendency of high-temperature fatigue strength of the composites will be realized through crack initiation and subsequent small crack growth behaviour. In the composites with larger particle sizes such as 20 lm and 60 lm, a major influence of temperature was to change the crack initiation process as summarized in Table 1. It is believed that crack initiation is significantly affected by factors such as the stress concentration around particles, the interface strength and the particle strength itself. At 150 °C, the softening of the matrix was less remarkable as shown in Fig. 12, thus the decrease in the interface strength would not be significant, resulting in particle fracture due to lowered fracture strength of particles with increasing particle size [17]. Since particle fracture was also observed at ambient temperature, the crack initiation mechanism is basically the same as that at ambient temperature. At 250 °C, on the other hand, cracks were initiated at the particle/matrix interfaces. As discussed previously, the interface strength would be reduced due to the large difference in the thermal-expansion coefficient between particle and matrix. Furthermore, it is believed that the sensitivity to stress concentration around particles or at the interfaces is lowered due to the remarkable softening of the matrix. These have led to the disappearance of the particle size dependence of crack initiation
10
–3
SiCp/2024Al composite Axial loading, R=–1
Crack growth rate da/dN (mm/cycle)
Surface crack length 2c (mm)
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10
–4
10
–5
10
–6
Temp.(ºC)
10
10
–7
R.T. 150 250
Unreinforced 5μmSiCp/Al 20μmSiCp/Al 60μmSiCp/Al
–8 –5
10 1/2 Kmax/E (m )
10
–4
Fig. 11. Crack growth behaviour characterized in terms of maximum stress intensity factor normalized with respect to elastic modulus for unreinforced alloy and composites at ambient and elevated temperatures.
at this temperature. In Fig. 2, 60 lm SiCp/Al composite exhibited slightly lower fatigue strength in long life regime at 250 °C. It is considered that the effect of the difference in the thermal-expan-
Y. Uematsu et al. / Composites Science and Technology 68 (2008) 2785–2791
Vickers hardness HV
200
150 ºC 250 ºC
Unreinforced 20μmSiC p /Al R.T. for 20 μmSiC p/Al
150 R.T. for unreinforced alloy
100
50 0
6
12 18 24 Exposure time t (h)
30
Fig. 12. Vickers hardness change with exposure time at elevated temperatures for unreinforced alloy and 20 lm SiCp/Al composite. Note that remarkable softening occurs at 250 °C in both materials.
sion coefficient between particle and matrix was slightly enhanced in large particles of 60 lm SiCp/Al. Temperature exerted a significant influence on small crack growth as shown in Figs. 10 and 11. At 150 °C, when elastic modulus was considered, the crack growth behaviour was the same as that at ambient temperature except for 60 lm SiCp/Al composite. In 60 lm SiCp/Al composite, numerous cracks were initiated, grew and coalesced. Furthermore, cracks grew with particle fracture. These have led to the inferior crack growth resistance to the other materials. At 250 °C, however, crack grows through the remarkably softened matrix and particle/matrix interfaces with low strength, resulting in little influence of reinforcement incorporation and particle size on small crack growth at 250 °C. It is concluded that the particle size dependence is covered up by such properties of the matrix and interface at this temperature. In addition, the remarkable softening and associated loss in strength of the matrix can induce considerable plastic deformation at the crack tip, which may be responsible for the faster crack growth rates of all materials observed at this temperature. 5. Conclusions 1. Regardless of reinforcement incorporation and particle size, fatigue strength decreased with increasing temperature, with a considerable decrease at 250 °C. 2. At ambient temperature, fatigue strength was the highest in the 5 lm SiCp/Al composite, then 20 lm SiCp/Al composite, the unreinforced alloy and 60 lm SiCp/Al composite in decreasing order. Such particle size dependence significantly decreased at 150 °C and almost disappeared at 250 °C. 3. Fatigue cracks initiated from defects or due to slip in the unreinforced alloy and at clustering of particles in 5 lm SiCp/Al composite independent of temperature, while the temperature
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dependence of crack initiation was observed in 20 lm SiCp/Al and 60 lm SiCp/Al composites. Cracks generated due to particle fracture in the former and particle/matrix decohesion in the latter. 4. The resistance to crack initiation depended on particle size at 150 °C, but all materials showed similar resistance to crack initiation regardless of particle size at 250 °C. 5. When crack growth rates were characterized in terms of maximum stress intensity factor normalized with respect to elastic modulus, crack growth rates were nearly the same between ambient temperature and 150 °C, but still an order faster at 250 °C than those temperatures.
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Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Fatigue behaviour of SiC-particulate-reinforced aluminium alloy composites with different particle sizes at elevated temperatures Y. Uematsu a,*, K. Tokaji a, M. Kawamura b a b
Department of Mechanical and Systems Engineering, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan NTN Corporation, 1-3-17 Kyomachibori, Nishi-ku, Osaka 550-0003, Japan
a r t i c l e
i n f o
Article history: Received 3 April 2008 Received in revised form 29 May 2008 Accepted 3 June 2008 Available online 11 June 2008 Keywords: A. Metal–matrix composite B. Fatigue B. High-temperature properties D. Fractography
a b s t r a c t Fully reversed axial fatigue tests have been performed at 150 °C (423 K) and 250 °C (523 K) using smooth specimens of SiC-particulate-reinforced aluminium alloy composites with different particle sizes at a constant wt.% of SiC particles. Regardless of particle size, fatigue strength decreased with increasing temperature with a remarkable reduction at 250 °C. The particle size dependence of fatigue strength was clearly seen at ambient temperature, while became small at 150 °C and almost disappeared at 250 °C. Crack initiation depended on temperature and particle size and small crack growth rates were an order faster at 250 °C than at ambient temperature and 150 °C in all materials studied. It was indicated that the softening and associated loss in strength of the matrix at elevated temperatures were the primary causes for the observed temperature and particle size dependence of fatigue behaviour. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction SiC-particulate-reinforced aluminium alloy composites (hereafter, SiCp/Al composites) posses many advantages such as high specific strength and stiffness and good wear resistance. In addition to those properties, they can be processed by conventional means such as forging, rolling, extrusion and subsequent machining. Therefore SiCp/Al composites are suitable for structural components, and they are further expected to be widely used in various industries because of weight and energy saving and high performance in machines and structures, where fatigue properties are critical. Various fatigue properties of SiCp/Al composites have been investigated extensively [1], most of which have been devoted to the characterization of the ambient temperature fatigue behaviour. Since it is believed that SiCp/Al composites possess superior hightemperature performance to the matrix alloy due to the presence of ceramic reinforcement, they are potential candidates for components exposed to elevated temperatures. However, there have been very limited studies on the high-temperature fatigue behaviour of SiCp/Al composites [2–5], in particular, crack initiation and small crack growth have not been studied in any great detail because of experimental difficulties for measurement. There are many factors influencing the fatigue properties of SiCp/Al composites, such as the matrix properties, and the size, shape, distribution and volume fraction of the reinforcement, in * Corresponding author. Tel.: +81 58 293 2501; fax: +81 58 230 1892. E-mail address: [email protected] (Y. Uematsu). 0266-3538/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2008.06.005
which particle size is one of the important microstructural variables. Therefore there are still recent works about the effect of particle size on plastic deformation [6,7], fracture toughness [8] and so on. Hence, the authors have recently studied the fatigue crack propagation and the fatigue behaviour of SiCp/2024Al composites with different particle sizes at ambient temperature [9–12]. However, it is expected that particle size would play different roles in fatigue behaviour at elevated temperatures. The purpose of the present study is to understand the high-temperature fatigue behaviour of SiCp/Al composites with different particle sizes at a constant wt.% of SiC particles. Fully reversed axial fatigue tests were performed at ambient temperature, 150 °C and 250 °C using smooth specimens of the unreinforced alloy and the SiCp/2024Al composites, and the effects of temperature and particle size on fatigue behaviour were discussed based on crack initiation, small crack growth and fractographic analysis. 2. Experimental details 2.1. Material Aluminium alloy 2024 fabricated by powder metallurgy was used in the unreinforced condition and with 10 wt.% SiCp (about 9.0 vol.%) reinforcement. The chemical composition (wt.%) of the matrix alloy is Si 0.13, Fe 0.24, Cu 4.57, Mn 0.63, Mg 1.65, Cr 0.01, Zn 0.091, Ti 0.02, balance Al. Gas atomized 2024 alloy powders were blended with SiC particles and then the mixed powders were cold compacted in an aluminium container. Subsequently, the degassing process was performed at 450 °C for 1 h in vacuum
Y. Uematsu et al. / Composites Science and Technology 68 (2008) 2785–2791
below 10 1b Torr. The same processing route was employed for the unreinforced alloy. Finally, extrusion at 430 °C was applied to produce bars with a diameter of 35 mm. The composites with the different nominal particle sizes of 5 lm, 20 lm and 60 lm were prepared, hereafter denoted in this paper as 5 lm SiCp/Al, 20 lm SiCp/Al and 60 lm SiCp/Al, respectively. It should be noted that the unreinforced alloy and the composites were prepared from different lots from those used in previous reports [9–12]. The microstructures of the unreinforced alloy and the composites can be found in previous reports [9–11], where SiC particles tended to be weakly aligned in the extrusion direction and the aspect ratio of particles was approximately 1.7. Any pores were not found in the microstructures of all materials. 2.2. Specimen preparation Fatigue specimens with a width of 8 mm, a thickness of 4 mm and a gauge length of 12 mm were machined from the as-received bars. A shallow notch with a depth of 0.4 mm was introduced in one side of the gauge section in order to facilitate the observation of crack initiation and subsequent small crack growth. The stress concentration factor of the notch is small (Kt = 1.06), thus the specimens can be regarded as smooth ones. All specimens were solution treated at 495 °C for 1 h, quenched in water and then aged at 190 °C to achieve the peak-aged condition for the matrix. The ageing time was 5 h for the unreinforced alloy and 5 lm SiCp/Al composite, 3 h for 20 lm SiCp/Al composite, and 2.5 h for 60 lm SiCp/Al composite. These ageing conditions are based on the previous work [9]. Before fatigue test, the surface of the gauge section was mechanically polished using progressively finer grades of emery paper. 2.3. Procedures Fully reversed axial fatigue tests were performed on a 19 kN capacity electro-servohydraulic fatigue testing machine operating at a sinusoidal frequency of 10 Hz. Test temperatures are ambient temperature, 150 °C and 250 °C. An induction heating system was used to control temperature of the specimen. Prior to fatigue test, temperature calibration was conducted between the centre of the specimen and the location of 20 mm away from the center. Based on this temperature calibration, the temperature at the latter position was controlled to the desired temperatures at the center of the specimen during experiment. The accuracy of temperature was within 1% of the desired temperatures. Crack initiation and subsequent crack growth were monitored with replication technique. Experiment was interrupted and the specimen was cooled down to ambient temperature and then replicas were taken before temperature rise. This procedure was repeated until crack length was reached a critical size. Prior to experiment, it was confirmed that there was no discernible difference in fatigue life between the interrupted and continuous tests. The crack length was measured by means of an optical microscope on a plastic replica and defined as the projected length vertical to the loading direction. After experiment, fracture surfaces were examined in detail using a scanning electron microscope (SEM) to identify crack initiation and growth mechanisms operated at elevated temperatures.
3. Results 3.1. Temperature dependence of tensile strength The tensile strengths, rB, of the unreinforced alloy and of the composites are shown in Fig. 1 as a function of temperature, where
600
Tensile strength σ B (MPa)
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SiC p/2024Al composite
500 400 300 Unreinforced 5μmSiC p/Al 20μmSiC p/Al 60μmSiC p/Al
200 100
0
100 200 Temperature (ºC)
300
Fig. 1. Temperature dependence of tensile strength in unreinforced alloy and SiCp/ Al composites.
tensile tests were conducted on fatigue testing machine using smooth fatigue specimens without notch. In this case, the yield strength was not measured, but it is considered that the tensile strength could be correlated with the yield strength. It can be seen that regardless of reinforcement incorporation and particle size, tensile strength decreases with increasing temperature with an abrupt decrease above 200 °C. Similar behaviour has been reported by Nair et al. in unreinforced 2024 alloy and SiCp/2024Al composite with 21 vol.% SiC [13]. In the temperature range studied, 60 lm SiCp/Al composite exhibits slightly lower tensile strengths than the other materials, but the tensile strengths of 5 lm SiCp/Al and 20 lm SiCp/Al composites are the same as that of the unreinforced alloy. The brittle fracture of large particles in 60 lm SiCp/Al might be detrimental to static strength, but the detail is unclear. Han et al. have also indicated that the tensile strengths of the composites with different particle sizes became nearly equal at elevated temperature [3]. Therefore, it is believed that the properties of the matrix itself may play a dominant role in determining high-temperature composite strength. The detailed analysis of the fracture surfaces is not performed because this paper aims at fatigue properties. Some fractographic analysis of the fracture under monotonic loading at elevated temperatures can be referred for example to [14]. 3.2. Fatigue behaviour 3.2.1. Fatigue strength The S–N diagram is represented in Fig. 2 for the unreinforced alloy and the composites at ambient and elevated temperatures. In both the unreinforced alloy and the composites, fatigue strength decreases with increasing temperature with a considerable reduction at 250 °C, which is the same trend as tensile strength. At ambient temperature, fatigue strength is the highest in 5 lm SiCp/Al composite, then 20 lm SiCp/Al composite, the unreinforced alloy, 60 lm SiCp/Al composite in decreasing order. This particle size dependence of fatigue strength is similar to that obtained in previous reports [9,11]. At 150 °C, the particle size dependence observed at ambient temperature can be still seen, but differences in fatigue strength among the unreinforced alloy, 5 lm SiCp/Al composite and 20 lm SiCp/Al composite become small. Furthermore, at 250 °C, the fatigue strengths for all materials are nearly the same, i.e. the particle size dependence almost disappears, but 60 lm SiCp/Al composite still exhibits slightly lower fatigue strength in long life regime. Fig. 3 shows the fatigue strengths characterized in terms of fatigue ratio, r/rB, i.e. stress amplitude normalized by tensile
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Stress amplitude σ (MPa)
250
SiC p/2024Al composite Temp.(ºC) R.T.150 250 Axial loading, R =–1 Unreinforced 5μmSiC p/Al 20μmSiC p/Al 60μmSiC p/Al
200 150 100 50 0 3 10
4
5
6
7
10 10 10 10 Number of cycles to failure Nf
10
8
Fig. 2. S–N diagram for unreinforced alloy and SiCp/Al composites at ambient and elevated temperatures.
0.8
Fatigue ratio σ/σB
0.7
SiC p/2024Al composite Axial loading, R=–1 Temp.(˚C) R.T.150 250 Unreinforced 5μmSiC p/Al 20μmSiC p/Al 60μmSiC p/Al
0.6 0.5 0.4 0.3 0.2 0.1 3 10
4
5
6
7
10 10 10 10 Number of cycles to failure Nf
10
8
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5 lm SiCp/Al. In 20 lm SiCp/Al composite, cracks are generated by cyclic slip deformation at ambient temperature, while due to particle fracture at 150 °C and at the particle/matrix interface at 250 °C. In 60 lm SiCp/Al composite, similar initiation behaviour can be seen at elevated temperatures, but the particle/matrix interface is the dominant crack initiation site at ambient temperature. Examples of matching SEM micrographs of fracture surfaces are shown in Figs. 5–8. Figs. 5 and 6 are crack initiation due to particle fracture (150 °C) and at the particle/matrix interface (250 °C), respectively, in 20 lm SiCp/Al composite. Figs. 7 and 8 are crack initiation due to due to particle fracture (150 °C) and at the particle/matrix interface (250 °C), respectively, in 60 lm SiCp/Al composite. It is evident in Figs. 5 and 7 that the crack was initiated due to particle fracture at 150 °C because a particle can be seen on a pair of fracture surfaces. In Figs. 6 and 8, a particle can be seen on one side of fracture surfaces (Figs. 6 and 8b), but the corresponding hole is observed on the opposite side of fracture surfaces (Figs. 6 and 8a), clearly indicating that the crack was generated at the particle/matrix interface at 250 °C. Based on detailed examination and analysis of the crack initiation behaviour for all fatigue-failed specimens, the predominant crack initiation mechanisms operated are summarized in Table 1 for each temperature and material. When two different mechanisms were identified, both are indicated in the table. The unreinforced alloy and 5 lm SiCp/Al composite show the same crack initiation mechanism regardless of temperature; slip or defects in the former and clustering or agglomeration of particles in the latter. On the contrary, in the composites with larger particle sizes, it should be noted that the crack initiation mechanism changes with temperature, where particle fracture and particle/matrix interface are the dominant mechanisms at 150 °C and 250 °C, respectively. Illustrated in Fig. 9 are the relationships between crack length, 2c, and number of cycles, N, at elevated temperatures in the unreinforced alloy and the composites. As indicated in previous report [9,11], at ambient temperature, the crack initiation resistance decreased with increasing particle size. In Fig. 9, similar behaviour can be seen at 150 °C, where crack initiation becomes faster with increasing particle size. At 250 °C, however, it becomes nearly the same for all materials, indicating that the particle size dependence of crack initiation observed at ambient temperature and 150 °C completely disappears at this temperature.
Fig. 3. Fatigue strength characterized in terms of fatigue ratio.
strength. Nieh et al. have indicated that the fatigue ratio at 107 cycles increased with increasing temperature in various Al–matrix composites [2]. In the present study, however, regardless of reinforcement incorporation and particle size, the relative fatigue strengths are the same between ambient temperature and 150 °C, but slightly lower at 250 °C, particularly in the composites. This suggests that the fatigue strength decreases more than the tensile strength and some localized damage relating to the presence of reinforcement would take place in crack initiation or small crack growth during fatigue testing at 250 °C. 3.2.2. Crack initiation Revealed in Fig. 4 are the typical examples of SEM micrographs showing crack initiation site of the composites at ambient and elevated temperatures. The crack initiation sites for the unreinforced alloy were found to be defects or due to cyclic slip deformation regardless of temperature and applied stress level. As can be seen in the micrographs, the crack initiation site for 5 lm SiCp/ Al composite is the clustering or agglomeration of particles at all temperatures. This clustering of particles was recognized only in
3.2.3. Small crack growth Crack growth rate, da/dN, is shown in Fig. 10 as a function of maximum stress intensity factor, Kmax, for the unreinforced alloy and the composites. Crack growth rate was calculated as a slop of a–N curve based on a polynomial approximation using five data points. Kmax was calculated using Newman–Raju formula [15], assuming a semicircular surface small crack. Regardless of reinforcement incorporation and particle size, crack growth rates become faster with increasing temperature. The difference in crack growth rate between ambient temperature and 150 °C is small, but the crack growth rates at 250 °C are an order of magnitude faster. At ambient temperature, the crack growth rates of the unreinforced alloy and 5 lm SiCp/Al composite are slightly faster than those of 20 lm SiCp/Al and 60 lm SiCp/Al composites. At 150 °C, there are no discernible differences among the unreinforced alloy, 5 lm SiCp/Al composite and 20 lm SiCp/Al composite, but 60 lm SiCp/Al composite shows slightly faster crack growth rates. At 250 °C, on the contrary, the da/dN–Kmax relationships for all materials are similar, indicating little influence of reinforcement incorporation and particle size. It is known that elastic modulus, E, is the most important material variables influencing fatigue crack propagation. Elastic
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Temperature
5μmSiCp/Al
60μmSiCp/Al
20μmSiCp/Al
R.T. 20μm
10μm
20μm
50μm
20μm
20μm
10μm
20μm
20μm
150 ºC
250 ºC
Fig. 4. SEM micrographs showing fracture surfaces near crack initiation site in SiCp/Al composites at ambient and elevated temperatures.
Fig. 5. Matching SEM micrographs of crack initiation site at 150 °C in 20 lm SiCp/Al composite (r = 160 MPa, Nf = 1.3 105).
Fig. 6. Matching SEM micrographs of crack initiation site at 250 °C in 20 lm SiCp/Al composite (r = 60 MPa, Nf = 1.6 105).
modulus decreases with increasing temperature, thus the small crack growth data were characterized in terms of Kmax normalized with respect to elastic modulus, Kmax/E, and the results are shown in Fig. 11. In the present study, the elastic modulus at each temperature was estimated based on the results in the Ref. [16]. As can be seen in the figure, the crack growth rates of the unreinforced alloy,
5 lm SiCp/Al composite and 20 lm SiCp/Al composite are nearly the same between ambient temperature and 150 °C, but 60 lm SiCp/Al composite still exhibits slightly faster crack growth rates at 150 °C. At 250 °C, the crack growth rates are still approximately an order of magnitude faster than those at ambient temperature and 150 °C.
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Fig. 7. Matching SEM micrographs of crack initiation site at 150 °C in 60 lm SiCp/Al composite (r = 160 MPa, Nf = 4.9 104).
Fig. 8. Matching SEM micrographs of crack initiation site at 250 °C in 60 lm SiCp/Al composite (r = 50 MPa, Nf = 2.7 105).
Table 1 Predominant crack initiation mechanisms depending on temperature and particle size Temperature (°C)
Unreinforced
5 lm SiCp/Al
20 lm SiCp/Al
60 lm SiCp/Al
R.T.
Slip or Defect
Particle cluster
Slip or particle fracture Particle fracture Interface
Slip or interface
150 250
Particle fracture Interface
4. Discussion 4.1. High-temperature fatigue strength of the composites As described in introduction, there have been limited studies on the high-temperature fatigue behaviour of SiCp/Al composites. Nieh et al. [2] and Han et al. [3] have studied the fatigue behaviour of SiCp/6090Al at 300 °C and the low cycle fatigue behaviour of SiCp/pure Al composite at 168 °C, respectively. Srivatsan and Al-Hajri have also investigated the fatigue behaviour of SiCp/ 7034Al composite at 120 °C [4]. LLorca have indicated the fatigue strength of SiCp/2080Al composites at 150 °C and 170 °C in his review for the fatigue behaviour of discontinuously-reinforced metal–matrix composites [5]. In those works, the fatigue strengths of the composites were found to be lowered at elevated temperatures independent of volume fraction and particle size of reinforcement and matrix alloy, as well as in the present study. However, the range of variables such as temperature and particle size was relatively narrow in those works. The present study clearly indicates that the improvement of the fatigue strength of the composites is not always achieved. As shown in Fig. 2, at ambient temperature, the composites with smaller particle sizes showed higher fatigue strength than the unrein-
forced alloy, but with increasing temperature, their fatigue strengths become nearly equal to those of the unreinforced alloy, i.e. the composites lose the advantage for improving fatigue strength by reinforcement incorporation at elevated temperatures. Therefore, it may be concluded that the matrix plays a dominant role in determining the high-temperature fatigue strength of the composites. Fig. 12 shows Vickers hardness change of the matrix with exposure time at elevated temperatures in the unreinforced alloy and 20 lm SiCp/Al composite, where measurements were done until 27 h corresponding to 106 fatigue cycles. Both the unreinforced alloy and the composite exhibit slightly lower hardness at 150 °C than at ambient temperature. This temperature is lower than the ageing temperature of the materials, but the matrix becomes overaged. At 250 °C, on the contrary, considerable decrease in hardness can be seen in both materials, indicating the remarkable softening and associated loss in strength of the matrix. Therefore, the matrix becomes more plastically deformable at elevated temperatures, particularly at 250 °C. The enhanced ambient temperature fatigue strength of the composites with smaller particles resulted primarily from the increased crack initiation resistance and early small crack growth resistance [11]. However, the effects of factors contributing to the improvement of the resistance would be minimized or eliminated at elevated temperatures. Crack initiation will be discussed in the following section. At ambient temperature, early small crack growth was significantly affected by particles where small cracks grew avoiding particles, i.e. hard particles acted as barriers to small crack growth [11]. With increasing temperature, it is assumed that the particle/matrix interfaces become easy to decohere, because of the large difference in the thermal-expansion coefficient between particle and matrix. The coefficients of particle and matrix are about 4.4 and 22.9 (10 6 °C 1), respectively. Therefore, particles no longer act as effective barriers to small crack growth.
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SiC p/2024Al composite
Unreinforced (σ=160MPa) 5μmSiCp/Al (σ=160MPa) 20μmSiCp/Al (σ=160MPa) 60μmSiCp/Al (σ=150MPa)
2.0
1.0
3.0
0
0.4 0.8 1.2 1.6 2.0 5 Number of cycles N (x10 )
2.4
SiC p/2024Al composite Temperature:250 ºC
Crack growth rate da/dN (mm/cycle)
Temperature:150 ºC
0
Surface crack length 2c (mm)
–3
10
3.0 SiC p/2024Al composite
2.0
1.0
–4
Axial loading R=–1
10
–5
10
–6
10
Temp.(ºC ) –7
10
250
R.T. 150
Unreinforced 5μmSiC p/Al 20μmSiCp/Al 60μmSiCp/Al
Unreinforced (σ=65MPa) 5μmSiCp/Al (σ=70MPa) 20μmSiCp/Al (σ=65MPa) 60μmSiCp/Al (σ=60MPa)
–8
10
0.3 1 2 Maximum stress intensity factor 1/2 Kmax (MPam )
10
Fig. 10. Relationship between crack growth rate and maximum stress intensity factor for unreinforced alloy and composites at ambient and elevated temperatures.
0
0
0.2 0.4 0.6 0.8 1.0 1.2 5 Number of cycles N (x10 )
1.4
Fig. 9. Surface crack length as a function of number of cycles for unreinforced alloy and composites: (a) 150 °C; (b) 250 °C.
4.2. Particle size dependence of high-temperature fatigue behaviour At ambient temperature, fatigue strength increased with decreasing particle size, but the particle size dependence of fatigue strength became small at 150 °C and almost disappeared at 250 °C. Such tendency of high-temperature fatigue strength of the composites will be realized through crack initiation and subsequent small crack growth behaviour. In the composites with larger particle sizes such as 20 lm and 60 lm, a major influence of temperature was to change the crack initiation process as summarized in Table 1. It is believed that crack initiation is significantly affected by factors such as the stress concentration around particles, the interface strength and the particle strength itself. At 150 °C, the softening of the matrix was less remarkable as shown in Fig. 12, thus the decrease in the interface strength would not be significant, resulting in particle fracture due to lowered fracture strength of particles with increasing particle size [17]. Since particle fracture was also observed at ambient temperature, the crack initiation mechanism is basically the same as that at ambient temperature. At 250 °C, on the other hand, cracks were initiated at the particle/matrix interfaces. As discussed previously, the interface strength would be reduced due to the large difference in the thermal-expansion coefficient between particle and matrix. Furthermore, it is believed that the sensitivity to stress concentration around particles or at the interfaces is lowered due to the remarkable softening of the matrix. These have led to the disappearance of the particle size dependence of crack initiation
10
–3
SiCp/2024Al composite Axial loading, R=–1
Crack growth rate da/dN (mm/cycle)
Surface crack length 2c (mm)
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10
–4
10
–5
10
–6
Temp.(ºC)
10
10
–7
R.T. 150 250
Unreinforced 5μmSiCp/Al 20μmSiCp/Al 60μmSiCp/Al
–8 –5
10 1/2 Kmax/E (m )
10
–4
Fig. 11. Crack growth behaviour characterized in terms of maximum stress intensity factor normalized with respect to elastic modulus for unreinforced alloy and composites at ambient and elevated temperatures.
at this temperature. In Fig. 2, 60 lm SiCp/Al composite exhibited slightly lower fatigue strength in long life regime at 250 °C. It is considered that the effect of the difference in the thermal-expan-
Y. Uematsu et al. / Composites Science and Technology 68 (2008) 2785–2791
Vickers hardness HV
200
150 ºC 250 ºC
Unreinforced 20μmSiC p /Al R.T. for 20 μmSiC p/Al
150 R.T. for unreinforced alloy
100
50 0
6
12 18 24 Exposure time t (h)
30
Fig. 12. Vickers hardness change with exposure time at elevated temperatures for unreinforced alloy and 20 lm SiCp/Al composite. Note that remarkable softening occurs at 250 °C in both materials.
sion coefficient between particle and matrix was slightly enhanced in large particles of 60 lm SiCp/Al. Temperature exerted a significant influence on small crack growth as shown in Figs. 10 and 11. At 150 °C, when elastic modulus was considered, the crack growth behaviour was the same as that at ambient temperature except for 60 lm SiCp/Al composite. In 60 lm SiCp/Al composite, numerous cracks were initiated, grew and coalesced. Furthermore, cracks grew with particle fracture. These have led to the inferior crack growth resistance to the other materials. At 250 °C, however, crack grows through the remarkably softened matrix and particle/matrix interfaces with low strength, resulting in little influence of reinforcement incorporation and particle size on small crack growth at 250 °C. It is concluded that the particle size dependence is covered up by such properties of the matrix and interface at this temperature. In addition, the remarkable softening and associated loss in strength of the matrix can induce considerable plastic deformation at the crack tip, which may be responsible for the faster crack growth rates of all materials observed at this temperature. 5. Conclusions 1. Regardless of reinforcement incorporation and particle size, fatigue strength decreased with increasing temperature, with a considerable decrease at 250 °C. 2. At ambient temperature, fatigue strength was the highest in the 5 lm SiCp/Al composite, then 20 lm SiCp/Al composite, the unreinforced alloy and 60 lm SiCp/Al composite in decreasing order. Such particle size dependence significantly decreased at 150 °C and almost disappeared at 250 °C. 3. Fatigue cracks initiated from defects or due to slip in the unreinforced alloy and at clustering of particles in 5 lm SiCp/Al composite independent of temperature, while the temperature
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dependence of crack initiation was observed in 20 lm SiCp/Al and 60 lm SiCp/Al composites. Cracks generated due to particle fracture in the former and particle/matrix decohesion in the latter. 4. The resistance to crack initiation depended on particle size at 150 °C, but all materials showed similar resistance to crack initiation regardless of particle size at 250 °C. 5. When crack growth rates were characterized in terms of maximum stress intensity factor normalized with respect to elastic modulus, crack growth rates were nearly the same between ambient temperature and 150 °C, but still an order faster at 250 °C than those temperatures.
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