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Effect of clustering on the mechanical properties of SiC particulate-reinforced aluminum alloy 2024 metal matrix composites
Materials Science and Engineering A347 (2003) 198 /204 www.elsevier.com/locate/msea
Effect of clustering on the mechanical properties of SiC particulatereinforced aluminum alloy 2024 metal matrix composites Soon-Jik Hong a, Hong-Moule Kim a, Dae Huh a, C. Suryanarayana b,*, Byong Sun Chun a a
Rapidly Solidified Materials Research Center (RASOM), Chungnam National University, Taedok Science Town, Taejon 305-764, South Korea b Department of Mechanical, Materials and Aerospace Engineering, University of Central Florida, Orlando, FL 32816-2450, USA Received 20 December 1999; received in revised form 29 July 2002
Abstract Al 2024 /SiC metal matrix composite (MMC) powders produced by centrifugal atomization were hot extruded to investigate the effect of clustering on their mechanical properties. Fracture toughness and tension tests were conducted on specimens reinforced with different volume fractions of SiC. A model was proposed to suggest that the strength of the MMCs could be estimated from the load transfer model approach that takes into consideration the extent of clustering. This model has been successful in predicting the experimentally observed strength and fracture toughness values of the Al 2024 /SiC MMCs. On the basis of experimental observations, it is suggested that the strength of particulate-reinforced MMCs may be calculated from the relation: sy /smVm/sr (Vr/Vc)/srVc, where s and V represent the yield strength and volume fraction, respectively, and the subscripts m, r, and c represent the matrix, reinforcement, and clusters, respectively. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Composites; Clustering; Mechanical properties; Fractography; Al 2024 alloy
1. Introduction Most research on metal matrix composites (MMCs) in recent years has been concentrated on the development of high performance continuous fiber-reinforced composites for specialized applications. In spite of their unique properties, such composites are expensive. Therefore, development of less-expensive composites for non-critical applications is desirable. Particulatereinforced MMCs are cost-effective alternatives and have the advantage of being machinable and workable using conventional processing methods. However, their poor mechanical properties such as low fracture strain and fracture toughness, which are important for the design of structural materials, have limited their widespread applications. One of the major areas of research in MMCs has been to study how the reinforcement
* Corresponding author. Tel.: /1-407-823-6662; fax: /1-407-8230208 E-mail address: [email protected] (C. Suryanarayana).
phase affects the failure mechanism(s) and hence controls the fracture toughness of these materials. A major potential problem in the application of SiC/Al composites is the possibility that the reaction between Al and SiC produces Al4C3, which weakens the interface depending on the temperature, environment, and other parameters [1]. The second problem of SiC/Al composites is that the microstructure is non-uniform with elongated clusters of particles present along the extrusion direction. The most significant detrimental property change may be the decrease in ductility and fracture toughness, which is true for all the aluminum MMCs and process histories; a major obstacle preventing their extensive use. The mechanism of reinforcement affecting the fracture toughness of MMCs is not well understood. Several models have been proposed to characterize the relationship between fracture toughness and microstructure [2]. The principal objective of this current investigation was to determine how the microstructural parameters, such as the particulate volume fraction and extent of clustering affect the tensile properties and fracture toughness of Al 2024 /SiC MMCs. Another objective
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was to shed more light on the failure mechanisms operative in the composites so as to aid in the correlation between microstructure and properties of such materials.
2. Experimental procedure The material used is an Al 2024 alloy reinforced with 3, 5, 7, or 10 vol.% of 10 mm diameter SiC particles. In producing the material, the Al 2024 alloy was melted and mixed with SiC powder by a stirrer. The mixture was then poured through a centrifugal atomizer to produce the atomized composite powder. The atomized powders were compacted to 95% of theoretical density and then degassed in a vacuum chamber for 1 h at an elevated temperature of 673 K. The billet was then subjected to hot extrusion at 673 K with an extrusion ratio of 13:1. Tensile tests were performed using an Instron mechanical testing machine to obtain the basic mechanical properties including 0.2% offset yield strength sy, ultimate tensile strength su, and fracture strain o f. The load was measured by a 5000 kg load cell and the deformation of the specimen was measured by a 2.54 cm extensometer. The specimens were loaded at a constant crosshead speed of 10 3cm s 1 until failure. The pure mode I fracture toughness, KIQ of the composites was measured by three point bending tests performed on an MTS servohydraulic machine. The specimen dimensions are shown in Fig. 1. The KIQ value was calculated using the equation: KIQ (PQ =BW 1=2 )f (a=W )
(1)
where PQ is the load where the initial drop in the load occurs during the compact tension test, B is the specimen thickness, W is the specimen width, and a is the crack length. f(a /W) is a complex function of a/W and its values for different values of a/W are listed in the ASTM specification E-399 [3]. This equation, established on the basis of linear elastic fracture mechanics, is valid when a sharp crack of adequate size exists in a specimen. This condition is accomplished by fatigue precracking the specimens with a stress intensity level of less than 60% of KIQ at the final stage. The specimen was then loaded with a crosshead speed of 0.01 mm s 1 until
Fig. 1. Dimensions of the fracture test specimen.
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the load-displacement curve deviated from linearity. Within this part a 95% secant line could be drawn to calculate the KIQ value. Fractographs of all the broken specimens tested under tension and three-point bending conditions were recorded using a JSM-6400 scanning electron microscope (SEM). A computerized image analyzer was used to obtain the distribution and volume fraction of SiC particulates/clusters in the as-processed condition.
3. Results 3.1. Tensile properties The composites produced exhibited a heterogeneous microstructure, with elongated clusters of particles forming along the extrusion direction, with the local volume of these clusters varying between 16% for 3 vol.% SiC and 32% for 10 vol.% SiC. However, it should be noted that the vol.% of the clusters has not increased regularly with increasing SiC content. As shown in Fig. 2, the vol.% of the clusters reached a minimum of 12% for the addition of 5 vol.% of SiC. Thus, the minimum extent of clustering seems to be obtained for 5 vol.% SiC. The microstructural features of the starting materials, in the form of extrusions, are listed in Table 1. These composites have excellent bonding between the SiC particulates and the aluminum alloy matrix, assumed to be associated with the formation of Al4C3, Al4SiC4, and CuAl2O4 [4] as intermediate phases at the interface. The 0.2% offset yield strength sy, ultimate tensile strength su, and fracture strain o f were calculated from the stress /strain curves, and these data are listed in Table 2. Each value listed is an average of at least two measurements, with a mean deviation for all tests of 2.7, 1.8, and 4.0% for sy, su, and o f, respectively. For comparison purposes, these mechanical properties for the Al 2024 matrix alloy also are included in Table 2 [5]. Two significant features of the stress /strain curve are that there is no necking before failure and that the
Fig. 2. Variation of the volume percentage of clusters as a function of the SiC content.
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Table 1 Specifications of the composites investigated Material
Volume % of cluster
Aspect ratio of the cluster (particle)
Particle thickness (mm)
Al Al Al Al
16 12 20 32
7.0 (1.5)
10
2024 /3 vol.% SiC 2024 /5 vol.% SiC 2024 /7 vol.% SiC 2024 /10 vol.% SiC
on the tensile properties of the composites. As shown in Fig. 3, the yield strength and ultimate tensile strength started to decrease at higher SiC contents, mostly due to the increased amount of clustering. This seems to be especially true for the composite containing 10 vol.% of SiC when the strength is expected to be much higher than at lower SiC levels. But, it is lower due to the increased amount of clustering.
fracture strain of the composites is much lower than that of the Al 2024 matrix. While the fracture strain was 0.21 for the un-reinforced Al 2024 alloy, it had decreased to 0.03 when 5 vol.% of SiC was added to it. The fracture strains for the composites ranged from 3.2 to 7.0% suggesting that the composite materials are semi-brittle and that only moderate plasticity occurs before failure. As expected, the yield strength of all the composites has significantly increased for all the composite specimens, with a 3 / increase over Al 2024 for the composites with ]/5 vol.% SiC. The volume fraction of the SiC particulates was found to have a significant effect on the tensile properties of the composites; both sy and su started to decrease beyond about 7 vol.% of SiC. Fig. 3 shows the variation of the 0.2% offset yield strength and ultimate tensile strength with the vol.% of SiC, for a constant particulate size of 10 mm. This figure clearly shows that the amount of the reinforcement plays a significant role in controlling the tensile properties of the composites. It is also important to note that the SiC particulate clusters also have a significant effect
3.2. Fracture toughness results The mode I stress intensity factor analysis was performed according to ASTM E-399. The KIQ values were calculated using Eq. (1), and the calculated values are listed in Table 2. Each value listed is an average of two measurements. The average spread of the two values was about 2%. For most cases KIQ satisfied the condition for the applicability of linear elastic fracture mechanics, viz., presence of a sharp crack of adequate size. Fig. 4 shows the effect of the vol.% of SiC (and the vol.% of clusters) on the conditional mode I fracture toughness (KIQ) indicating that KIQ decreases with
Fig. 3. Variation of yield strength and ultimate tensile strength of the composites as a function of the volume percentage of SiC, for a constant particulate size of 10 mm.
Fig. 4. Mode I fracture toughness and vol.% of clusters of the Al 2024 /SiC composites as a function of the volume percentage of SiC.
Table 2 Tensile properties and fracture toughness of the composites Material
sy (MPa)
su (MPa)
of
Crack length (mm)
KIQ (MPa m1/2)
Al Al Al Al Al
75 175 214 221 210
185 264 320 374 308
0.210 0.051 0.032 0.070 0.066
/ 14.97 14.75 14.58 15.14
/ 20.16 19.02 18.42 14.67
2024 2024 /3 vol.% SiC 2024 /5 vol.% SiC 2024 /7 vol.% SiC 2024 /10 vol.% SiC
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increasing volume fraction of SiC. As expected, the fracture toughness decreased with an increase in the vol.% of SiC and hence the tensile strength of the samples. A similar correlation between the tensile properties and fracture toughness was reported earlier for Al /6Si composites reinforced with alumina particulates [6]. It may also be noted from Fig. 4 that the decrease of conditional mode I fracture toughness with increasing cluster vol.% was minimal up to about 7 vol.% SiC (when the vol.% of the clusters was about 20%), but beyond this value, the fracture toughness decreased significantly. For example, for the Al 2024 /10 vol.% SiC composite, the fracture toughness was only 14.7 MPaâm; associated with an increased strength due to SiC and decreased strength due to clustering. The tensile strength could have been higher and the fracture toughness lower, if the clustering was not there.
3.3. Fractography
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surfaces have also cracked near the interface. When examined at higher magnifications, the fractured SiC particles show a clear crack running through the center of the particle as shown in Fig. 6. Fig. 7 shows a region of clustered SiC particles on the fracture surface where damage accumulation ahead of the crack tends to occur more easily. A few secondary branches of the main crack were observed on the fracture surface of the specimen and this was confirmed by viewing the region in a direction perpendicular to the fracture surface.
4. Discussion 4.1. Strength In predicting the strength of the particulate-reinforced MMCs, the simplest load flow stress model might be the rule of mixtures: sy sm Vm sr Vr
(2)
Even though the composites did not exhibit much ductility on a macroscopic scale (Table 2), SEM fractographs indicated that the fracture occurred by a locally ductile mechanism. Typical fracture surfaces (Fig. 5) consisted of a bimodal distribution of dimples*/larger dimples associated with the SiC particulates and smaller dimples associated with the ductile failure of the Al 2024 alloy matrix. Similar observations have also been reported earlier by other investigators [5 /7]. In most cases the larger dimples contained SiC particles and were about the same size as the particles responsible for their formation. Decohesion from the matrix initiates at the matrix/SiC interface. The edges of the dimples around SiC particles are elongated and a small fraction of the SiC particles on the fracture
where sy is the predicted yield strength, sm and sr are the individual yield strengths of the matrix and reinforcement, respectively, and Vm and Vr are the volume fractions of the matrix and the reinforcement, respectively. The proportion of the external load borne by the individual constituents can be estimated by volume averaging the load, and the external applied load will then be equal to the sum of the volume averaged loads [2]. This model has been very successful in predicting the elastic modulus of the MMCs, but is only moderately successful for strength predictions. The second model would be related to dispersion hardening, with plastic incompatibility, that can be expressed, for example, by the Ashby geometrically necessary dislocation concept [8], leading to excess dislocations in the near-particle region. The plastic-
Fig. 5. Scanning electron micrograph showing a typical fracture surface. One can notice large dimples associated with the SiC particles and small dimples related to the ductile fracture of the matrix.
Fig. 6. A magnified scanning electron micrograph showing fractured SiC particles.
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Fig. 7. Scanning electron micrograph showing clustering of SiC particulates in the composite.
deformation-induced dislocations would become dominant when the plastic strain exceeds the thermal mismatch strain. The two effects would act in parallel and may be combined. Wu and Lavernia [6] combined the effects of load transfer and matrix strengthening to the Al /4Si/TiB2 system, and obtained relatively good agreement with the experimental results. However, for the relatively larger particle size as obtained in the present work, the single dislocation, Orowan-type hardening would be completely negligible. Hence, we tried to use the simplest load transfer model to explain the results. Use of the load transfer model to calculate the yield strength and tensile strength (3.5 /4.0 GPa for SiC) yields results that are significantly different from the experimental data. The calculations are compared with the experimental results of strength versus vol.% of SiC in Fig. 8. It may be noted that the calculated strengths do not agree with the experimentally obtained values, especially at higher SiC contents. While the calculated values increase continuously with the vol.% of SiC, the
Fig. 8. Comparison of experimental and calculated strength values as a function of the volume percentage of SiC in the composites.
experimental results show a drop beyond about 7 vol.% SiC. In view of this discrepancy, a modified strengthening model is developed in the present work, taking clustering into consideration. It is known that the extrusion process produces elongated clusters of particles in the extrusion direction [6,9]. Further, the distribution of these clusters is also non-uniform. As can be seen in Fig. 9, these clustered particles are the sites for damage accumulation ahead of the crack. Cluster fracture is the dominant mode of fracture, with the clusters perpendicular to the loading direction. Cluster fracture occurs at an early stage of the tensile loading process, and cracks in the fractured clusters grow to final fracture [6,10]. Thus, clustering of the reinforcement in the composite makes a negative contribution to the strength of the particulate-reinforced MMCs. Therefore, we suggest that the strength of particulate reinforced MMCs may be calculated from the relation: sy sm Vm sr (Vr Vc )sr Vc
(3)
where Vc is the volume fraction of the clusters. Accordingly, as a simplified model, the strength of the MMCs can be estimated from the load transfer model taking clustering into consideration. Expressed differently, Eq. (3) can be rewritten as: sy sm Vm sr Vr 2sr Vc
(4)
Considering V as the total volume of the composite, it can be written down as V /Vm/Vr. If P is the load borne by an individual component, then the following expressions can be set up: Vm (1Pr =100)V ; Vr (Pr =100)V ; Vc (Pr Pc =1002 )V :
Fig. 9. Fracture surface of the Al 2024 /SiC composite.
S.-J. Hong et al. / Materials Science and Engineering A347 (2003) 198 /204
Then, Eq. (4) can be expressed as: sy sm (1Pr =100)(sr Pr =100)(12Pc =100)
(5)
where Pm /100Vm/V , Pr 100Pm ; and Pc 100Vc =Vr : The strength values calculated using Eq. (5) are compared with the experimental results in Fig. 10. As can be seen, reasonably good agreement is noticed with the experimental results. 4.2. Fracture Reinforcements contribute to failure by particle cracking, cluster cracking, interface cracking, and matrix/particle debonding. Failure also results from matrix damage, like void formation and matrix cracking. On the fracture surface, ductile dimples and particles in the voids are commonly observed. As indicated earlier, the predominant fracture mode of particulate-reinforced MMCs is particle and cluster cracking, which occurs at an early stage of loading. Since the ductility of the composites is low, it is reasonable to assume that the cracks through the fractured clusters and particles obey the fracture mechanics approach, and have a plane strain plastic zone [10,11]. The plastic zone size increases with increasing load, and failure of the composite occurs when the plastic zones of adjacent cracks coalesce. At the same time, the stress intensity factor KIQ reaches the plane strain fracture toughness of the composite KIC. Hahn and Rosenfield [12] calculated the fracture toughness of Al alloys with large Fe- or Si-rich inclusions in terms of the volume fraction and proposed that KIC 8/Vr1/6. The variation of KIQ with Vr suggests that
Fig. 10. Comparison of the experimental and calculated (according to Eq. (5)) strength values as a function of the volume percentage of SiC in the composites.
203
the toughness of the composites is limited by Vr as suggested elsewhere [10]. This leads to the following equation: KIQ sY [3pt(rr =Vr )1=2 ]1=2
(6)
where rr is the aspect ratio of the SiC particulate, t is the thickness and sY is the effective yield strength of the matrix calculated as sY /(sy/su)/2 which turns out to be 219 and 297 MPa, for the 2024/3 vol.% SiC and the Al 2024 /7 vol.% SiC composites, respectively. Based on this model, the fracture toughness calculated using Eq. (6) is compared with the experimental results as a function of the vol.% of SiC in Fig. 11. Whereas the experimental results show that the fracture toughness decreases with increasing vol.% of SiC, the calculated values do not show a continuous decrease; the fracture toughness increases slightly up to about 7 vol.% SiC, and then starts to decrease. Further, the experimental values are about four times as much as the values calculated from Eq. (6). In view of this discrepancy, a modified model is developed in the present work, taking the clusters into consideration. It may be safely assumed that fracture preferentially initiates in the clustered reinforcements in particulate-reinforced MMCs. Metallographic and fractographic analyses confirm that clustered regions were the preferred sites for damage initiation during deformation in an aluminum alloy reinforced with SiC particles. It is shown that the micromechanisms of fracture are significantly influenced by the degree of clustering in the composites. Therefore, on the basis of experimental observations, we suggest that the fracture toughness of the composites in particulate-reinforced MMCs may be calculated by introducing the volume fraction of clusters into Eq. (6) and obtaining: KIQ sY [(3pt)f(rr =Vr )(rc =Vc )g1=2 ]1=2
(7)
Fig. 11. Comparison of the experimental and calculated (according to Eqs. (6) and (7)) fracture toughness values as a function of the volume percentage of SiC in the composites.
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where Vc is the volume fraction of SiC particulate clusters and rc is the aspect ratio of the cluster. The results obtained using Eq. (7) are compared with the experimental results in Fig. 11. This model successfully predicts the fracture toughness of MMCs, although the calculated fracture toughness values are not always exactly the same as those observed experimentally. The experimental and the calculated results show that the fracture toughness decreases with increasing volume fraction of the particulates and clusters. Since this model assumes that all the particulates and clusters will crack or debond ahead of the crack tip, which has fractographic evidence to support it, the optimum structure would be a more uniform particulate distribution. The desired optimum cluster volume fraction could be achieved by decreasing the volume fraction of the reinforcement or increasing the particle size up to a limiting value. 4.3. Crack path In the present experiments, although the cracks exhibited a random total path, damage accumulation was observed to occur in the highly clustered regions along parts of the fracture path. The selection of damage accumulation in locally clustered regions showed that the gross fracture path was essentially random but may be related to the orientation and volume fraction of the particulate along the total crack path. It was observed that alternating ‘bands’ of high and low volume fraction reinforcement regions are present in the direction of crack propagation. This requires that the crack passes through some low cluster volume fraction regions in order to progress. The growth of the crack could be controlled due to the presence of a compliant layer on the specimen. The present work further illustrates that the examination of fractured surfaces alone may be insufficient to determine whether fracture is initiating or propagating along a path of clustered particles. Thus, as an extension of this work, subsequent work will focus on the relationship between clustered regions and damage accumulation to evaluate the effects of changes in clustering on the fracture properties.
5. Conclusions Tension tests on Al 2024/(3 /10 vol.%) SiC composites showed that the yield strength and ultimate tensile strength increased and the fracture strain decreased due to the addition of SiC. The rule of mixtures could roughly be used to calculate the strength of the composites. Although the strength of the MMCs increased generally with increasing volume fraction of
the reinforcements, the actual increase was less than anticipated due, among other factors, to the presence of clustered particulate. Therefore, a model was proposed to suggest that the strength and fracture toughness of the MMCs could be estimated from the load transfer model approach that takes into consideration the effect of clustering. This model was successful in predicting the strength and fracture toughness values of the composites that were in good agreement with those observed experimentally. It has also been shown that by optimizing the volume fraction of the reinforcement and the clusters, high strength and reasonably good fracture toughness values could be obtained. For the Al 2024/SiC system, the optimum values appear to be 5 /7 vol.% SiC, with a cluster volume of about 15 /20%. Processing methods that produce microstructures with a more uniform distribution of reinforcements could potentially result in composites with improved mechanical properties.
Acknowledgements The work reported in this paper is supported by the US National Science Foundation, Arlington, VA and Korea Science and Engineering Foundation, Taejon, Korea under the US-Korea Cooperative Science Project Grant Award No. INT-9910531. The authors are grateful to Dr. Thomas Klassen of the GKSS Research Center, Geesthacht, Germany, for his technical help and useful discussions.
References [1] M. Taya, R.J. Arsenault, Metal Matrix Composites, Pergamon Press, New York, 1989. [2] T.W. Clyne, P.J. Withers, An Introduction To Metal Matrix Composites, Cambridge University Press, Cambridge, UK, 1993. [3] ASTM Annual Book of Standards, Amer. Soc. Test. Mater., Philadelphia, PA, E-399 (1987) 417 /427. [4] F.M. Hosking, F. Folgar Portillo, R. Wunderlin, R. Mehrabian, J. Mater. Sci. 17 (1982) 477. [5] S.V. Kamat, J.P. Hirth, R. Mehrabian, Acta Metall. 37 (1989) 2395 /2402. [6] Y. Wu, E.J. Lavernia, Scripta Metall. 27 (1992) 173 /178. [7] M. Manoharan, J.J. Lewandowski, Mater. Sci. Eng. A150 (1992) 179 /186. [8] M.F. Ashby, in: A. Kelly, R.B. Nicholson (Eds.), Strengthening Methods in Crystals, Elsevier, Amsterdam, 1971, p. 137. [9] H.J. Kim, T. Kobayashi, H.S. Yoon, Mater. Sci. Eng. A154 (1992) 35 /41. [10] D. Zhao, F.R. Tuler, Eng. Fract. Mech. 47 (1994) 303 /308. [11] G.R. Irwin, J. Appl. Mechanics 24 (1957) 361 /364. [12] G.T. Hahn, A.R. Rosenfield, Metall. Trans. A. 6A (1975) 653 / 667.
Effect of clustering on the mechanical properties of SiC particulatereinforced aluminum alloy 2024 metal matrix composites Soon-Jik Hong a, Hong-Moule Kim a, Dae Huh a, C. Suryanarayana b,*, Byong Sun Chun a a
Rapidly Solidified Materials Research Center (RASOM), Chungnam National University, Taedok Science Town, Taejon 305-764, South Korea b Department of Mechanical, Materials and Aerospace Engineering, University of Central Florida, Orlando, FL 32816-2450, USA Received 20 December 1999; received in revised form 29 July 2002
Abstract Al 2024 /SiC metal matrix composite (MMC) powders produced by centrifugal atomization were hot extruded to investigate the effect of clustering on their mechanical properties. Fracture toughness and tension tests were conducted on specimens reinforced with different volume fractions of SiC. A model was proposed to suggest that the strength of the MMCs could be estimated from the load transfer model approach that takes into consideration the extent of clustering. This model has been successful in predicting the experimentally observed strength and fracture toughness values of the Al 2024 /SiC MMCs. On the basis of experimental observations, it is suggested that the strength of particulate-reinforced MMCs may be calculated from the relation: sy /smVm/sr (Vr/Vc)/srVc, where s and V represent the yield strength and volume fraction, respectively, and the subscripts m, r, and c represent the matrix, reinforcement, and clusters, respectively. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Composites; Clustering; Mechanical properties; Fractography; Al 2024 alloy
1. Introduction Most research on metal matrix composites (MMCs) in recent years has been concentrated on the development of high performance continuous fiber-reinforced composites for specialized applications. In spite of their unique properties, such composites are expensive. Therefore, development of less-expensive composites for non-critical applications is desirable. Particulatereinforced MMCs are cost-effective alternatives and have the advantage of being machinable and workable using conventional processing methods. However, their poor mechanical properties such as low fracture strain and fracture toughness, which are important for the design of structural materials, have limited their widespread applications. One of the major areas of research in MMCs has been to study how the reinforcement
* Corresponding author. Tel.: /1-407-823-6662; fax: /1-407-8230208 E-mail address: [email protected] (C. Suryanarayana).
phase affects the failure mechanism(s) and hence controls the fracture toughness of these materials. A major potential problem in the application of SiC/Al composites is the possibility that the reaction between Al and SiC produces Al4C3, which weakens the interface depending on the temperature, environment, and other parameters [1]. The second problem of SiC/Al composites is that the microstructure is non-uniform with elongated clusters of particles present along the extrusion direction. The most significant detrimental property change may be the decrease in ductility and fracture toughness, which is true for all the aluminum MMCs and process histories; a major obstacle preventing their extensive use. The mechanism of reinforcement affecting the fracture toughness of MMCs is not well understood. Several models have been proposed to characterize the relationship between fracture toughness and microstructure [2]. The principal objective of this current investigation was to determine how the microstructural parameters, such as the particulate volume fraction and extent of clustering affect the tensile properties and fracture toughness of Al 2024 /SiC MMCs. Another objective
0921-5093/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 2 ) 0 0 5 9 3 - 2
S.-J. Hong et al. / Materials Science and Engineering A347 (2003) 198 /204
was to shed more light on the failure mechanisms operative in the composites so as to aid in the correlation between microstructure and properties of such materials.
2. Experimental procedure The material used is an Al 2024 alloy reinforced with 3, 5, 7, or 10 vol.% of 10 mm diameter SiC particles. In producing the material, the Al 2024 alloy was melted and mixed with SiC powder by a stirrer. The mixture was then poured through a centrifugal atomizer to produce the atomized composite powder. The atomized powders were compacted to 95% of theoretical density and then degassed in a vacuum chamber for 1 h at an elevated temperature of 673 K. The billet was then subjected to hot extrusion at 673 K with an extrusion ratio of 13:1. Tensile tests were performed using an Instron mechanical testing machine to obtain the basic mechanical properties including 0.2% offset yield strength sy, ultimate tensile strength su, and fracture strain o f. The load was measured by a 5000 kg load cell and the deformation of the specimen was measured by a 2.54 cm extensometer. The specimens were loaded at a constant crosshead speed of 10 3cm s 1 until failure. The pure mode I fracture toughness, KIQ of the composites was measured by three point bending tests performed on an MTS servohydraulic machine. The specimen dimensions are shown in Fig. 1. The KIQ value was calculated using the equation: KIQ (PQ =BW 1=2 )f (a=W )
(1)
where PQ is the load where the initial drop in the load occurs during the compact tension test, B is the specimen thickness, W is the specimen width, and a is the crack length. f(a /W) is a complex function of a/W and its values for different values of a/W are listed in the ASTM specification E-399 [3]. This equation, established on the basis of linear elastic fracture mechanics, is valid when a sharp crack of adequate size exists in a specimen. This condition is accomplished by fatigue precracking the specimens with a stress intensity level of less than 60% of KIQ at the final stage. The specimen was then loaded with a crosshead speed of 0.01 mm s 1 until
Fig. 1. Dimensions of the fracture test specimen.
199
the load-displacement curve deviated from linearity. Within this part a 95% secant line could be drawn to calculate the KIQ value. Fractographs of all the broken specimens tested under tension and three-point bending conditions were recorded using a JSM-6400 scanning electron microscope (SEM). A computerized image analyzer was used to obtain the distribution and volume fraction of SiC particulates/clusters in the as-processed condition.
3. Results 3.1. Tensile properties The composites produced exhibited a heterogeneous microstructure, with elongated clusters of particles forming along the extrusion direction, with the local volume of these clusters varying between 16% for 3 vol.% SiC and 32% for 10 vol.% SiC. However, it should be noted that the vol.% of the clusters has not increased regularly with increasing SiC content. As shown in Fig. 2, the vol.% of the clusters reached a minimum of 12% for the addition of 5 vol.% of SiC. Thus, the minimum extent of clustering seems to be obtained for 5 vol.% SiC. The microstructural features of the starting materials, in the form of extrusions, are listed in Table 1. These composites have excellent bonding between the SiC particulates and the aluminum alloy matrix, assumed to be associated with the formation of Al4C3, Al4SiC4, and CuAl2O4 [4] as intermediate phases at the interface. The 0.2% offset yield strength sy, ultimate tensile strength su, and fracture strain o f were calculated from the stress /strain curves, and these data are listed in Table 2. Each value listed is an average of at least two measurements, with a mean deviation for all tests of 2.7, 1.8, and 4.0% for sy, su, and o f, respectively. For comparison purposes, these mechanical properties for the Al 2024 matrix alloy also are included in Table 2 [5]. Two significant features of the stress /strain curve are that there is no necking before failure and that the
Fig. 2. Variation of the volume percentage of clusters as a function of the SiC content.
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Table 1 Specifications of the composites investigated Material
Volume % of cluster
Aspect ratio of the cluster (particle)
Particle thickness (mm)
Al Al Al Al
16 12 20 32
7.0 (1.5)
10
2024 /3 vol.% SiC 2024 /5 vol.% SiC 2024 /7 vol.% SiC 2024 /10 vol.% SiC
on the tensile properties of the composites. As shown in Fig. 3, the yield strength and ultimate tensile strength started to decrease at higher SiC contents, mostly due to the increased amount of clustering. This seems to be especially true for the composite containing 10 vol.% of SiC when the strength is expected to be much higher than at lower SiC levels. But, it is lower due to the increased amount of clustering.
fracture strain of the composites is much lower than that of the Al 2024 matrix. While the fracture strain was 0.21 for the un-reinforced Al 2024 alloy, it had decreased to 0.03 when 5 vol.% of SiC was added to it. The fracture strains for the composites ranged from 3.2 to 7.0% suggesting that the composite materials are semi-brittle and that only moderate plasticity occurs before failure. As expected, the yield strength of all the composites has significantly increased for all the composite specimens, with a 3 / increase over Al 2024 for the composites with ]/5 vol.% SiC. The volume fraction of the SiC particulates was found to have a significant effect on the tensile properties of the composites; both sy and su started to decrease beyond about 7 vol.% of SiC. Fig. 3 shows the variation of the 0.2% offset yield strength and ultimate tensile strength with the vol.% of SiC, for a constant particulate size of 10 mm. This figure clearly shows that the amount of the reinforcement plays a significant role in controlling the tensile properties of the composites. It is also important to note that the SiC particulate clusters also have a significant effect
3.2. Fracture toughness results The mode I stress intensity factor analysis was performed according to ASTM E-399. The KIQ values were calculated using Eq. (1), and the calculated values are listed in Table 2. Each value listed is an average of two measurements. The average spread of the two values was about 2%. For most cases KIQ satisfied the condition for the applicability of linear elastic fracture mechanics, viz., presence of a sharp crack of adequate size. Fig. 4 shows the effect of the vol.% of SiC (and the vol.% of clusters) on the conditional mode I fracture toughness (KIQ) indicating that KIQ decreases with
Fig. 3. Variation of yield strength and ultimate tensile strength of the composites as a function of the volume percentage of SiC, for a constant particulate size of 10 mm.
Fig. 4. Mode I fracture toughness and vol.% of clusters of the Al 2024 /SiC composites as a function of the volume percentage of SiC.
Table 2 Tensile properties and fracture toughness of the composites Material
sy (MPa)
su (MPa)
of
Crack length (mm)
KIQ (MPa m1/2)
Al Al Al Al Al
75 175 214 221 210
185 264 320 374 308
0.210 0.051 0.032 0.070 0.066
/ 14.97 14.75 14.58 15.14
/ 20.16 19.02 18.42 14.67
2024 2024 /3 vol.% SiC 2024 /5 vol.% SiC 2024 /7 vol.% SiC 2024 /10 vol.% SiC
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increasing volume fraction of SiC. As expected, the fracture toughness decreased with an increase in the vol.% of SiC and hence the tensile strength of the samples. A similar correlation between the tensile properties and fracture toughness was reported earlier for Al /6Si composites reinforced with alumina particulates [6]. It may also be noted from Fig. 4 that the decrease of conditional mode I fracture toughness with increasing cluster vol.% was minimal up to about 7 vol.% SiC (when the vol.% of the clusters was about 20%), but beyond this value, the fracture toughness decreased significantly. For example, for the Al 2024 /10 vol.% SiC composite, the fracture toughness was only 14.7 MPaâm; associated with an increased strength due to SiC and decreased strength due to clustering. The tensile strength could have been higher and the fracture toughness lower, if the clustering was not there.
3.3. Fractography
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surfaces have also cracked near the interface. When examined at higher magnifications, the fractured SiC particles show a clear crack running through the center of the particle as shown in Fig. 6. Fig. 7 shows a region of clustered SiC particles on the fracture surface where damage accumulation ahead of the crack tends to occur more easily. A few secondary branches of the main crack were observed on the fracture surface of the specimen and this was confirmed by viewing the region in a direction perpendicular to the fracture surface.
4. Discussion 4.1. Strength In predicting the strength of the particulate-reinforced MMCs, the simplest load flow stress model might be the rule of mixtures: sy sm Vm sr Vr
(2)
Even though the composites did not exhibit much ductility on a macroscopic scale (Table 2), SEM fractographs indicated that the fracture occurred by a locally ductile mechanism. Typical fracture surfaces (Fig. 5) consisted of a bimodal distribution of dimples*/larger dimples associated with the SiC particulates and smaller dimples associated with the ductile failure of the Al 2024 alloy matrix. Similar observations have also been reported earlier by other investigators [5 /7]. In most cases the larger dimples contained SiC particles and were about the same size as the particles responsible for their formation. Decohesion from the matrix initiates at the matrix/SiC interface. The edges of the dimples around SiC particles are elongated and a small fraction of the SiC particles on the fracture
where sy is the predicted yield strength, sm and sr are the individual yield strengths of the matrix and reinforcement, respectively, and Vm and Vr are the volume fractions of the matrix and the reinforcement, respectively. The proportion of the external load borne by the individual constituents can be estimated by volume averaging the load, and the external applied load will then be equal to the sum of the volume averaged loads [2]. This model has been very successful in predicting the elastic modulus of the MMCs, but is only moderately successful for strength predictions. The second model would be related to dispersion hardening, with plastic incompatibility, that can be expressed, for example, by the Ashby geometrically necessary dislocation concept [8], leading to excess dislocations in the near-particle region. The plastic-
Fig. 5. Scanning electron micrograph showing a typical fracture surface. One can notice large dimples associated with the SiC particles and small dimples related to the ductile fracture of the matrix.
Fig. 6. A magnified scanning electron micrograph showing fractured SiC particles.
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Fig. 7. Scanning electron micrograph showing clustering of SiC particulates in the composite.
deformation-induced dislocations would become dominant when the plastic strain exceeds the thermal mismatch strain. The two effects would act in parallel and may be combined. Wu and Lavernia [6] combined the effects of load transfer and matrix strengthening to the Al /4Si/TiB2 system, and obtained relatively good agreement with the experimental results. However, for the relatively larger particle size as obtained in the present work, the single dislocation, Orowan-type hardening would be completely negligible. Hence, we tried to use the simplest load transfer model to explain the results. Use of the load transfer model to calculate the yield strength and tensile strength (3.5 /4.0 GPa for SiC) yields results that are significantly different from the experimental data. The calculations are compared with the experimental results of strength versus vol.% of SiC in Fig. 8. It may be noted that the calculated strengths do not agree with the experimentally obtained values, especially at higher SiC contents. While the calculated values increase continuously with the vol.% of SiC, the
Fig. 8. Comparison of experimental and calculated strength values as a function of the volume percentage of SiC in the composites.
experimental results show a drop beyond about 7 vol.% SiC. In view of this discrepancy, a modified strengthening model is developed in the present work, taking clustering into consideration. It is known that the extrusion process produces elongated clusters of particles in the extrusion direction [6,9]. Further, the distribution of these clusters is also non-uniform. As can be seen in Fig. 9, these clustered particles are the sites for damage accumulation ahead of the crack. Cluster fracture is the dominant mode of fracture, with the clusters perpendicular to the loading direction. Cluster fracture occurs at an early stage of the tensile loading process, and cracks in the fractured clusters grow to final fracture [6,10]. Thus, clustering of the reinforcement in the composite makes a negative contribution to the strength of the particulate-reinforced MMCs. Therefore, we suggest that the strength of particulate reinforced MMCs may be calculated from the relation: sy sm Vm sr (Vr Vc )sr Vc
(3)
where Vc is the volume fraction of the clusters. Accordingly, as a simplified model, the strength of the MMCs can be estimated from the load transfer model taking clustering into consideration. Expressed differently, Eq. (3) can be rewritten as: sy sm Vm sr Vr 2sr Vc
(4)
Considering V as the total volume of the composite, it can be written down as V /Vm/Vr. If P is the load borne by an individual component, then the following expressions can be set up: Vm (1Pr =100)V ; Vr (Pr =100)V ; Vc (Pr Pc =1002 )V :
Fig. 9. Fracture surface of the Al 2024 /SiC composite.
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Then, Eq. (4) can be expressed as: sy sm (1Pr =100)(sr Pr =100)(12Pc =100)
(5)
where Pm /100Vm/V , Pr 100Pm ; and Pc 100Vc =Vr : The strength values calculated using Eq. (5) are compared with the experimental results in Fig. 10. As can be seen, reasonably good agreement is noticed with the experimental results. 4.2. Fracture Reinforcements contribute to failure by particle cracking, cluster cracking, interface cracking, and matrix/particle debonding. Failure also results from matrix damage, like void formation and matrix cracking. On the fracture surface, ductile dimples and particles in the voids are commonly observed. As indicated earlier, the predominant fracture mode of particulate-reinforced MMCs is particle and cluster cracking, which occurs at an early stage of loading. Since the ductility of the composites is low, it is reasonable to assume that the cracks through the fractured clusters and particles obey the fracture mechanics approach, and have a plane strain plastic zone [10,11]. The plastic zone size increases with increasing load, and failure of the composite occurs when the plastic zones of adjacent cracks coalesce. At the same time, the stress intensity factor KIQ reaches the plane strain fracture toughness of the composite KIC. Hahn and Rosenfield [12] calculated the fracture toughness of Al alloys with large Fe- or Si-rich inclusions in terms of the volume fraction and proposed that KIC 8/Vr1/6. The variation of KIQ with Vr suggests that
Fig. 10. Comparison of the experimental and calculated (according to Eq. (5)) strength values as a function of the volume percentage of SiC in the composites.
203
the toughness of the composites is limited by Vr as suggested elsewhere [10]. This leads to the following equation: KIQ sY [3pt(rr =Vr )1=2 ]1=2
(6)
where rr is the aspect ratio of the SiC particulate, t is the thickness and sY is the effective yield strength of the matrix calculated as sY /(sy/su)/2 which turns out to be 219 and 297 MPa, for the 2024/3 vol.% SiC and the Al 2024 /7 vol.% SiC composites, respectively. Based on this model, the fracture toughness calculated using Eq. (6) is compared with the experimental results as a function of the vol.% of SiC in Fig. 11. Whereas the experimental results show that the fracture toughness decreases with increasing vol.% of SiC, the calculated values do not show a continuous decrease; the fracture toughness increases slightly up to about 7 vol.% SiC, and then starts to decrease. Further, the experimental values are about four times as much as the values calculated from Eq. (6). In view of this discrepancy, a modified model is developed in the present work, taking the clusters into consideration. It may be safely assumed that fracture preferentially initiates in the clustered reinforcements in particulate-reinforced MMCs. Metallographic and fractographic analyses confirm that clustered regions were the preferred sites for damage initiation during deformation in an aluminum alloy reinforced with SiC particles. It is shown that the micromechanisms of fracture are significantly influenced by the degree of clustering in the composites. Therefore, on the basis of experimental observations, we suggest that the fracture toughness of the composites in particulate-reinforced MMCs may be calculated by introducing the volume fraction of clusters into Eq. (6) and obtaining: KIQ sY [(3pt)f(rr =Vr )(rc =Vc )g1=2 ]1=2
(7)
Fig. 11. Comparison of the experimental and calculated (according to Eqs. (6) and (7)) fracture toughness values as a function of the volume percentage of SiC in the composites.
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where Vc is the volume fraction of SiC particulate clusters and rc is the aspect ratio of the cluster. The results obtained using Eq. (7) are compared with the experimental results in Fig. 11. This model successfully predicts the fracture toughness of MMCs, although the calculated fracture toughness values are not always exactly the same as those observed experimentally. The experimental and the calculated results show that the fracture toughness decreases with increasing volume fraction of the particulates and clusters. Since this model assumes that all the particulates and clusters will crack or debond ahead of the crack tip, which has fractographic evidence to support it, the optimum structure would be a more uniform particulate distribution. The desired optimum cluster volume fraction could be achieved by decreasing the volume fraction of the reinforcement or increasing the particle size up to a limiting value. 4.3. Crack path In the present experiments, although the cracks exhibited a random total path, damage accumulation was observed to occur in the highly clustered regions along parts of the fracture path. The selection of damage accumulation in locally clustered regions showed that the gross fracture path was essentially random but may be related to the orientation and volume fraction of the particulate along the total crack path. It was observed that alternating ‘bands’ of high and low volume fraction reinforcement regions are present in the direction of crack propagation. This requires that the crack passes through some low cluster volume fraction regions in order to progress. The growth of the crack could be controlled due to the presence of a compliant layer on the specimen. The present work further illustrates that the examination of fractured surfaces alone may be insufficient to determine whether fracture is initiating or propagating along a path of clustered particles. Thus, as an extension of this work, subsequent work will focus on the relationship between clustered regions and damage accumulation to evaluate the effects of changes in clustering on the fracture properties.
5. Conclusions Tension tests on Al 2024/(3 /10 vol.%) SiC composites showed that the yield strength and ultimate tensile strength increased and the fracture strain decreased due to the addition of SiC. The rule of mixtures could roughly be used to calculate the strength of the composites. Although the strength of the MMCs increased generally with increasing volume fraction of
the reinforcements, the actual increase was less than anticipated due, among other factors, to the presence of clustered particulate. Therefore, a model was proposed to suggest that the strength and fracture toughness of the MMCs could be estimated from the load transfer model approach that takes into consideration the effect of clustering. This model was successful in predicting the strength and fracture toughness values of the composites that were in good agreement with those observed experimentally. It has also been shown that by optimizing the volume fraction of the reinforcement and the clusters, high strength and reasonably good fracture toughness values could be obtained. For the Al 2024/SiC system, the optimum values appear to be 5 /7 vol.% SiC, with a cluster volume of about 15 /20%. Processing methods that produce microstructures with a more uniform distribution of reinforcements could potentially result in composites with improved mechanical properties.
Acknowledgements The work reported in this paper is supported by the US National Science Foundation, Arlington, VA and Korea Science and Engineering Foundation, Taejon, Korea under the US-Korea Cooperative Science Project Grant Award No. INT-9910531. The authors are grateful to Dr. Thomas Klassen of the GKSS Research Center, Geesthacht, Germany, for his technical help and useful discussions.
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