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In situ transmission electron microscopy observation of the initiation and growth of microcracks in an SiC whiskerAl composite
Materials Science and Engineering, A189 (1994) 235-239
235
In situ transmission electron microscopy observation of the initiation
and growth of microcracks in an SiC whisker-A1 composite Z. R. Liu, D. Z. Wang, C. K. Yao, J. Liu and L. Geng Centre for Metal Matrix Composites, College of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001 (People ~ Republic of China) (Received l)ccember 30, 1993: in revised form March 15, 1994]
Abstract The initiation and growth of microcracks in a SiC whisker (SiC,,)-AI composite have been studied by means of the transmission electron microscope equipped with a tensile stage. It was found that microcrack initiation sources mainly included (1) the whiskers damaged and/or fractured during the preparation of the composite, (2) uninfiltrated zones, (3) SiC,-AI interfaces and (4) the matrix. Only the microcrack that nucleated in the matrix grew to become the major crack leading to specimen failure although many microcracks occurred in the early stages of straining. The whiskers and grain boundaries in the composite played an important role in impeding the growth of the cracks. When a crack was perpendicular to the frontal whisker, the crack tip would blunt near the longitudinal interface and it then advanced by bypassing the whisker end; when a crack was parallel to the frontal whisker, the crack would blunt near the whisker end and then propagated along the near-interface region of one side of this whisker.
1. Introduction SiC whisker (SiCw)-A1 composites have attracted a great deal of interest in the aerospace industry for their unique combination of properties such as light weight, high stiffness and high strength. However, these materials also exhibit low ductility and fracture toughness which limit their use in many structural applications. Hence, it is very important to study the deformation behaviour and the mechanism of crack initiation and growth in SiCw-A1 composites. The available results show that the deformation of SiCw-AI composites is very localized, and the level of localization increases with increasing volume fraction of SiCw [1]. Generally, crack initiation sources include whisker defects [2], inclusions [3], intermetallics [4] and interfaces [3]. Whiskers in the composites are not only a crack initiation source but also a main type of obstacle to the growth of cracks [5]. Unfortunately, previous work was mainly based on the result of scanning electron microscopy (SEM) observation, and the details of the effects of whiskers and grain boundaries in the composites on their deformation and fracture could hardly be observed nor could the dislocation activity during the deformation be detected. The aim of this paper is to investigate the deformation behaviour and the initiation and growth of microcracks in an SiCw-A1 composite using trans(1921-5093/94/$7.0(1 SSDI (1921-5093(94)09580-P
mission electron microscopy (TEM) straining techniques.
2. Experimental methods The SiCw-A1 composite was fabricated by a squeeze-casting method. The volume fraction of SiC~ is 19% and the matrix is commercial pure (99.75%) A1. The as-cast composite was hot extruded to a plate at 480 °C with an extrusion ratio of 1 to 27. Specimens were cut out of the plate parallel to the extrusion direction. After mechanical polishing, they were dimpled and ion milled. The specimens were then strained on the tensile stage in the TEM. The deformation and the initiation and growth of microcracks in the composite were observed in situ.
3. Experimental results
3.1. Observation of the dislocations formed during the straining process The dislocation density in the matrix of the composite before the straining (Fig. l(a)) is far lower than that after the straining (Figs. l(b) and l(c)). Dislocations are generated at grain boundaries and at SiCw-AI interfaces in the composite during the straining process. It © 1994 - Elsevier Sequoia. All rights reserved
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ln-situ TEM ofmicrocracks in SiC,,-AI
Fig. 1. A dislocation before and after straining of the specimen: (a) low dislocation density in the matrix before the straining; (b) dislocation pile-up at grain boundaries after straining; (c) dislocation pile-up around whiskers after straining (w, whisker).
Fig. 2. Microcrack (mc) initiation sources: (a) fracture of whisker; (b) uninfiltrated zone; (c) debonding of SiCw-A1 interface; (d) cracking of marix.
Fig. 3. Interaction between a crack and a grain boundary: (b), (c) further propagation of the crack in (a). can be observed that a dislocation starts at one interface (or grain boundary), then slips to another interface (or grain boundary) and finally will be impeded by the nearby whisker, forming a dislocation pile-up. A great number of dislocation pile-ups occur at the grain boundaries and around the whiskers. 3.2. Initiation of microcracks The experimental results indicate that the initiation of microcracks can be caused by the fracture of whiskers, the uninfiltrated zones, the debonding of
SiCw-A1 interfaces and the crack of the matrix, as shown in Figs. 2(a), 2(b), 2(c) and 2(d) respectively. It is found that only the microcrack caused by matrix cracking can propagate and form the major crack which will lead to fracture of the specimen. 3.3. Growth of microcracks Microcracks, especially those in the matrix, will propagate when the applied stress increases to a critical value. Figure 3 shows a crack tip meeting a grain boundary parallel to the applied stress axis. Firstly the
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ln-situ TEM o[microcracks in SiC,,-Al
crack tip is blunted at the grain boundary (Fig. 3(a)). Then a new microcrack nucleates in the grain on the right-hand side of the grain boundary. Finally the microcrack advances continuously, resulting in fracture of the grain boundary (Fig. 3(c)). In addition, a crack tip blunted at a grain boundary can also propagate along the grain boundary.
Fig. 4. A crack is blunted near the interface and then advances by bypassing the whisker end. The crack propagates by linking the major crack and nearby microcracks.
When a crack meets a whisker which is perpendicular to the crack, it blunts near the interface and then advances by bypassing the whisker end (Fig. 4). Figure 5 shows that a crack in the thicker region of the specimen is inhibited by a whisker which is parallel to the crack. It is blunted near the whisker end and then propagates along the two sides of the whisker, forming a branched crack. Finally, the crack advances along the weaker side (the top side of Fig. 5(b)) as the applied load reaches a critical value. The major crack advances by crack linkage (Fig. 4). In the early stages of crack propagation, a film of aluminium can often be found adhering to the exposed whisker. The main crack propagates mainly in the matrix near the SiCw-A1 interfaces, namely the nearinterface region. In the late stages approaching fracture, however, the quantity of aluminium adhering to the exposed whiskers decreases (Fig. 6). In addition, Fig. 6 shows that the matrix in the fractured surface layer is ruptured to thin needles or sharp edges during the crack propagation. When a crack approaches perpendicular to the frontal whisker, the whisker will be exposed on the crack path by bypassing of the crack (Fig. 6(a)). If a crack tip is parallel to the frontal whisker, the crack will
Fig. 5. Interaction between a crack and a whisker: (b) further propagation of the crack in (a).
1.0prn Fig. 6. Micrographs of the fractured surface.
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In-situ TEM of microcracks in SiCw-AI
advance along only one side of the whisker (whisker a in Fig. 6(b)). Along the crack path, the quantity of the exposed whiskers is far smaller than the average volume fraction of SiCw in the composite.
4. Discussion 4.1. Deformation characteristic
The deformation in the SiCw-A1 composite is heterogeneous. Dislocations are generated mainly at the SiCw-A1 interfaces and at the grain boundaries. This indicates that the SiCw-AI interfaces will deform first during the straining process. This kind of deformation cannot spread out beyond the nearby whiskers because they impede deformation. In addition, as reported in ref. 1, the deformation of SiCw-AI composite is very localized because of the inhomogeneous distribution of the reinforcement SiCw. 4.2. Microcrack initiation sources
It can be deduced from previous work that microcracks are generally initiated at whisker ends owing to the thermal residual stress in the composites. Since previous work was mainly based on SEM observations, it cannot explain the fracture mechanism of the composites in detail. The present work, which is inconsistent with the work of Doong et al. [6], indicates that some SiC whiskers will fracture in the early stages of straining because of the existence of whiskers cracked or damaged during the processes of squeeze casting and extrusion. These fractured whiskers have been kept in a closed state before straining because of their thermal residual compressive stress. When the specimen has an applied stress, even the applied stress is far lower than the fracture strength of the whisker, the damaged whiskers will also crack and the previously fractured whiskers will open up to become microcracks. In whisker-rich regions, the interfaces (especially the whisker ends) bearing a larger shear stress [7] can be easily debonded to form microcracks owing to the larger localized deformation in the regions [1] and the mutual deformation constraint between the whiskers and the matrix. The uninfiltrated zone which occurred during the squeeze-casting process is a microcrack itself. Moreover, microcracks are easier to initiate in the matrix in the whisker-rich regions since the deformation of the matrix here is always larger than that outside these regions. 4.3. The growth of microcracks
Only the microcrack which has nucleated in the matrix grows to become a major crack, which will lead to the specimen failure, although many microcracks occur in the early stages of straining. The whiskers are
under a compressive state and the matrix in the vicinity of whiskers has a tensile stress owing to the large thermal residual stress in the composite [8]. The initiation of microcracks caused by the fracture of the whiskers or the debonding of the SiCw-AI interfaces will relax the thermal residual tensile stress. On the contrary, in the region between the whiskers the matrix contains a lower dislocation density than adjacent to the SiCw, i.e. in this region the matrix is weaker. When the tensile stress exceeds the fracture strength of the matrix in front of a microcrack tip, the microcrack which has nucleated in the matrix will propagate and form the major crack, leading to fracture of the specimen. The grain boundaries and SiCw in the SiCw-AI composite play important roles in impeding crack propagation. When a crack meets a grain boundary which is perpendicular to the crack, the crack tip is blunted because the grain boundary impedes it. In the meantime, a new microcrack will be nucleated on the right-hand side of the grain boundary by the stress concentration in front of the crack tip. Then the new microcrack advances. Grain boundaries in the composite are obstacles to the growth of cracks. In the early propagation stages of cracks, a film of aluminium can always be found adhering to the exposed whiskers along the crack edge. This indicates that the bonding strengh of the SiCw-A1 interfaces in the composite made by the squeeze-casting method is higher than the yield stress of the matrix, consistent with the previous observation of Arsenault and Flom [9]. In addition, the temperaure change during the preparation of the composite together with the large difference between the coefficients of thermal expansion (CTEs) of SiC and A1 will give rise to a CTE mismatch strain which will induce a high dislocation density in the near-interface region. When a crack approaches near a whisker, the matrix near the nearinterface region will plastically deform and the crack will advance along the near-interface region. The matrix in the fractured surface layer is ruptured to thin needles or sharp edges during the crack propagation. The characteristic of the plastic deforamtion for the matrix in the SiCw-AI composite is similar to that of pure aluminium. This indicates that only the plastic deformation of the matrix in the near-interface region will be limited by the whiskers if the matrix is easily deformable. For the above reasons, when a crack is parallel to the frontal whisker, the crack will propagate along the near-interface region of one side of the whisker. If a crack meets a whisker which is perpendicular to the crack, the crack will advance by bypassing the whisker. The exposed whiskers on the fractured surface cannot be pulled out. The crack will bypass the whisker which
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ln-situ TEM ()f microcracks in SiC~,.-AI
is perpendicular to the crack because the stress concentration in the region in front of the crack tip, which can be relaxed more easily owing to the plastic deformation of the matrix, is not sufficient to crack the matrix on the other side of the whisker. Since the major crack advances mainly in a path through the matrix and along the near-interface regions, the number of exposed whiskers on the fractured surface is far lower than the average volume fraction of SiC whiskers in the composite.
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strain. Only the microcrack caused by the matrix cracking can advance to form the major crack. (3) The whiskers and the grain boundaries in the composite play an important role in impeding the growth of cracks. The major crack propagates mainly in the matrix and along the near-interface regions, and a film of aluminium will be left on the whiskers exposing along the crack path. The interfaces in the SiCw-A1 composite fabricated by the squeeze-casting method is high strength bonded.
5. Conclusions (1) During the straining, dislocations are generated at grain boundaries and a SiCw-Al interfaces. The spread of dislocations will be limited by the nearby whiskers. The deformation in the composite is very localized. (2) There exist four crack initiation sources in the SiCw-AI composite fabricated by the squeeze-casting method. The damaged and/or fractured whiskers caused during the fabrication of the composite will result in microcracks. Microcracks can also be initiated by the debonding of SiCw-A1 interfaces. The uninfiltrated zone is a kind of microcrack itself. In addition, microcracks can easily develop in the matrix in the whisker-rich regions since the matrix there has a larger
References 1 R.J. Arsenault and N. Shi, Mater. Sci. Eng., AI31 ( 1991 ) 55. 2 S. B. Wu and R. J. Arsenault, Matep: Sci. Eng., A 138(1991) 227. 3 M. Manoharan and J. J. Lcwandowski, Mater. Sci. Eng., A 150 (1992) 179. 4 D.L. Davidson, Metall. Trans. A, 22 ( 1991 ) 1 l 3. 5 N. Sato and T. Kurauchi, J. Compos. Mater., 22 (1988) 850. 6 S. H. Doong, T. C. Lee, I. M. Robertson and H. K. Birnbaum, Scr. Memll., 23(1989) 1413. 7 C. R Jiang and L. Cao, ('hin. ,1. Met. Sci. Teclmol., 5 (1989) 43(/. 8 E. U. Lee, Memll. Trans. A, 23(1992) 2205. 9 R. J. Arscnault and Y. Flom, Strucure and D@~rmation of Boundaries, Metallurgical Society of AIME, Warrendale, PA, 1986, p. 261.
235
In situ transmission electron microscopy observation of the initiation
and growth of microcracks in an SiC whisker-A1 composite Z. R. Liu, D. Z. Wang, C. K. Yao, J. Liu and L. Geng Centre for Metal Matrix Composites, College of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001 (People ~ Republic of China) (Received l)ccember 30, 1993: in revised form March 15, 1994]
Abstract The initiation and growth of microcracks in a SiC whisker (SiC,,)-AI composite have been studied by means of the transmission electron microscope equipped with a tensile stage. It was found that microcrack initiation sources mainly included (1) the whiskers damaged and/or fractured during the preparation of the composite, (2) uninfiltrated zones, (3) SiC,-AI interfaces and (4) the matrix. Only the microcrack that nucleated in the matrix grew to become the major crack leading to specimen failure although many microcracks occurred in the early stages of straining. The whiskers and grain boundaries in the composite played an important role in impeding the growth of the cracks. When a crack was perpendicular to the frontal whisker, the crack tip would blunt near the longitudinal interface and it then advanced by bypassing the whisker end; when a crack was parallel to the frontal whisker, the crack would blunt near the whisker end and then propagated along the near-interface region of one side of this whisker.
1. Introduction SiC whisker (SiCw)-A1 composites have attracted a great deal of interest in the aerospace industry for their unique combination of properties such as light weight, high stiffness and high strength. However, these materials also exhibit low ductility and fracture toughness which limit their use in many structural applications. Hence, it is very important to study the deformation behaviour and the mechanism of crack initiation and growth in SiCw-A1 composites. The available results show that the deformation of SiCw-AI composites is very localized, and the level of localization increases with increasing volume fraction of SiCw [1]. Generally, crack initiation sources include whisker defects [2], inclusions [3], intermetallics [4] and interfaces [3]. Whiskers in the composites are not only a crack initiation source but also a main type of obstacle to the growth of cracks [5]. Unfortunately, previous work was mainly based on the result of scanning electron microscopy (SEM) observation, and the details of the effects of whiskers and grain boundaries in the composites on their deformation and fracture could hardly be observed nor could the dislocation activity during the deformation be detected. The aim of this paper is to investigate the deformation behaviour and the initiation and growth of microcracks in an SiCw-A1 composite using trans(1921-5093/94/$7.0(1 SSDI (1921-5093(94)09580-P
mission electron microscopy (TEM) straining techniques.
2. Experimental methods The SiCw-A1 composite was fabricated by a squeeze-casting method. The volume fraction of SiC~ is 19% and the matrix is commercial pure (99.75%) A1. The as-cast composite was hot extruded to a plate at 480 °C with an extrusion ratio of 1 to 27. Specimens were cut out of the plate parallel to the extrusion direction. After mechanical polishing, they were dimpled and ion milled. The specimens were then strained on the tensile stage in the TEM. The deformation and the initiation and growth of microcracks in the composite were observed in situ.
3. Experimental results
3.1. Observation of the dislocations formed during the straining process The dislocation density in the matrix of the composite before the straining (Fig. l(a)) is far lower than that after the straining (Figs. l(b) and l(c)). Dislocations are generated at grain boundaries and at SiCw-AI interfaces in the composite during the straining process. It © 1994 - Elsevier Sequoia. All rights reserved
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ln-situ TEM ofmicrocracks in SiC,,-AI
Fig. 1. A dislocation before and after straining of the specimen: (a) low dislocation density in the matrix before the straining; (b) dislocation pile-up at grain boundaries after straining; (c) dislocation pile-up around whiskers after straining (w, whisker).
Fig. 2. Microcrack (mc) initiation sources: (a) fracture of whisker; (b) uninfiltrated zone; (c) debonding of SiCw-A1 interface; (d) cracking of marix.
Fig. 3. Interaction between a crack and a grain boundary: (b), (c) further propagation of the crack in (a). can be observed that a dislocation starts at one interface (or grain boundary), then slips to another interface (or grain boundary) and finally will be impeded by the nearby whisker, forming a dislocation pile-up. A great number of dislocation pile-ups occur at the grain boundaries and around the whiskers. 3.2. Initiation of microcracks The experimental results indicate that the initiation of microcracks can be caused by the fracture of whiskers, the uninfiltrated zones, the debonding of
SiCw-A1 interfaces and the crack of the matrix, as shown in Figs. 2(a), 2(b), 2(c) and 2(d) respectively. It is found that only the microcrack caused by matrix cracking can propagate and form the major crack which will lead to fracture of the specimen. 3.3. Growth of microcracks Microcracks, especially those in the matrix, will propagate when the applied stress increases to a critical value. Figure 3 shows a crack tip meeting a grain boundary parallel to the applied stress axis. Firstly the
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ln-situ TEM o[microcracks in SiC,,-Al
crack tip is blunted at the grain boundary (Fig. 3(a)). Then a new microcrack nucleates in the grain on the right-hand side of the grain boundary. Finally the microcrack advances continuously, resulting in fracture of the grain boundary (Fig. 3(c)). In addition, a crack tip blunted at a grain boundary can also propagate along the grain boundary.
Fig. 4. A crack is blunted near the interface and then advances by bypassing the whisker end. The crack propagates by linking the major crack and nearby microcracks.
When a crack meets a whisker which is perpendicular to the crack, it blunts near the interface and then advances by bypassing the whisker end (Fig. 4). Figure 5 shows that a crack in the thicker region of the specimen is inhibited by a whisker which is parallel to the crack. It is blunted near the whisker end and then propagates along the two sides of the whisker, forming a branched crack. Finally, the crack advances along the weaker side (the top side of Fig. 5(b)) as the applied load reaches a critical value. The major crack advances by crack linkage (Fig. 4). In the early stages of crack propagation, a film of aluminium can often be found adhering to the exposed whisker. The main crack propagates mainly in the matrix near the SiCw-A1 interfaces, namely the nearinterface region. In the late stages approaching fracture, however, the quantity of aluminium adhering to the exposed whiskers decreases (Fig. 6). In addition, Fig. 6 shows that the matrix in the fractured surface layer is ruptured to thin needles or sharp edges during the crack propagation. When a crack approaches perpendicular to the frontal whisker, the whisker will be exposed on the crack path by bypassing of the crack (Fig. 6(a)). If a crack tip is parallel to the frontal whisker, the crack will
Fig. 5. Interaction between a crack and a whisker: (b) further propagation of the crack in (a).
1.0prn Fig. 6. Micrographs of the fractured surface.
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In-situ TEM of microcracks in SiCw-AI
advance along only one side of the whisker (whisker a in Fig. 6(b)). Along the crack path, the quantity of the exposed whiskers is far smaller than the average volume fraction of SiCw in the composite.
4. Discussion 4.1. Deformation characteristic
The deformation in the SiCw-A1 composite is heterogeneous. Dislocations are generated mainly at the SiCw-A1 interfaces and at the grain boundaries. This indicates that the SiCw-AI interfaces will deform first during the straining process. This kind of deformation cannot spread out beyond the nearby whiskers because they impede deformation. In addition, as reported in ref. 1, the deformation of SiCw-AI composite is very localized because of the inhomogeneous distribution of the reinforcement SiCw. 4.2. Microcrack initiation sources
It can be deduced from previous work that microcracks are generally initiated at whisker ends owing to the thermal residual stress in the composites. Since previous work was mainly based on SEM observations, it cannot explain the fracture mechanism of the composites in detail. The present work, which is inconsistent with the work of Doong et al. [6], indicates that some SiC whiskers will fracture in the early stages of straining because of the existence of whiskers cracked or damaged during the processes of squeeze casting and extrusion. These fractured whiskers have been kept in a closed state before straining because of their thermal residual compressive stress. When the specimen has an applied stress, even the applied stress is far lower than the fracture strength of the whisker, the damaged whiskers will also crack and the previously fractured whiskers will open up to become microcracks. In whisker-rich regions, the interfaces (especially the whisker ends) bearing a larger shear stress [7] can be easily debonded to form microcracks owing to the larger localized deformation in the regions [1] and the mutual deformation constraint between the whiskers and the matrix. The uninfiltrated zone which occurred during the squeeze-casting process is a microcrack itself. Moreover, microcracks are easier to initiate in the matrix in the whisker-rich regions since the deformation of the matrix here is always larger than that outside these regions. 4.3. The growth of microcracks
Only the microcrack which has nucleated in the matrix grows to become a major crack, which will lead to the specimen failure, although many microcracks occur in the early stages of straining. The whiskers are
under a compressive state and the matrix in the vicinity of whiskers has a tensile stress owing to the large thermal residual stress in the composite [8]. The initiation of microcracks caused by the fracture of the whiskers or the debonding of the SiCw-AI interfaces will relax the thermal residual tensile stress. On the contrary, in the region between the whiskers the matrix contains a lower dislocation density than adjacent to the SiCw, i.e. in this region the matrix is weaker. When the tensile stress exceeds the fracture strength of the matrix in front of a microcrack tip, the microcrack which has nucleated in the matrix will propagate and form the major crack, leading to fracture of the specimen. The grain boundaries and SiCw in the SiCw-AI composite play important roles in impeding crack propagation. When a crack meets a grain boundary which is perpendicular to the crack, the crack tip is blunted because the grain boundary impedes it. In the meantime, a new microcrack will be nucleated on the right-hand side of the grain boundary by the stress concentration in front of the crack tip. Then the new microcrack advances. Grain boundaries in the composite are obstacles to the growth of cracks. In the early propagation stages of cracks, a film of aluminium can always be found adhering to the exposed whiskers along the crack edge. This indicates that the bonding strengh of the SiCw-A1 interfaces in the composite made by the squeeze-casting method is higher than the yield stress of the matrix, consistent with the previous observation of Arsenault and Flom [9]. In addition, the temperaure change during the preparation of the composite together with the large difference between the coefficients of thermal expansion (CTEs) of SiC and A1 will give rise to a CTE mismatch strain which will induce a high dislocation density in the near-interface region. When a crack approaches near a whisker, the matrix near the nearinterface region will plastically deform and the crack will advance along the near-interface region. The matrix in the fractured surface layer is ruptured to thin needles or sharp edges during the crack propagation. The characteristic of the plastic deforamtion for the matrix in the SiCw-AI composite is similar to that of pure aluminium. This indicates that only the plastic deformation of the matrix in the near-interface region will be limited by the whiskers if the matrix is easily deformable. For the above reasons, when a crack is parallel to the frontal whisker, the crack will propagate along the near-interface region of one side of the whisker. If a crack meets a whisker which is perpendicular to the crack, the crack will advance by bypassing the whisker. The exposed whiskers on the fractured surface cannot be pulled out. The crack will bypass the whisker which
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ln-situ TEM ()f microcracks in SiC~,.-AI
is perpendicular to the crack because the stress concentration in the region in front of the crack tip, which can be relaxed more easily owing to the plastic deformation of the matrix, is not sufficient to crack the matrix on the other side of the whisker. Since the major crack advances mainly in a path through the matrix and along the near-interface regions, the number of exposed whiskers on the fractured surface is far lower than the average volume fraction of SiC whiskers in the composite.
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strain. Only the microcrack caused by the matrix cracking can advance to form the major crack. (3) The whiskers and the grain boundaries in the composite play an important role in impeding the growth of cracks. The major crack propagates mainly in the matrix and along the near-interface regions, and a film of aluminium will be left on the whiskers exposing along the crack path. The interfaces in the SiCw-A1 composite fabricated by the squeeze-casting method is high strength bonded.
5. Conclusions (1) During the straining, dislocations are generated at grain boundaries and a SiCw-Al interfaces. The spread of dislocations will be limited by the nearby whiskers. The deformation in the composite is very localized. (2) There exist four crack initiation sources in the SiCw-AI composite fabricated by the squeeze-casting method. The damaged and/or fractured whiskers caused during the fabrication of the composite will result in microcracks. Microcracks can also be initiated by the debonding of SiCw-A1 interfaces. The uninfiltrated zone is a kind of microcrack itself. In addition, microcracks can easily develop in the matrix in the whisker-rich regions since the matrix there has a larger
References 1 R.J. Arsenault and N. Shi, Mater. Sci. Eng., AI31 ( 1991 ) 55. 2 S. B. Wu and R. J. Arsenault, Matep: Sci. Eng., A 138(1991) 227. 3 M. Manoharan and J. J. Lcwandowski, Mater. Sci. Eng., A 150 (1992) 179. 4 D.L. Davidson, Metall. Trans. A, 22 ( 1991 ) 1 l 3. 5 N. Sato and T. Kurauchi, J. Compos. Mater., 22 (1988) 850. 6 S. H. Doong, T. C. Lee, I. M. Robertson and H. K. Birnbaum, Scr. Memll., 23(1989) 1413. 7 C. R Jiang and L. Cao, ('hin. ,1. Met. Sci. Teclmol., 5 (1989) 43(/. 8 E. U. Lee, Memll. Trans. A, 23(1992) 2205. 9 R. J. Arscnault and Y. Flom, Strucure and D@~rmation of Boundaries, Metallurgical Society of AIME, Warrendale, PA, 1986, p. 261.