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Fracture energy of lamellar eutectic
.%&wing
Fracture Mechanics.
1972,
FRACTURE
Vol.4. pp. 129-13I. Pergamon Press.
ENERGY
Printed inGreat Britain
OF LAMELLAR
EUTECTIC
K. PIEKARSKI and M. HELMER University of Waterloo, Mechanical Engineering Department, Ontario, Canada Abstract-The
two forms of Lamellar AI-&AI, eutectics were tested for fracture energy. It was found that randomly oriented eutectic absorbed more energy in fracture tests than a unidirectionally solidified ahoy. The fractured surface was examined under scanning electron microscope revealing different kinds of deformation. In both cases shearing of a soft aluminum matrix between hard CuAI, plates was observed. Fractured surface showed pull-out of hard lamellae from a soft matrix. The randomly oriented eutectic displayed additional mechanism of fracture. Complete cells slipped out of a soft aluminum which segregated itself at cell boundaries. The larger amounts of fracture energy in this case is contributed to the additional plastic deformation occurring at large surfaces of cell boundaries.
INTRODUCTION already exists a large volume of literature concerned with the properties of unidirectional eutectics considered as a composite material. The obvious advantage implied is the greater toughness of such a material compared with randomly solidified eutectics. The toughness of a composite material depends on the mechanism of deformation which leads to fracture. Materials having hard and soft phases are generally tough because deformation occurs by shear of a soft phase between two hard phases. The shear mechanism results in fracture because hard fibres or lamellae are extracted from a soft matrix. The objective of the present study was the measurement of energy of fracture of Al-CuAl, eutectics, using the method of Tattersal and Tapin[4], to see how this energy is related to the mode of fracture propagation, THERE
MATERIAL Two kinds of eutectic alloys were prepared: A unidirectional solidified eutectic produced by a continuous casting method and a random eutectic obtained by casting in an air-cooled graphite mold. The preliminary metallographic examination revealed that a unidirectional eutectic had a much finer grain or cell size. The size of cells varied from 100 to 200 or.while the spacings between lamellae were between 1 and 2 p (Figs. la and b). The cells in a randomly oriented eutectic were at least 3-4 times as large and the lamellae were about twice as coarse (Figs. 2a and b). There was also another basic difference between the two kinds of structure. The cell boundaries in the unidirectional material appeared dark under examination with an optical microscope while a random eutectic displayed a white phase segregated at the cell boundaries. This phase also appeared to be much thicker than the dark phase. An examination with an electron probe revealed that the light phase material possessed a very high aluminum content. EXPERIMENTAL RESULTS Specimens were machined from the centre portion of cast rods of both alloys and were then prepared for fracture test as shown in Fig. 3. When the crack was propagating from the apex of a triangular cross section at control!ed strain rates, the typical 129 E.F.M.VoL 4 No. 1-I
130
K. PlEKARSKl
and M. HELMER
Fig. 3. Specimen for the work of fracture deflection (Dimensions in mm). stress-strain curves appeared as shown in Fig. 4. The loading of specimens was performed at the rate of 0402 in./min. The results of the tests are summarized in Table 1. Table I Work of fracture (mean) kg cm/cm2 Directional eutectic Random eutectic The difference between times the standard error.
1.177 1.919 the means is three
Figure 5 shows fractured surface of directionally oriented eutectic. The cell boundaries can be traced by the difference of orientation of lamellae. The whole surface of the specimen did not indicate any deformation at the cell boundaries. Figure 6 is the higher magnification of the same specimen as Fig. 5. It illustrates the mechanism of fracture which occurred by the pull of hard lamellae out of a soft matrix.
1
Fig. 4. Typical load deflection curve for the bending test.
-a
‘4 =
Fig.
1. Unidirectionally
solidified
Al-CuAI, eutectic. structure X 500.
(a) Cell
structure
X 50. (b) Lamellar
-._-.
“_
”
-
_.----
- ---
Fig. 5. Fractured surface of directionally solidified eutectic x 550,
Fig. 6. Detail of surface from Fig. 3 showing pull-out fracture of eutectic.
Fig. 7. Fractural surface of randomly solidified eutectic showing that deformation occurred at cell boundaries.
Fracture energy of lamellar eutectic
131
This is precisely the type of fracture which should require the absorption of large amounts of energy. Figure 7 shows fractured surface of randomly oriented eutectic. The lamellar structure is coarser requiring only half the magnification to resolve it. On the lamellar scale the type of fracture seems to be the same. The photomicrograph in Fig. 6 also being typical for some portions of randomly oriented eutectic. In addition to this mode of fracture, all specimens with random eutectic demonstrated another mechanism. Figure 7 illustrates this mechanism. Complete cells slipped out of a soft matrix which segregated itself at cell boundaries. DISCUSSION Toughness of composite materials is usually attributed to the pull-out mechanism of fracture. Toughness of Al-CuAl, eutectic is very low compared with other metals. According to Tattersal and Tapin[4], the toughness of copper and steel is 247 kg cm/cm2 and brass 148 kg cm/cm2 which are three orders of magnitude higher than Al-CuAI, eutectic. However. despite its low toughness, it should be noted that in this study two mechanisms of deformation have been observed: pull-out of lamellae and pull-out of whole cells. the latter mechanism requiring about twice the work of fracture of the former. The existence of two different mechanisms has also been observed on fractured surfaces of bonePI. It may be postulated that in order to optimize future properties of eutectic composites, the size and the number of shearing interfaces should be taken into consideration as well as their existence. Acknowledgements use of their material.
-Thanks to Dr. H. W. Kerr and Mr. Scott Lawson of the University of Waterloo for the equipment
and advise on growing
unidirectional
eutectics.
REFERENCES [II K. W.
Hertzberg, F. D. Lemkey and J. Ford, Mechanical behaviour of lamellar Al - CuAI, and whisker type Al - AI,Ni unidirectionally solidified eutectic alloys. Trans. metal/. Sm. AIME. 233, (1965). solidified AI-CuAI, 121 F. W. Crossman, A. S. Yue and A. I. Vidoz, Tensile properties of unidirectionally eutectic composites. Trans. meraN. Sot. AIME, 245 (1969). r31 K. Piekarski, Fracture of bone. J. appl. Phys., 41,2 15 (1970). 141 H. G. Tattersal and G. Tapin, The work of fracture and its measurement in metals. ceramics and other materials. J. Mater. Sci. 10,296 (1966). (Received
10 Srpfernber
1970)
Fracture Mechanics.
1972,
FRACTURE
Vol.4. pp. 129-13I. Pergamon Press.
ENERGY
Printed inGreat Britain
OF LAMELLAR
EUTECTIC
K. PIEKARSKI and M. HELMER University of Waterloo, Mechanical Engineering Department, Ontario, Canada Abstract-The
two forms of Lamellar AI-&AI, eutectics were tested for fracture energy. It was found that randomly oriented eutectic absorbed more energy in fracture tests than a unidirectionally solidified ahoy. The fractured surface was examined under scanning electron microscope revealing different kinds of deformation. In both cases shearing of a soft aluminum matrix between hard CuAI, plates was observed. Fractured surface showed pull-out of hard lamellae from a soft matrix. The randomly oriented eutectic displayed additional mechanism of fracture. Complete cells slipped out of a soft aluminum which segregated itself at cell boundaries. The larger amounts of fracture energy in this case is contributed to the additional plastic deformation occurring at large surfaces of cell boundaries.
INTRODUCTION already exists a large volume of literature concerned with the properties of unidirectional eutectics considered as a composite material. The obvious advantage implied is the greater toughness of such a material compared with randomly solidified eutectics. The toughness of a composite material depends on the mechanism of deformation which leads to fracture. Materials having hard and soft phases are generally tough because deformation occurs by shear of a soft phase between two hard phases. The shear mechanism results in fracture because hard fibres or lamellae are extracted from a soft matrix. The objective of the present study was the measurement of energy of fracture of Al-CuAl, eutectics, using the method of Tattersal and Tapin[4], to see how this energy is related to the mode of fracture propagation, THERE
MATERIAL Two kinds of eutectic alloys were prepared: A unidirectional solidified eutectic produced by a continuous casting method and a random eutectic obtained by casting in an air-cooled graphite mold. The preliminary metallographic examination revealed that a unidirectional eutectic had a much finer grain or cell size. The size of cells varied from 100 to 200 or.while the spacings between lamellae were between 1 and 2 p (Figs. la and b). The cells in a randomly oriented eutectic were at least 3-4 times as large and the lamellae were about twice as coarse (Figs. 2a and b). There was also another basic difference between the two kinds of structure. The cell boundaries in the unidirectional material appeared dark under examination with an optical microscope while a random eutectic displayed a white phase segregated at the cell boundaries. This phase also appeared to be much thicker than the dark phase. An examination with an electron probe revealed that the light phase material possessed a very high aluminum content. EXPERIMENTAL RESULTS Specimens were machined from the centre portion of cast rods of both alloys and were then prepared for fracture test as shown in Fig. 3. When the crack was propagating from the apex of a triangular cross section at control!ed strain rates, the typical 129 E.F.M.VoL 4 No. 1-I
130
K. PlEKARSKl
and M. HELMER
Fig. 3. Specimen for the work of fracture deflection (Dimensions in mm). stress-strain curves appeared as shown in Fig. 4. The loading of specimens was performed at the rate of 0402 in./min. The results of the tests are summarized in Table 1. Table I Work of fracture (mean) kg cm/cm2 Directional eutectic Random eutectic The difference between times the standard error.
1.177 1.919 the means is three
Figure 5 shows fractured surface of directionally oriented eutectic. The cell boundaries can be traced by the difference of orientation of lamellae. The whole surface of the specimen did not indicate any deformation at the cell boundaries. Figure 6 is the higher magnification of the same specimen as Fig. 5. It illustrates the mechanism of fracture which occurred by the pull of hard lamellae out of a soft matrix.
1
Fig. 4. Typical load deflection curve for the bending test.
-a
‘4 =
Fig.
1. Unidirectionally
solidified
Al-CuAI, eutectic. structure X 500.
(a) Cell
structure
X 50. (b) Lamellar
-._-.
“_
”
-
_.----
- ---
Fig. 5. Fractured surface of directionally solidified eutectic x 550,
Fig. 6. Detail of surface from Fig. 3 showing pull-out fracture of eutectic.
Fig. 7. Fractural surface of randomly solidified eutectic showing that deformation occurred at cell boundaries.
Fracture energy of lamellar eutectic
131
This is precisely the type of fracture which should require the absorption of large amounts of energy. Figure 7 shows fractured surface of randomly oriented eutectic. The lamellar structure is coarser requiring only half the magnification to resolve it. On the lamellar scale the type of fracture seems to be the same. The photomicrograph in Fig. 6 also being typical for some portions of randomly oriented eutectic. In addition to this mode of fracture, all specimens with random eutectic demonstrated another mechanism. Figure 7 illustrates this mechanism. Complete cells slipped out of a soft matrix which segregated itself at cell boundaries. DISCUSSION Toughness of composite materials is usually attributed to the pull-out mechanism of fracture. Toughness of Al-CuAl, eutectic is very low compared with other metals. According to Tattersal and Tapin[4], the toughness of copper and steel is 247 kg cm/cm2 and brass 148 kg cm/cm2 which are three orders of magnitude higher than Al-CuAI, eutectic. However. despite its low toughness, it should be noted that in this study two mechanisms of deformation have been observed: pull-out of lamellae and pull-out of whole cells. the latter mechanism requiring about twice the work of fracture of the former. The existence of two different mechanisms has also been observed on fractured surfaces of bonePI. It may be postulated that in order to optimize future properties of eutectic composites, the size and the number of shearing interfaces should be taken into consideration as well as their existence. Acknowledgements use of their material.
-Thanks to Dr. H. W. Kerr and Mr. Scott Lawson of the University of Waterloo for the equipment
and advise on growing
unidirectional
eutectics.
REFERENCES [II K. W.
Hertzberg, F. D. Lemkey and J. Ford, Mechanical behaviour of lamellar Al - CuAI, and whisker type Al - AI,Ni unidirectionally solidified eutectic alloys. Trans. metal/. Sm. AIME. 233, (1965). solidified AI-CuAI, 121 F. W. Crossman, A. S. Yue and A. I. Vidoz, Tensile properties of unidirectionally eutectic composites. Trans. meraN. Sot. AIME, 245 (1969). r31 K. Piekarski, Fracture of bone. J. appl. Phys., 41,2 15 (1970). 141 H. G. Tattersal and G. Tapin, The work of fracture and its measurement in metals. ceramics and other materials. J. Mater. Sci. 10,296 (1966). (Received
10 Srpfernber
1970)