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Scripta Metallurgica et Materialia, Vol. 32, No. 6, pp. 799-804,1995 Copyright 0 1995 Elsevier Science Ltd Printed in the USA. All rights reserved 0956-716X/95 $9.50 + .OO
DEFORMATION
BEHAVIOR OF AN ALUMINUM-SILICON CARBIDE METAL MATRIX COMPOSITE IN PLANE STRESS/PLANE STRAIN F. Irizarry-Lago and P.A. Sundaram Department of Mechanical Engineering University of Puerto Rico Mayaguez, PR 00660 (Received August30,1994) (Revised October10,1994) Introduction
The deformation behavior of Al/Sic metal matrix composites (MMCs) has been studied quite extensively. The high increased strength observed in short fiber reinforced Al/Sic MMCs is attributed to a high dislocation density in the ductile aluminum matrix (1). In fact a high dislocation density was found by Vogelsang (2) in the aluminum matrix as a result of cooling from 743 to 300 K. These dislocations can be attributed to the difference in the coefficient of thermal expansion (CTE) in the fiber and the matrix. The influence of the CTE on the strength of particulate reinforced MMCs has been considered by Arsenault and Shi (3). Christian and Suresh (4) and Taya (5). Current research is primarily concerned with the thermally-induced residual stresses generated in composites resulting from differences in CTE between the matrix and the fiber or particles. Residual stresses can also be generated by mechanical means. Shi et al. (6) studied the deformation-induced changes of the matrix residual stresses in a whisker reinforced Al/Sic MMC. It Wats found that the changes are asymmetric in response to a uniaxial external tensile or compressive load applied along the longitudinal whisker axis. Arsenault and Taya (7) indicated that the yield stress of the composite under tension is different from that under uniaxial compression. The effect of microstructural parameters on the fracture mechanism in particle reinforced MlMCs was investigated by Kim et al. (8). It was found that the fracture of the Sic particles contributed to the overall fracture process. The critical particle diameter for fracture of these particles decreased as the volume fraction of Sic particles increased. Kim et al. (9) studied the fracture mechanisms in whisker reinforced Al/Sic MMCs and emphasized the role of matrix inter-metallic particles, inhomogeneous distribution of whiskers and whisker breakage during the fracture process. Wu and Arsenault (10) have proposed four distinct separation fracture modes for Al/Sic composites and determined experimentally that the mode of fracl:ure was through the matrix. Wang et al. (11) used finite elements to study the stress distribution in a particulate reinforced Al/SIC MMC subjected to external loads. It was shown that the non-uniform stress distribution in the material was a result of a diversity of factors including the particle distribution and the casting process. Also, the biaxial stress state around a particle or inside a particle cluster may change the Von Mises stress and result in plastic deformation features such as strain localization. As the applied stress is increased, the state of triaxial stress Inside a particle cluster appears to promote early particle cracking, interface debonding, and void formation in the ductile matrix. To add to the wide variety of data available in the literature, a further study of the deformation behavior of the Al/Sic MMC was conducted. The goal of the research presented
799
DEFORMATION OF COMPOSITE
800
Vol. 32, No. 6
in this paper is to characterize the mechanical behavior of the Al/Sic composite under a plane stress/plane strain deformation field. Also, the important role of the particle-matrix interface ln the deformation and fracture process under the above deformation field will be investigated. Experimental Procedure In this research, the deformation behavior of an Al-20% volume fraction SIC particle MMC will be studied using a Clausing specimen (Fig. 1). The specimen when subjected to a uniaxial load develops a plane stress/plane strain deformation stress state because of its special geometry (12). The composite material was received in the form of extruded bars of 50 mm diameter. The Clausing specimens were machined using electric discharge machining (EDM), with the loading axis in the direction of extrusion. TWO specimens were loaded monotonically to fracture to generate corresponding stress-strain curves and establish the baseline mechanical behavior. TWO other specimens were subjected to incremental loading. Strain markers were made in the gage sections of these specimens as shown in Fig. 1. The initial gage length, width and thickness of these specimens were measured. Optical micrographs at magnifications of 5X and 100X were taken of the specimen gage section and the center of the specimen gage section respectively. The specimens were loaded in a mechanical testing machine to just above the yield point and then unloaded. The displacement in the longitudinal direction, the width and the thickness were measured once again. Optical micrographs of the gage section were again taken at 5X and 100X respectively. This loading-unloading cycle along with the measurements and micrographs was repeated for small strain Increments until the specimen fractured. The purpose of this method of testing was to determine when and where cracks resulting from interfacial debonding originated.
Results and Discussion The true stress-strain and the do/d& versus o plots for the Clausing specimens are shown in Figs. 2 and 3. As can be observed, the curves of the incrementally loaded specimens coincide to a fair extent with those of the monotonically loaded specimens. A simple power law equation of the type o = K&n,where K and n are the strength coefficient and the work hardening coefficient respectively, was used to fit the true stress-strain data. The values of K and n obtained are tabulated with other relevant mechanical property data in Table I. At the present time we are unable to explain the low n value for specimen #4. TABLE 1. Mechanical Property Data for the Specimens Tested Specimen No.
Loading Method
1 2 3 4
Monotonic Monotonic Incremental Incremental
Tensile Strength (MPa) 300 319 299 313
Elongation (o/o) 8.03 7.48 3.94 6.22
(MKpal 880 879 720 520
n 0.32 0.29 0.27 0.18
The specimen behavior during deformation is typically that of a Clauslng specimen (13). However, the lateral contraction in the width direction is very small (Fig. 4) compared to data for other materials such as a brass and AISI 1090 spheroidized steel (13.14). This is because of the inherent brittle nature of the Al/Sic composite. The lateral strain values are plotted
Vol. 32, No. 6
801
DEFORMATION OF COMPOSITE
with respect to the longitudinal strain in Fig. 5. As is expected, the contraction in the thickness dtrection is greater than in the lateral direction. While there is some lateral contraction, there is hardly any necking in the thickness direction (see Fig. 4~). Some interesting features are seen in the photomicrographs taken from the center of the specimen gage section (Fig. 6). Microcracks are seen to develop parallel to the tensile axis at low strains (Fig. 6b). Some other microcracks are seen, perpendicular to the tensile axis in the later stages of deformation (Fig. 6~). The latter feature is to be expected because of the brittle nature of the composite. The microcracks parallel to the tensile axis are somewhat curious and it is believed that the lateral constraint developed by the Clausing specimen (13) is the cause for the nucleation of these microcracks. The lateral constraint restricts deformation lin the width direction and hence causes microcracks to nucleate in a direction parallel to the loading axis. With increasing deformation, the growth of the microcracks in the direction perpendicular to the loading axis dominates compared to those in the parallel direction resulting in fracture (Fig. 6d). The former do not seem to play a role in the final fracture of the specimen while the latter possibly contribute to final fracture. Fracture of the specimens takes place in the center of the gage section as is expected (Fig. 4d). The role of the particle-matrix interface in the nucleation of growth of the microcracks is not clear at the present time. Detailed evaluation using scanning electron microscopy has to be performed to obtain such information. Conclusions 1. The deformation behavior of an Al-20 vol.%(SiC$ composite was studied under plane stress/plane strain conditions. The deformation behavior is well defined by a simple power law type of equation. :: Longitudinal microcracks form at low strains in the direction of the loading axis as a result of tlhe lateral constraint developed because of the geometry of the Clausing specimen. Microcracks perpendicular to the loading direction are formed at larger strains. Final fracture is caused by the perpendicular cracks. Acknowledgment The authors are grateful to ALCOA (Aluminum Al/Sic composite material for this research.
Company
of America)
for supplying
the
References 1. R.J. Arsenault and R.M. Fisher, Scripta Metall., 17, 67-71 (1983). 2. M. Vogelsang, R.M. Fisher and R.J. Arsenault, Metall. Trans. 17A, 379-389 (1986). 3. R.J. Arsenault and N. Shi, Mater. Sci. & Eng., 81, 175-187 (1986). 4. T. Christian and S. Suresh, Acta Metall., 36, 1691-1704 (1988). 5. M. Taya, IK. E. Lulay and D.J. Lloyd, Acta Metall., 39. 73-87 (1991). 6. N. Shi, R.J. Arsenault. A.D. Krawitz and L.F. Smith, Metall. Trans. 24A, 187-196 (1986). 7. R.J. Arsenault and M. Taya, Acta Metall., 35, 651-659 (1987). 8. H.J. Kim, T. Kobayashi and H.S. Yoon, Mater. Sci. & Eng., 54, 35-41 (1992). 9. Y.H. Kim, S.H. Lee and N.J. Kim, Metall. Trans. 23A, 2589-2596 (1992). 10. S.B. Wu and R.J. Arsenault. Mater. Sci. & Eng., 38, 227-235 (1991). 11. Z. Wang, T.K. Chen and D.J. Lloyd, Metall. Trans. 24A, 197-207 (1993). 12. D.P. Clausing, Technical Report on Project No. 35.066-001(2)., U.S. Steel Research Laboratory, ( 1972). 13. P.A. Sundaram, D.Rodriguez and S. Santiago, Scripta Metall., 30, 95-100 (1994). 14. A.E. Valkonen. Ph.D Dissertation, The Ohio State University (1987).
Vol. 32, No. 6
DEFORMATION OF COMPOSITE
802
.* Nomi.w.4
D~menr~ons(mm)
L,
= 75.0
T
~6.3
I-T
2.
D
l
Engineering Strain
FIG. 1. Geometry and dimensions of the Clawing specimen. The loading axis is vertical.
FIG. 2. True stress-strain curves for the various specimens.
FIG. 3. do/d& versus d plots for the various specimens.
FIG. 5. Longitudinal strain versus lateral strain plots for Specimen 4.
Vol. 32,No.6
DEFORMATIONOFCO~S~
(d)
FIG. 4. Surface topography of the gage section at various strains (a) & = 0, (b) E = 0.01, (c) & = 0.037 and (d) at fracture. The loading axis is horizontal.
803
804
DEFORMA‘IIONOFCOMPOS~
(4
Vol.32,No.6
(b)
FIG. 6. Observations of microcracks at various strains (al & = 0, (bl E = 0.005, (c) E = 0.011 and (d) at fracture. The loading axis is horizontal.
DEFORMATION
BEHAVIOR OF AN ALUMINUM-SILICON CARBIDE METAL MATRIX COMPOSITE IN PLANE STRESS/PLANE STRAIN F. Irizarry-Lago and P.A. Sundaram Department of Mechanical Engineering University of Puerto Rico Mayaguez, PR 00660 (Received August30,1994) (Revised October10,1994) Introduction
The deformation behavior of Al/Sic metal matrix composites (MMCs) has been studied quite extensively. The high increased strength observed in short fiber reinforced Al/Sic MMCs is attributed to a high dislocation density in the ductile aluminum matrix (1). In fact a high dislocation density was found by Vogelsang (2) in the aluminum matrix as a result of cooling from 743 to 300 K. These dislocations can be attributed to the difference in the coefficient of thermal expansion (CTE) in the fiber and the matrix. The influence of the CTE on the strength of particulate reinforced MMCs has been considered by Arsenault and Shi (3). Christian and Suresh (4) and Taya (5). Current research is primarily concerned with the thermally-induced residual stresses generated in composites resulting from differences in CTE between the matrix and the fiber or particles. Residual stresses can also be generated by mechanical means. Shi et al. (6) studied the deformation-induced changes of the matrix residual stresses in a whisker reinforced Al/Sic MMC. It Wats found that the changes are asymmetric in response to a uniaxial external tensile or compressive load applied along the longitudinal whisker axis. Arsenault and Taya (7) indicated that the yield stress of the composite under tension is different from that under uniaxial compression. The effect of microstructural parameters on the fracture mechanism in particle reinforced MlMCs was investigated by Kim et al. (8). It was found that the fracture of the Sic particles contributed to the overall fracture process. The critical particle diameter for fracture of these particles decreased as the volume fraction of Sic particles increased. Kim et al. (9) studied the fracture mechanisms in whisker reinforced Al/Sic MMCs and emphasized the role of matrix inter-metallic particles, inhomogeneous distribution of whiskers and whisker breakage during the fracture process. Wu and Arsenault (10) have proposed four distinct separation fracture modes for Al/Sic composites and determined experimentally that the mode of fracl:ure was through the matrix. Wang et al. (11) used finite elements to study the stress distribution in a particulate reinforced Al/SIC MMC subjected to external loads. It was shown that the non-uniform stress distribution in the material was a result of a diversity of factors including the particle distribution and the casting process. Also, the biaxial stress state around a particle or inside a particle cluster may change the Von Mises stress and result in plastic deformation features such as strain localization. As the applied stress is increased, the state of triaxial stress Inside a particle cluster appears to promote early particle cracking, interface debonding, and void formation in the ductile matrix. To add to the wide variety of data available in the literature, a further study of the deformation behavior of the Al/Sic MMC was conducted. The goal of the research presented
799
DEFORMATION OF COMPOSITE
800
Vol. 32, No. 6
in this paper is to characterize the mechanical behavior of the Al/Sic composite under a plane stress/plane strain deformation field. Also, the important role of the particle-matrix interface ln the deformation and fracture process under the above deformation field will be investigated. Experimental Procedure In this research, the deformation behavior of an Al-20% volume fraction SIC particle MMC will be studied using a Clausing specimen (Fig. 1). The specimen when subjected to a uniaxial load develops a plane stress/plane strain deformation stress state because of its special geometry (12). The composite material was received in the form of extruded bars of 50 mm diameter. The Clausing specimens were machined using electric discharge machining (EDM), with the loading axis in the direction of extrusion. TWO specimens were loaded monotonically to fracture to generate corresponding stress-strain curves and establish the baseline mechanical behavior. TWO other specimens were subjected to incremental loading. Strain markers were made in the gage sections of these specimens as shown in Fig. 1. The initial gage length, width and thickness of these specimens were measured. Optical micrographs at magnifications of 5X and 100X were taken of the specimen gage section and the center of the specimen gage section respectively. The specimens were loaded in a mechanical testing machine to just above the yield point and then unloaded. The displacement in the longitudinal direction, the width and the thickness were measured once again. Optical micrographs of the gage section were again taken at 5X and 100X respectively. This loading-unloading cycle along with the measurements and micrographs was repeated for small strain Increments until the specimen fractured. The purpose of this method of testing was to determine when and where cracks resulting from interfacial debonding originated.
Results and Discussion The true stress-strain and the do/d& versus o plots for the Clausing specimens are shown in Figs. 2 and 3. As can be observed, the curves of the incrementally loaded specimens coincide to a fair extent with those of the monotonically loaded specimens. A simple power law equation of the type o = K&n,where K and n are the strength coefficient and the work hardening coefficient respectively, was used to fit the true stress-strain data. The values of K and n obtained are tabulated with other relevant mechanical property data in Table I. At the present time we are unable to explain the low n value for specimen #4. TABLE 1. Mechanical Property Data for the Specimens Tested Specimen No.
Loading Method
1 2 3 4
Monotonic Monotonic Incremental Incremental
Tensile Strength (MPa) 300 319 299 313
Elongation (o/o) 8.03 7.48 3.94 6.22
(MKpal 880 879 720 520
n 0.32 0.29 0.27 0.18
The specimen behavior during deformation is typically that of a Clauslng specimen (13). However, the lateral contraction in the width direction is very small (Fig. 4) compared to data for other materials such as a brass and AISI 1090 spheroidized steel (13.14). This is because of the inherent brittle nature of the Al/Sic composite. The lateral strain values are plotted
Vol. 32, No. 6
801
DEFORMATION OF COMPOSITE
with respect to the longitudinal strain in Fig. 5. As is expected, the contraction in the thickness dtrection is greater than in the lateral direction. While there is some lateral contraction, there is hardly any necking in the thickness direction (see Fig. 4~). Some interesting features are seen in the photomicrographs taken from the center of the specimen gage section (Fig. 6). Microcracks are seen to develop parallel to the tensile axis at low strains (Fig. 6b). Some other microcracks are seen, perpendicular to the tensile axis in the later stages of deformation (Fig. 6~). The latter feature is to be expected because of the brittle nature of the composite. The microcracks parallel to the tensile axis are somewhat curious and it is believed that the lateral constraint developed by the Clausing specimen (13) is the cause for the nucleation of these microcracks. The lateral constraint restricts deformation lin the width direction and hence causes microcracks to nucleate in a direction parallel to the loading axis. With increasing deformation, the growth of the microcracks in the direction perpendicular to the loading axis dominates compared to those in the parallel direction resulting in fracture (Fig. 6d). The former do not seem to play a role in the final fracture of the specimen while the latter possibly contribute to final fracture. Fracture of the specimens takes place in the center of the gage section as is expected (Fig. 4d). The role of the particle-matrix interface in the nucleation of growth of the microcracks is not clear at the present time. Detailed evaluation using scanning electron microscopy has to be performed to obtain such information. Conclusions 1. The deformation behavior of an Al-20 vol.%(SiC$ composite was studied under plane stress/plane strain conditions. The deformation behavior is well defined by a simple power law type of equation. :: Longitudinal microcracks form at low strains in the direction of the loading axis as a result of tlhe lateral constraint developed because of the geometry of the Clausing specimen. Microcracks perpendicular to the loading direction are formed at larger strains. Final fracture is caused by the perpendicular cracks. Acknowledgment The authors are grateful to ALCOA (Aluminum Al/Sic composite material for this research.
Company
of America)
for supplying
the
References 1. R.J. Arsenault and R.M. Fisher, Scripta Metall., 17, 67-71 (1983). 2. M. Vogelsang, R.M. Fisher and R.J. Arsenault, Metall. Trans. 17A, 379-389 (1986). 3. R.J. Arsenault and N. Shi, Mater. Sci. & Eng., 81, 175-187 (1986). 4. T. Christian and S. Suresh, Acta Metall., 36, 1691-1704 (1988). 5. M. Taya, IK. E. Lulay and D.J. Lloyd, Acta Metall., 39. 73-87 (1991). 6. N. Shi, R.J. Arsenault. A.D. Krawitz and L.F. Smith, Metall. Trans. 24A, 187-196 (1986). 7. R.J. Arsenault and M. Taya, Acta Metall., 35, 651-659 (1987). 8. H.J. Kim, T. Kobayashi and H.S. Yoon, Mater. Sci. & Eng., 54, 35-41 (1992). 9. Y.H. Kim, S.H. Lee and N.J. Kim, Metall. Trans. 23A, 2589-2596 (1992). 10. S.B. Wu and R.J. Arsenault. Mater. Sci. & Eng., 38, 227-235 (1991). 11. Z. Wang, T.K. Chen and D.J. Lloyd, Metall. Trans. 24A, 197-207 (1993). 12. D.P. Clausing, Technical Report on Project No. 35.066-001(2)., U.S. Steel Research Laboratory, ( 1972). 13. P.A. Sundaram, D.Rodriguez and S. Santiago, Scripta Metall., 30, 95-100 (1994). 14. A.E. Valkonen. Ph.D Dissertation, The Ohio State University (1987).
Vol. 32, No. 6
DEFORMATION OF COMPOSITE
802
.* Nomi.w.4
D~menr~ons(mm)
L,
= 75.0
T
~6.3
I-T
2.
D
l
Engineering Strain
FIG. 1. Geometry and dimensions of the Clawing specimen. The loading axis is vertical.
FIG. 2. True stress-strain curves for the various specimens.
FIG. 3. do/d& versus d plots for the various specimens.
FIG. 5. Longitudinal strain versus lateral strain plots for Specimen 4.
Vol. 32,No.6
DEFORMATIONOFCO~S~
(d)
FIG. 4. Surface topography of the gage section at various strains (a) & = 0, (b) E = 0.01, (c) & = 0.037 and (d) at fracture. The loading axis is horizontal.
803
804
DEFORMA‘IIONOFCOMPOS~
(4
Vol.32,No.6
(b)
FIG. 6. Observations of microcracks at various strains (al & = 0, (bl E = 0.005, (c) E = 0.011 and (d) at fracture. The loading axis is horizontal.