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SiC metal matrix composites
h4aterialaRsscarch Bulletin, Vol. 30, No. 8,99. 1023-1030.1995 Copyright 0 1995 Elsevisr Science Ltd Printed in the USA. All rightr mrerved 0025~5408/95 $9.50 + .OO
Pergamon
EFFECT OF INCREASE JN HETEROGENEOUS NUCLXATION SITES ON THE AGING BEHAVIOR OF 6061/SiC METAL MATRIX COMl’OSlTES
M. Gupta and M.K. Surappa* Department of Mechanical and Production Engineering National University of Singapore, 10 Kent Ridge Crescent, Singapore 05 11 ‘Department of Metallurgy, Indian Institute of Science, Bangalore, India 5600 12 (Received March 3,199s; Refereed)
ABSTRACT In this study, aging characteristics of an aluminum based metallic matrix reinforced with various volume fraction of silicon carbide was investigated. The results showed an increase in aging kinetics with an increase in volume &action of Sic particulates. Results of microstructural characterization studies exhibited the presence of solute rich zone in the near vicinity of Sic particulates and the nucleation of secondary phases both at and in near vicinity of Sic particulates. The results of accelerated aging kinetics thus observed were rationalized in terms of an increase in the heterogeneous nucleation volume in the metallic matrix and the constitutional characteristics of the Al/Sic inter-facial region. MATERIALS INDEX: nucleation
Metal matrix composites, Aging, Heterogeneous
ODUCTION The innate ability of metal matrix composites (MMCs) to combine the properties of ceramic phase with that of the metallic phase has been instrumental in the insurgence of extensive research activities $1 over the world (1). The suitability of these composite materials, however, lies in the judicious selection of synthesizing/processing technique, matrix material and ceramic reinforcement. Amongst the various matrix-reinforcement combinations, silicon carbide (Sic) reinforced ahnninum matrices have been widely acknowledged as the potential candidates for the weight critical automobile and aerospace applications (2). The addition of 1023
1024
M. GUPTA et al.
Vol. 30, No. 8
Sic particulates has been correlated with an increase in mechanical properties such as yield strength, ultimate tensile strength and elastic modulus of the aluminum based metallic matrices (2). This improvement can be ascribed, in part, to the particulates-assisted microstrnctural strengthening. In related studies, for example, investigators have reported the change in microstructural features such as dislocation density (3), grain size (4), precipitation behavior (5) and excess solid solubility (6) as a result of the presence of ceramic particulates. The presence of ceramic reinforcement, in addition, has also been correlated with the kinetics of microstructural evolution during aging step of the conventional T6 heat treatment (4,5,7). The change in aging kinetics of the metallic matrix as a result of the presence of a fixed volume fraction of ceramic particulates, for example, has been reported by various investigators (4,5,7). However, no systematic studies have been carried out to correlate the effect of volume fraction of Sic particulates with the increase in heterogeneous nucleation sites in the metallic matrix and the ensuing accelerated aging characteristic of the reinforced metallic matrix. Accordingly, the present study was undertaken to provide insight into the effect of volume fraction of the incorporated ceramic reinforcement on the aging response of an aluminum based metallic matrix processed using casting route. Particular emphasis was placed to correlate the aging results obtained on the reinforced samples with the microstructural characteristics.
The starting matrix material used in the present study was AA 606 1 aluminum alloy containing (in wt. %): 0.6Si - 1.05Mg - 0.46Fe - 0.25Cu - 0.25Cr - Al (bal.). Silicon carbide (a-Sic) particulates with an average size of 54 pm were selected as the reinforcement phase. Three casting experiments were made to synthesize unreinforced alloy and composites containing 10 (V, = 0.086) and 15 (V, = 0.130) wt. % Sic part&dates. The synthesis of metal matrix composites used in the present study was carried out according to the following procedure. The metal ingots, prior to melting, were cleaned and melted under a cover of nitrogen gas in order to mmimize the oxidation of molten metal. Sic particulates, preheated to 900 “C, were then added into the molten metal stirred using an impeller. The melt was alloyed with small amounts of Mg and Zr (Mg+Zr < 1 wt. %) in order to improve the wettability of Sic particulates. The composite melt thus obtained was poured into cylindrical cast iron moulds (75 mm diameter and 150 mm height). For the purpose of comparison, the base alloy was cast after adding similar amounts of magnesium and zirconium (Mg+Zr < 1%) into 606 1 Al alloy melt following the similar casting parameters. The as-cast cylindrical bars were homogenized at540”Cfor3hoursandthenextrndedusinganextrusionratioof10:1 at500°Cinto16mm diameter rods in order to close the residual porosity. Aging studies were carried out on extruded unreinforced and reinforced samples (16 mm diameter x 10 mm height) in order to find out the time required to attain peak hardness. It essentially involved solutionizing of samples for one hour at 530 “C, quenching in cold water followed by isothermal aging at 177 “C for various intervals of time. Brine11 hardness measurements were made using 5 mm diameter indentor with a 500 kg load. Microsuucmml characterization studies were primarily conducted using a JEOL scanning electron microscope (SEM) equipped with EDS (Energy Dispersive Spectroscopy) on the plastic mounted and metallographically polished composite samples in order to examine the
HETEROG~OUS
Vol. 30,No. 8
NUCLFATlON SITES
1025
precipitation behavior and segregation of alloying elements in the inter-facial region between the Al alloy matrix and ceramic particulate.
The results of the aging studies conducted on the monolithic alloy and composites containing 8.6 and 13 vol. % Sic particulates are shown in Fig. 1. The results revealed that aging time for peak hardness for monolithic alloy was 8 hours when compared with 6 hours for 606 118.6 vol. % Sic and 2 hours for 6061/l 3 vol. % Sic composite samples. The results of scamring electron microscopy carried out on the composites samples revealed: a) absence of voids or other discontinuities in the matrix and particulate-matrix interface, b) good interfacial bonding between dispersed Sic particulates and the matrix, and c) pretkrential presence of Mg rich precipitates at and in near vicinity of Sic-matrix interface (see Fig. 2). The identity of Mg-rich precipitates was conformed using EDS point analyses. In addition, the results of EDS analyses conducted on 6061 /13 .Ovol. % Sic samples in both as-extruded and peak aged conditions revealed enrichment of Mg and Si in the near vicinity of Sic particulates when compared to the bulk material composition. The variation in weight percent silicon observed at various distances ti-om the Sic-matrix interface, for example, is graphically represented in Fig. 3.
125
I
2.5
I
5
El
6061
0
6W/S.6
A
6061113 vol. 96 Sic
I
7.5
I
10
vol.% SE
I
12.5
Aging Time (Hrs) FIG. 1 Graphical repmsentation of aging studies conducted on unreinforced 6061 ahoy, 606V8.6 vol. % SIC and 6061113 vol.% Sic. All materials were solution-treated at 530 “C for 1 h.
1026
Vol. 30: No. 8
M. GUPTA et al.
FIG. 2 Representative SEM micrograph taken from the 606 l/l 3 vol. % SIC composite sample showing the presence of Mg-rich intermetallic phase at the 606 l/SIC interface (marked A) and in the near vicinity of Sic particulate (marked B).
8 4
Cl
Peakaged
0
As-extruded
Distance from interface (microns) FIG. 3 Graphical representation of the segregation pattern of Si at 6061/SiC interfacial regim o&erved in as-exiruded and peak aged 6061113 vol. % Sic samples.
1027
HETFXOGENEOLJS NUCLEATlON SITEB
Vol. 30, Not.8
Matrix Heterogenous nucleation zone I Plastic zone
Sic
FIG. 4 !khemaGcillustration representing the heterogeneous nucleation zone around Sic patticulates in the metallic matrix.
DISCUSSION The 6061 Al-Mg-Si alloy is one of the most common aluminum based alloy used as metallic matrix in discontinuously reinforced aluminum based metal matrix composites ($8). In order to provide insight into the aging results obtained in the present study, some background information on the precipitation sequence of 6061 Al alloy is provided here. The precipitation sequence of reinforced 606 1 Al alloy have been shown to be identical to those of unreinforced 6061 Al alloy (5,7,9) namely: supersaturated solid solution --> needle shaped G.P. zones --> needle shaped p ” semicoherent phase --> rod shaped p’ transition phase (primary strengthening phase) --> equilibrium and incoherent p (MgrSi) phase. In this work, the results of the aging studies revealed an increase in aging kinetics of the metahic matrix with an increase in volume fraction of Sic particulates. The accelerated aging behavior observed in the present study can be attributed to the enhanced heterogeneous nucleation of the strengthening phases in the metallic matrix during the aging step of the conventional T6 heat treatment. The enhanced heterogeneous nucleation of the strengthening phases in the metallic matrix can primarily be attributed to: a) the pres’ence of Sic particulates, b) formation of defect-rich interfacial region in the near vicinity of Sic particulates, and
TABLE 1 Results of the Heterogeneous Volume Computations.
6061 6061 6061
0.000 0.086 0.130
-0.204 0.308
-0.710 0.562
8 6 2
1028
M GUPTA et al.
Vol. 30, No. 8
c) modification of microstructural features in the metallic matrix due to the presence of Sic particulates The ability of the Sic particulates to act as heterogeneous nucleation sites is consistent with the microstructural characterization studies revealing the presence of Mg-rich phases located at the Al/Sic interface (see Fig. 2). Moreover, the present results are also consistent with the research findings of other investigators showing convincingly the precipitation of secondaty phases at the Sic interface in (Al-Zn-Mg) alloy (10). The heterogeneous nucleation capability of inter-facial region formed in the near vicinity of Sic particulates can be attributed, in part, to the high dislocation density present in the composite matrix arising due to the mismatch between the coefficients of thermal expansion of the metal matrix and the ceramic reinforcement (l-4,6,10-12). This increase in dislocation density promotes the dislocationassisted diffusion of the alloying elements from the adjacent dislocation lean areas of the matrix resulting into the solute enrichment in the interfacial region thus making the compositional requirement for the precipitation more favorable. The solute enrichment results obtained in the present study are also consistent with the transmission electron microscopy results of other investigators conducted on spray deposited AlCuBiC metal matrix composites (4). Furthermore, the accelerated aging kinetics observed in the present study can also be attributed to an increase in the heterogeneous nucleation volume formed aromd Sic particulates with an increase in the volume fraction of Sic particulates (see Fig. 4). The volume traction of heterogeneous nucleation volume around SIC particulates, Vohv, and the mmaining matrix volume, V, Iy can be computed from the volume fraction of particulates, V, using the following expressions suggested by Wu and Lavernia (13) Vbhv=(R3-1)Vf V,,=
1 -R’V,
(1) (2)
where R is the ratio of the size of the heterogeneous nucleation zone to that of the reinforcement particulate. On the basis of the work discussed elsewhere (14) the value of R = 1.5 was used to calculate V, ,” for the SIC reinforced metal matrix composites used in the present study. The results of this calculation, summarized in Table 1, indicate an increase in the V,, with an increase in Vrof the Sic particulates. This heterogeneous nucleation volume or plastic zone (13) formed around Sic particulates as a result of the variation in coefficient of thermal expansion between the Sic particulates and the metallic matrix plays an instrumental role in heterogeneous nucleation of strengthening phases. The heterogeneous nucleation capability of the plastic zone can primarily be attributed to the presence of high dislocation density and relatively fmer subgrain size (3,15). The heterogeneous nucleation of matrix precipitates on the dislocation, for example, is favored since it lowers the elastic strain energy Bssociated with the dislocations (16). In addition, it has also been suggested that the distortion field surrounding edge dislocations reduces the energy barrier for nucleation of the g ’ phase, efficiently increasing aging kinetics (7,17, 18). This is also consistent with the microstructural characterization results obtained in the present study indicating the presence of Mg-rich precipitates in the near vicinity of Sic particulates (see Fig. 2). In addition, the presence of precipitates in the dislocation rich areas in the near vicinity of Sic particulates have also been reported by other investigators (4). Moreover, in related studies, Song and
Vol. 30, No. 8
HETEROGENEOUS NUCLEATION SITES
1029
Baker (5) attributed the accelerated aging observed in powder metallurgy processed AA 606 l/l 5 vol.% Sic composite to the lower activation energy required for the aging process arising as a result of the dislocation-assisted nucleation process in the composites. Finally, the accelerated aging kinetics observed in the present study can also be attributed to an increase in subgrain and grain boundary area in the metallic matrix. The decrease in subgrain and grain size of the metallic matrix as a result of the presence of ceramic reinforcement has previously been established by various investigators (4,15). An increase in grain boundary area in the matrix, for example, will assist in increasing the frequency of nucleation of the strengthening phases as a result of the reduced activation barrier for the heterogeneous nucleation (16). The experimental confirmation to this phenomenon has been established previously in case of Al based matrices (4,19). The results of this study thus clearly indicate that the increase in aging kinetics of the metallic matrix is due to the coupled influence of the presence of Sic particulates, formation of heterogeneous nucleation volume and the microstructural changes of the matrix in increasing the heterogeneous nucleation sites for the precipitation of the strengthening phases in the bulk matrix. CONCLUThe present investigation illustrate an increase in aging kinetics of the metallic matrix with an increase in volume fraction of Sic particulates. The accelerated aging kinetics was rationalized emphasizing particularly the pivotal role of Sic particulates in increasing the number of heterogeneous nucleation sites for the matrix precipitates in the bulk matrix. In particular, the present study established the role of Sic par&dates and the heterogeneous nucleation volume formed around Sic particulates in nucleating the strengthening phases. OWLEDGMENTS The authors wish to acknowledge Mr Thomas Tan, Mr Tung Siew Kong and Mr Boon Heng (National University of Singapore, Singapore) for their valuable assistance in experimental part of this study and for many useful discussions.
1. LA. Ibrahim, F.A. Mohamed and E.J. Lavernia, J. Mat. Sci. & 1137(1991). 2. A.L. Geiger and J.A. Walker, J. Met. Q(8), 8(1991). 3. R.J. Arsenault and N. Shi, Mat. Sci. and Eng. 175(1981). 4. M. Gupta, T.S. Srivatsan, F.A. Mohamed and E.J. Lavemia, J. Mat. Sci. 28,2245(1993). 5. Y. Song and T.N. Baker, Mat. Sci. and Tech. u 406(1994). 6.
M. Gupta, J. Juarez-Islas, W.E. Frazier, F.A. Mohamed and E.J. Lavemia, Metall. Trans. B & 719(1992).
7. Y. WuandE.J.Lavemia, J.Met.G(8), 16(1991). 8. M.J. Hadianfard, Y.W. Mai and J.C. Healy, J. Mat. Sci. 28,3665(1993). 9. H.J. Rack and R.W. Krenzer, Metall. Trans. AS, 335(1977). 10. D.J. Lloyd,Int. Mater. Rev. B(l),
l(1994).
11. W.S. :M.illerand F.J. Humphreys, Ser. Metall. et Mater. a
33(1991).
1030
12. 13. 14. 15. 16.
M. GUPTA et al.
Vol. 30, No. 8
R.J. Arsenault, Ser. Metall. et Mater. Z 2617(1991). Y. Wu and E.J. Lavemia, Ser. Metall. et Mater. 22,173(1992). J.K. b, Y.Y. Earmme, H.I. Aaroson and KC. Russell, Metall. Trans. A fi 1837(1980). R.J. Arsenault, L. Wang and C.R. Feng,ActaMetall. et Mater. a47(1991). P.G. Shewmon, Transformations in Metals, p. 214, p. 309. McGraw Hill Book Company, USA( 1969). 17. I. Dutta and D.L. Bourell, Mater. Sci. Eng. m 67(1989). 18. I. Dutta and D.L. Bourell, Acta Met. a 2041(1990). 19. T.S. Srivatsan, M. Gupta, F.A. Mohamed and E.J. Lavemia, Aluminium &, 494(1992).
Pergamon
EFFECT OF INCREASE JN HETEROGENEOUS NUCLXATION SITES ON THE AGING BEHAVIOR OF 6061/SiC METAL MATRIX COMl’OSlTES
M. Gupta and M.K. Surappa* Department of Mechanical and Production Engineering National University of Singapore, 10 Kent Ridge Crescent, Singapore 05 11 ‘Department of Metallurgy, Indian Institute of Science, Bangalore, India 5600 12 (Received March 3,199s; Refereed)
ABSTRACT In this study, aging characteristics of an aluminum based metallic matrix reinforced with various volume fraction of silicon carbide was investigated. The results showed an increase in aging kinetics with an increase in volume &action of Sic particulates. Results of microstructural characterization studies exhibited the presence of solute rich zone in the near vicinity of Sic particulates and the nucleation of secondary phases both at and in near vicinity of Sic particulates. The results of accelerated aging kinetics thus observed were rationalized in terms of an increase in the heterogeneous nucleation volume in the metallic matrix and the constitutional characteristics of the Al/Sic inter-facial region. MATERIALS INDEX: nucleation
Metal matrix composites, Aging, Heterogeneous
ODUCTION The innate ability of metal matrix composites (MMCs) to combine the properties of ceramic phase with that of the metallic phase has been instrumental in the insurgence of extensive research activities $1 over the world (1). The suitability of these composite materials, however, lies in the judicious selection of synthesizing/processing technique, matrix material and ceramic reinforcement. Amongst the various matrix-reinforcement combinations, silicon carbide (Sic) reinforced ahnninum matrices have been widely acknowledged as the potential candidates for the weight critical automobile and aerospace applications (2). The addition of 1023
1024
M. GUPTA et al.
Vol. 30, No. 8
Sic particulates has been correlated with an increase in mechanical properties such as yield strength, ultimate tensile strength and elastic modulus of the aluminum based metallic matrices (2). This improvement can be ascribed, in part, to the particulates-assisted microstrnctural strengthening. In related studies, for example, investigators have reported the change in microstructural features such as dislocation density (3), grain size (4), precipitation behavior (5) and excess solid solubility (6) as a result of the presence of ceramic particulates. The presence of ceramic reinforcement, in addition, has also been correlated with the kinetics of microstructural evolution during aging step of the conventional T6 heat treatment (4,5,7). The change in aging kinetics of the metallic matrix as a result of the presence of a fixed volume fraction of ceramic particulates, for example, has been reported by various investigators (4,5,7). However, no systematic studies have been carried out to correlate the effect of volume fraction of Sic particulates with the increase in heterogeneous nucleation sites in the metallic matrix and the ensuing accelerated aging characteristic of the reinforced metallic matrix. Accordingly, the present study was undertaken to provide insight into the effect of volume fraction of the incorporated ceramic reinforcement on the aging response of an aluminum based metallic matrix processed using casting route. Particular emphasis was placed to correlate the aging results obtained on the reinforced samples with the microstructural characteristics.
The starting matrix material used in the present study was AA 606 1 aluminum alloy containing (in wt. %): 0.6Si - 1.05Mg - 0.46Fe - 0.25Cu - 0.25Cr - Al (bal.). Silicon carbide (a-Sic) particulates with an average size of 54 pm were selected as the reinforcement phase. Three casting experiments were made to synthesize unreinforced alloy and composites containing 10 (V, = 0.086) and 15 (V, = 0.130) wt. % Sic part&dates. The synthesis of metal matrix composites used in the present study was carried out according to the following procedure. The metal ingots, prior to melting, were cleaned and melted under a cover of nitrogen gas in order to mmimize the oxidation of molten metal. Sic particulates, preheated to 900 “C, were then added into the molten metal stirred using an impeller. The melt was alloyed with small amounts of Mg and Zr (Mg+Zr < 1 wt. %) in order to improve the wettability of Sic particulates. The composite melt thus obtained was poured into cylindrical cast iron moulds (75 mm diameter and 150 mm height). For the purpose of comparison, the base alloy was cast after adding similar amounts of magnesium and zirconium (Mg+Zr < 1%) into 606 1 Al alloy melt following the similar casting parameters. The as-cast cylindrical bars were homogenized at540”Cfor3hoursandthenextrndedusinganextrusionratioof10:1 at500°Cinto16mm diameter rods in order to close the residual porosity. Aging studies were carried out on extruded unreinforced and reinforced samples (16 mm diameter x 10 mm height) in order to find out the time required to attain peak hardness. It essentially involved solutionizing of samples for one hour at 530 “C, quenching in cold water followed by isothermal aging at 177 “C for various intervals of time. Brine11 hardness measurements were made using 5 mm diameter indentor with a 500 kg load. Microsuucmml characterization studies were primarily conducted using a JEOL scanning electron microscope (SEM) equipped with EDS (Energy Dispersive Spectroscopy) on the plastic mounted and metallographically polished composite samples in order to examine the
HETEROG~OUS
Vol. 30,No. 8
NUCLFATlON SITES
1025
precipitation behavior and segregation of alloying elements in the inter-facial region between the Al alloy matrix and ceramic particulate.
The results of the aging studies conducted on the monolithic alloy and composites containing 8.6 and 13 vol. % Sic particulates are shown in Fig. 1. The results revealed that aging time for peak hardness for monolithic alloy was 8 hours when compared with 6 hours for 606 118.6 vol. % Sic and 2 hours for 6061/l 3 vol. % Sic composite samples. The results of scamring electron microscopy carried out on the composites samples revealed: a) absence of voids or other discontinuities in the matrix and particulate-matrix interface, b) good interfacial bonding between dispersed Sic particulates and the matrix, and c) pretkrential presence of Mg rich precipitates at and in near vicinity of Sic-matrix interface (see Fig. 2). The identity of Mg-rich precipitates was conformed using EDS point analyses. In addition, the results of EDS analyses conducted on 6061 /13 .Ovol. % Sic samples in both as-extruded and peak aged conditions revealed enrichment of Mg and Si in the near vicinity of Sic particulates when compared to the bulk material composition. The variation in weight percent silicon observed at various distances ti-om the Sic-matrix interface, for example, is graphically represented in Fig. 3.
125
I
2.5
I
5
El
6061
0
6W/S.6
A
6061113 vol. 96 Sic
I
7.5
I
10
vol.% SE
I
12.5
Aging Time (Hrs) FIG. 1 Graphical repmsentation of aging studies conducted on unreinforced 6061 ahoy, 606V8.6 vol. % SIC and 6061113 vol.% Sic. All materials were solution-treated at 530 “C for 1 h.
1026
Vol. 30: No. 8
M. GUPTA et al.
FIG. 2 Representative SEM micrograph taken from the 606 l/l 3 vol. % SIC composite sample showing the presence of Mg-rich intermetallic phase at the 606 l/SIC interface (marked A) and in the near vicinity of Sic particulate (marked B).
8 4
Cl
Peakaged
0
As-extruded
Distance from interface (microns) FIG. 3 Graphical representation of the segregation pattern of Si at 6061/SiC interfacial regim o&erved in as-exiruded and peak aged 6061113 vol. % Sic samples.
1027
HETFXOGENEOLJS NUCLEATlON SITEB
Vol. 30, Not.8
Matrix Heterogenous nucleation zone I Plastic zone
Sic
FIG. 4 !khemaGcillustration representing the heterogeneous nucleation zone around Sic patticulates in the metallic matrix.
DISCUSSION The 6061 Al-Mg-Si alloy is one of the most common aluminum based alloy used as metallic matrix in discontinuously reinforced aluminum based metal matrix composites ($8). In order to provide insight into the aging results obtained in the present study, some background information on the precipitation sequence of 6061 Al alloy is provided here. The precipitation sequence of reinforced 606 1 Al alloy have been shown to be identical to those of unreinforced 6061 Al alloy (5,7,9) namely: supersaturated solid solution --> needle shaped G.P. zones --> needle shaped p ” semicoherent phase --> rod shaped p’ transition phase (primary strengthening phase) --> equilibrium and incoherent p (MgrSi) phase. In this work, the results of the aging studies revealed an increase in aging kinetics of the metahic matrix with an increase in volume fraction of Sic particulates. The accelerated aging behavior observed in the present study can be attributed to the enhanced heterogeneous nucleation of the strengthening phases in the metallic matrix during the aging step of the conventional T6 heat treatment. The enhanced heterogeneous nucleation of the strengthening phases in the metallic matrix can primarily be attributed to: a) the pres’ence of Sic particulates, b) formation of defect-rich interfacial region in the near vicinity of Sic particulates, and
TABLE 1 Results of the Heterogeneous Volume Computations.
6061 6061 6061
0.000 0.086 0.130
-0.204 0.308
-0.710 0.562
8 6 2
1028
M GUPTA et al.
Vol. 30, No. 8
c) modification of microstructural features in the metallic matrix due to the presence of Sic particulates The ability of the Sic particulates to act as heterogeneous nucleation sites is consistent with the microstructural characterization studies revealing the presence of Mg-rich phases located at the Al/Sic interface (see Fig. 2). Moreover, the present results are also consistent with the research findings of other investigators showing convincingly the precipitation of secondaty phases at the Sic interface in (Al-Zn-Mg) alloy (10). The heterogeneous nucleation capability of inter-facial region formed in the near vicinity of Sic particulates can be attributed, in part, to the high dislocation density present in the composite matrix arising due to the mismatch between the coefficients of thermal expansion of the metal matrix and the ceramic reinforcement (l-4,6,10-12). This increase in dislocation density promotes the dislocationassisted diffusion of the alloying elements from the adjacent dislocation lean areas of the matrix resulting into the solute enrichment in the interfacial region thus making the compositional requirement for the precipitation more favorable. The solute enrichment results obtained in the present study are also consistent with the transmission electron microscopy results of other investigators conducted on spray deposited AlCuBiC metal matrix composites (4). Furthermore, the accelerated aging kinetics observed in the present study can also be attributed to an increase in the heterogeneous nucleation volume formed aromd Sic particulates with an increase in the volume fraction of Sic particulates (see Fig. 4). The volume traction of heterogeneous nucleation volume around SIC particulates, Vohv, and the mmaining matrix volume, V, Iy can be computed from the volume fraction of particulates, V, using the following expressions suggested by Wu and Lavernia (13) Vbhv=(R3-1)Vf V,,=
1 -R’V,
(1) (2)
where R is the ratio of the size of the heterogeneous nucleation zone to that of the reinforcement particulate. On the basis of the work discussed elsewhere (14) the value of R = 1.5 was used to calculate V, ,” for the SIC reinforced metal matrix composites used in the present study. The results of this calculation, summarized in Table 1, indicate an increase in the V,, with an increase in Vrof the Sic particulates. This heterogeneous nucleation volume or plastic zone (13) formed around Sic particulates as a result of the variation in coefficient of thermal expansion between the Sic particulates and the metallic matrix plays an instrumental role in heterogeneous nucleation of strengthening phases. The heterogeneous nucleation capability of the plastic zone can primarily be attributed to the presence of high dislocation density and relatively fmer subgrain size (3,15). The heterogeneous nucleation of matrix precipitates on the dislocation, for example, is favored since it lowers the elastic strain energy Bssociated with the dislocations (16). In addition, it has also been suggested that the distortion field surrounding edge dislocations reduces the energy barrier for nucleation of the g ’ phase, efficiently increasing aging kinetics (7,17, 18). This is also consistent with the microstructural characterization results obtained in the present study indicating the presence of Mg-rich precipitates in the near vicinity of Sic particulates (see Fig. 2). In addition, the presence of precipitates in the dislocation rich areas in the near vicinity of Sic particulates have also been reported by other investigators (4). Moreover, in related studies, Song and
Vol. 30, No. 8
HETEROGENEOUS NUCLEATION SITES
1029
Baker (5) attributed the accelerated aging observed in powder metallurgy processed AA 606 l/l 5 vol.% Sic composite to the lower activation energy required for the aging process arising as a result of the dislocation-assisted nucleation process in the composites. Finally, the accelerated aging kinetics observed in the present study can also be attributed to an increase in subgrain and grain boundary area in the metallic matrix. The decrease in subgrain and grain size of the metallic matrix as a result of the presence of ceramic reinforcement has previously been established by various investigators (4,15). An increase in grain boundary area in the matrix, for example, will assist in increasing the frequency of nucleation of the strengthening phases as a result of the reduced activation barrier for the heterogeneous nucleation (16). The experimental confirmation to this phenomenon has been established previously in case of Al based matrices (4,19). The results of this study thus clearly indicate that the increase in aging kinetics of the metallic matrix is due to the coupled influence of the presence of Sic particulates, formation of heterogeneous nucleation volume and the microstructural changes of the matrix in increasing the heterogeneous nucleation sites for the precipitation of the strengthening phases in the bulk matrix. CONCLUThe present investigation illustrate an increase in aging kinetics of the metallic matrix with an increase in volume fraction of Sic particulates. The accelerated aging kinetics was rationalized emphasizing particularly the pivotal role of Sic particulates in increasing the number of heterogeneous nucleation sites for the matrix precipitates in the bulk matrix. In particular, the present study established the role of Sic par&dates and the heterogeneous nucleation volume formed around Sic particulates in nucleating the strengthening phases. OWLEDGMENTS The authors wish to acknowledge Mr Thomas Tan, Mr Tung Siew Kong and Mr Boon Heng (National University of Singapore, Singapore) for their valuable assistance in experimental part of this study and for many useful discussions.
1. LA. Ibrahim, F.A. Mohamed and E.J. Lavernia, J. Mat. Sci. & 1137(1991). 2. A.L. Geiger and J.A. Walker, J. Met. Q(8), 8(1991). 3. R.J. Arsenault and N. Shi, Mat. Sci. and Eng. 175(1981). 4. M. Gupta, T.S. Srivatsan, F.A. Mohamed and E.J. Lavemia, J. Mat. Sci. 28,2245(1993). 5. Y. Song and T.N. Baker, Mat. Sci. and Tech. u 406(1994). 6.
M. Gupta, J. Juarez-Islas, W.E. Frazier, F.A. Mohamed and E.J. Lavemia, Metall. Trans. B & 719(1992).
7. Y. WuandE.J.Lavemia, J.Met.G(8), 16(1991). 8. M.J. Hadianfard, Y.W. Mai and J.C. Healy, J. Mat. Sci. 28,3665(1993). 9. H.J. Rack and R.W. Krenzer, Metall. Trans. AS, 335(1977). 10. D.J. Lloyd,Int. Mater. Rev. B(l),
l(1994).
11. W.S. :M.illerand F.J. Humphreys, Ser. Metall. et Mater. a
33(1991).
1030
12. 13. 14. 15. 16.
M. GUPTA et al.
Vol. 30, No. 8
R.J. Arsenault, Ser. Metall. et Mater. Z 2617(1991). Y. Wu and E.J. Lavemia, Ser. Metall. et Mater. 22,173(1992). J.K. b, Y.Y. Earmme, H.I. Aaroson and KC. Russell, Metall. Trans. A fi 1837(1980). R.J. Arsenault, L. Wang and C.R. Feng,ActaMetall. et Mater. a47(1991). P.G. Shewmon, Transformations in Metals, p. 214, p. 309. McGraw Hill Book Company, USA( 1969). 17. I. Dutta and D.L. Bourell, Mater. Sci. Eng. m 67(1989). 18. I. Dutta and D.L. Bourell, Acta Met. a 2041(1990). 19. T.S. Srivatsan, M. Gupta, F.A. Mohamed and E.J. Lavemia, Aluminium &, 494(1992).