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AA 2024 composites
Journal of Materials Processing Technology 95 (1999) 210±215
The effects of SiC particle volume fraction on the microstructure and hot workability of SiCp/AA 2024 composites Byung-Chul Koa, Gyeong-Su Parkb, Yeon-Chul Yooa,* a Department of Metallurgical Engineering, Inha University, 253, Yonghyeon-dong, Nam-ku, Inchon, 402-751, South Korea Analytical Engineering Lab, Corporate TCS Center, Samsung Advanced Institute of Technology, Suwon, 440-600, South Korea
b
Received 7 May 1998
Abstract The hot workability of AA 2024 composites reinforced with different volume fractions (5, 10, 15, 20, and 30 vol%) of SiCp was investigated within the temperature range 320±5008C at a strain-rate of 1.0 sÿ1. Dynamic recrystallization (DRX) or dynamic recovery (DRV) of the composites was studied from the ¯ow curves and deformed microstructures. The effect of the SiCp volume fraction on the critical strain for the onset of DRX was analyzed from the relationship between the work-hardening rate and effective strain during hot deformation. The addition of the SiCp to an AA 2024 matrix alloy increased the dislocation density, resulting in a high ¯ow stress and a low critical strain for the DRX of the composites. Upon increasing the SiCp volume fraction, the ¯ow stress and the temperature showing the highest failure strain increased. Also, the difference in ¯ow stress amongst the composites decreased with increasing deformation temperature. This result has been discussed in relation to dislocation density and stacking fault in the matrix of the composites. # 1999 Elsevier Science S.A. All rights reserved. Keywords: Hot restoration; Dynamic recrystallization; Work-hardening rate; Critical strain; Flow stress; Flow strain
1. Introduction Because Al-based composites reinforced with SiC whisker (SiCw) or SiC particle (SiCp) show a high strength, excellent wear resistance, and good damage tolerance, they are currently under investigation for industrial applications [1,2]. To improve the hot workability of the composites during forming processes such as rolling, extrusion, and forging, it is necessary to identify the effect of reinforcements on their hot-restoration, ¯ow stress, and ¯ow strain. It has been reported that SiCp reinforcements in the composites increase the hot-strength and reduce the hotductility of the composites because they increase the initial high dislocation density caused by the difference in thermal expansion coef®cient (CTE) of the matrix/reinforcements and by the constraints of plastic ¯ow arising from their relative rigidity during hot deformation [3±5]. Also, the hot deformation behaviours of Al-based composites are quite different from those of the unreinforced monolithic alloy [4± 7]. For example, the reinforcement in the Al-based compo*Corresponding author. Tel.: +82-32-860-7535; fax: +82-32-862-5546 E-mail address: [email protected] (Y.-C. Yoo)
sites contributes to a development of a small equi-axed substructure compared to that of the unreinforced monolithic alloy [6]. The addition of SiCw to the AA 2124 matrix alloy decreases the critical strain for the onset of DRX by increasing the dislocation density during hot deformation [7]. Further, the addition of SiCp to the AA 2024 matrix leads to different strain-rate sensitivity of the composites, which increases with decrease of the SiCp size [8]. Moreover, SiCw are more effective for the improvement of the high temperature ¯ow stress of the composites than is SiCp, resulting in a dynamically recrystallized grain with small sub-grains [9]. In this study, the hot deformation behaviour, such as hot restoration, ¯ow stress, and ¯ow strain, of AA 2024 composites reinforced with different volume fractions of SiCp is investigated by means of the torsion test. 2. Experimental 2.1. Composites fabrication To investigate the hot deformation behaviour of AA 2024 composites reinforced with different volume fractions (5,
0924-0136/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 9 9 ) 0 0 2 9 2 - 7
B.-C. Ko et al. / Journal of Materials Processing Technology 95 (1999) 210±215 Table 1 Specification for the AA 2024 composites reinforced with different volume fractions of Sicp
SiCp/Al 2024 composites
Nomination
SiCp volume fraction
Composite Composite Composite Composite Composite
5 10 15 20 30
P5 P10 P15 P20 P30
10, 15, 20, and 30 vol%) of SiCp, the composites were fabricated by the powder metallurgy route. All the composites fabricated in this study are summarized in Table 1. The materials used in this work are SiCp reinforcements (8 mm) and AA 2024 alloy powders (44 mm diameter). For the fabrication of the composites, the Al 2024 powders and SiCp reinforcements were ball-milled for 72 h and the resulting mixtures compacted by hot-pressing at 5208C and 120 MPa in vacuum before cooling. The hot-pressed billets with a diameter of 50 mm and a length of 50 mm were hotextruded at an extrusion ratio of 25:1 at 4308C. 2.2. Hot torsion tests Torsion specimens with a gauge length of 10 mm and a diameter of 7 mm were machined from the extruded bars. The samples for torsion tests were heated to 320±5008C using a dual elliptical radiant furnace and then kept at this temperature for 10 min. The torsion tests were conducted at a strain rate of 1.0 sÿ1. The axis of rotation was parallel to the extrusion axis with the shear direction at right angles to it. Subsequently, the torsion-tested specimens were quenched immediately into water at 258C. The effective stress () and effective strain () of the composites were calculated by the von Mises criterion [10] using the torque
211
moment (M) and angular displacement () measured from the torsion tests. The microstructures of the extruded composites were observed by optical microscopy (OM). Transmission electron microscopy (TEM) was used to examine the microstructures of the hot-deformed composites. Also, to make clear the microstructure of the hot-deformed composites at the atomic scale, a high-resolution electron microscope Ê ) was utilized. (JEM-ARM1250, 1250 kV, resolution: 1.0 A The specimens for TEM observation were sectioned perpendicularly to the extrusion axis. The TEM specimens were mechanically ground to 80 mm and then jet-polished with a solution of 25% nitric acid and 75% methanol using a potential of 30 V and 60 mA at about ÿ408C. 3. Results and discussion 3.1. Microstructure of the extrudates Fig. 1(a)±(d) show the optical micrographs of the hotextruded Composites P5, P10, P15, and P30 along the extrusion direction, respectively. Upon increasing the SiCp volume fraction, the interparticle spacing between the SiCp decreases and the matrix grains change from elongated to equi-axed form. Elongated matrix grains, of 7.1 mm, are frequently observed along the extrusion direction in Composite P5 (Fig. 1(a)). For Composite P15, small equi-axed matrix grains are observed around the SiCp. The SiCp reinforcements in Composite P15 are relatively homogeneously distributed in the matrix, the grain size of which is 5.6 mm. (Fig. 1(c)). It is also found that the inter-particle spacing between the SiCp is 8.4 mm. For Composite P30, the matrix grain size and the interparticle spacing are found to be 4.1 and 4.3 mm, respectively (Fig. 1(d)). The
Fig. 1. Optical micrographs of the extruded Composites: (a) P5; (b) P10; (c) P15; and (d) P30; along the extrusion direction.
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B.-C. Ko et al. / Journal of Materials Processing Technology 95 (1999) 210±215
Fig. 2. Grain size and interparticle spacing (d) of the composites as a function of SiCp volume fraction.
matrix grain size in the SiCp-rich region is smaller than that in the SiCp-poor region. The degree of SiCp clustering, resulting in SiCp rich/poor regions, increases with increase in the SiCp volume fraction. These results indicate that the inter-particle spacing, and the matrix grain size and shape vary with the SiCp volume fraction. The grain size and interparticle spacing (d) of the composites as a function of the SiCp volume fraction (Fv) are presented in Fig. 2. 3.2. Flow curves of the composites Fig. 3(a) shows the ¯ow curves of the composites and monolithic AA 2024 alloy deformed at 3708C. The ¯ow curves of all of the composites showed a single peak followed by a low steady state due to DRX. As the volume fraction of SiCp increases, the ¯ow stress of the composites increases and the failure strain decreases. Composite P30 exhibits the highest ¯ow stress (171.1 MPa) at 3708C. The ¯ow stress (116.2 MPa) of the AA 2024 monolithic alloy is lower than that of the composites, but the failure strain (22.1) is much higher than that of the composites. It is found that the difference in ¯ow stresses between Composites P20 and P30 becomes small, Composite P30 showing a higher work-hardening behaviour than that of the other composites and monolithic AA 2024 alloy. The variation of the ¯ow stress of all of the composites with deformation temperature is presented in Fig. 3(b). Upon increasing the deformation temperature, the ¯ow
Fig. 3. Presenting: (a) effective stress ()±effective strain () curves of the AA 2024 composites reinforced with different volume fractions of SiCp (Fv) deformed at 3708C; and (b) the variation of the flow stress of the composites with deformation temperature.
stress of Composite P30 decreases rapidly compared to that of the other composites. At temperatures above 4308C, the difference in ¯ow stress amongst the composites is small because the dislocations can climb at the matrix/reinforcement interfaces during hot deformation [11]. It is found that the work- hardening rate of the composites and monolithic AA 2024 alloy deformed at 4308C were lower than that when deformed at 3708C. Therefore, with increasing deformation temperature, the critical volume fraction of SiCp contributing to a strengthening of the composites decreases. This result implies that at high temperature of 4808C, the difference in ¯ow stresses amongst the composites is quite small and there would be little SiCp strengthening at higher temperatures. 3.3. Critical strain and work-hardening rate Table 2 shows the work-hardening rate (d/d) of composites deformed at 3708C. The work-hardening rate
Table 2 Work-hardening rate of the AA 2024 composites reinforced with different volume fractions of SiCp deformed at various temperatures. Temperature (8C)
Work-hardening rate (MPa) (d/d, 0.05) Matrix
Composite P5
Composite P10
Composite P15
Composite P20
Composite P30
320 370 430 480 500
1178.9 1575.6 696.2 578.1 ±
1858.2 1785.6 1118.9 697.9 518.8
2000.1 1889.5 1189.1 789.5 567.8
2675.2 2214.9 1255.5 805.4 678.5
2755.5 2376.6 1381.3 877.5 744.3
2898.4 2358.6 1182.2 687.7 538.3
B.-C. Ko et al. / Journal of Materials Processing Technology 95 (1999) 210±215
213
Table 3 Critical strain for the onset of DRX of the composites reinforced with different volume fractions of SiCp deformed at various temperatures. Temperature (8C) 320 370 430 480 500
Critical strain (c) Matrix
Composite P5
Composite P10
Composite P15
Composite P20
Composite P30
0.123 0.121 0.115 0.094 ±
0.112 0.110 0.108 0.089 ±
0.090 0.089 0.085 0.074 0.066
0.078 0.071 0.065 0.063 0.055
0.075 0.070 0.063 0.062 0.058
0.071 0.070 0.064 0.062 0.059
is calculated from the ¯ow curves under an equivalent deformation strain of 0.05 using the method reported by Ryan and McQueen [12]. The critical strain (c) for the onset of DRX is determined from the relationship between the work-hardening rate and effective strain using the ¯ow curve in Fig. 2 [8,11]. Composite P20, amongst the composites used, shows the highest work-hardening rate. The value of c of the composites deformed at 4308C is smaller than that of the composites deformed at 3708C. This is because with increasing deformation temperature, the work-hardening of the composites decreases due to an increase in grain size and a decrease in dislocation density of the matrix. Thus, it is found that the value of c for DRX of the composites decreases with increasing deformation temperature. Results of the value of c of the composites according to the SiCp volume fraction and deformation temperature are shown in Table 3. The values of c of Composite P20 deformed at 3208C, 3708C, 4308C, 4808C, and 5008C are 0.075, 0.070, 0.063, 0.062, and 0.058, respectively: these are smaller than those of Composites P5, P10, and P15 and are quite similar to those of Composite P30. This result suggests that Composite P20 is the most effective for the nucleation for DRX at an earlier strain stage during hot deformation. This is because, even though Composite P20 shows a relatively non-uniform distribution of SiCp, the matrix grains are smaller and the dislocation density is higher relative to those of the other Composites, P5, P10, and P15 and monolithic AA 2024 matrix alloy. Upon increasing the SiCp volume fraction from 20 to 30, the degree of SiCp clustering increases, leading to a decrease in ability of load transfer from the matrix to the SiCp during hot deformation. Therefore, Composite P30 may give rise to a lower work-hardening rate than Composite P20. 4. Microstructure Fig. 4(a) and (b) show the TEM bright ®eld images of Composite P15 prior to hot deformation and Composite P10 deformed at 3208C, respectively. In Fig. 4(a), many dislocations are observed near to the SiCp and the dislocation density increases with increasing SiCp volume fraction. The high dislocations around the SiCp can promote the nucleation of DRX, giving rise to small sub-grains during hot deformation. This suggests that the dynamically recrystal-
lized grains are in¯uenced signi®cantly by the SiCp volume fraction of the composites. In Fig. 4(b), it is found that small sub-grains with 0.5 mm are formed around the SiCp during deformation. The DRX grains are predominantly observed around the SiCp. The sub-grain size of Composite P10 is larger than that (0.3 mm) of Composite P15 (Fig. 4(c)). Fig. 4(c) and (d) show the TEM bright ®eld images of Composite P15 deformed at 3708C and 4808C, respectively. Upon increasing the deformation temperature, the dislocation density decreases and the grain size increases for all of the composites. At 4808C, dislocation sub-structures are frequently observed near to the SiCp (Fig. 4(d)). Fig. 5(a) and (b) show HREM images of stacking faults, observed at the helical dislocation, in the matrix of Composites P5 and P15 deformed at 4808C, respectively. It is found that the width of the stacking faults of these two composites is 2 times that of an inter-planar spacing distance of {111} planes. On the other hand, the stacking faults are more frequently observed in Composite P15 than in Composite P5. It seems that the difference of the SiCp volume fraction affects the density of stacking faults and helical dislocations. Thus, the SiCp in Composite P15 may cause many helical dislocations compared to Composite P5 during hot deformation. Previously, the present authors reported that helical dislocations were frequently observed in the matrix of the composites, caused by the difference in thermal expansion coef®cient (CTE) between the Al 2024 matrix and SiC reinforcements [8]. Thus, it can be expected that with increasing SiCp volume fraction, there would be many stacking faults in the matrix of Composite P15 compared to Composite P5. The high density of stacking faults in Composite P15 would make the composite cross slip easily, giving rise to a decrease in ¯ow stress. These results also imply that with increasing deformation temperature, the difference in ¯ow stress between the composites decreases. Therefore, the ¯ow stress of Composite P15 would be similar to that of Composite P5 at 4808C. 4.1. Failure strain Fig. 6 shows the failure strain of the composites as a function of deformation temperature. The failure strain increases with increasing deformation temperature and then decreases. The highest failure strains of Composites P5, P10, P15, P20, and P30 are obtained at 3708C, 4308C,
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B.-C. Ko et al. / Journal of Materials Processing Technology 95 (1999) 210±215
Fig. 4. TEM bright field images of: (a) Composite P15 prior to hot deformation; (b) Composite P10 deformed at 3208C; and Composite P15 deformed at (c) 3708C and (d) 4808C, respectively.
Fig. 5. HREM images of stacking faults in the matrix of the Composites: (a) P5; and (b) P15; deformed at 4808C.
B.-C. Ko et al. / Journal of Materials Processing Technology 95 (1999) 210±215
Fig. 6. Failure strain of the AA 2024 composites reinforced with different volume fractions of SiCp deformed at various temperatures.
4808C, 4808C, and 4808C, respectively. It has been reported that superplastic-like behaviour or the highest failure strain of Al-based composite reinforced with SiCw, appeared at around the solidus temperature. For examples, the large tensile elongation of 400% was obtained in the SiCw/Al 6061 composite at 6008C [13], which is above the solidus temperature. Also, the highest failure strain was obtained in the SiCw/Al 2124 composite at 5008C [7], which is below the solidus temperature. The solidus temperatures of monolithic Al 6061 and Al 2024 alloys have been reported as 5828C and 5088C, respectively. The highest failure strain of the monolithic AA 2024 alloy is obtained at 3708C. At the same time, for Composites P5, P10, and P15, the temperature showing the highest failure strain moves to a higher value, implying that different temperatures should be considered, to improve the hotductility of the composites during hot deformation. It is also found that amongst the composites used, the highest failure strain is obtained in Composite P15: this is because the SiCp are uniformly distributed in the equi-axed matrix grain. 5. Conclusions From the ¯ow curves and hot-deformed microstructures, it was found that DRX was responsible for the hot restoration of the composites. Upon increasing the SiCp volume fraction, the ¯ow stress increased and the failure strain decreased. In addition, the temperature showing the highest
215
failure strain increased to a higher value. The difference in ¯ow stresses of the composites reinforced with 20 and 30 vol% of SiCp deformed at 3708C was small. The work-hardening rate of the composites did not increase linearly with increase in the SiCp volume fraction. The composite reinforced with 20 vol% of SiCp showed the highest work-hardening rate. Also, the critical strain for onset of DRX of the composites decreased with increase in the SiCp volume fraction, except for the composite reinforced with 30 vol% of SiCp. Also, upon increase the SiCp volume fraction, the degree of SiCp clustering increased, resulting in a decrease of load transfer from the matrix to the SiCp. Therefore, the critical strain for DRX of the composite reinforced with 20 vol% of SiCp was similar to that of the composite reinforced with 30 vol% of SiCp. Acknowledgements This work was supported by Korea Ministry of Education Research Fund for Advanced Materials in 1997. References [1] T.F. Klimowicz, J. Met. 46 (1994) 49. [2] D.M. Schuster, M.D. Skibo, R.S. Bruski, R. Provencher, G. Riverin, J. Met. 45 (1993) 26. [3] X. Xia, P. Sakaris, H.J. McQueen, Mater. Sci. Technol. 10 (1994) 487. [4] F.J. Humphreys, W.S. Miller, M.R. Djazeb, Mater. Sci. and Technol. 6 (1994) 1157. [5] S.I. Hong, G.T. Gray III, J.J. Lewandowski, Acta Metall. Mater. 41(8) (1993) 2337. [6] G. Gonzalez-Doncel, O.D. Sherby, Acta Metall. Mater. 41(10) (1993) 2797. [7] Y.C. Yoo, J.S. Jeon, H.I. Lee, Comp. Sci. and Technol. 57 (1997) 651. [8] B.C. Ko, K. Park, Y.C. Yoo, Mater. Sci. and Technol. 14(8) (1998) 765. [9] B.C. Ko, Y.C. Yoo, Comp. Sci. and Technol. 58 (1998) 479. [10] K. Mills, J.R. Davis, J.D. Destefani, D.A. Dieterich, G.M. Crankovic, H.J. Frissel, D.M. Jenkins, W.H. Cubberly, R.L. Stedfeld, Metals Handbook: Mechanical Testing, vol. 8, ASM, Metals Park, OH, 1985, p.154. [11] F.J. Humphrey, P.N. Kalu, Acta Metall. 35(12) (1987) 2815. [12] N.D. Ryan, H.J. McQueen, High Temp. Technol. 8(1) (1990) 27. [13] B.Q. Han, K.C. Chan, Scripta Metall. 36(5) (1997) 593.
The effects of SiC particle volume fraction on the microstructure and hot workability of SiCp/AA 2024 composites Byung-Chul Koa, Gyeong-Su Parkb, Yeon-Chul Yooa,* a Department of Metallurgical Engineering, Inha University, 253, Yonghyeon-dong, Nam-ku, Inchon, 402-751, South Korea Analytical Engineering Lab, Corporate TCS Center, Samsung Advanced Institute of Technology, Suwon, 440-600, South Korea
b
Received 7 May 1998
Abstract The hot workability of AA 2024 composites reinforced with different volume fractions (5, 10, 15, 20, and 30 vol%) of SiCp was investigated within the temperature range 320±5008C at a strain-rate of 1.0 sÿ1. Dynamic recrystallization (DRX) or dynamic recovery (DRV) of the composites was studied from the ¯ow curves and deformed microstructures. The effect of the SiCp volume fraction on the critical strain for the onset of DRX was analyzed from the relationship between the work-hardening rate and effective strain during hot deformation. The addition of the SiCp to an AA 2024 matrix alloy increased the dislocation density, resulting in a high ¯ow stress and a low critical strain for the DRX of the composites. Upon increasing the SiCp volume fraction, the ¯ow stress and the temperature showing the highest failure strain increased. Also, the difference in ¯ow stress amongst the composites decreased with increasing deformation temperature. This result has been discussed in relation to dislocation density and stacking fault in the matrix of the composites. # 1999 Elsevier Science S.A. All rights reserved. Keywords: Hot restoration; Dynamic recrystallization; Work-hardening rate; Critical strain; Flow stress; Flow strain
1. Introduction Because Al-based composites reinforced with SiC whisker (SiCw) or SiC particle (SiCp) show a high strength, excellent wear resistance, and good damage tolerance, they are currently under investigation for industrial applications [1,2]. To improve the hot workability of the composites during forming processes such as rolling, extrusion, and forging, it is necessary to identify the effect of reinforcements on their hot-restoration, ¯ow stress, and ¯ow strain. It has been reported that SiCp reinforcements in the composites increase the hot-strength and reduce the hotductility of the composites because they increase the initial high dislocation density caused by the difference in thermal expansion coef®cient (CTE) of the matrix/reinforcements and by the constraints of plastic ¯ow arising from their relative rigidity during hot deformation [3±5]. Also, the hot deformation behaviours of Al-based composites are quite different from those of the unreinforced monolithic alloy [4± 7]. For example, the reinforcement in the Al-based compo*Corresponding author. Tel.: +82-32-860-7535; fax: +82-32-862-5546 E-mail address: [email protected] (Y.-C. Yoo)
sites contributes to a development of a small equi-axed substructure compared to that of the unreinforced monolithic alloy [6]. The addition of SiCw to the AA 2124 matrix alloy decreases the critical strain for the onset of DRX by increasing the dislocation density during hot deformation [7]. Further, the addition of SiCp to the AA 2024 matrix leads to different strain-rate sensitivity of the composites, which increases with decrease of the SiCp size [8]. Moreover, SiCw are more effective for the improvement of the high temperature ¯ow stress of the composites than is SiCp, resulting in a dynamically recrystallized grain with small sub-grains [9]. In this study, the hot deformation behaviour, such as hot restoration, ¯ow stress, and ¯ow strain, of AA 2024 composites reinforced with different volume fractions of SiCp is investigated by means of the torsion test. 2. Experimental 2.1. Composites fabrication To investigate the hot deformation behaviour of AA 2024 composites reinforced with different volume fractions (5,
0924-0136/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 9 9 ) 0 0 2 9 2 - 7
B.-C. Ko et al. / Journal of Materials Processing Technology 95 (1999) 210±215 Table 1 Specification for the AA 2024 composites reinforced with different volume fractions of Sicp
SiCp/Al 2024 composites
Nomination
SiCp volume fraction
Composite Composite Composite Composite Composite
5 10 15 20 30
P5 P10 P15 P20 P30
10, 15, 20, and 30 vol%) of SiCp, the composites were fabricated by the powder metallurgy route. All the composites fabricated in this study are summarized in Table 1. The materials used in this work are SiCp reinforcements (8 mm) and AA 2024 alloy powders (44 mm diameter). For the fabrication of the composites, the Al 2024 powders and SiCp reinforcements were ball-milled for 72 h and the resulting mixtures compacted by hot-pressing at 5208C and 120 MPa in vacuum before cooling. The hot-pressed billets with a diameter of 50 mm and a length of 50 mm were hotextruded at an extrusion ratio of 25:1 at 4308C. 2.2. Hot torsion tests Torsion specimens with a gauge length of 10 mm and a diameter of 7 mm were machined from the extruded bars. The samples for torsion tests were heated to 320±5008C using a dual elliptical radiant furnace and then kept at this temperature for 10 min. The torsion tests were conducted at a strain rate of 1.0 sÿ1. The axis of rotation was parallel to the extrusion axis with the shear direction at right angles to it. Subsequently, the torsion-tested specimens were quenched immediately into water at 258C. The effective stress () and effective strain () of the composites were calculated by the von Mises criterion [10] using the torque
211
moment (M) and angular displacement () measured from the torsion tests. The microstructures of the extruded composites were observed by optical microscopy (OM). Transmission electron microscopy (TEM) was used to examine the microstructures of the hot-deformed composites. Also, to make clear the microstructure of the hot-deformed composites at the atomic scale, a high-resolution electron microscope Ê ) was utilized. (JEM-ARM1250, 1250 kV, resolution: 1.0 A The specimens for TEM observation were sectioned perpendicularly to the extrusion axis. The TEM specimens were mechanically ground to 80 mm and then jet-polished with a solution of 25% nitric acid and 75% methanol using a potential of 30 V and 60 mA at about ÿ408C. 3. Results and discussion 3.1. Microstructure of the extrudates Fig. 1(a)±(d) show the optical micrographs of the hotextruded Composites P5, P10, P15, and P30 along the extrusion direction, respectively. Upon increasing the SiCp volume fraction, the interparticle spacing between the SiCp decreases and the matrix grains change from elongated to equi-axed form. Elongated matrix grains, of 7.1 mm, are frequently observed along the extrusion direction in Composite P5 (Fig. 1(a)). For Composite P15, small equi-axed matrix grains are observed around the SiCp. The SiCp reinforcements in Composite P15 are relatively homogeneously distributed in the matrix, the grain size of which is 5.6 mm. (Fig. 1(c)). It is also found that the inter-particle spacing between the SiCp is 8.4 mm. For Composite P30, the matrix grain size and the interparticle spacing are found to be 4.1 and 4.3 mm, respectively (Fig. 1(d)). The
Fig. 1. Optical micrographs of the extruded Composites: (a) P5; (b) P10; (c) P15; and (d) P30; along the extrusion direction.
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B.-C. Ko et al. / Journal of Materials Processing Technology 95 (1999) 210±215
Fig. 2. Grain size and interparticle spacing (d) of the composites as a function of SiCp volume fraction.
matrix grain size in the SiCp-rich region is smaller than that in the SiCp-poor region. The degree of SiCp clustering, resulting in SiCp rich/poor regions, increases with increase in the SiCp volume fraction. These results indicate that the inter-particle spacing, and the matrix grain size and shape vary with the SiCp volume fraction. The grain size and interparticle spacing (d) of the composites as a function of the SiCp volume fraction (Fv) are presented in Fig. 2. 3.2. Flow curves of the composites Fig. 3(a) shows the ¯ow curves of the composites and monolithic AA 2024 alloy deformed at 3708C. The ¯ow curves of all of the composites showed a single peak followed by a low steady state due to DRX. As the volume fraction of SiCp increases, the ¯ow stress of the composites increases and the failure strain decreases. Composite P30 exhibits the highest ¯ow stress (171.1 MPa) at 3708C. The ¯ow stress (116.2 MPa) of the AA 2024 monolithic alloy is lower than that of the composites, but the failure strain (22.1) is much higher than that of the composites. It is found that the difference in ¯ow stresses between Composites P20 and P30 becomes small, Composite P30 showing a higher work-hardening behaviour than that of the other composites and monolithic AA 2024 alloy. The variation of the ¯ow stress of all of the composites with deformation temperature is presented in Fig. 3(b). Upon increasing the deformation temperature, the ¯ow
Fig. 3. Presenting: (a) effective stress ()±effective strain () curves of the AA 2024 composites reinforced with different volume fractions of SiCp (Fv) deformed at 3708C; and (b) the variation of the flow stress of the composites with deformation temperature.
stress of Composite P30 decreases rapidly compared to that of the other composites. At temperatures above 4308C, the difference in ¯ow stress amongst the composites is small because the dislocations can climb at the matrix/reinforcement interfaces during hot deformation [11]. It is found that the work- hardening rate of the composites and monolithic AA 2024 alloy deformed at 4308C were lower than that when deformed at 3708C. Therefore, with increasing deformation temperature, the critical volume fraction of SiCp contributing to a strengthening of the composites decreases. This result implies that at high temperature of 4808C, the difference in ¯ow stresses amongst the composites is quite small and there would be little SiCp strengthening at higher temperatures. 3.3. Critical strain and work-hardening rate Table 2 shows the work-hardening rate (d/d) of composites deformed at 3708C. The work-hardening rate
Table 2 Work-hardening rate of the AA 2024 composites reinforced with different volume fractions of SiCp deformed at various temperatures. Temperature (8C)
Work-hardening rate (MPa) (d/d, 0.05) Matrix
Composite P5
Composite P10
Composite P15
Composite P20
Composite P30
320 370 430 480 500
1178.9 1575.6 696.2 578.1 ±
1858.2 1785.6 1118.9 697.9 518.8
2000.1 1889.5 1189.1 789.5 567.8
2675.2 2214.9 1255.5 805.4 678.5
2755.5 2376.6 1381.3 877.5 744.3
2898.4 2358.6 1182.2 687.7 538.3
B.-C. Ko et al. / Journal of Materials Processing Technology 95 (1999) 210±215
213
Table 3 Critical strain for the onset of DRX of the composites reinforced with different volume fractions of SiCp deformed at various temperatures. Temperature (8C) 320 370 430 480 500
Critical strain (c) Matrix
Composite P5
Composite P10
Composite P15
Composite P20
Composite P30
0.123 0.121 0.115 0.094 ±
0.112 0.110 0.108 0.089 ±
0.090 0.089 0.085 0.074 0.066
0.078 0.071 0.065 0.063 0.055
0.075 0.070 0.063 0.062 0.058
0.071 0.070 0.064 0.062 0.059
is calculated from the ¯ow curves under an equivalent deformation strain of 0.05 using the method reported by Ryan and McQueen [12]. The critical strain (c) for the onset of DRX is determined from the relationship between the work-hardening rate and effective strain using the ¯ow curve in Fig. 2 [8,11]. Composite P20, amongst the composites used, shows the highest work-hardening rate. The value of c of the composites deformed at 4308C is smaller than that of the composites deformed at 3708C. This is because with increasing deformation temperature, the work-hardening of the composites decreases due to an increase in grain size and a decrease in dislocation density of the matrix. Thus, it is found that the value of c for DRX of the composites decreases with increasing deformation temperature. Results of the value of c of the composites according to the SiCp volume fraction and deformation temperature are shown in Table 3. The values of c of Composite P20 deformed at 3208C, 3708C, 4308C, 4808C, and 5008C are 0.075, 0.070, 0.063, 0.062, and 0.058, respectively: these are smaller than those of Composites P5, P10, and P15 and are quite similar to those of Composite P30. This result suggests that Composite P20 is the most effective for the nucleation for DRX at an earlier strain stage during hot deformation. This is because, even though Composite P20 shows a relatively non-uniform distribution of SiCp, the matrix grains are smaller and the dislocation density is higher relative to those of the other Composites, P5, P10, and P15 and monolithic AA 2024 matrix alloy. Upon increasing the SiCp volume fraction from 20 to 30, the degree of SiCp clustering increases, leading to a decrease in ability of load transfer from the matrix to the SiCp during hot deformation. Therefore, Composite P30 may give rise to a lower work-hardening rate than Composite P20. 4. Microstructure Fig. 4(a) and (b) show the TEM bright ®eld images of Composite P15 prior to hot deformation and Composite P10 deformed at 3208C, respectively. In Fig. 4(a), many dislocations are observed near to the SiCp and the dislocation density increases with increasing SiCp volume fraction. The high dislocations around the SiCp can promote the nucleation of DRX, giving rise to small sub-grains during hot deformation. This suggests that the dynamically recrystal-
lized grains are in¯uenced signi®cantly by the SiCp volume fraction of the composites. In Fig. 4(b), it is found that small sub-grains with 0.5 mm are formed around the SiCp during deformation. The DRX grains are predominantly observed around the SiCp. The sub-grain size of Composite P10 is larger than that (0.3 mm) of Composite P15 (Fig. 4(c)). Fig. 4(c) and (d) show the TEM bright ®eld images of Composite P15 deformed at 3708C and 4808C, respectively. Upon increasing the deformation temperature, the dislocation density decreases and the grain size increases for all of the composites. At 4808C, dislocation sub-structures are frequently observed near to the SiCp (Fig. 4(d)). Fig. 5(a) and (b) show HREM images of stacking faults, observed at the helical dislocation, in the matrix of Composites P5 and P15 deformed at 4808C, respectively. It is found that the width of the stacking faults of these two composites is 2 times that of an inter-planar spacing distance of {111} planes. On the other hand, the stacking faults are more frequently observed in Composite P15 than in Composite P5. It seems that the difference of the SiCp volume fraction affects the density of stacking faults and helical dislocations. Thus, the SiCp in Composite P15 may cause many helical dislocations compared to Composite P5 during hot deformation. Previously, the present authors reported that helical dislocations were frequently observed in the matrix of the composites, caused by the difference in thermal expansion coef®cient (CTE) between the Al 2024 matrix and SiC reinforcements [8]. Thus, it can be expected that with increasing SiCp volume fraction, there would be many stacking faults in the matrix of Composite P15 compared to Composite P5. The high density of stacking faults in Composite P15 would make the composite cross slip easily, giving rise to a decrease in ¯ow stress. These results also imply that with increasing deformation temperature, the difference in ¯ow stress between the composites decreases. Therefore, the ¯ow stress of Composite P15 would be similar to that of Composite P5 at 4808C. 4.1. Failure strain Fig. 6 shows the failure strain of the composites as a function of deformation temperature. The failure strain increases with increasing deformation temperature and then decreases. The highest failure strains of Composites P5, P10, P15, P20, and P30 are obtained at 3708C, 4308C,
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Fig. 4. TEM bright field images of: (a) Composite P15 prior to hot deformation; (b) Composite P10 deformed at 3208C; and Composite P15 deformed at (c) 3708C and (d) 4808C, respectively.
Fig. 5. HREM images of stacking faults in the matrix of the Composites: (a) P5; and (b) P15; deformed at 4808C.
B.-C. Ko et al. / Journal of Materials Processing Technology 95 (1999) 210±215
Fig. 6. Failure strain of the AA 2024 composites reinforced with different volume fractions of SiCp deformed at various temperatures.
4808C, 4808C, and 4808C, respectively. It has been reported that superplastic-like behaviour or the highest failure strain of Al-based composite reinforced with SiCw, appeared at around the solidus temperature. For examples, the large tensile elongation of 400% was obtained in the SiCw/Al 6061 composite at 6008C [13], which is above the solidus temperature. Also, the highest failure strain was obtained in the SiCw/Al 2124 composite at 5008C [7], which is below the solidus temperature. The solidus temperatures of monolithic Al 6061 and Al 2024 alloys have been reported as 5828C and 5088C, respectively. The highest failure strain of the monolithic AA 2024 alloy is obtained at 3708C. At the same time, for Composites P5, P10, and P15, the temperature showing the highest failure strain moves to a higher value, implying that different temperatures should be considered, to improve the hotductility of the composites during hot deformation. It is also found that amongst the composites used, the highest failure strain is obtained in Composite P15: this is because the SiCp are uniformly distributed in the equi-axed matrix grain. 5. Conclusions From the ¯ow curves and hot-deformed microstructures, it was found that DRX was responsible for the hot restoration of the composites. Upon increasing the SiCp volume fraction, the ¯ow stress increased and the failure strain decreased. In addition, the temperature showing the highest
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failure strain increased to a higher value. The difference in ¯ow stresses of the composites reinforced with 20 and 30 vol% of SiCp deformed at 3708C was small. The work-hardening rate of the composites did not increase linearly with increase in the SiCp volume fraction. The composite reinforced with 20 vol% of SiCp showed the highest work-hardening rate. Also, the critical strain for onset of DRX of the composites decreased with increase in the SiCp volume fraction, except for the composite reinforced with 30 vol% of SiCp. Also, upon increase the SiCp volume fraction, the degree of SiCp clustering increased, resulting in a decrease of load transfer from the matrix to the SiCp. Therefore, the critical strain for DRX of the composite reinforced with 20 vol% of SiCp was similar to that of the composite reinforced with 30 vol% of SiCp. Acknowledgements This work was supported by Korea Ministry of Education Research Fund for Advanced Materials in 1997. References [1] T.F. Klimowicz, J. Met. 46 (1994) 49. [2] D.M. Schuster, M.D. Skibo, R.S. Bruski, R. Provencher, G. Riverin, J. Met. 45 (1993) 26. [3] X. Xia, P. Sakaris, H.J. McQueen, Mater. Sci. Technol. 10 (1994) 487. [4] F.J. Humphreys, W.S. Miller, M.R. Djazeb, Mater. Sci. and Technol. 6 (1994) 1157. [5] S.I. Hong, G.T. Gray III, J.J. Lewandowski, Acta Metall. Mater. 41(8) (1993) 2337. [6] G. Gonzalez-Doncel, O.D. Sherby, Acta Metall. Mater. 41(10) (1993) 2797. [7] Y.C. Yoo, J.S. Jeon, H.I. Lee, Comp. Sci. and Technol. 57 (1997) 651. [8] B.C. Ko, K. Park, Y.C. Yoo, Mater. Sci. and Technol. 14(8) (1998) 765. [9] B.C. Ko, Y.C. Yoo, Comp. Sci. and Technol. 58 (1998) 479. [10] K. Mills, J.R. Davis, J.D. Destefani, D.A. Dieterich, G.M. Crankovic, H.J. Frissel, D.M. Jenkins, W.H. Cubberly, R.L. Stedfeld, Metals Handbook: Mechanical Testing, vol. 8, ASM, Metals Park, OH, 1985, p.154. [11] F.J. Humphrey, P.N. Kalu, Acta Metall. 35(12) (1987) 2815. [12] N.D. Ryan, H.J. McQueen, High Temp. Technol. 8(1) (1990) 27. [13] B.Q. Han, K.C. Chan, Scripta Metall. 36(5) (1997) 593.