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Effects of thermal residual stress creeping on microstructure and tensile properties of SiC whisker reinforced aluminum matrix composite
Materials Science and Engineering A356 (2003) 17 /22 www.elsevier.com/locate/msea
Effects of thermal residual stress creeping on microstructure and tensile properties of SiC whisker reinforced aluminum matrix composite W.D. Fei *, M. Hu, C.K. Yao School of Materials Science and Engineering, Harbin Institute of Technology. P.O. Box 405, Harbin 150001, PR China Received 20 November 2001; received in revised form 11 October 2002
Abstract After quenching from 600 8C, the isothermal treatment at 80 8C for SiC whisker reinforced aluminum composite was carried out. The results indicated that the dislocation state in the matrix was changed greatly, especially in the region adjacent to the interface between SiC whisker and matrix. The change of dislocation state was considered as the result of creeping caused by thermal residual stress (TRS). It was found that the thermal residual stress creeping (TRSC) treatment is a new method to reduce effectively the TRS, and increase the strengths, including micro yield strength (s0.005), yield strength (s0.2), ultimate tensile strength (UTS), of SiCw/Al composite. The mechanism of TRSC of SiCw/Al composite was given in detail. After TRSC treatment for 500 h, the UTS of pure aluminum matrix composite was up to 330 MPa. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Thermal residual stress; Yield strength; Tensile strength
1. Introduction It is known that the thermal residual stress (TRS) caused by mismatch of coefficients of thermal expansion (CTEs) between the reinforcement and matrix is inherent and has some undesirable effects on the tensile properties of SiC whisker reinforced aluminum (SiCw/ Al) composite [1,2], especially on tensile yield strength [3,4]. The lower tensile yield strength resulting from residual stress limits the application of the composite in many precision structural parts. Owing to the complexity of material system and the importance of TRS itself in metal matrix composite (MMC), many researches on TRS of MMCs have been undertaken in the last two decades. On the basis of Eshely’s equivalent inclusion theory [5], many studies on TRS in MMCs were reported [6,7]. Wither et al. [8] calculated the TRS in short fiber reinforced MMC, and the results indicated that the average TRS in the matrix
* Corresponding author. Fax: /86-451-6413922. E-mail address: [email protected] (W.D. Fei).
is proportional to the temperature change of composites and the difference of CTEs between matrix and reinforcement. The presence of TRS results in that the microstructure of matrix of MMCs is quite different from that of unreinforced metals. The greatest difference is that the dislocation density in the matrix of MMCs is very high, and its state is very complicated. The highly densified dislocation in the matrix is mainly caused by the following two reasons. First, as the greater the temperature change, the higher the TRS, the TRS in the matrix of MMC can exceed the yield strength of the matrix, when the composite is cooled from higher temperature, and dislocation generation takes place due to the plastic deformation of the matrix [9,10]. Second, the geometryneeded dislocations can also be induced when the composite is cooled from high temperature [11]. Recently, our researches [12 /15] indicated that both the TRS and dislocation in the matrix of SiCw/Al composite are sensitive to the thermal history of the composite, and the TMS and dislocation density in the matrix of SiCw/ Al composite can be reduced greatly when the composite is cooled slowly from a higher temperature. It was also
0921-5093/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-5093(02)00827-4
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found that although the TMS was higher, the higher ultimate tensile strength (UTS) was obtained in the quenched SiCw/Al composite because of higher density of dislocation in the matrix, which suggested that dislocation density in the matrix played a very important role in the strength of SiCw/Al composite. Therefore, although the theoretical and numerical studies can describe the TRS in SiCw/Al composite clearly, it is very difficult to predict accurately the TMS and mechanical properties of the composites subjected to various heat treatments. Although the TRS of composites can be decreased by slowly cooling from high temperature [12], the technique has two unacceptable disadvantages: one is that the aging strengthening can not be realized in the composite with aging strengthening matrix; the other is that the technique reduces greatly the dislocation density of the matrixes although it reduces the TRS of SiCw/Al composites. Because SiCw/Al composite has many excellent properties, such as high specific strength, specific modulus, good thermal conductivity and low thermal expansion, many potential applications in the field of aerospace are expected. So it is very important to find out a technique to reduce the TMS and keep higher density of dislocation in the matrix of SiCw/Al composite for the optimization the tensile properties of SiCw/ Al composite, which is the main aim of the present study. In the present study, after quenching, an SiCw/Al composite was isothermally treated at 80 8C for various times, and the TMS, microstructure and tensile properties were investigated. The results indicated that the creeping process of matrix took place during the treatment under the action of TRS, so the treatment is referred to as thermal residual stress creeping (TRSC) treatment.
h. That the temperature of TRSC treatment was carried out at 80 8C is due to the following reasons. First, when the composite was treated at the temperature above 100 8C for longer time, the recovery degree of the matrix of SiCw/Al composite is higher, and the dislocation density of the matrix is lower [16]. When the temperature of TRSC treatment is lower, the creeping process of matrix hardly occurs. Second, if one wants to extend the treatment to the composite with aging strengthening matrix, the temperature of TRSC treatment should be far below the aging temperature of the composite. The XRD experiments were carried out using an X’pert Analytical Powo 3040 type X-ray diffractometer with the radiation of Cu-Ka, the voltage of 40 kV and the current of 40 mA. The methods given in the Refs. [17] and [18] were employed for the measurement of TRS and dislocation density in the matrix of the composite, respectively. The accuracy of TRS measurement given in Ref. [17] is about 15 MPa, and it will be found in the present study that the accuracy of the measurement is enough to characterize the change tendency of TRS during TRSC treatment. The accuracy of dislocation density by XRD method was discussed in Ref. [18], the purpose of the XRD method employed here is to examine the change tendency of dislocation density during TRSC treatment. The Microstructures of SiCw/Al composite before and after TRSC treatment were evaluated on a type CM-12 transmission electron microscope (TEM). Thin foils for TEM observation were prepared by an ion milling. The tensile tests were done using a type CSS 44000 electron tensile machine. The dimension of tensile specimens is shown in Fig. 1. The tensile rate was 0.5 mm s 1. The work hardening indexes of specimens before and after TRSC treatment were obtained by the true stress /strain curves.
2. Experiments The materials used were b-SiC whisker and commercially pure aluminum. The composite was fabricated by squeezing cast technique, and the volume fraction of SiC whisker was 20%. For simplification, the pure aluminum was selected as the matrix, in the case of absence of second phase, the effects of TRS and dislocation in matrix on the tensile properties of SiCw/Al composite can be analyzed. As mentioned in Refs. [12,15], quenching treatment can induce higher density of dislocation in the matrix of SiCw/Al composite. To obtain high density of dislocation in the matrix, the composite was treated at 600 8C for 2 h, then quenched into water at room temperature. After quenching, the specimen was isothermally treated at 80 8C (TRSC treatment) for different times and cooled in air. The longest holding time is about 500
3. Results 3.1. Thermal residual stresses and dislocations in the matrix The TRS in the matrix of the quenched SiCw/Al composite with TRSC treatment time is shown in Fig. 2.
Fig. 1. Schematic diagram of tensile specimen.
W.D. Fei et al. / Materials Science and Engineering A356 (2003) 17 /22
Fig. 2. Dependence of TRS in the matrix of the quenched SiCw/Al composite on the holding time at 80 8C.
It can be found that TRS in the matrix of the composite decreases with TRSC treatment time increasing, which suggests that TRSC treatment is an effective technique to reduce the TRS in SiCw/Al composite. Fig. 3 shows the dislocation configuration in the matrix of the composite before TRSC treatment. In the figure, the highly densified, curved and closely tangled dislocations are observed in the matrix. The results are quite in agreement with the previous study [12,15]. Certainly, the high-density dislocation can enhance the strength of the matrix and increase the strength of the composite. Fig. 4 presents the dislocation density measured by XRD in the matrix of the composite after TRSC treatment time. Only an average dislocation density can be obtained by XRD measurement, so the results shown in Fig. 4 suggests that no obvious change of average dislocation density can be found during TRSC treatment. It can be concluded that the TRSC treatment at 80 8C can reduce the TRS effectively, and the higher density of dislocation in the matrix can be sustained in the SiCw/Al composite. To analyze the mechanism of effect of TRSC treatment on the TRS, the TEM observation of the disloca-
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Fig. 4. Dependence of dislocation density of SiCw/Al composite on the holding time at 80 8C.
tion in the matrix was carried out. Fig. 5 shows the dislocation configurations adjacent to SiCw /Al interfaces in the matrix of the composite after TRSC treatment. It can be found that the density of dislocation in the region of the matrix adjacent to SiCw /Al interface decreased obviously. As shown in Fig. 5, the subgrains or dislocation walls can be clearly seen in the region of the matrix nearby the interfaces when TRSC treatment time is longer, which suggests that the recovery process takes place during the TRSC treatment. From the comparison among Fig. 5(a /c), it can be found that the degree of recovery of the matrix adjacent to the interface increases with TRSC treatment time increasing. Fig. 6 gives the microstructures of the matrix far away from SiCw /Al interfaces. It can be seen that there are still many curved and tangled dislocations in the matrix. After TRSC treatment for shorter time, such as 70 and 150 h, the dislocation density and state are quite similar to those before TRSC treatment, as shown in Fig. 5(a and b). The above observation of the microstructures in the matrix of the composite after TRSC treatment suggests that the recovery of the matrix mainly takes place nearby the SiCw /Al interfaces. Because TRSC treatment temperature is far below the recovery temperature of aluminum [19], the recovery of matrix is considered as the results of creeping caused by the larger TRS in the matrix of the specimen, which is discussed in detail in Section 4 of the paper. Because of short range of TRS in the matrix, it is easy to understand that the recovery or creeping process presents mainly nearby the SiCw /Al interfaces. Because of the above reasons, the treatment presented is called TRSC treatment [20]. 3.2. Tensile properties
Fig. 3. Dislocation configurations in the matrix of the quenched SiCw/ Al composite before TRSC treatment.
Fig. 7 shows the micro yield strengths (s0.005), yield strength (s0.2) and UTS of SiCw/Al composites before and after TRSC treatment for 500 h. It can be seen that the micro yield strength, yield strength and UTS of the composite after TRSC treatment are enhanced com-
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Fig. 5. Dislocation configurations adjacent to the interfaces in the matrix of SiCw/Al composite after TRSC treatment for different times, (a) 70 h; (b) 150 h; (c) 480 h.
Fig. 6. Dislocation configurations far from the interfaces in the matrix of SiCw/Al composite after TRSC treatment for different times, (a) 70 h; (b) 150 h; (c) 480 h.
pared with those before TRSC treatment. The maximum increment of strengths can be more than 40 MPa. The UTS of SiC whisker reinforced pure aluminum matrix
Fig. 7. Micro yield strength (s0.005), yield strength (s0.2) and UTS (sb) of composite during different deformation stages.
composite is obtained as high as 330 MPa after TRSC treatment for 500 h. In order to investigate the work hardening behaviors of the quenched SiCw/Al composites before and after TRSC treatment, the work hardening indexes were calculated within the true strain regions from 5 /10 5 to 2 /10 3 (micro deformation stage) and from 2 / 10 3 to 2 /10 2 (macro deformation stage), respectively, and the results are shown in Fig. 8. It can be seen that the work hardening index of the composite in the micro deformation stage after TRSC treatment is smaller than that before TRSC treatment, while the work hardening index in the macro deformation stage after TRSC treatment is larger than that before TRSC treatment.
W.D. Fei et al. / Materials Science and Engineering A356 (2003) 17 /22
Fig. 8. Work hardening indexes of SiCw/Al composite during different deformation stages.
4. Discussion 4.1. Thermal residual stress and microstructure The higher density of dislocation in the quenching SiCw/Al composite was discussed by many researches [12,14]. When the composite is cooled from high temperature, two processes may take place in the matrix. One is the dislocation generation resulting from relaxation of the TRS, the other is the dislocation annihilation resulting from the recovery or recrystallization of matrix when the specimens are cooled from high temperature. The final dislocation state in the matrix of the composite at room temperature is determined by competition of the above two processes. Because the cooling rate of quenching process is so fast that the recovery in the matrix cannot happen sufficiently, which results in that almost dislocations generated by relaxation of TRS remains. Therefore, highly densified dislocation is obtained in the matrix of quenched composite. After TRSC treatment for long time, the TRS in the matrix of the composite is reduced greatly (Fig. 2), and the typical recovery microstructure of the matrix nearby the SiCw /Al interfaces can be clearly observed. The results may be related to the creeping relaxation of TRS. For the creeping of alloys, the active energy of creeping can be reduced by a larger external stress [21]. Although no external stress is applied on the composite during the isothermal treatment, the larger TRS in the matrix plays the similar role with the external stress. In this case, the creeping process may take place during the treatment with the help of TRS in the matrix and the thermal activation. Although the treatment temperature is far below the recovery temperature of aluminum, the clear recovery structure in the matrix adjacent to the SiCw /Al interfaces is found after the treatment. In fact, the TRS in the matrix nearby the SiCw /Al interfaces is very high, and the elastic energy in this region is also very high. Therefore, the creeping of TRS and the recovery in the matrix can occur when the composite is isothermally treated for longer time. Because of the higher stacking fault energy of pure aluminum, the cross sliding of dislocations in the matrix
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take place easily, which results in the formation of clear subgrains in the region of the matrix nearby the SiCw / Al interfaces. As is well known, the TRS in the matrix of SiCw/Al composite has a very short range, which decreases rapidly with the increasing of distance from the interface. Because the creeping and recovery of the matrix are caused by the larger TRS in the matrix, one can hope that the creeping and recovery processes are mainly in the region nearby the interface, as shown in Fig. 5 and Fig. 6. When the creeping process occurs, the TRS is reduced, and it is easy to understand that the TRS decreases with the TRSC time increasing, as shown in Fig. 2. It is worth noting that the creeping found in the matrix of the quenched SiCw/Al composite during the TRSC treatment is different from the conventional creeping in alloys. Firstly, the stress to cause the creeping is not an external stress but an internal residual stress. Secondly, the stress is inhomogeneous or short range, which concentrates on the vicinity surrounding whiskers. Thirdly, the stress in the creeping process is not a constant, which is decreasing gradually due to the creeping when the recovery takes place in the vicinities surrounding whiskers, and the plastic deformation of matrix caused by creeping is also decreasing gradually in the regions. Because of much lower TRS in the regions of the matrix far away from whiskers, the creeping in the regions, if it occurs, can not reach the stage of recovery creeping at lower temperature, which results in the dislocation density in the region increasing slightly or does not change, as shown in Fig. 6. Therefore, the change of average dislocation density in the matrix of the composite after TRSC treatment is not great, as shown in Fig. 3 and Fig. 6.
4.2. Tensile properties Because TRSC treatment temperature is lower, one can hope that the interface strength and whisker distribution do not change during the treatment, the tensile properties of SiCw/Al composite are mainly determined by the dislocation in the matrix and TRS in the composite. It is known that the dislocation density and state nearby the interfaces have a great influence on the micro yield behaviors of the composite [22]. Meanwhile the TRS also affects the micro yield and yield behaviors of SiCw/Al composite [14]. As shown in Fig. 5 and Fig. 7, although the dislocation density adjacent to the interface in the matrix of the composite decreases, the yield strengths of the composite is enhanced after the treatment, which may suggest that the TRS plays a more important role in the micro yield behaviors of the composites.
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The work hardening index depends upon the resistance of dislocation movement [22], so both the TRS and dislocation state in the matrix affect the work hardening behavior of SiCw/Al composite. Because there exists great stress concentration in the matrix nearby the interfaces or the tip of SiC whisker, the initial stage of the plastic deformation, i.e. the micro deformation, usually begins from the matrix nearby the tip or interfaces of SiC whiskers [23]. For the quenched composite before TRSC treatment, the dislocation density is very high and the dislocation state is complicated near the interfaces in the matrix, especially at the tip of whiskers. Although the larger TRS may be helpful for the tensile deformation, the highly densified dislocation adjacent to the interface may provide great resistance for the sequent dislocation movement, which results in the higher work hardening index at micro deformation stage because the micro deformation is corresponding to the short range slide of dislocation nearby the interface. For the composite after TRSC treatment, the dislocation density adjacent to the interfaces in the matrix is lower, which may result in the lower work hardening index of the composite at micro deformation stage. At the macro deformation stage, the work hardening index is mainly determined by the resistance of long range slide of dislocations in the matrix. After TRSC treatment, the TRS is decreased greatly, meanwhile the dislocation density in the matrix far from SiCw /Al interface changes little, which make higher resistance of long range slide of dislocation, and result in the higher work hardening index for the composite after TRSC treatment. Cao [24] indicated that the micro crack could nucleate at the tip of the whisker in SiCw/Al composite even if the stress is smaller than the yield strength (s0.2). The lower TRS in the matrix may reduce the nucleating probability of micro crack, which may enhance the UTS of the composite. Therefore, the UTS of the composite after TRSC treatment is higher than that before TRSC treatment, as shown in Fig. 7.
5. Conclusions (1) The dislocation state in the matrix of the composite with quenching treatment is highly densified, curved, closely tangled, and the TRSs in the composite are larger. (2) When the quenched composite is treated at 80 8C for longer time, the creeping in the matrix occurs, and
subgrains form adjacent to the SiCw /Al interface. With TRSC treatment time increasing, the total dislocation density changes little, but TRS in the matrix decreases greatly. (3) After TRSC treatment, the micro yield strength s0.005, yield strength s0.2, UTS of the composite with quenching treatment are larger than those before TRSC treatment. (4) The maximum value of UTS of SiC whisker reinforced pure aluminum matrix composite can reach 330 MPa after TRS creeping.
References [1] H. Zhang, P.M. Anderson, S.D. Daehn, Metall. Mater. Trans. 25A (11) (1992) 415. [2] L.C. Davis, J.E. Allison, Metall. Mater. Trans. 24A (11) (1993) 2487. [3] R.J. Arsenault, S.B. Wu, Mater. Sci. Eng. 96 (1987) 77. [4] J.M. Papazian, P.N. Adler, Metall. Trans. 21A (1990) 401. [5] J.D. Eshely, Proceedings of the Royal Society, London Series, 24A, 1957, p. 376. [6] R.J. Arsenault, M. Taya, Acta Metall. 35 (A) (1987) 651. [7] M. Taya, R.J. Arsenaut, Scripta Metall. 21 (1987) 349. [8] P.J. Wither, W.M. Stobbs, O.B. Pedersen, Acta Mater. 37 (1989) 3061. [9] J.K. Lee, Y.Y. Earmme, H.I. Aaronson, et al., Metall. Trans. 11A (11) (1837). [10] R.J. Arsenault, N. Shi, Mater. Sci. Eng. 81 (1987) 175. [11] M.F. Ashby, Phil. Mag. 21 (1970) 399 /411. [12] J. Hu, R.S. Lou, W.D. Fei, C.K. Yao, J. Mater. Sci. Lett. 18 (1999) 1525. [13] W.D. Fei, M. Hu, C.K. Yao, Mater. Chem. Phys. 77 (2002) 882 / 888. [14] M. Hu, J. Hu, W.D. Fei, C.K. Yao, Chin. J. Nonferrous Metals 11 (S1) (2001) 125 /128. [15] M. Hu, W.D. Fei, C.K. Yao, J. Mater. Sci. Lett. 20 (2001) 643 / 645. [16] M. Hu, Ph. D. Thesis, Harbin Institute of Technology, 2002. [17] W.D. Fei, Q.Y. Liu, C.K. Yao, Mater. Sci. Eng. 17 (8) (2001) 912 /916. [18] Y.M. Wang, in: S.T. Huang (Ed.), X-ray Diffraction of Solid, High Education Press, Beijing, China, 1985, p. 54. [19] C. Li, Metallurgy of Metals, Machinery Industry Press, Beijing, 1996, p. 173. [20] M. Hu, W.D. Fei, C.K. Yao, Mater. Sci. Tech. S1 (1) (2001) 212 / 214 (in Chinese). [21] D.C. Dunand, A. Mortensen, Acta Metall. Mater. 39 (2) (1991) 127 /139. [22] C.H. Jiang, Ph. D. Thesis, Harbin Institute of Technology, 2000, p. 90. [23] B. Prangnell, T. Downes, W.M. Stobbs, P.J. Withers, Acta Metal. Mater. 17A (1986) 379. [24] L. Cao, Dissertation for the Doctoral Degree in Engineering, Harbin Institute of Technology, 1993, p. 50.
Effects of thermal residual stress creeping on microstructure and tensile properties of SiC whisker reinforced aluminum matrix composite W.D. Fei *, M. Hu, C.K. Yao School of Materials Science and Engineering, Harbin Institute of Technology. P.O. Box 405, Harbin 150001, PR China Received 20 November 2001; received in revised form 11 October 2002
Abstract After quenching from 600 8C, the isothermal treatment at 80 8C for SiC whisker reinforced aluminum composite was carried out. The results indicated that the dislocation state in the matrix was changed greatly, especially in the region adjacent to the interface between SiC whisker and matrix. The change of dislocation state was considered as the result of creeping caused by thermal residual stress (TRS). It was found that the thermal residual stress creeping (TRSC) treatment is a new method to reduce effectively the TRS, and increase the strengths, including micro yield strength (s0.005), yield strength (s0.2), ultimate tensile strength (UTS), of SiCw/Al composite. The mechanism of TRSC of SiCw/Al composite was given in detail. After TRSC treatment for 500 h, the UTS of pure aluminum matrix composite was up to 330 MPa. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Thermal residual stress; Yield strength; Tensile strength
1. Introduction It is known that the thermal residual stress (TRS) caused by mismatch of coefficients of thermal expansion (CTEs) between the reinforcement and matrix is inherent and has some undesirable effects on the tensile properties of SiC whisker reinforced aluminum (SiCw/ Al) composite [1,2], especially on tensile yield strength [3,4]. The lower tensile yield strength resulting from residual stress limits the application of the composite in many precision structural parts. Owing to the complexity of material system and the importance of TRS itself in metal matrix composite (MMC), many researches on TRS of MMCs have been undertaken in the last two decades. On the basis of Eshely’s equivalent inclusion theory [5], many studies on TRS in MMCs were reported [6,7]. Wither et al. [8] calculated the TRS in short fiber reinforced MMC, and the results indicated that the average TRS in the matrix
* Corresponding author. Fax: /86-451-6413922. E-mail address: [email protected] (W.D. Fei).
is proportional to the temperature change of composites and the difference of CTEs between matrix and reinforcement. The presence of TRS results in that the microstructure of matrix of MMCs is quite different from that of unreinforced metals. The greatest difference is that the dislocation density in the matrix of MMCs is very high, and its state is very complicated. The highly densified dislocation in the matrix is mainly caused by the following two reasons. First, as the greater the temperature change, the higher the TRS, the TRS in the matrix of MMC can exceed the yield strength of the matrix, when the composite is cooled from higher temperature, and dislocation generation takes place due to the plastic deformation of the matrix [9,10]. Second, the geometryneeded dislocations can also be induced when the composite is cooled from high temperature [11]. Recently, our researches [12 /15] indicated that both the TRS and dislocation in the matrix of SiCw/Al composite are sensitive to the thermal history of the composite, and the TMS and dislocation density in the matrix of SiCw/ Al composite can be reduced greatly when the composite is cooled slowly from a higher temperature. It was also
0921-5093/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-5093(02)00827-4
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found that although the TMS was higher, the higher ultimate tensile strength (UTS) was obtained in the quenched SiCw/Al composite because of higher density of dislocation in the matrix, which suggested that dislocation density in the matrix played a very important role in the strength of SiCw/Al composite. Therefore, although the theoretical and numerical studies can describe the TRS in SiCw/Al composite clearly, it is very difficult to predict accurately the TMS and mechanical properties of the composites subjected to various heat treatments. Although the TRS of composites can be decreased by slowly cooling from high temperature [12], the technique has two unacceptable disadvantages: one is that the aging strengthening can not be realized in the composite with aging strengthening matrix; the other is that the technique reduces greatly the dislocation density of the matrixes although it reduces the TRS of SiCw/Al composites. Because SiCw/Al composite has many excellent properties, such as high specific strength, specific modulus, good thermal conductivity and low thermal expansion, many potential applications in the field of aerospace are expected. So it is very important to find out a technique to reduce the TMS and keep higher density of dislocation in the matrix of SiCw/Al composite for the optimization the tensile properties of SiCw/ Al composite, which is the main aim of the present study. In the present study, after quenching, an SiCw/Al composite was isothermally treated at 80 8C for various times, and the TMS, microstructure and tensile properties were investigated. The results indicated that the creeping process of matrix took place during the treatment under the action of TRS, so the treatment is referred to as thermal residual stress creeping (TRSC) treatment.
h. That the temperature of TRSC treatment was carried out at 80 8C is due to the following reasons. First, when the composite was treated at the temperature above 100 8C for longer time, the recovery degree of the matrix of SiCw/Al composite is higher, and the dislocation density of the matrix is lower [16]. When the temperature of TRSC treatment is lower, the creeping process of matrix hardly occurs. Second, if one wants to extend the treatment to the composite with aging strengthening matrix, the temperature of TRSC treatment should be far below the aging temperature of the composite. The XRD experiments were carried out using an X’pert Analytical Powo 3040 type X-ray diffractometer with the radiation of Cu-Ka, the voltage of 40 kV and the current of 40 mA. The methods given in the Refs. [17] and [18] were employed for the measurement of TRS and dislocation density in the matrix of the composite, respectively. The accuracy of TRS measurement given in Ref. [17] is about 15 MPa, and it will be found in the present study that the accuracy of the measurement is enough to characterize the change tendency of TRS during TRSC treatment. The accuracy of dislocation density by XRD method was discussed in Ref. [18], the purpose of the XRD method employed here is to examine the change tendency of dislocation density during TRSC treatment. The Microstructures of SiCw/Al composite before and after TRSC treatment were evaluated on a type CM-12 transmission electron microscope (TEM). Thin foils for TEM observation were prepared by an ion milling. The tensile tests were done using a type CSS 44000 electron tensile machine. The dimension of tensile specimens is shown in Fig. 1. The tensile rate was 0.5 mm s 1. The work hardening indexes of specimens before and after TRSC treatment were obtained by the true stress /strain curves.
2. Experiments The materials used were b-SiC whisker and commercially pure aluminum. The composite was fabricated by squeezing cast technique, and the volume fraction of SiC whisker was 20%. For simplification, the pure aluminum was selected as the matrix, in the case of absence of second phase, the effects of TRS and dislocation in matrix on the tensile properties of SiCw/Al composite can be analyzed. As mentioned in Refs. [12,15], quenching treatment can induce higher density of dislocation in the matrix of SiCw/Al composite. To obtain high density of dislocation in the matrix, the composite was treated at 600 8C for 2 h, then quenched into water at room temperature. After quenching, the specimen was isothermally treated at 80 8C (TRSC treatment) for different times and cooled in air. The longest holding time is about 500
3. Results 3.1. Thermal residual stresses and dislocations in the matrix The TRS in the matrix of the quenched SiCw/Al composite with TRSC treatment time is shown in Fig. 2.
Fig. 1. Schematic diagram of tensile specimen.
W.D. Fei et al. / Materials Science and Engineering A356 (2003) 17 /22
Fig. 2. Dependence of TRS in the matrix of the quenched SiCw/Al composite on the holding time at 80 8C.
It can be found that TRS in the matrix of the composite decreases with TRSC treatment time increasing, which suggests that TRSC treatment is an effective technique to reduce the TRS in SiCw/Al composite. Fig. 3 shows the dislocation configuration in the matrix of the composite before TRSC treatment. In the figure, the highly densified, curved and closely tangled dislocations are observed in the matrix. The results are quite in agreement with the previous study [12,15]. Certainly, the high-density dislocation can enhance the strength of the matrix and increase the strength of the composite. Fig. 4 presents the dislocation density measured by XRD in the matrix of the composite after TRSC treatment time. Only an average dislocation density can be obtained by XRD measurement, so the results shown in Fig. 4 suggests that no obvious change of average dislocation density can be found during TRSC treatment. It can be concluded that the TRSC treatment at 80 8C can reduce the TRS effectively, and the higher density of dislocation in the matrix can be sustained in the SiCw/Al composite. To analyze the mechanism of effect of TRSC treatment on the TRS, the TEM observation of the disloca-
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Fig. 4. Dependence of dislocation density of SiCw/Al composite on the holding time at 80 8C.
tion in the matrix was carried out. Fig. 5 shows the dislocation configurations adjacent to SiCw /Al interfaces in the matrix of the composite after TRSC treatment. It can be found that the density of dislocation in the region of the matrix adjacent to SiCw /Al interface decreased obviously. As shown in Fig. 5, the subgrains or dislocation walls can be clearly seen in the region of the matrix nearby the interfaces when TRSC treatment time is longer, which suggests that the recovery process takes place during the TRSC treatment. From the comparison among Fig. 5(a /c), it can be found that the degree of recovery of the matrix adjacent to the interface increases with TRSC treatment time increasing. Fig. 6 gives the microstructures of the matrix far away from SiCw /Al interfaces. It can be seen that there are still many curved and tangled dislocations in the matrix. After TRSC treatment for shorter time, such as 70 and 150 h, the dislocation density and state are quite similar to those before TRSC treatment, as shown in Fig. 5(a and b). The above observation of the microstructures in the matrix of the composite after TRSC treatment suggests that the recovery of the matrix mainly takes place nearby the SiCw /Al interfaces. Because TRSC treatment temperature is far below the recovery temperature of aluminum [19], the recovery of matrix is considered as the results of creeping caused by the larger TRS in the matrix of the specimen, which is discussed in detail in Section 4 of the paper. Because of short range of TRS in the matrix, it is easy to understand that the recovery or creeping process presents mainly nearby the SiCw /Al interfaces. Because of the above reasons, the treatment presented is called TRSC treatment [20]. 3.2. Tensile properties
Fig. 3. Dislocation configurations in the matrix of the quenched SiCw/ Al composite before TRSC treatment.
Fig. 7 shows the micro yield strengths (s0.005), yield strength (s0.2) and UTS of SiCw/Al composites before and after TRSC treatment for 500 h. It can be seen that the micro yield strength, yield strength and UTS of the composite after TRSC treatment are enhanced com-
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Fig. 5. Dislocation configurations adjacent to the interfaces in the matrix of SiCw/Al composite after TRSC treatment for different times, (a) 70 h; (b) 150 h; (c) 480 h.
Fig. 6. Dislocation configurations far from the interfaces in the matrix of SiCw/Al composite after TRSC treatment for different times, (a) 70 h; (b) 150 h; (c) 480 h.
pared with those before TRSC treatment. The maximum increment of strengths can be more than 40 MPa. The UTS of SiC whisker reinforced pure aluminum matrix
Fig. 7. Micro yield strength (s0.005), yield strength (s0.2) and UTS (sb) of composite during different deformation stages.
composite is obtained as high as 330 MPa after TRSC treatment for 500 h. In order to investigate the work hardening behaviors of the quenched SiCw/Al composites before and after TRSC treatment, the work hardening indexes were calculated within the true strain regions from 5 /10 5 to 2 /10 3 (micro deformation stage) and from 2 / 10 3 to 2 /10 2 (macro deformation stage), respectively, and the results are shown in Fig. 8. It can be seen that the work hardening index of the composite in the micro deformation stage after TRSC treatment is smaller than that before TRSC treatment, while the work hardening index in the macro deformation stage after TRSC treatment is larger than that before TRSC treatment.
W.D. Fei et al. / Materials Science and Engineering A356 (2003) 17 /22
Fig. 8. Work hardening indexes of SiCw/Al composite during different deformation stages.
4. Discussion 4.1. Thermal residual stress and microstructure The higher density of dislocation in the quenching SiCw/Al composite was discussed by many researches [12,14]. When the composite is cooled from high temperature, two processes may take place in the matrix. One is the dislocation generation resulting from relaxation of the TRS, the other is the dislocation annihilation resulting from the recovery or recrystallization of matrix when the specimens are cooled from high temperature. The final dislocation state in the matrix of the composite at room temperature is determined by competition of the above two processes. Because the cooling rate of quenching process is so fast that the recovery in the matrix cannot happen sufficiently, which results in that almost dislocations generated by relaxation of TRS remains. Therefore, highly densified dislocation is obtained in the matrix of quenched composite. After TRSC treatment for long time, the TRS in the matrix of the composite is reduced greatly (Fig. 2), and the typical recovery microstructure of the matrix nearby the SiCw /Al interfaces can be clearly observed. The results may be related to the creeping relaxation of TRS. For the creeping of alloys, the active energy of creeping can be reduced by a larger external stress [21]. Although no external stress is applied on the composite during the isothermal treatment, the larger TRS in the matrix plays the similar role with the external stress. In this case, the creeping process may take place during the treatment with the help of TRS in the matrix and the thermal activation. Although the treatment temperature is far below the recovery temperature of aluminum, the clear recovery structure in the matrix adjacent to the SiCw /Al interfaces is found after the treatment. In fact, the TRS in the matrix nearby the SiCw /Al interfaces is very high, and the elastic energy in this region is also very high. Therefore, the creeping of TRS and the recovery in the matrix can occur when the composite is isothermally treated for longer time. Because of the higher stacking fault energy of pure aluminum, the cross sliding of dislocations in the matrix
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take place easily, which results in the formation of clear subgrains in the region of the matrix nearby the SiCw / Al interfaces. As is well known, the TRS in the matrix of SiCw/Al composite has a very short range, which decreases rapidly with the increasing of distance from the interface. Because the creeping and recovery of the matrix are caused by the larger TRS in the matrix, one can hope that the creeping and recovery processes are mainly in the region nearby the interface, as shown in Fig. 5 and Fig. 6. When the creeping process occurs, the TRS is reduced, and it is easy to understand that the TRS decreases with the TRSC time increasing, as shown in Fig. 2. It is worth noting that the creeping found in the matrix of the quenched SiCw/Al composite during the TRSC treatment is different from the conventional creeping in alloys. Firstly, the stress to cause the creeping is not an external stress but an internal residual stress. Secondly, the stress is inhomogeneous or short range, which concentrates on the vicinity surrounding whiskers. Thirdly, the stress in the creeping process is not a constant, which is decreasing gradually due to the creeping when the recovery takes place in the vicinities surrounding whiskers, and the plastic deformation of matrix caused by creeping is also decreasing gradually in the regions. Because of much lower TRS in the regions of the matrix far away from whiskers, the creeping in the regions, if it occurs, can not reach the stage of recovery creeping at lower temperature, which results in the dislocation density in the region increasing slightly or does not change, as shown in Fig. 6. Therefore, the change of average dislocation density in the matrix of the composite after TRSC treatment is not great, as shown in Fig. 3 and Fig. 6.
4.2. Tensile properties Because TRSC treatment temperature is lower, one can hope that the interface strength and whisker distribution do not change during the treatment, the tensile properties of SiCw/Al composite are mainly determined by the dislocation in the matrix and TRS in the composite. It is known that the dislocation density and state nearby the interfaces have a great influence on the micro yield behaviors of the composite [22]. Meanwhile the TRS also affects the micro yield and yield behaviors of SiCw/Al composite [14]. As shown in Fig. 5 and Fig. 7, although the dislocation density adjacent to the interface in the matrix of the composite decreases, the yield strengths of the composite is enhanced after the treatment, which may suggest that the TRS plays a more important role in the micro yield behaviors of the composites.
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The work hardening index depends upon the resistance of dislocation movement [22], so both the TRS and dislocation state in the matrix affect the work hardening behavior of SiCw/Al composite. Because there exists great stress concentration in the matrix nearby the interfaces or the tip of SiC whisker, the initial stage of the plastic deformation, i.e. the micro deformation, usually begins from the matrix nearby the tip or interfaces of SiC whiskers [23]. For the quenched composite before TRSC treatment, the dislocation density is very high and the dislocation state is complicated near the interfaces in the matrix, especially at the tip of whiskers. Although the larger TRS may be helpful for the tensile deformation, the highly densified dislocation adjacent to the interface may provide great resistance for the sequent dislocation movement, which results in the higher work hardening index at micro deformation stage because the micro deformation is corresponding to the short range slide of dislocation nearby the interface. For the composite after TRSC treatment, the dislocation density adjacent to the interfaces in the matrix is lower, which may result in the lower work hardening index of the composite at micro deformation stage. At the macro deformation stage, the work hardening index is mainly determined by the resistance of long range slide of dislocations in the matrix. After TRSC treatment, the TRS is decreased greatly, meanwhile the dislocation density in the matrix far from SiCw /Al interface changes little, which make higher resistance of long range slide of dislocation, and result in the higher work hardening index for the composite after TRSC treatment. Cao [24] indicated that the micro crack could nucleate at the tip of the whisker in SiCw/Al composite even if the stress is smaller than the yield strength (s0.2). The lower TRS in the matrix may reduce the nucleating probability of micro crack, which may enhance the UTS of the composite. Therefore, the UTS of the composite after TRSC treatment is higher than that before TRSC treatment, as shown in Fig. 7.
5. Conclusions (1) The dislocation state in the matrix of the composite with quenching treatment is highly densified, curved, closely tangled, and the TRSs in the composite are larger. (2) When the quenched composite is treated at 80 8C for longer time, the creeping in the matrix occurs, and
subgrains form adjacent to the SiCw /Al interface. With TRSC treatment time increasing, the total dislocation density changes little, but TRS in the matrix decreases greatly. (3) After TRSC treatment, the micro yield strength s0.005, yield strength s0.2, UTS of the composite with quenching treatment are larger than those before TRSC treatment. (4) The maximum value of UTS of SiC whisker reinforced pure aluminum matrix composite can reach 330 MPa after TRS creeping.
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