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6061Al composite during compression at temperatures below and above the solidus of the matrix alloy
Materials Chemistry and Physics 96 (2006) 2–8
Microstructural changes of SiCw/6061Al composite during compression at temperatures below and above the solidus of the matrix alloy G.S. Wang ∗ , L. Geng School of Materials Science and Engineering, Harbin Institute of Technology, P.O. Box 433, Harbin 150001, China Received 13 April 2005; received in revised form 24 May 2005; accepted 12 June 2005
Abstract Elevated temperature compressive behavior of a 6061 aluminum alloy matrix composite reinforced with 20 vol.% -SiC whisker was investigated at the strain rate from 0.016 to 1.0 s−1 . The deformation temperatures were below and above the solidus temperature of the composite, which was determined by differential scanning calorimeter (DSC). It was found that the flow stress of the composite decreased with increasing compressive temperature. The superplasticity occurred in the composite during compression at 580 ◦ C with the strain rate 0.37 s−1 . The microstructure comprising whisker distribution and dislocation density of the matrix as well as interfacial reaction between the matrix alloy and the whisker of the composite before and after compression was observed and analyzed by using SEM and TEM. © 2005 Elsevier B.V. All rights reserved. Keywords: Composite materials; Deformation; SEM; TEM
1. Introduction Whisker reinforced aluminum alloys exhibit high specific strength, high specific elastic modulus and good wear resistance, and they are relatively stable at elevated temperatures. High temperature plastic forming is one of the most important techniques in SiCw/Al composite application. Recently, it was found that SiCw/Al composites exhibit a high-strainrate superplasticity at temperatures close to or slightly above the solidus of the matrix aluminum alloy [1,2]. Many attempts have been made to understand the tensile plastic deformation behavior of various discontinuously reinforced aluminum alloy composites [3–5]. However, few studies have been conducted on the compression behavior of the composites at such a high temperature range, which seems more important in application of high temperature plastic forming. Optimum superplasticity is often attained in metals at a very low strain rate, of the order of 10−3 s−1 . However, it was demonstrated by Nieh and Wadsworth [1] that it is possible ∗
Corresponding author. Tel.: +86 451 86418836; fax: +86 451 86413922. E-mail address: [email protected] (G.S. Wang).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.06.026
to achieve superplastic-like flow and tensile elongation of up to 300% in a 20 vol.% SiCw/2024Al composite at 525 ◦ C using a high strain rate of 3.3 × 10−1 s−1 . Such a high strain rate superplasticity provides a practical route for the plastic forming of SiCw/Al composites. So far, most of the research work on high-strain-rate superplasticity focuses on tensile tests. Recently, some research work on hot compression of aluminum matrix composites was carried out [6,7]. The emphasis of the literatures is on the hot compression behavior and its relevant parameters of SiCw/Al composite [6,7]. The objective of this paper is to investigate the microstructural changes of SiCw/6061Al during hot compression.
2. Experimental materials and procedures The SiCw/6061Al composite was made by the squeeze casting technique using the, -SiC whisker (TWS-100) as a reinforcing element and the commercial 6061 alloy as a base metal. The whisker used here has a diameter of 0.5–1.0 m and a length of 20–30 m and the whisker volume fraction
G.S. Wang, L. Geng / Materials Chemistry and Physics 96 (2006) 2–8
(Vf ) is 0.20. The compression specimens were in the shape of solid cylinders with a diameter of 6 mm and a length of 9 mm. In the present investigation, the compressive deformation was carried out over a wide temperature range of 300–620 ◦ C, including the temperatures below and above the solidus temperature of the composite. The solidus temperature of the composite was determined by using differential scanning calorimeter (DSC). Compressive deformation behavior of the composite at elevated temperatures was studied on Gleeble 1500 Thermal Simulator. The microstructure of the composite was observed and analyzed by using SEM and TEM. The calculation of dislocation density of the matrix in the composite was examined by using XRD.
3. Results and discussion The DSC result shows that the partial melting temperature of the present composite is 579.5 ◦ C and that of 6061Al alloy is 589.4 ◦ C, respectively. The partial melting temperature of the composite is a little lower than that of matrix alloy. The possible reason is that there more interfaces exist in the SiCw/66061Al composite due to the addition of SiC whiskers. Mabuchi et al. found that Mg and Cu segregation occur at the interface between the matrix and Si3 N4 whisker or AlN particle and pointed out that segregation of Mg or Cu can reduce the solidus temperature at the interface between the matrix and the reinforcement [8]. Based on the DSC result, the compressive tests were conducted at 300, 540, 580 and 620 ◦ C, respectively. Microstructure of as cast SiCw/6061Al composite is shown in Fig. 1. It can be seen that the whiskers distribute randomly in the matrix alloy. Fig. 1(b) shows that the grain is equiaxed in shape with size about 1–2 m and many dislocations generated during fabrication exist in the grain of the matrix of the SiCw/6061Al composite.
3
Fig. 2 shows the true stress-true strain curves of the SiCw/6061Al composite and the 6061Al alloy compressed with a strain rate of 0.37 s−1 at different temperatures. It can be seen that the flow stresses of the SiCw/6061Al composite are much higher than that of 6061Al alloy at lower temperature (300 ◦ C), but the difference becomes quite small at higher temperatures (580 and 620 ◦ C). The strength of the composite decreased with increasing compressive temperature. The dislocation density of matrix alloy is high when the SiCw/6061Al composite was compressed at 300 ◦ C. The stable dislocation jam was formed in the matrix alloy due to the interactions of dislocations during compression. And the bonding effect of the matrix alloy to SiC whisker is great, so it is very difficult for SiC whisker to rotate in the matrix alloy. But it is very easy for the SiC whiskers to fracture during compression at 300 ◦ C. The load transferred from the matrix alloy to the SiC whiskers is larger and the load carried by whiskers is larger also. So the flow stress for the composite is very high when tested at 300 ◦ C. It has been reported that fiber fracture affects the mechanical response of the composite because it can cause a decreasing load carrying ability and, hence, lead to catastrophic stress relaxation in the matrix [9]. It was also pointed out any increase of matrix yield strength will cause fracture of fibers to smaller fragment length [9]. The smaller length of whiskers will affect the properties of the composite after compressed seriously. With increasing compression temperatures, such as 540 and 580 ◦ C, the matrix alloy of the SiCw/6061Al composite becomes more and more soft, especially at 580 and 620 ◦ C, there is a little liquid appeared in the composite. Compared with the composite compressed at 300 ◦ C, the dislocation density of matrix alloy becomes lower and the load transferred from the matrix alloy to the whiskers drops. In addition, it is very easy for the whiskers to rotate and not to break in the matrix alloy. The whiskers rotated to the plane vertical to the compression direction. So the load carried by the whiskers
Fig. 1. Microstructure of as cast SiCw/6061A1 composite: (a) SEM and (b) TEM.
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Fig. 2. True stress–strain curves of SiCw/6061A1 and 6061A1 alloy compressed at different temperatures (˙ε = 0.37 s−1 ): (a) 300 ◦ C, (b) 540 ◦ C, (c) 580 ◦ C and (d) 620 ◦ C.
decreases. Then the flow stress for the composite become lower at higher compression temperatures. The SiC whiskers can keep larger length in the composite after compressed at higher temperatures. It is very beneficial for the compressed SiCw/6061Al composite to obtain high mechanical properties. In conclusion, the flow stress for the composite decreases with increasing compression temperatures. It is worth to mention that the superplasticity occurred in the composite during compression at 580 ◦ C with the strain rate 0.37 s−1 . The maximum deformability of the composite was obtained under the above compression condition. The critical compression reduction (the maximum value of reduction in height of the sample when no microcracks could be seen by naked eye during compression) of the composite can be reach 83%. Several possible deformation processes, including grain boundary sliding, interfacial sliding at liquid phase are expected to take place in the composite during hot compression [10]. Fig. 3 shows the SEM microstructure of the SiCw/6061Al composite compressed at different temperatures. It can be seen that cavities are formed easily when the temperature is much higher than the solidus temperature of the matrix or much lower than it, and the cavities are located at the interfaces between the reinforcement and the matrix. But,
when the compression temperature is near the solidus of the matrix, such as 580 ◦ C, cavitation is seldom seen. This is because when the composite is compressed at 540 ◦ C, it leads to stress concentration at the interfaces of the composite and gives rise to cavity nucleation. As a result of potential nucleation sites increases and consequently the number of cavities increases with increasing strain. The presence of a liquid phase at grain boundaries with high misfit angles, and at reinforcement–matrix interfaces, in the composite has been proved by in situ observation using transmission electron microscopy (TEM) at elevated temperature [11]. Therefore, the stress concentration level at the interface containing a liquid phase may be much lower than in interfaces without a liquid phase. Therefore, the accommodation required to relax the stress concentration by sliding can be much more easily achieved through rapid mass transport within a liquid phase. However, at 620 ◦ C, cavity nucleation occurs much more easily at these interfaces, since the continuous liquid phase in the composite leads to a weak bonding force between the liquid and the solid. In addition, the increasing amount of the liquid phase tends to aggregate or is even squeezed out of the samples. The liquid phase is solidified after compression and some casting defects (cavities) may appear. All this will affect the deformability of SiCw/66061Al composite. In gen-
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Fig. 3. SEM microstructure of SiCw/6061A1 composites compressed at different temperatures (˙ε = 0.37 s−1 , ε = 0.8): (a) 540 ◦ C, (b) 580 ◦ C and (c) 620 ◦ C.
eral, the maximum deformability of the composite is obtained when the compression is at 580 ◦ C, which has been reported by the literature [12]. The dislocations and grain size and shape in the matrix of the composite after compression at different temperatures were shown in Fig. 4. It can be seen from Fig. 4(a) that the dislocation density of the SiCw/6061Al composite after compression at 540 ◦ C increased sharply compared with the composite as cast. The dislocation density is very high, 1011 cm−2 , which is determined by XRD. The SiCw/6061Al composite sample was tested by XRD and the peaks of each phase in the composite were obtained firstly. And then using the software programmed by Prof. Wang Yumin in Jilin University, the dislocation density was calculated and obtained. The details of calculation process were well documented in the literature [13]. The main equations used in the determination were as follows: h(x) =
g(y)f (x − y)dy Q
(1)
Here, h(x) is the actual line shape of SiCw/6061Al composite sample tested by XRD, g(y) is the width of line shape caused by geometry factors, f(y) is the width of line shape caused by physical factors, and Q is proportional factor and Q can be omitted when only the shape changes can be researched. f(x), g(x), h(x) could be proceeded with Fourier transformation during the region of −Q/2, Q/2 in the s = 2 sin θ/λ space: i2πny F (n)exp − f (y) = Q n 2πny 2πny = Fr (n)cos + Fi (n)sin (2) Q Q n i2πn y g(z) = G(n )exp − Q n 2πn y 2πn y Gr (n )cos + Gi (n )sin = Q Q n (3)
G.S. Wang, L. Geng / Materials Chemistry and Physics 96 (2006) 2–8
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Fig. 4. The TEM micrographs of the matrix microstructure of SiCw/6061A1 compressed at different temperatures (˙ε = 0.37 s−1 , ε = 0.8): (a) 540 ◦ C, (b) 580 ◦ C and (c) 620 ◦ C.
h(x) =
H(n )exp −
i2πn y
Q 2πn y 2πn y Hr (n )cos = + Hi (n )sin Q Q n (4) n
Here, Fr (n), Fi (n), Gr (n ), Gi (n ), Hr (n ), Hi (n ) are solid part and empty part of f(x), g(x), h(x) Fourier outspread coefficients, respectively: Fr (n) =
Hr (n )Gr (n ) + Hi (n )Gi (n ) G2r (n ) + G2i (n )
(5)
Put Eqs. (2)–(4) into Eq. (1), we can obtain: Hr (n )Gr (n ) − Hi (n )Gi (n ) Fi (n) = G2r (n ) + G2i (n )
Let A(L) = Fr (L), B(L) = Fi (L), we can know from literature [14], the symmetry of line shape was reflected by A(L) and it is related to grain size Deff , root-mean-square of second type strain SMR. The non-symmetry of line shape was reflected by B(L) and it is related to fault energy. Therefore, A(L) = AD (L)Asm (L)
(7)
Here, AD (L) is the width of line shape caused by grain size. Asm (L) is the width of line shape caused by microscopic strain ε2L corresponds to m grade reflection. L Deff 2π2 L2 ε2L s Am (L) = exp − 2 dm
AD (L) = A −
(6) Here, A is bending effect constant.
(8)
(9)
G.S. Wang, L. Geng / Materials Chemistry and Physics 96 (2006) 2–8
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Fig. 5. The interfaces of SiCw/6061A1 composite compressed at different temperatures (˙ε = 0.37 s−1 , ε = 0.8): (a) 540 ◦ C, (b) 580 ◦ C and (c) 620 ◦ C.
According to the above equations, the information of crystalline defects could be obtained. At the same time, combining the curve of standard sample researched by Prof. Wang Yuming, the grain size and the dislocation density of the composite was achieved. It is assumed that such high dislocation density may be attributed to the stress concentration caused by the deformation. There was no obvious difference in grain size of the composite before and after compression at 540 ◦ C. Compressed at 580 ◦ C, the grain of the composite keeps equiaxed in shape still and the dislocation density of the composite is much lower than that of at 540 ◦ C. This result is agreement with that a little liquid appeared in the grain boundary or the interface could relax the stress concentration in the composite. The grain size increased obviously and the dislocation density decreased clearly in the composite compressed at 620 ◦ C compared with its counterparts. The interfaces of the SiCw/6061Al composite after compression at different temperatures are shown in Fig. 5.
At 540 ◦ C, Fig. 5(a), TEM examination reveals that a clean interface exists between the SiC whisker and aluminum matrix. No any interface reaction products could be seen. However, compressed at 580 ◦ C, some discontinuous interface reactants appeared on the surface of SiC whiskers. The interface reactant was determined to be Al4 C3 by XRD and EDAX. The thick and continuous interface reactant layers were produced in the composite during compression at 620 ◦ C. The serious interface reaction between the whisker and the matrix could affect the mechanical properties of the SiCw/6061Al composite greatly. It is suggested that 620 ◦ C is unfit for deforming of the SiCw/6061Al composite.
4. Conclusion The partial melting temperature of the SiCw/6061Al composite is a little lower than that of 6061Al matrix. The
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flow stress of the SiCw/6061Al composite and the difference between the composite and the matrix alloy both decreased with increasing temperatures. The superplasticity occurred in the composite during compression at 580 ◦ C with the strain rate 0.37 s−1 . There was no obvious difference in the grain size and shape among the composites compressed at 540 and 580 ◦ C and before compression. The grain size increased sharply and the dislocation density decreased clearly of the composite compressed at 620 ◦ C. No any interface reactant could be seen in the SiCw/6061Al composite compressed at 540 ◦ C. The interface reaction product was discontinuous and continuous in the composite when the compression is at 580 and 620 ◦ C, respectively. Acknowledgements The authors are grateful for the finance support of the National Nature Science Foundation of People’s Republic of China under grant no. 50071018, and the National Key Project for Basic Research of China (973 Project) under the number of G2000067206.
References [1] T.G. Nieh, C.A. Henshall, J. Wadsworth, Scripta Metall. 18 (1984) 1405. [2] J.-h. Kim, D.-n. Lee, K.-h. Oh, Scripta Metall. Mater. 29 (1993) 377. [3] S.-w. Lim, Y. Nshida, Scripta Metall. Mater. 32 (1995) 1911. [4] M. Mabuchi, K. Higashi, T.G. Langdon, Acta Metall. Mater. 12 (1994) 1739. [5] I. Tsunemichi, T. Isao, A. Kyosuke, G. L’Esperance, H. Bande, Scripta Mater. 34 (1996) 1627. [6] G.S. Wang, L. Geng, Z.Z. Zheng, D.Z. Wang, C.K. Yao, Mater. Chem. Phys. 70 (2001) 164. [7] L. Geng, S. Ochiai, J.Q. Hu, C.K. Yao, Mater. Sci. Eng. A246 (1998) 302. [8] M. Mabuchi, K. Higashi, Y. Okada, S. Tanimura, T. Imai, K. Kubo, Scripta Metall. Mater. 25 (1991) 2517. [9] M. Guden, I.W. Hall, Mater. Sci. Eng. A232 (1997) 1. [10] T. Imai, I. Tochigi, K. Ai, Scripta Mater. 34 (1996) 1627. [11] J. Koike, M. Mabuchi, K. Higashi, Acta Metall. Mater. 43 (1995) 199. [12] L. Geng, G.S. Wang, J. Zhang, T.Q. Lei, J. Mater. Sci. 37 (2002) 3987. [13] F.E. Teng, Y.M. Wang, X.L. Jiang, Structure Analysis of X Ray and Measuring of Properties of Materials, Publishing House of Science, Beijing, 1997, pp. 158–163. [14] S.T. Huang, Solid X Ray (second part), Publishing House of High Education, Beijing, 1998, pp. 162–215.
Microstructural changes of SiCw/6061Al composite during compression at temperatures below and above the solidus of the matrix alloy G.S. Wang ∗ , L. Geng School of Materials Science and Engineering, Harbin Institute of Technology, P.O. Box 433, Harbin 150001, China Received 13 April 2005; received in revised form 24 May 2005; accepted 12 June 2005
Abstract Elevated temperature compressive behavior of a 6061 aluminum alloy matrix composite reinforced with 20 vol.% -SiC whisker was investigated at the strain rate from 0.016 to 1.0 s−1 . The deformation temperatures were below and above the solidus temperature of the composite, which was determined by differential scanning calorimeter (DSC). It was found that the flow stress of the composite decreased with increasing compressive temperature. The superplasticity occurred in the composite during compression at 580 ◦ C with the strain rate 0.37 s−1 . The microstructure comprising whisker distribution and dislocation density of the matrix as well as interfacial reaction between the matrix alloy and the whisker of the composite before and after compression was observed and analyzed by using SEM and TEM. © 2005 Elsevier B.V. All rights reserved. Keywords: Composite materials; Deformation; SEM; TEM
1. Introduction Whisker reinforced aluminum alloys exhibit high specific strength, high specific elastic modulus and good wear resistance, and they are relatively stable at elevated temperatures. High temperature plastic forming is one of the most important techniques in SiCw/Al composite application. Recently, it was found that SiCw/Al composites exhibit a high-strainrate superplasticity at temperatures close to or slightly above the solidus of the matrix aluminum alloy [1,2]. Many attempts have been made to understand the tensile plastic deformation behavior of various discontinuously reinforced aluminum alloy composites [3–5]. However, few studies have been conducted on the compression behavior of the composites at such a high temperature range, which seems more important in application of high temperature plastic forming. Optimum superplasticity is often attained in metals at a very low strain rate, of the order of 10−3 s−1 . However, it was demonstrated by Nieh and Wadsworth [1] that it is possible ∗
Corresponding author. Tel.: +86 451 86418836; fax: +86 451 86413922. E-mail address: [email protected] (G.S. Wang).
0254-0584/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2005.06.026
to achieve superplastic-like flow and tensile elongation of up to 300% in a 20 vol.% SiCw/2024Al composite at 525 ◦ C using a high strain rate of 3.3 × 10−1 s−1 . Such a high strain rate superplasticity provides a practical route for the plastic forming of SiCw/Al composites. So far, most of the research work on high-strain-rate superplasticity focuses on tensile tests. Recently, some research work on hot compression of aluminum matrix composites was carried out [6,7]. The emphasis of the literatures is on the hot compression behavior and its relevant parameters of SiCw/Al composite [6,7]. The objective of this paper is to investigate the microstructural changes of SiCw/6061Al during hot compression.
2. Experimental materials and procedures The SiCw/6061Al composite was made by the squeeze casting technique using the, -SiC whisker (TWS-100) as a reinforcing element and the commercial 6061 alloy as a base metal. The whisker used here has a diameter of 0.5–1.0 m and a length of 20–30 m and the whisker volume fraction
G.S. Wang, L. Geng / Materials Chemistry and Physics 96 (2006) 2–8
(Vf ) is 0.20. The compression specimens were in the shape of solid cylinders with a diameter of 6 mm and a length of 9 mm. In the present investigation, the compressive deformation was carried out over a wide temperature range of 300–620 ◦ C, including the temperatures below and above the solidus temperature of the composite. The solidus temperature of the composite was determined by using differential scanning calorimeter (DSC). Compressive deformation behavior of the composite at elevated temperatures was studied on Gleeble 1500 Thermal Simulator. The microstructure of the composite was observed and analyzed by using SEM and TEM. The calculation of dislocation density of the matrix in the composite was examined by using XRD.
3. Results and discussion The DSC result shows that the partial melting temperature of the present composite is 579.5 ◦ C and that of 6061Al alloy is 589.4 ◦ C, respectively. The partial melting temperature of the composite is a little lower than that of matrix alloy. The possible reason is that there more interfaces exist in the SiCw/66061Al composite due to the addition of SiC whiskers. Mabuchi et al. found that Mg and Cu segregation occur at the interface between the matrix and Si3 N4 whisker or AlN particle and pointed out that segregation of Mg or Cu can reduce the solidus temperature at the interface between the matrix and the reinforcement [8]. Based on the DSC result, the compressive tests were conducted at 300, 540, 580 and 620 ◦ C, respectively. Microstructure of as cast SiCw/6061Al composite is shown in Fig. 1. It can be seen that the whiskers distribute randomly in the matrix alloy. Fig. 1(b) shows that the grain is equiaxed in shape with size about 1–2 m and many dislocations generated during fabrication exist in the grain of the matrix of the SiCw/6061Al composite.
3
Fig. 2 shows the true stress-true strain curves of the SiCw/6061Al composite and the 6061Al alloy compressed with a strain rate of 0.37 s−1 at different temperatures. It can be seen that the flow stresses of the SiCw/6061Al composite are much higher than that of 6061Al alloy at lower temperature (300 ◦ C), but the difference becomes quite small at higher temperatures (580 and 620 ◦ C). The strength of the composite decreased with increasing compressive temperature. The dislocation density of matrix alloy is high when the SiCw/6061Al composite was compressed at 300 ◦ C. The stable dislocation jam was formed in the matrix alloy due to the interactions of dislocations during compression. And the bonding effect of the matrix alloy to SiC whisker is great, so it is very difficult for SiC whisker to rotate in the matrix alloy. But it is very easy for the SiC whiskers to fracture during compression at 300 ◦ C. The load transferred from the matrix alloy to the SiC whiskers is larger and the load carried by whiskers is larger also. So the flow stress for the composite is very high when tested at 300 ◦ C. It has been reported that fiber fracture affects the mechanical response of the composite because it can cause a decreasing load carrying ability and, hence, lead to catastrophic stress relaxation in the matrix [9]. It was also pointed out any increase of matrix yield strength will cause fracture of fibers to smaller fragment length [9]. The smaller length of whiskers will affect the properties of the composite after compressed seriously. With increasing compression temperatures, such as 540 and 580 ◦ C, the matrix alloy of the SiCw/6061Al composite becomes more and more soft, especially at 580 and 620 ◦ C, there is a little liquid appeared in the composite. Compared with the composite compressed at 300 ◦ C, the dislocation density of matrix alloy becomes lower and the load transferred from the matrix alloy to the whiskers drops. In addition, it is very easy for the whiskers to rotate and not to break in the matrix alloy. The whiskers rotated to the plane vertical to the compression direction. So the load carried by the whiskers
Fig. 1. Microstructure of as cast SiCw/6061A1 composite: (a) SEM and (b) TEM.
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Fig. 2. True stress–strain curves of SiCw/6061A1 and 6061A1 alloy compressed at different temperatures (˙ε = 0.37 s−1 ): (a) 300 ◦ C, (b) 540 ◦ C, (c) 580 ◦ C and (d) 620 ◦ C.
decreases. Then the flow stress for the composite become lower at higher compression temperatures. The SiC whiskers can keep larger length in the composite after compressed at higher temperatures. It is very beneficial for the compressed SiCw/6061Al composite to obtain high mechanical properties. In conclusion, the flow stress for the composite decreases with increasing compression temperatures. It is worth to mention that the superplasticity occurred in the composite during compression at 580 ◦ C with the strain rate 0.37 s−1 . The maximum deformability of the composite was obtained under the above compression condition. The critical compression reduction (the maximum value of reduction in height of the sample when no microcracks could be seen by naked eye during compression) of the composite can be reach 83%. Several possible deformation processes, including grain boundary sliding, interfacial sliding at liquid phase are expected to take place in the composite during hot compression [10]. Fig. 3 shows the SEM microstructure of the SiCw/6061Al composite compressed at different temperatures. It can be seen that cavities are formed easily when the temperature is much higher than the solidus temperature of the matrix or much lower than it, and the cavities are located at the interfaces between the reinforcement and the matrix. But,
when the compression temperature is near the solidus of the matrix, such as 580 ◦ C, cavitation is seldom seen. This is because when the composite is compressed at 540 ◦ C, it leads to stress concentration at the interfaces of the composite and gives rise to cavity nucleation. As a result of potential nucleation sites increases and consequently the number of cavities increases with increasing strain. The presence of a liquid phase at grain boundaries with high misfit angles, and at reinforcement–matrix interfaces, in the composite has been proved by in situ observation using transmission electron microscopy (TEM) at elevated temperature [11]. Therefore, the stress concentration level at the interface containing a liquid phase may be much lower than in interfaces without a liquid phase. Therefore, the accommodation required to relax the stress concentration by sliding can be much more easily achieved through rapid mass transport within a liquid phase. However, at 620 ◦ C, cavity nucleation occurs much more easily at these interfaces, since the continuous liquid phase in the composite leads to a weak bonding force between the liquid and the solid. In addition, the increasing amount of the liquid phase tends to aggregate or is even squeezed out of the samples. The liquid phase is solidified after compression and some casting defects (cavities) may appear. All this will affect the deformability of SiCw/66061Al composite. In gen-
G.S. Wang, L. Geng / Materials Chemistry and Physics 96 (2006) 2–8
5
Fig. 3. SEM microstructure of SiCw/6061A1 composites compressed at different temperatures (˙ε = 0.37 s−1 , ε = 0.8): (a) 540 ◦ C, (b) 580 ◦ C and (c) 620 ◦ C.
eral, the maximum deformability of the composite is obtained when the compression is at 580 ◦ C, which has been reported by the literature [12]. The dislocations and grain size and shape in the matrix of the composite after compression at different temperatures were shown in Fig. 4. It can be seen from Fig. 4(a) that the dislocation density of the SiCw/6061Al composite after compression at 540 ◦ C increased sharply compared with the composite as cast. The dislocation density is very high, 1011 cm−2 , which is determined by XRD. The SiCw/6061Al composite sample was tested by XRD and the peaks of each phase in the composite were obtained firstly. And then using the software programmed by Prof. Wang Yumin in Jilin University, the dislocation density was calculated and obtained. The details of calculation process were well documented in the literature [13]. The main equations used in the determination were as follows: h(x) =
g(y)f (x − y)dy Q
(1)
Here, h(x) is the actual line shape of SiCw/6061Al composite sample tested by XRD, g(y) is the width of line shape caused by geometry factors, f(y) is the width of line shape caused by physical factors, and Q is proportional factor and Q can be omitted when only the shape changes can be researched. f(x), g(x), h(x) could be proceeded with Fourier transformation during the region of −Q/2, Q/2 in the s = 2 sin θ/λ space: i2πny F (n)exp − f (y) = Q n 2πny 2πny = Fr (n)cos + Fi (n)sin (2) Q Q n i2πn y g(z) = G(n )exp − Q n 2πn y 2πn y Gr (n )cos + Gi (n )sin = Q Q n (3)
G.S. Wang, L. Geng / Materials Chemistry and Physics 96 (2006) 2–8
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Fig. 4. The TEM micrographs of the matrix microstructure of SiCw/6061A1 compressed at different temperatures (˙ε = 0.37 s−1 , ε = 0.8): (a) 540 ◦ C, (b) 580 ◦ C and (c) 620 ◦ C.
h(x) =
H(n )exp −
i2πn y
Q 2πn y 2πn y Hr (n )cos = + Hi (n )sin Q Q n (4) n
Here, Fr (n), Fi (n), Gr (n ), Gi (n ), Hr (n ), Hi (n ) are solid part and empty part of f(x), g(x), h(x) Fourier outspread coefficients, respectively: Fr (n) =
Hr (n )Gr (n ) + Hi (n )Gi (n ) G2r (n ) + G2i (n )
(5)
Put Eqs. (2)–(4) into Eq. (1), we can obtain: Hr (n )Gr (n ) − Hi (n )Gi (n ) Fi (n) = G2r (n ) + G2i (n )
Let A(L) = Fr (L), B(L) = Fi (L), we can know from literature [14], the symmetry of line shape was reflected by A(L) and it is related to grain size Deff , root-mean-square of second type strain SMR. The non-symmetry of line shape was reflected by B(L) and it is related to fault energy. Therefore, A(L) = AD (L)Asm (L)
(7)
Here, AD (L) is the width of line shape caused by grain size. Asm (L) is the width of line shape caused by microscopic strain ε2L corresponds to m grade reflection. L Deff 2π2 L2 ε2L s Am (L) = exp − 2 dm
AD (L) = A −
(6) Here, A is bending effect constant.
(8)
(9)
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Fig. 5. The interfaces of SiCw/6061A1 composite compressed at different temperatures (˙ε = 0.37 s−1 , ε = 0.8): (a) 540 ◦ C, (b) 580 ◦ C and (c) 620 ◦ C.
According to the above equations, the information of crystalline defects could be obtained. At the same time, combining the curve of standard sample researched by Prof. Wang Yuming, the grain size and the dislocation density of the composite was achieved. It is assumed that such high dislocation density may be attributed to the stress concentration caused by the deformation. There was no obvious difference in grain size of the composite before and after compression at 540 ◦ C. Compressed at 580 ◦ C, the grain of the composite keeps equiaxed in shape still and the dislocation density of the composite is much lower than that of at 540 ◦ C. This result is agreement with that a little liquid appeared in the grain boundary or the interface could relax the stress concentration in the composite. The grain size increased obviously and the dislocation density decreased clearly in the composite compressed at 620 ◦ C compared with its counterparts. The interfaces of the SiCw/6061Al composite after compression at different temperatures are shown in Fig. 5.
At 540 ◦ C, Fig. 5(a), TEM examination reveals that a clean interface exists between the SiC whisker and aluminum matrix. No any interface reaction products could be seen. However, compressed at 580 ◦ C, some discontinuous interface reactants appeared on the surface of SiC whiskers. The interface reactant was determined to be Al4 C3 by XRD and EDAX. The thick and continuous interface reactant layers were produced in the composite during compression at 620 ◦ C. The serious interface reaction between the whisker and the matrix could affect the mechanical properties of the SiCw/6061Al composite greatly. It is suggested that 620 ◦ C is unfit for deforming of the SiCw/6061Al composite.
4. Conclusion The partial melting temperature of the SiCw/6061Al composite is a little lower than that of 6061Al matrix. The
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flow stress of the SiCw/6061Al composite and the difference between the composite and the matrix alloy both decreased with increasing temperatures. The superplasticity occurred in the composite during compression at 580 ◦ C with the strain rate 0.37 s−1 . There was no obvious difference in the grain size and shape among the composites compressed at 540 and 580 ◦ C and before compression. The grain size increased sharply and the dislocation density decreased clearly of the composite compressed at 620 ◦ C. No any interface reactant could be seen in the SiCw/6061Al composite compressed at 540 ◦ C. The interface reaction product was discontinuous and continuous in the composite when the compression is at 580 and 620 ◦ C, respectively. Acknowledgements The authors are grateful for the finance support of the National Nature Science Foundation of People’s Republic of China under grant no. 50071018, and the National Key Project for Basic Research of China (973 Project) under the number of G2000067206.
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