Materials Science and Engineering B 177 (2012) 873–876
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Short communication
Anisotropic thermal expansion behaviors of copper matrix in -eucryptite/copper composite Lidong Wang a , Zongwei Xue a , Yingjie Qiao b , W.D. Fei a,c,∗ a b c
School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China College of Materials Science and Chemical Engineering, Harbin Engineering University 150001, China School of Mechanical Engineering, Qinghai University, Xining 810016, China
a r t i c l e
i n f o
Article history: Received 11 October 2011 Received in revised form 31 January 2012 Accepted 18 March 2012 Available online 4 April 2012 Keywords: -Eucryptite Metal Matrix Composite Thermal expansion behavior Deformation Twinning
a b s t r a c t A -eucryptite/copper composite was fabricated by spark plasma sintering process. The thermal expansion behaviors of Cu matrix of the composite were studied by in situ X-ray diffraction during heating process. The results show that Cu matrix exhibits anisotropic thermal expansion behaviors for different crystallographic directions, the expansion of Cu{1 1 1} plane is linear in the temperature range from 20 ◦ C to 300 ◦ C and the expansion of Cu{2 0 0} is nonlinear with a inflection at about 180 ◦ C. The microstructures of Cu matrix before and after thermal expansion testing were investigated using transmission electronic microscope. The anisotropic thermal expansion behavior is related to the deformation twinning formed in the matrix during heating process. At the same time, the deformation twinning of Cu matrix makes the average coefficient of thermal expansion of the composite increase. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Metal matrix composites (MMCs) with low coefficient of thermal expansion (CTE), high thermal and electric conductivities for the application of electronic packaging have attracted extensive research interests [1,2]. In order to meet the low CTE demand in application, MMCs generally have high volume fraction of reinforcements, which inevitability induces the deterioration of thermal and electronic conductivities of MMCs [3–5]. Recently, -eucryptite (LiAlSiO4 , denoted as Euc) with large anisotropic CTE (˛a = 7.26 × 10−6 /◦ C, ˛c = −16.35 × 10−6 /◦ C) [6] and near-zero volume CTE over the temperature range of 30–1100 ◦ C [7,8], has been chosen as the reinforcements for Al and Cu matrix composites. These researches have showed that the MMCs reinforced by the ceramic with negative CTE can obtain low CTE and good thermal conductivity simultaneously with lower volume fraction of reinforcements [9,10]. Due to the importance of the CTE of MMCs in application, many models have been proposed to predict the CTE of MMCs so far, such as Tunner’s model [11], Kenner’s model [12], Schapery’s model [13]. However, these models can only give the boundary of CTE of MMCs [14], but cannot accurately predict the CTE. The main rea-
∗ Corresponding author at: School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China. Fax: +86 451 86413908. E-mail address:
[email protected] (W.D. Fei). 0921-5107/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2012.03.036
son is that these models were established on the foundation of the thermoelasticity theory. According to the researches of Wang et al. [15] and Fei et al. [16], the plastic deformation of metal matrix resulted from the relaxation of thermal mismatch stress (TMS) during heating process has obvious affect on the CTE of MMCs. In this paper, Euc particles reinforced Cu matrix composite (Euc/Cu composite) was produced by spark plasma sintering (SPS). The thermal expansion behavior of different crystalline plane of Cu matrix has been studied by means of in situ X-ray diffraction (XRD) during heating process. The effect of microstructure evolution of Cu matrix on its thermal expansion behavior has been analyzed. 2. Experimental The Euc powder used in this study was produced by a method developed in our laboratory [17]. Pure copper powder (30 m) was supplied by Harbin Dong Da High-Tech Materials (Group) Corp. Ltd. Euc particles were firstly coated with Ag by an electroless plating method. Then the pure copper powder and Ag-coated Euc powder were mechanically mixed in a planetary mill for 4 h. Finally, Euc/Cu composite with 45% volume fraction of Euc particles was fabricated by SPS at 750 ◦ C in a graphite die under a uniaxial pressure of 50 MPa for 5 min. Microstructures were investigated on a Hitachi H3000 scanning electron microscope (SEM) and a FEI Tecnai G2 F30 transmission electron microscope (TEM). The phase composition of the composite was analyzed on a Philips X’Pert XRD with Cu K␣ radiation.
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Fig. 1. XRD patterns of the Euc/Cu composite before and after CTE measurement.
Specimens for TEM observations were thinned by ion milling. In situ XRD experiments were performed on a Rigaku D/max 2500 diffractometer with Cu K␣ radiation under a 40 kV voltage, 250 mA current and a step size of 0.05◦ ; a Rigaku temperature attachment was used to control the temperature of sample in the heating process. The K␣2 diffraction peaks were stripped by using the software provided by Rigaku Company. And the peaks were fitted by Gaussian function to search diffraction peaks. Finally, the interplanar spacings were calculated by Bragg’s law: 2d sin = , where d is the interplanar spacing, is the angle between the wavevector of the incident plane wave and the lattice planes, is wavelength of X-ray. The CTE measurements were carried out on a Netzsch DIL 402 dilatometer with a heating rate of 2 ◦ C/min. 3. Results and discussion Fig. 1 shows the XRD spectra of Euc/Cu composite before and after CTE measurement. The diffraction peaks are readily indexed to Euc (JCPDS No. 73-2328), Cu (JCPDS No. 04-0836) and Ag (JCPDS No. 87-0720), and the XRD spectra before and after CTE measurement are almost same, which suggests that no serious interfacial reaction has happened in the process of sintering and heating. Meanwhile, the ratios of the relative intensities of Cu diffraction peaks are very close to those of Cu powder (JCPDF No. 04-0836), which implies that there is no preferential orientation or texture in the Cu matrix. Fig. 2(a) shows the in situ XRD peaks of Cu{1 1 1} and Cu{2 0 0} for the composite at different temperatures. It is clear that the peaks of both Cu{1 1 1} and Cu{2 0 0} shift to lower degree with increasing temperature. Based on the results of in situ XRD in Fig. 2(a), the interplanar spacings of Cu{1 1 1} and Cu{2 0 0} in the composite are calculated using Bragg’s law. Fig. 2(b) and (c) exhibit the temperature dependences of dCu{111} and dCu{200} of Cu matrix during heating process. According to Fig. 2(b) and (c), we find that the thermal expansion behaviors of dCu{111} and dCu{200} are evidently different: the expansion of dCu{111} is linear from 20 ◦ C to 300 ◦ C, but the expansion of dCu{200} is nonlinear with an inflection point at about 180 ◦ C. Because of the close correlation between the thermal expansion behavior of MMCs and the microstructure evolution of metal matrix, TEM analyses of the composite before and after CTE measurement were carried out for understanding the anisotropic thermal expansion behavior in the Cu matrix of the composite. The TEM observations of Euc/Cu composite before and after CTE measurement are shown in Fig. 3. The interface of the composite is very clear and smooth and high density dislocations can be found in Cu matrix before the CTE measurement as shown in
Fig. 2. (a) In situ XRD peaks of Cu{1 1 1} and Cu{2 0 0} for the composite at different temperatures; (b) and (c) temperature dependences of dCu{111} and dCu{200} in the Euc/Cu composite measured by in situ XRD during heating process.
Fig. 3(a). Fig. 3(b) shows the TEM image of the composite after CTE measurement. The inserted image is the selected area diffraction pattern (SADP) of Cu matrix, and the mechanical twins in the Cu matrix near the interface can be observed in the composite. The results indicate that the TMS in the as-sintered composite is basically relaxed by the dislocation generation of Cu matrix when the composite is cooled from the sintering temperature to room temperature. However, TMS can be relaxed by the deformation twinning of Cu matrix to decrease the elastic energy of the composite during heating process of the CTE measurement.
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Fig. 3. TEM images of Euc/Cu composite (a) before CTE measurement and (b) after CTE measurement (the inserted image is the SADP of the matrix).
The above results in Fig. 1 show that Cu matrix has no preferential orientation. In addition, there are no specific orientation relationship between Euc particles and Cu matrix in the composite prepared by powder metallurgy. In this situation, even if the CTE values of dCu{111} and dCu{200} are different, dCu{111} and dCu{200} will exhibit similar linear thermal expansion behavior. As shown in Fig. 2, when the temperature is below 180 ◦ C, the linear expansion behaviors of dCu{111} and dCu{200} are similar; when the temperature exceeds 180 ◦ C, the expansion rate of dCu{200} decreases, but the expansion rate of dCu{111} remains unchanging. Combining the TEM results, we think that the nonlinear expansion behavior of dCu{200} is dependent on the deformation twinning of Cu matrix in the composite, and the detailed explanations are presented as follows. Firstly, Cu is a metal with low stacking fault energy, so it is prone to deformation twinning at low stress. Because the yield strength
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of Cu matrix in the heating process decreases gradually, the TMS of the composite may be relaxed by deformation twinning of the matrix, since the TMS in the Euc/Cu composite may be very high because of large difference of thermal expansion between Euc and copper. A similar phenomenon has been found in aluminum matrix composite [16]. Secondly, Cu with the face-centered cubic (FCC) structure has ¯ twin variants. Generally, the variants of these twelve {1 1 1}1 1 2 twins are random and equivalent in monolithic Cu. Owning to the large anisotropic CTE of Euc, the TMS of Cu grains adjacent to Euc particles in the Euc/Cu composite is also anisotropic. For this reason, the twinning direction of Cu matrix should be determined by the minimum of elastic energy in the composite. Finally, the TMS in MMCs is dependent on the difference both in CTE and in Young’s modulus between the reinforcement and matrix [18]. It is easy to understand that the maximum TMS exists along the Euc 0 0 1 direction because the CTE of Euc along the direction is as low as about −16.75 × 10−6 ◦ C−1 . Because the higher TMS means the higher elastic energy, the higher elastic energy along the Euc 0 0 1 direction can be reduced when the Cu grain direction with lower Young’s modulus is parallel to Euc 0 0 1 direction. Since the Young’s modulus of Cu along 1 0 0 directions is the lowest compared with other directions, the twinning caused by the relaxation of TMS can decrease the angle between Euc 0 0 1 and Cu 1 0 0 direction, On the basis of above analysis, the schematic of deformation twinning of Cu matrix in the composite is shown in Fig. 4. As the critical stress for twinning of matrix decreases with the temperature increasing. In this case, the matrix twinning takes place at the certain temperature (about 180 ◦ C) to relax the thermal mismatch stress. Generally, no certain orientation relationship exists between Cu matrix and Euc particle in the composite prepared by powder metallurgy technology, so various orientations of twin variants are possible. However, the orientations of twins near Euc particle are affected by the residual stress or elastic energy although the twin orientations. The effect of thermal mismatch stress on the twin orientations mainly results from the anisotropy of elastic properties of Cu matrix, which is schematically shown in Fig. 4. The CTE of Euc along Euc 0 0 1 direction is negative, which leads to the highest thermal mismatch along the direction. The deformation twinnings of Cu grains near Euc particles preferentially take place when the lowest elastic modulus is close to Euc 0 0 1 direction, among the equivalent twin variants of Cu grains. As shown in Fig. 4, the angle of Cu 1 0 0 direction of a Cu grain to Euc 0 0 1 direction of Euc particle is ˛ (˛ values for different Cu grains are different) before twin formation. Because of the elastic modulus along Cu 1 0 0 direction of Cu is the lowest, the twin variant orientation with the lower value of angle, ˛ , is preferential in equivalent twin variants, where ˛ is the angle between the twin Cu 1 0 0 direction and the Euc 0 0 1 direction. Correspondingly, the thermal mismatch stress or elastic energy can be reduced more effectively. With the decrease of the angle between Euc 0 0 1 and Cu 1 0 0, the thermal expansion of dCu{200} is restricted more effectively by Euc along Euc 0 0 1 direction. Consequently, the expansion rate of dCu{200} is decreasing after 180 ◦ C. The average CTEs of the composite obtained by the two CTE measurements in succession in the temperature range from 50 ◦ C to 180 ◦ C are presented in Fig. 5, which shows that the CTE of the composite in the first testing is larger than that in the second testing. The effect can be understood on the basis of the relaxation rate of the TMS in the MMCs since higher relaxation rate of compression TMS will result in the increased CTE of the MMCs [16]. During CTE testing, the 1 0 0 direction of Cu matrix tends to be parallel to the Euc 0 0 1 direction due to the twinning of Cu matrix for the first CTE testing, so there are higher relaxation rate of compression TMS in the composite in the second testing, which induces the larger CTE of the composite in the second testing.
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Fig. 4. Schematic of twinning mechanism of Cu matrix in the Euc/Cu composite.
expansion of Cu matrix and the increase of the average CTE of the composite. Acknowledgements This work was supported by National Natural Foundation of China (No. 50801016), National Basic Research Program of China (No. 2011CB612200), and Harbin Science and Technology Research Funds for innovation talents (No. 2011RFXXG025). References
Fig. 5. Average CTEs of the composite in the temperature range from 50 ◦ C to 180 ◦ C obtained by two CTE testings in succession.
4. Conclusions (1) The thermal expansion behaviors of Cu{1 1 1} and Cu{2 0 0} plane in the Euc/Cu composite are different. The former exhibits linear expansion behavior in the temperature range from 20 ◦ C to 300 ◦ C, and the later exhibits nonlinear expansion with an inflection point at 180 ◦ C. (2) The microstructures of Cu matrix in the Euc/Cu composite have been changed by the relaxation of TMS during heating process. Before CTE measurement, the substructure of Cu matrix has high density dislocations; after CTE measurement, the deformation twins can be observed. (3) The decrease of the angle between Cu 1 0 0 direction and Euc 0 0 1 in the Euc/Cu composite is beneficial to relax the TMS in the composite, which may cause the anisotropic thermal
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