Preparation and characterization of copper matrix composites by reaction sintering of the Cu–Mg–B system

Preparation and characterization of copper matrix composites by reaction sintering of the Cu–Mg–B system

Journal of Alloys and Compounds 466 (2008) 87–91 Preparation and characterization of copper matrix composites by reaction sintering of the Cu–Mg–B sy...

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Journal of Alloys and Compounds 466 (2008) 87–91

Preparation and characterization of copper matrix composites by reaction sintering of the Cu–Mg–B system D.B. Liu a,∗ , M.F. Chen a , A. Rauf b , C.X. Cui c , J.J. Tan a a

b

Institute of Material, Tianjin University of Technology, TianJin 300191, China National Center of Nanotechnology, Pakistan Institute of Engineering & Applied Science, Islamabad 45650, Pakistan c Institute of Material, Hebei University of Technology, TianJin 300293, China Received 11 September 2007; received in revised form 6 November 2007; accepted 7 November 2007 Available online 17 November 2007

Abstract Mg–B intermetallics reinforced copper matrix composites were prepared by reaction sintering of the Cu–Mg–B system. The sintering was performed at the temperature as determined from the result of the differential thermal analysis (DTA). The phases formed in the samples were identified by X-ray diffraction (XRD). The microstructures were observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). These results indicate that some Mg–B intermetallic involving MgB2 , MgB4 and MgB6 were produced in the copper matrix. The interface between the in situ formed Mg–B intermetallic particles and matrix is clean and well bonded. Compared with the MgB2P /Cu composites fabricated by ex-situ powder metallurgy, the composites fabricated by reaction sintering technology exhibit higher density, hardness and lower electrical conductivity. © 2007 Elsevier B.V. All rights reserved. Keywords: Copper matrix composites; Reaction sintering; Fabrication; Characterization

1. Introduction Boride ceramics possess many desirable properties, such as high hardness, low density, high melting temperature, high modulus and high corrosion resistance [1]. These outstanding features make them to be potential reinforcement candidates in copper matrix composites. For example, many of the studies have been focused on TiB2 /Cu composites due to the availability and relatively low fabrication cost [2,3]. Another boride ceramic is MgB2 which was discovered as superconductor by Akimitsu et al. in 2001 [4]. A series of studies had been carried out regarding its thermal and electrical transport properties, the dope effect, the isotope effect, etc. [5–7]. However, keeping in view its mechanical and physical properties of MgB2 at normal state, such as its lower density (2.1 g/cm3 ), lower line expansion coefficient (8.1 × 10−6 K−1 ) and lower electrical resistance (80–220 ␮·cm) [8], it can be used as reinforcement for copper metal matrix composites. Some researchers have used MgB2 as reinforcement for Mg matrix and ∗

Corresponding author. Tel.: +86 22 23678581; fax: +86 22 23679345. E-mail address: [email protected] (D.B. Liu).

0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.11.022

Al2 O3 matrix composites [9,10]. In the present study, MgB2 reinforced copper matrix composites have been prepared by means of reaction sintering from Cu, B and Mg system. The microstructure and properties of the composites have been studied and discussed. 2. Experiment materials and methods Copper powder (99.0% purity, ∼60 ␮m), magnesium powder (99.9% purity, ∼29 ␮m)and amorphous boron powder (99.0% purity, ∼10 ␮m) were mixed in weight proportions corresponding to the following reaction (1) and (2), labeled respectively as 10 wt% (Mg + 2B)–Cu system and 20 wt% (Mg + 2B)–Cu system. Mg + 2B + 90 wt%Cu → MgB2 + 90 wt%Cu

(1)

Mg + 2B + 80 wt%Cu → MgB2 + 80 wt%Cu

(2)

Powder blends were mixed by ball milling for 10 h. The mixtures were cold pressed into pellets by applying a pressure of 500 MPa. Then these pellets were placed into a small sealed stainless steel tube. Before sintering, the temperatures at which reactions and phase transformations could occurred in the mixed powder were determined by differential thermal analysis (DTA). During DTA, the powder blends were heated in argon atmosphere in the furnace of the analyzer, where the temperature was raised from ambient to 1000 ◦ C at a rate of 10 ◦ C/min. Sintering was performed according to the results obtained from DTA.

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Fig. 1. DTA of 10 wt% (Mg + 2B)–Cu system. Phases formation in the samples were identified by X-ray diffraction (XRD). The microstructures of these samples were observed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The hardness of the samples was evaluated by Vickers hardness tests. Each hardness value recorded was an average of five distinct indented measurements made on the sample surface. The density of the composites was obtained by the Archimedean principle. The electrical conductivity of the composites was measured using eddy current electrical conductivity apparatus.

3. Results and discussion 3.1. DTA analysis Fig. 1 is the DTA curve of 10 wt% (Mg + 2B)–Cu system, in which a number of exothermic peaks can be seen. A gentle first exothermic peak appears at 563.48 ◦ C, which corresponds to

the solid–solid reaction between Mg and B forming MgB2 . This temperature is higher than the previously reported temperature of 530 ◦ C for the corresponding solid reaction [11]. This may be because of two reasons, firstly, due to the relatively higher heating rate compared to our previous study and secondly, the copper particle which is the major portion in Cu–Mg–B systems plays the role of chemical dilution, hindering the reaction between Mg and B. There is an endothermic peak at 653.16 ◦ C corresponding to the melting of magnesium. As Mg melts, the reaction between the liquid Mg and B powder became faster than that in the solid–solid reaction stage due to the fluidity of liquid phase (Mg) which facilitates diffusion of atom and enlarging the contact area of the reactants. So, the second exothermic peak at 681.25 ◦ C corresponding to the reaction between liquid Mg and B forming MgB2 is more obvious than the first exothermic peak. The third exothermic peak located at 742.23 ◦ C is corresponding to the reaction between liquid Mg and Cu forming Mg–Cu intermetallics, including Mg2 Cu and MgCu2 .The melting temperature of the Mg2 Cu is 568 ◦ C, so at 742.23 ◦ C it cannot be stable. Moreover, the Gibbs free energy of MgCu2 is lower than that of Mg2 Cu [12], so the formation of the former phase is more dominant. Hence the stable Mg–Cu intermetallics phase at 742.23 ◦ C seems be MgCu2 . In the light of the transitions measured in the DTA analysis, the sintering was proceeded in the following way. The cylindrical pellets (19 mm diameter and 10 mm height) were heated first to 600 ◦ C and held at this temperature for 2 h in order to make Mg diffuse sufficiently in the pellet so that the solid–solid reaction with B could occur. Then, the temperature was raised to 900 ◦ C and held for 2 h for the densification of the pellets.

Fig. 2. SEM micrographs and element distribution maps of 10 wt% MgB2 /Cu after reaction sintering: (a) SEM micrographs of composites; (b) Cu elemental distribution; (c) Mg elemental distribution; (d) B elemental distribution.

D.B. Liu et al. / Journal of Alloys and Compounds 466 (2008) 87–91

Fig. 3. The XRD pattern of 10 wt% (Mg + B)–Cu system after reaction sintering.

3.2. Microstructure and formation phase Fig. 2(a) shows the SEM microstructure of 10 wt% (Mg + 2B)–Cu system after sintering. It can be seen that some rectangular particles, which are darker than the bright copper background, are clearly dispersed in the copper matrix. Cu, Mg and B elemental distribution maps are shown in Fig. 2(b)–(d). It can be seen that the elemental distributions of Cu and Mg are in contrast to each other (Fig. 2(b) and (c)), while those of B and Mg element are indistinguishable (Fig. 2(c) and (d)). It clearly indicates that the rectangular particles formed in copper matrix are Mg–B intermetallics. Fig. 3 is the XRD pattern of the 10 wt% (Mg + 2B)–Cu system after sintering. In addition to the major Cu phase, peaks of the

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MgO and MgCu2 were also detected. The existence of MgO is due to the oxidation of Mg in the raw materials as well as the deoxidization of CuO by Mg on the surface of copper powder during the sintering. It is interesting that no evident Mg–B phase peak could be detected by the XRD, although the rectangular Mg–B intermetallic particles can be observed in copper matrix by SEM as shown in Fig. 2. The possible reasons are their low content in the composites and due to the low structure factor of Mg–B intermetallic, its low relative intensity could not be detected by XRD analysis [13]. Moreover, the occurrence of MgCu2 is attributed to the reaction between Mg and Cu. Being the major portion in Cu–Mg–B system copper will be largely in contact with the magnesium forming the solid solution or MgCu2 intermetallic. When sintering temperature exceeds the melting point of MgCu2 (797 ◦ C), it may decompose into Mg and Cu. However, Mg will re-react with Cu to form MgCu2 by eutectic reaction in the following cooling process. Fig. 4(a) shows the SEM microstructure of 20 wt% (Mg + 2B)–Cu system after sintering. In contrast to the surface microstructure of 10 wt% (Mg + 2B)–Cu system (Fig. 2(a)), some irregular particles dispersed in the copper matrix can be seen. Fig. 4(b)–(d) refers to the Cu, Mg and B elemental distribution respectively, clearly indicate that the irregular particles are also Mg–B intermetallic. Fig. 5 is the XRD pattern of the 20 wt% (Mg + 2B)–Cu system after sintering. Different from the XRD pattern of the 10 wt% (Mg + 2B)–Cu system, the peak of MgB2 , MgB4 , MgB6 and CuB24 phase are observed in addition to the peak of Cu, MgO and MgCu2 phases of Fig. 3. This implies that the Mg–B intermetallic formed in copper matrix are MgB2 , MgB4 or MgB6 phase, however, they cannot be identified individually. It is interest-

Fig. 4. SEM micrographs and element distribution maps of 20 wt% MgB2 /Cu after reaction sintering: (a) SEM micrographs of composites (b) Cu elemental distribution (c) Mg elemental distribution (d) B elemental distribution.

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D.B. Liu et al. / Journal of Alloys and Compounds 466 (2008) 87–91 Table 1 Physics-mechanics properties of Cu–10 wt% (Mg + 2B) and 10 wt% MgB2P /Cu

Fig. 5. The XRD pattern of 20 wt% (Mg + B)–Cu system after reaction sintering.

ing that though the magnesium and boron powders were mixed by the stoichiometric of MgB2 , but only little amount of MgB2 could be detected. This may be explained by following reasons. Firstly, the volatility of Mg at 900 ◦ C resulted in Mg deficiency which favored the formation of phases such as MgB4 and MgB6 as evident from some unreacted Mg was found on the side of stainless steel tube after sintering. Secondly, during the sintering process, some amount of Mg may diffuse into copper forming solid solution or MgCu2 by the chemical reaction, which may also result in the Mg deficiency. Thirdly, some MgB2 are decomposed into MgB4 at 900 ◦ C [14]. Fourthly, the aggregation of B powder in some local region causes the stoichiometric ratio of Mg and B to be 1:2 + x. To compensate the Mg deficiency, we can increase the proportion of Mg in the starting materials, however, higher Mg will favor reaction with Cu to form MgCu2 due to high affinity between Mg and Cu. To control the reaction between Mg and Cu and maintain more Mg to react with B, further work will be carried out in our group. 3.3. Characteristics of interface Fig. 6(a) is the amplificatory image of Fig. 4(a). It can be seen that Mg–B intermetallic particles have a good interface bond-

Material

Relative density (%)

Hardness (HV)

Electrical conductivity (IACS%)

Cu–10 wt% (Mg + 2B) 10 wt% MgB2P /Cu

91 87

128 117

28 61

ing with copper matrix. The result of TEM in Fig. 6 (b) also indicates the interface is clean and trim. Line scanning image in Fig. 6(a) shows an interesting phenomenon that the Mg element varies gradually from the edge to center in the particle. In other words, the amount of Mg element at the interface is lesser than at the center of the Mg–B compound particle. The reason for this is that Mg–B intermetallic particles reacted with copper matrix during sintering and consequently the Mg atom in the Mg–B intermetallic diffuse into copper matrix to form MgCu2 whose diffusivity reduces as it approaches the center within the particle. The line scanning image (Fig. 6(a)) also indicated that the particle is a multiple phase rather than a single phase. 3.4. Mechanical and electrical properties of 10 wt% (Mg + 2B)–Cu composites Table 1 shows the mechanical and electrical properties of 10 wt% (Mg + 2B)–Cu composite fabricated by reaction sintering and 10 wt% MgB2P /Cu composite fabricated by ex-situ powder metallurgy. The former exhibits higher density and hardness, however, the electrical conductivity is considerably lower than that of later. During reaction sintering the liquid Mg phase can increase the relative density of composites according to the theory of liquid phase sintering [15]. As compared with 10 wt% MgB2P /Cu composites, the Cu–10 wt% (Mg + 2B) composites have higher amount of MgCu2 phase and other impurity phases which increase the scattering surfaces for the conduction electrons and consequently reduce the electrical conductivity while increasing the hardness of the composites.

Fig. 6. (a) Line scanning image and (b) TEM image of interface area of 20 wt% (Mg + B)–Cu system after reaction sintering.

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4. Conclusions Cu, Mg and B powder were used to prepare the MgB2 particles reinforced copper matrix composites by means of reaction sintering technology. However, the quantity of MgB2 was minor in the composites due to the Mg deficiency. The interface between Mg–B compound particles and matrix is clean and well bonded. Because of the interface reaction between the Mg–B intermetallic and copper matrix, the amount of Mg element at the interface is less than that at the center of Mg–B compound particles. MgCu2 phase was also unavoidably present in the composite due to the high affinity between Mg and Cu. Compared with the 10 wt% MgB2P /Cu composite fabricated by ex-situ powder metallurgy, the Cu–10wt% (Mg + 2B) composite fabricated by reaction sintering exhibits higher density and hardness, however, the electrical conductivity of the later is considerably lower than that of the former composite. Acknowledgements The work is supported by China National Science Foundation (50471048) and Tianjin High University and the

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