收稿日期：22 September 2013
基金：financially supported by the Special Foundation for Technology Research and Development in Research Institute of China(No.2011DIA5k023);
Microstructure of phases in a Cu–Zr alloy
Li-Jun Peng Xu-Jun Mi Bai-Qing Xiong Hao-Feng Xie Guo-Jie Huang
State Key Laboratory of Nonferrous Metals and Processes,General Research Institute for Nonferrous Metals
Abstract：
The morphology and crystallography of phases in the Cu-0.12% Zr alloy were investigated by scanning electron microscope(SEM), transmission electron microscope(TEM), and high-resolution transmission electron microscope(HRTEM). The results show that the as-cast microstructure of Cu–Zr alloy is mainly Cu matrix and eutectic structure which consist of Cu and Cu5Zr phases with a fine lamellar structure. The disk-shaped and plateliked Cu5Zr phases with fcc structure are found in the matrix, in which habit plane is parallel to {111}a plane of the matrix.Between the copper matrix and Cu5Zr phase,there exists an orientation relationship of [112]a|||| [011]Cu5Zr;(111)a||||(111)Cu5Zr. The space structure model of Cu5Zr phase can be established.
Keyword：
Cu–Zr alloy; Microstructure; Crystal structure; Morphology; Habit plane;
Author： Xu-Jun Mi,e-mail:mxj@grinm.com; Li-Jun Peng,e-mail:penglijun198677@163.com;
Received： 22 September 2013
1 Introduction
Age-hardenable Cu–Zr alloys are used in numerous applications where an excellent combination of mechanical properties, high electronic conductivity, and good resistance to fatigue is required. Thus, they are attractive candidates for railway contact wire [1, 2], electrodes for spot welding [3, 4], and heat exchangers [5]. The excellent strength (compared with pure copper) is attributed to the formation of the fine and uniform distribution CuxZr precipitation, whereas the high conductivity is due to the extremely low solubility of Zr in Cu matrix at room temperature [6].
Over the past several decades, many investigations have been focused on types, crystallographic structure, and morphology of the Zr-rich phases in the binary Cu–Zr system or ternary Cu–Cr–Zr system, but there are few unanimous agreements on the phases of Cu–Zr alloy. The Cu3Zr was reported to be the first equilibrium phase by Kawakatsu et al. [7] in 1967 through diffusion couple method. In addition, At least three other precipitation products were also found in Cu-Zr based alloys: asorthorhombic Cu4Zr (believed as Cu9Zr2) phase with a = 0.504 nm, b = 0.492 nm, and c = 0.664 nm, hcp Cu51Zr14(believed as Cu3Zr) phase with a = 1.125 nm and c = 0.8275 nm, and fcc Cu5Zr phases with a = 0.687 nm [8–13]. Recently, some researchers tried to identify the relative stability of intermetallic phases in the binary Cu–Zr system by thermodynamic calculation and first principles [14–16]. On the basis of these studies, it is concluded that phases are not completely characterized and not well understood. Therefore, the morphology and crystallography of phases in the Cu–Zr alloy were investigated in detail by scanning electron microscope (SEM), transmission electron microscope (TEM), and high resolution transmission electron microscope (HRTEM) in the present study.
2 Experimental
The experimental material Cu-0.12Zr (wt%) was prepared with electrolytic copper and copper-10 wt% zirconium master alloy in a vacuum-induction melting furnace, and then cast into cylindrical ingots of 33 mm in diameter. The specimens were homogenized at 900 °C for 12 h, coldrolled and drawn to 5 mm in diameter. The drawn specimens were solution-treated at 950 °C for 1 h, waterquenched, and aged at 450 °C for 8 h. The microstructure of the alloy was examined with a Quanta 200F field emission-environment SEM. TEM samples were prepared by double jet electropolishing techniques using a 25 % nitric acid in methanol solution at about -40 °C. TEM observations were carried out on a JEM 2100F TEM operating at 200 k V.
Fig.1 BSE images of a as-cast microstructure of Cu–Zr alloy, b stripe-shaped phases, c rob-shaped phases, and d triangle-shaped phases
Fig.2 TEM images of as-cast alloy: a eutectic structure, b fine lamellar structure, and c SAED pattern of fine lamellar structure
3 Results
Figure 1 shows the as-cast microstructures (BSE image) of Cu–Zr alloy. It can be seen that the as-cast microstructure of Cu–Zr alloy is mainly Cu matrix and white phases. Three different kinds of the white phases are shown in Fig. 1, such as stripe-shaped (Fig. 1b), rod-shaped (Fig. 1c), and triangle-shaped (Fig. 1d) exist in the alloy. According to the TEM analysis, white phase is a typical eutectic structure which is composed of Cu and fine CuxZr (x = 1–5) lamellar structure. From the selected area electron diffraction (SAED) patterns of the lamellar structure, it is complex since there are numerous weak spots in addition to the strong spots, which may be attributed to the faults present in the lamellar structure. By considering the strong spots only, the crystal structure of lamellar structure is identified as fcc with lattice constant of 0.682 nm. Therefore, it is confirmed as Cu5Zr phase, which is consistent with the result in Ref. [3]. The TEM images of eutectic structure are shown in Fig. 2.
When the specimen was homogenized at 900 °C for 12 h, the eutectic structure is almost dissolved into the matrix. The homogenized specimens were then cold-rolled, drawn, solution-treated at 950 °C for 1 h, water-quenched, and aged at 450°C for 8 h; some phases tend to precipitate out, aligned along a certain direction. Owing to high degrees of symmetry in the copper matrix, different shapes of precipitates are observed. Figure 3 shows the TEM images of an aged Cu–Zr alloy. A large number of phases (plate-like and disk-shaped) with a diameter of 50 to 100 nm are obviously found in the matrix, as shown in Fig. 3a and b, respectively. From Fig. 3a, the plate-like phases precipitate along on the (111)aand (111)aplane of the matrix, respectively, which is parallel to the fault plane. In addition, disk-shaped phases are also observed and angle between the two phases is about 70°. It can be deduced that their habit planes would be (111)aand (111)a. So the habit plane of Zr-rich phase is {111}aplane of the Cu matrix. From the SAED patterns of the alloy, it is obtained that thecrystal structure of plate-like phase is identified as fcc with lattice constant of 0.687 nm and it is confirmed as Cu5Zr phase, which has semi-coherent with matrix and the orientation relationship with matrix, e.g., ?112ak ?011Cu5Zr; ?111?ak ?111?Cu5Zr, as shown in Fig. 3b. According to the HRTEM and fast Fourier transformation (FFT) results of the phase, it can be seen that the crystal structure of the Cu5Zr phase is fcc and the phase is parallel to ?111?aplane, which is in accordance with Fig. 3b.
Fig.3 TEM images of Cu–Zr alloy aged at 450 °C for 8 h: a bright field image with zone axis of [011]a, b bright field image with zone axis of [112]a, c SAED pattern of b with zone axis of [112]a, and d HRTEM image along [112]adirection of alloy and FFT
Fig.4 Space structure of Cu5Zr phase
4 Discussion
As described above, the morphology and crystallography of Cu5Zr phases are plated or disk-shaped and face-centered cubic structure. The habit plane of these phase is parallel to {111}aplane of copper matrix, based on the TEM and HRTRM observation. Therefore, the space structure model of Cu5Zr phases can be established, as shown in Fig. 4. Four equivalent {111}aplanes, namely: (111)a,(111)a; (111)a, and (111)a, and disk-shaped Cu5Zr phases are shown in Fig. 4. Different shapes of precipitates are obtained along the different directions. For example, the morphology of Cu5Zr phases on the (111)aand (111)aof the Cu matrix is plate-shaped along the [011]adirection, while on the (111)aand (111)ais disk-shaped. The morphology of Cu5Zr phases on the (111)aof the Cu matrix is plate-shaped along the [112]adirection, while the others are disk-shaped, which is in accordance with analysis of TEM.
5 Conclusion
In the alloy, the as-cast microstructure of Cu–Zr alloy is mainly Cu matrix and eutectic structure which consist of Cu and Cu5Zr phases with a fine lamellar structure. TEM and HRTEM results show that plate-like or disk-shaped Cu5Zr phases with fcc structure are found in matrix, in which habit plane is parallel to {111}aplane of the Cu matrix. The orientation relationship with exists between the Cu and Cu5Zr phases. The space structure model of Cu5Zr phase is established.
