Introduction

Age-hardened Cu-base alloys are most widely used in the electrical connection, high-strength springs, electrical contacts, and corrosion- and wear-resistant materials (Ref 1–3). Among these copper alloys, Cu-Be alloy has an excellent combination of electrical conductivity and mechanical properties. Nevertheless, its toxicity brings about environmental hazards during the fabrication process. In addition, poor anti-stress relaxation property at elevated temperatures hinders its applications (Ref 4). Subsequently, it is of significance to seek a new material to replace Cu-Be alloy. Currently, Cu-Ti alloy is regarded as the most potential substitute for age-hardened Cu-Be elastic alloy since its mechanical strength is comparable to that of Cu-Be alloy. However, Cu-Ti alloy has a quite low electrical conductivity due to the presence of solute Ti atoms in the copper matrix (Ref 5–10). Hence, it is necessary to improve the electrical conductivity of Cu-Ti alloy without deteriorating mechanical strength. Up to now, a few attempts have been made to improve the electrical conductivity of binary Cu-Ti alloys by reducing solute Ti atoms in the copper matrix. Semboshi et al. (Ref 11) reported that the electrical conductivity of Cu-1 at.% Ti alloy aged in a hydrogen atmosphere can reach up to 27.84 MS/m, which is increased by 167% compared to that of the same alloy aged in a vacuum. The improved electrical conductivity is ascribed to the precipitation of titanium hydride, which reduces the concentration of Ti in the matrix more efficiently than aging in a vacuum. However, the formation of the brittle TiH2 phase results in a sharp decrease in the hardness of Cu-1 at.%Ti alloy aged in the hydrogen atmosphere in comparison to the vacuum aging treatment. Konno et al. (Ref 12) revealed that the electrical conductivity of Cu-3 at.%Ti alloy with 4 at.%Al addition is 3.48 MS/m, which is about 6 times larger than that without Al addition, whereas the peak hardness decreases from 280 to 180 HV. Markandeya et al. (Ref 13) found that Cd addition decreases the electrical conductivity of Cu-3Ti alloy though the yield strength and tensile strength increase considerably. Lebreton et al. (Ref 14) studied the microstructural evolution and hardness of Cu-Ti-Sn alloys during aging, and good hardness values were obtained, but the effect of Sn addition on the electrical conductivity of Cu-Ti was unclear, and no data were reported on the electrical conductivity of Cu-Ti-Sn alloy in their work. Subsequently, it is necessary to clarify the effect of Sn addition on the electrical conductivity of Cu-Ti alloy. In the present work, Cu-3Ti-2Sn and Cu-2Ti-2Sn alloys were studied by structural observation and electrical and mechanical measurements. The electrical conductivity was examined as a function of aging time to discuss the relationship between microstructural evolution and electrical conductivity of the Cu-Ti-Sn alloy. The purpose is to get a better understanding of microstructural evolution during aging process and to develop a new alloy with a better combination of strength and electrical conductivity in order to extend their applicability among the electronic engineering fields.

Experimental

Button ingots with a nominal composition of Cu-3Ti-2Sn and Cu-2Ti-2Sn alloys were prepared from 99.9% electrolytic copper, 99.9% titanium, and 99.999% tin by arc-melting in a non-consumable vacuum furnace. Each ingot was melted at least four times to guarantee its homogeneity. The ingots were cut into blocks, which were subsequently solution-treated at 800 °C for 4 h, followed by water quenching. Finally, the aging treatments were performed in a furnace under an argon atmosphere at 450 °C for 0.5, 1, 4, and 8 h, respectively. Electrical conductivity was measured by a 7501A eddy current gage. Vickers hardness (HV) test was performed on a TUKON2100 microhardness tester at the load of 50 g holding for 15 s. The measured value is the average of five readings taken for each measurement. Light optical metallographic analysis was done on mechanically polished and etched samples. The etchant used was a mixture of 5 g FeCl3, 15 mL HCl, and 100 mL distilled water. The microstructures were examined in a GX71 optical microscope. The specimens for TEM observation were cut from the aged samples using a Buehler’s Isomet low-speed saw and then mechanically polished to obtain the slices with a thickness of 50 μm. Disks of 3 mm diameter were punched out from the slices, followed by thinning in an M691 ion milling at 4.5 kV. The thin foils were examined by a JEM-2100 transmission electron microscope (TEM) operated at 120 kV.

Results and Discussion

Microstructures and Properties of the As-cast Cu-Ti-Sn Alloys

Figure 1 shows the XRD pattern of the as-cast Cu-3Ti-2Sn alloy. It is learnt that the secondary phase is CuSn3Ti5 intermetallic compound. It is interesting to notice that the phase is different from the result reported by Lebreton et al. (Ref 14). They thought that the secondary phase is in the presence of the ternary eutectic intermetallic CuTi3Sn5 in the as-cast state. This is probably caused by the different Sn content. The microstructures of the as-cast Cu-3Ti-2Sn and Cu-2Ti-2Sn alloys are shown in Fig. 2(a) and (b), respectively. As seen from Fig. 2, CuSn3Ti5 phase distributes inside the copper grain and at the grain boundary in the as-cast ternary Cu-Ti-Sn alloys. The average grain size of Cu-3Ti-2Sn measured utilizing the linear intercept method is 37 μm, which is less than that of Cu-2Ti-2Sn alloy (50 μm). The measured electrical conductivity and hardness value of the as-cast Cu-3Ti-2Sn alloy are 4.4 MS/m and 98.6 HV, respectively, while Cu-2Ti-2Sn alloy has the electrical conductivity of 7.8 MS/m and the hardness value of 93.2 HV. It suggests that more Ti content is favorable for the improvement on the hardness, but deteriorates the electrical conductivity of the as-cast Cu-Ti-Sn alloys.

Fig. 1
figure 1

XRD pattern of the as-cast Cu-3Ti-2Sn alloy

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Fig. 2
figure 2

As-cast microstructures of Cu-3Ti-2Sn (a) and Cu-2Ti-2Sn alloys (b)

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Microstructural Evolution of the Aged Cu-3Ti-2Sn and Cu-2Ti-2Sn Alloys

Figure 3 and 4 show the microstructures of Cu-3Ti-2Sn and Cu-2Ti-2Sn alloys after various aging times, respectively. As seen from Fig. 3(a) and 4(a), a small amount of fine precipitates form in the copper matrix after aging at 450 °C for 1 h. It is also noticed that there are still some residues of CuSn3Ti5 intermetallic compound even after solution treatment. With the increase of aging time, larger numbers of the precipitates present in the copper matrix and at grain boundaries, see Fig. 3(b) and 4(b). At 8 h, fine precipitates are uniformly distributed in the copper matrix and at grain boundaries, as shown in Fig. 3(c) and 4(c).

Fig. 3
figure 3

Microstructures of Cu-3Ti-2Sn aged at 450 °C for 1 h (a), 4 h (b), and 8 h (c)

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Fig. 4
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Microstructures of Cu-2Ti-2Sn aged at 450 °C for 1 h (a), 4 h (b), and 8 h (c)

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TEM Characterization

To understand the precipitation behavior of Cu-Ti-Sn alloy during aging process, TEM analysis was performed on the aged Cu-3Ti-2Sn and Cu-2Ti-2Sn alloys. Figure 5(a) to (c) shows the bright-field image (BF), the selected area diffraction (SAD) pattern, and its schematic diagram for Cu-3Ti-2Sn alloy aged at 450 °C for 1 h, respectively. A number of nano-sized particle precipitates can be observed in the copper matrix, as shown in Fig. 5(a). It can be identified from Fig. 5(b) and (c) that the fine precipitates are β′-Cu4Ti phase, which has a body-centered tetragonal crystal structure with lattice parameters a = 0.4530 nm and c = 1.2930 nm. The precipitation of the fine, coherent, metastable β′-Cu4Ti phase plays a predominant role in age hardening of Cu-Ti alloys (Ref 2).

Fig. 5
figure 5

TEM images of Cu-3Ti-2Sn alloy after aging at 450 °C for 1 h. (a) BF image, (b) SAD, and (c) Schematic of SAD pattern

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TEM observation was also performed on the Cu-2Ti-2Sn alloy aged at 450 °C for 4 h. Figure 6(a) to (d) shows the bright-field image (BF), dark-field image (DF), selected area diffraction (SAD) pattern, and its schematic diagram, respectively. It can be seen from Fig. 6(a) and (b) that a number of vermicular secondary particles ranging from 20 to 150 nm precipitate discontinuously in the copper matrix. It is demonstrated from Fig. 6(c) and (d) that the fine precipitates are β-Cu3Ti phase, which belongs to orthorhombic crystal structure with the lattice parameters a = 0.5162 nm, b = 0.4347 nm, and c = 0.4531 nm. The finding is in agreement with the literatures reported by Nagarjuna et al. (Ref 2) and Markandeya et al. (Ref 10, 13). They thought that the coherent metastable β′-Cu4Ti phase precipitates at the early stage of aging for Cu-Ti alloys, but the prolonged aging time gives rise to the formation of incoherent equilibrium β-Cu3Ti phase.

Fig. 6
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TEM images of Cu-2Ti-2Sn alloy aged at 450 °C for 4 h. (a) BF, (b) DF, (c) SAD, and (d) Schematic of SAD

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In the aged Cu-2Ti-2Sn alloy, it can also be seen that there is a small amount of large particles. Figure 7(a) to (c) shows the bright-field image of Cu-2Ti-2Sn alloy aged at 450 °C for 4 h, selected area diffraction (SAD) pattern, and its schematic diagram, respectively. It can be seen that there is the presence of an undissolved particle, see Fig. 7(a). From Fig. 7(b) and (c), it can be further demonstrated that the particle is in CuSn3Ti5 phase having a hexagonal structure with the lattice parameters a = 0.8174 nm, b = 0.8174 nm, and c = 0.5577 nm. This is in good accordance with the result of XRD. Hence, the ternary Sn element is existed as a CuSn3Ti5 intermetallic compound.

Fig. 7
figure 7

TEM images of particle in the Cu-2Ti-2Sn alloy aged at 450 °C for 4 h. (a) BF, (b) SAD, and (c) Schematic of SAD

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The Electrical Conductivity and Hardness of the Aged Cu-3Ti-2Sn and Cu-2Ti-2Sn Alloys

The effect of aging time on the electrical conductivity of aged Cu-3Ti-2Sn and Cu-2Ti-2Sn alloys is presented in Fig. 8. For both Cu-3Ti-2Sn and Cu-2Ti-2Sn alloys, the electrical conductivity is significantly improved with the increase of aging time. The electrical conductivity of Cu-3Ti-2Sn alloy can reach 8.4 MS/m after aging at 450 °C for 0.5 h. After aging at 450 °C for 1 h, the electrical conductivity has a slight increase for both Cu-3Ti-2Sn and Cu-2Ti-2Sn alloys. After aging at 450 °C for 4 h, the electrical conductivity of Cu-3Ti-2Sn is quite close to that of Cu-2Ti-2Sn alloy. At 8 h, the electrical conductivity of Cu-3Ti-2Sn and Cu-2Ti-2Sn alloys reaches up to 23.1 and 21.5 MS/m, respectively. This can be explained as follows. Although the electrical conductivity is sensitive to a number of microstructural factors including vacancy concentration, solute concentration in the matrix, precipitate size, and precipitate volume fraction, the electrical conductivity primarily depends on the solubility of alloying elements in the matrix (Ref 15–17). At the initial stage of secondary aging, large supersaturated concentrations of Ti and Sn solute atoms in the Cu matrix affect the precipitation kinetics behavior and increase the formation rates of precipitates. Eventually, these solute atoms in the Cu matrix are greatly reduced by the precipitation of the second phases during aging, which decrease lattice distortion and electron scattering remarkably, giving rise to the increase of the electrical conductivity. However, the concentrations of Ti and Sn elements in the Cu matrix reduce progressively with further aging and the amount of precipitates decrease correspondingly. Hence, a slow increment occurs in the electrical conductivity after aging for 4 h.

Fig. 8
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The effect of aging time on the electrical conductivity and hardness of Cu-Ti and Cu-Ti-Sn alloys

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As seen from Fig. 8, it is evident that the hardness increases and then decreases with the increase of aging time for both the alloys. At 0.5 h, the hardness of Cu-3Ti-2Sn alloy is 176 HV, which is increased by 66% in comparison with that of Cu-2Ti-2Sn alloy (110 HV). After aging for 1 h, Cu-3Ti-2Sn alloy reaches its peak hardness (188 HV), then the hardness of Cu-3Ti-2Sn alloy decreases progressively. Nevertheless, Cu-2Ti-2Sn alloy reaches its peak hardness of 125.5 HV after aging for 4 h. This change is closely related to the formation of precipitated phases and phase transformation. Since higher Ti content for Cu-3Ti-2Sn alloy affects the precipitation kinetics and increases the precipitation rate, β′-Cu4Ti phase precipitates from supersaturate Cu matrix more quickly at the early stage of aging in comparison with that of Cu-2Ti-2Sn alloy. Moreover, β′-Cu4Ti phase is coherent with the Cu matrix (Ref 18). Thus, Cu-3Ti-2Sn alloy can reach its peak hardness value earlier than Cu-2Ti-2Sn alloy. However, further aging gives rise to the phase transformation from β′-Cu4Ti to incoherent equilibrium Cu3Ti (Ref 19) and the precipitate coarsening which does not act as an effective barrier against the dislocation motion, thus resulting in the decrease of the hardness.

Conclusions

  1. 1.

    The as-cast microstructure of Cu-Ti-Sn alloy consists of α-Cu(Ti,Sn) and primary CuSn3Ti5 intermetallic compound. CuSn3Ti5 phase has a hexagonal structure with the lattice parameters a = 0.81737 nm, b = 0.81737 nm, and c = 0.55773 nm.

  2. 2.

    With the increase of aging time, the hardness increases and then decreases, while the electrical conductivity progressively increases. In the range of experiments, the optimum aging process which can achieve good comprehensive properties is at 450 °C for 8 h. The electrical conductivity and hardness of Cu-3Ti-2Sn alloy are 23.1 MS/m and 134.5 HV, respectively, and those of Cu-2Ti-2Sn alloy are 21.5 MS/m and 119.3 HV, respectively.

  3. 3.

    The presence of CuSn3Ti5 phase reduces the solute Ti content in the copper, giving rise to the increase of the electrical conductivity for Cu-Ti-Sn alloys.

  4. 4.

    Both Cu-3Ti-2Sn and Cu-2Ti-2Sn alloys can be hardened by the precipitation of metastable and coherent β′-Cu4Ti phase. The decrease of hardness after prolonged aging time is due to the formation of incoherent equilibrium β-Cu3Ti phase and the coarsening of precipitates.