Synthesis and properties of 0.3%Y₂O₃/0.3%La₂O₃/0.3%Al₂O₃/Cu composite

1 Introduction

Copper exhibits excellent ductility and electrical conductivity while metallic oxide is very strong and hard [1]. The metallic oxide dispersion strengthened copper composite bonds both high strength and good electrical conductivity well [2, 3]. So, the copper composite has been given more attention for its high strength, excellent electrical conductivity, and resistance to softening at high temperature [4]. Due to good mechanical and electrical properties, the copper composite is usually used as a contact material in place of silver, electrode material, and crystallizer material in continuous casting equipments [5–8]. Recently many researchers have been interested on how to fabricate the metallic oxide dispersion strengthened copper composite more efficiently using powder metallurgy, internal oxidation, casting, and hot pressing method [9–18]. Shi reported that the Al₂O₃/Cu composite prepared by internal oxidation shows high strength and high conductivity [19]. Shu found that the Al₂O₃ and Y₂O₃ particles dispersion strengthened copper composites with excellent properties can be obtained by rapidly solidified centrifugal spray deposition technique [20].

In order to overcome the shortcoming due to only one kind of metallic oxide dispersing in the matrix, in the present work, the 0.3%Y₂O₃/0.3%La₂O₃/0.3%Al₂O₃/Cu multiphase oxide particles reinforced composite was fabricated by spray deposition, internal oxidation, and finally treated by annealing at different temperatures. Its microstructures and properties after annealing were investigated.

2 Experimental

The 0.3%Y₂O₃/0.3%La₂O₃/0.3%Al₂O₃/Cu composite was prepared by oxygen and nitrogen atomization spray deposition using the machine PCWF1-50 that was manufactured by Kunming Institute of Precious Metals, Yunnan, PR China. Table 1 shows the experimental parameters concerned with spray deposition forming. In order to promote internal oxidation process, the deposits were oxidized in the box furnace at 900°C for 2.5 h, and rolled with 40%, and finally treated by annealing at 700°C, 800°C, 900°C, and 1000°C for 2 h. The metallography of the composite was observed through an optical microscope Leica DM400M (Leica Microsystems, Wetzlar, Germany). The fracture appearance was characterized by a scanning electron microscope (Hitachi S-3400N, Hitachi High-Technologies Europe GmbH, Krefeld, Germany). The hardness of the composite was tested by a micro-hardness testing instrument HMV-FA2 (Shimadzu, Shanghai, China). The tensile testing was held by AG-IC10KV (Shimadzu, Shanghai, China). The electrical conductivity of the composite was measured by a machine FQR7501 (Kunming Institute of Precious Metals, Kunming, China). The arc erosion experiment was held by JF04C (Jjinfeng, Kunming, China) and the arc erosion experimental parameters are shown in Table 2.

3 Results and discussion

3.1 Microstructure of the composite

Figure 1 shows the microstructures of the 0.3%Y₂O₃/0.3%La₂O₃/0.3%Al₂O₃/Cu composite treated by annealing at different temperatures. The metallic oxide particles exist diffusely in the matrix grains with equixial shape. There are no metallic oxide particles gathering in the matrix. With an increase in the annealing temperature, the grains grow up obviously. An increase in the temperature promotes the diffusion process and leads to the migration of the grain boundaries. When the annealing temperature reaches 1000°C, the matrix grains begin to appear as annealing twins. The twins come from the occasional stacking fault of the (111) crystal plane and grow up through the migration of the high angle grain boundaries. As shown in Figure 1D, the annealing twins can restrain the recrystallization process and refine the grains of the composite.

Figure 1

Metallography of the composite treated by annealing at (A) 700°C; (B) 800°C; (C) 900°C; and (D) 1000°C for 2 h.

3.3 Fracture appearance analysis

Figure 2 shows the tensile fracture appearance of the 0.3%Y₂O₃/0.3%La₂O₃/0.3%Al₂O₃/Cu composite treated by annealing at 700°C for 2 h. The tensile fracture morphology shows a huge number of large and deep dimples. It is shown that the severe plastic deformation occurs before breaking failure of the composite. It can be also seen from Figure 2 that although the particles are occasionally separated and pulled out, tear ridges are left behind in the large and deep dimples. This demonstrates that the particles combine firmly with the copper matrix. The fracture mode of the composite is the microporous polycondensation plastic fracture. As shown in Figure 3, the scanning electron microscopy scanning electron microscopy (SEM) microarea element analysis of the white spot in Figure 2 proves that the particles in the dimples are metallic oxide dispersion strengthening phases.

Figure 2

Fracture appearance of the composite.

Figure 3

SEM microarea element analysis of the white spot in Figure 2.

3.4 Arc erosion surface analysis

Figure 4 shows the arc erosion surface of the 0.3%Y₂O₃/0.3%La₂O₃/0.3%Al₂O₃/Cu composite treated by annealing at 700°C for 2 h. The arc erosion surface shows a large number of paste-like coagulum and bubbles. Because of the sticky liquid metal and the fast solidification, there is no time for the liquid to spread out. The paste-like coagulum is formed. As shown in Figure 4, we can clearly see the sags and crests on the surface of the coagulum. This is due to the liquid metal splash and the liquid bridge breaking formed by the action of the arc. With the melt of the copper composite, the oxygen in the air quickly dissolves in the liquid copper matrix. Due to the fast solidification, the dissolved oxygen cannot be immediately discharged and then the bubbles are formed.

Figure 4

Arc erosion microarea surface of (A) anode and (B) cathode of the composite.

4 Conclusions

In the present study, the 0.3%Y₂O₃/0.3%La₂O₃/0.3%Al₂O₃/Cu composite was prepared by oxygen and nitrogen atomization spray deposition technique, internal oxidation, and annealing at different temperatures. The experimental results show that with an increase in the annealing temperature, the grains of the composite grow up obviously. When the annealing temperature reaches 1000°C, the matrix grains begin to appear as annealing twins. As the annealing temperature increases, the electrical conductivity of the composite increases gradually while the hardness and the tensile strength decrease. The tensile fracture morphology of the composite treated by annealing at 700°C for 2 h shows a huge number of large and deep dimples. The fracture mode of the composite is the microporous polycondensation plastic fracture and arc erosion surface shows a large number of paste-like coagulum and bubbles due to the fast solidification of the liquid metal.