Microstructure and Liquid Phase Separation of CuCr Alloys Treated by High Current Pulsed Electron Beam

Due to very limited mutual solubility between Cu and Cr at room temperature, Cu-Cr alloys have been developed to meet increasing industrial requirements for its outstanding combination of high mechanical strength and good electrical conductivities1-5. Because of high withstand voltage and exceptional current breaking capacity, Cu-Cr alloys have been considered as the best contact material for medium voltage vacuum interrupter4. Wang et al.2 reported that the grain size and distribution of Cr phases was critical to reduce chopping currents and sustain high voltage breakdown strength capacity. The finer the Cr-rich phase was, the better the electric contact materials properties could be achieved. In order to further improve electrical and mechanical properties, many efforts have been focused on refining and homogenizing the Cr-rich phase. Rapid solidification is believed to be able to refine microstructure and induce supersaturated solid solution, and thus was used by many researchers to prepare Cu-Cr alloys. To date, various methods of rapid solidification have been successfully explored, such as melt spinning4,5, splat quenching6, gas atomizing and electromagnetic levitation7. In these work, fine Cr-rich spheroids were detected in Cu matrix and attributed to occurrence of the liquid phase separation during rapid solidification. As a promising high-energy beam surface modification technique, high current pulsed electron beam (HCPEB) has been developed in recent years8. Because of good thermal conductivity of metal substrates, main features of HCPEB include rapid heating, melting, vaporization and cooling. Besides, stress waves, shock waves, vibration and other physical phenomena can also be induced on the material surface, which results in extraordinary modification effects, such as improved strength, microhardness, wear resistance and corrosion resistant properties9-11. Therefore, by using HCPEB surface modification techniques, many researchers had studied the surface strengthening and surface alloying of steels, magnesium alloys, aluminium alloys, etc11. The failure of vacuum interrupter usually occurs on the surface of Cu-Cr contact materials during usage. However, there are few efforts concentrated on the surface modification of the Cu-Cr alloys. As far as the studies of HCPEB concerned, only Lamperti et al.12,13 reported the surface microstructure of HCPEB-treated Cu-30Cr alloy by use of secondary ion mass spectrometry and atomic force microscopy. However, mechanism of the craters and cracks has not been analyzed. In the current investigation, the microstructure and crater evolution under HCPEB treatment on the surface modification in CuCr25 and CuCr50 alloys were studied with different pulse numbers.


Introduction
Due to very limited mutual solubility between Cu and Cr at room temperature, Cu-Cr alloys have been developed to meet increasing industrial requirements for its outstanding combination of high mechanical strength and good electrical conductivities [1][2][3][4][5] .Because of high withstand voltage and exceptional current breaking capacity, Cu-Cr alloys have been considered as the best contact material for medium voltage vacuum interrupter 4 .Wang et al. 2 reported that the grain size and distribution of Cr phases was critical to reduce chopping currents and sustain high voltage breakdown strength capacity.The finer the Cr-rich phase was, the better the electric contact materials properties could be achieved.In order to further improve electrical and mechanical properties, many efforts have been focused on refining and homogenizing the Cr-rich phase.Rapid solidification is believed to be able to refine microstructure and induce supersaturated solid solution, and thus was used by many researchers to prepare Cu-Cr alloys.To date, various methods of rapid solidification have been successfully explored, such as melt spinning 4,5 , splat quenching 6 , gas atomizing and electromagnetic levitation 7 .In these work, fine Cr-rich spheroids were detected in Cu matrix and attributed to occurrence of the liquid phase separation during rapid solidification.
As a promising high-energy beam surface modification technique, high current pulsed electron beam (HCPEB) has been developed in recent years 8 .Because of good thermal conductivity of metal substrates, main features of HCPEB include rapid heating, melting, vaporization and cooling.
Besides, stress waves, shock waves, vibration and other physical phenomena can also be induced on the material surface, which results in extraordinary modification effects, such as improved strength, microhardness, wear resistance and corrosion resistant properties [9][10][11] .Therefore, by using HCPEB surface modification techniques, many researchers had studied the surface strengthening and surface alloying of steels, magnesium alloys, aluminium alloys, etc 11 .
The failure of vacuum interrupter usually occurs on the surface of Cu-Cr contact materials during usage.However, there are few efforts concentrated on the surface modification of the Cu-Cr alloys.As far as the studies of HCPEB concerned, only Lamperti et al. 12,13 reported the surface microstructure of HCPEB-treated Cu-30Cr alloy by use of secondary ion mass spectrometry and atomic force microscopy.However, mechanism of the craters and cracks has not been analyzed.In the current investigation, the microstructure and crater evolution under HCPEB treatment on the surface modification in CuCr25 and CuCr50 alloys were studied with different pulse numbers.

Experimental
The CuCr25 (75wt.%Cuand 25wt.%Cr)master material was prepared by vacuum induction melting ( VIM ) while the CuCr50 master (50wt.%Cuand 50wt.%Cr)material was prepared by infiltrating.After polishing, the CuCr50 alloy was deposited with a Cr film using FJL560A ultrahigh vacuum magnetron sputtering equipment.An "RITM-2M" type HCPEB machine was applied with pulse numbers of 30 and 50.The HCPEB treatment was performed under the following conditions: the electron energy 30 keV, the frequency 0.1 Hz, pulse duration 2 μs, and the vacuum 6×10 −4 Pa.The microstructure and surface morphology after HCPEB treatment were analyzed by JEOL JSM-6460LV scanning electron microscope (SEM).The microstructure of cross-section was etched by FeCl 3 (5g) HCl (10 mL) + H 2 O (90 mL) solution.by arrow D) and irregular Cr-rich flows (marked by arrow E) occurr.A large number of Cr-rich spheroids with different diameters distribute on the surface.The dimension of Cr-rich spheroids varies from about 100 nm to more than 2 μm. Figure 2c and Figure 2d show the cross-section microstructures of the HCPEB-treated CuCr25 alloy.As can be seen from Figure 2c, a remelting layer with about 3-5 μm in thickness exists.The depth of the craters is about 10 μm (denoted by the arrow F in Figure 2d).

Experimental Results
Figure 3 illustrates the microstructure of the CuCr50 alloys after HCPEB treatment.As shown in Figure 3a, after 50-pulse HCPEB treatment, the number of cracks in Cr particles increases, and Cr phase in the Cu-rich matrix is further refined, and the interface of the Cr-rich particles and the Cu-rich phase becomes unclear.Meanwhile, a lot of small Cr-rich spheres appear in the Cu-rich matrix.The cross-section microstructures of alloys are shown in Figure 3b and Figure 3c.After the HCPEB treatment, the thickness of remelting layer in CuCr50 alloys increases to about 3~5 μm.As revealed by the arrows A in Figure 3b and B in Figure 3c, larger craters than cracks in CuCr50 alloy are about 5~10μm in the distance from the surface.
The microstructure of the CuCr50 alloys coated with Cr films after 50-pulse HCPEB treatment is shown in Figure 4.The cross-section microstructure of the magnetron sputtered Cr film can be seen in Figure 4a.It can be found that Cr layer with a thickness of 2μm is uniformly coated on the alloy surface (marked by arrow A).The surface microstructure of the CuCr50 coated with Cr after 50-pulse HCPEB treatment is shown in Figure 4b.Many cracks appear in Cr particles and the Cr phases are refined.The interfaces between the Cr particles and the Cu matrix become unclear.As indicated by arrow B, a lot of small Cr-rich spheroids exist in the Cu-rich matrix.The cross-section microstructures of CuCr50 alloys after HCPEB treatment are shown in Figure 4c and Figure 4d.The remelting layer with 3~5 μm in thickness occur in HCPEB-treated CuCr50 alloy (marked by arrow C in Figure 4c).As revealed by the arrow D in Figure 4d, the larger craters than cracks in the Cr particles are about 5~10 μm in the distance from the surface.

Discussion
Similar to the Cu-Co 14 and Cu-Fe 15 systems, the Cu-Cr system is a liquid metastable binary system with a large positive heat of mixing.Müller 16 suggested that the Cu-Cr system falls into a monotectic type with a stable miscibility gap.Recently, Zhou et al. 8 proposed a novel monotectic type Cu-Cr phase diagram, which displays a stable miscibility gap over compositions between 50.8 and 83.2 wt.% Cr at a monotectic temperature of 2020 K, where there were liquid spinodal lines and dash lines indicated the MG (binodal lines), respectively.When the Cu-Cr melt was cooled below a certain temperature, the liquid would enter the MG zone and be separated into Cu-rich liquid and Cr-rich liquid, thus leading to the liquid phase separation 8 .Our previous studies experimentally confirmed that the liquid phase separation will occur in the rapidly solidification Cu-Cr alloys by melt spinning, splat quenching and electromagnetic levitation 4,6,7 .
The electron beam will disappear suddenly at the end of every pulse, suggesting a extremely short duration.It means that the melt will cool down and solidify rapidly due to the excellent heat conduction ability of the matrix of Cu-Cr alloy.Because of the heat generated by HCPEB bombardment, substantial thermal stress will be produced, thus making it possible to induce cracks in Cr particles 17 (as shown in Figure 2a, Figure 3a and Figure 4b).When the electron beams irradiate the original chapped Cr phases again, coarse craters or cavities will occur in Cu-Cr alloys (marked by arrows A in Figure 2a and F in Figure 2d, A and B in Figure 3, and D in Figure 4d).Moreover, as shown in Figure 2a, splashing phenomenon and cracks are found in the crater.The craters are the consequence of the eruption of melted metal pools 18 .In addition, as shown in Figure 3a and Figure 4b, since the heat of HCPEB can make the alloy surface remelt and solidify repeatedly, the interface between Cr-rich particles and Cu-rich matrix become fuzzy.In this study, as shown in Figures 2c, 3b and 4c, the remelting layer with about 3-5 μm in thickness is formed in Cu-Cr alloys due to the ultra high cooling rate and the extremely large supercooling degree during HCPEB treatment.As a result, all the Cu-Cr melts in the composition range enter the MG zone.Therefore, the Cu-Cr alloy will not be heated uniformly by the electron beam, which will cause a flow of the melt and the grown Cr-rich droplets will be elongated and swirled (marked arrow D and E in Figure 2b).Another reason for the formation of the Cu-Cr remelting layer is splashing.Therefore, Cr-rich sheets (marked by the arrow C in Figure 2a) also occur in the craters due to the spraying of the molten pool, the high cooling rate and limited melt flow.The cooling rate of the alloy droplets is lower than that of the melts in the pool, and the liquid phase separation and the liquid flow proceed simultaneously.In accordance with previously reported results 2,19 , small Cr-rich droplets form in Cu-Cr remelting layer continuously and some extremely fine Cr-rich particles always appear during the liquid phase separation process.
Figure 5 shows a schematic illustration of the HCPEB-treated Cu-Cr alloy in this experiment.When the switches of HCPEB equipment are turned on, the surface of Cu-Cr alloy is melted and lots of molten pools are formed (as can be seen in Figure 5b).The previous investigation by Zou et al. 17 revealed that the highest temperature appeared in the subsurface of alloys after HCPEB treatment.Due to high temperature remelting of subsurface in Cu-rich matrix and Cr particles, as indicated by arrows A in Figure 3b, B in Figure 3c, and D in Figure 4d, the relatively large craters occurred in the subsurface of Cu-Cr alloy under the action of thermal stress.The depth of the molten pool is about 5-10 μm, which provides direct evidences to the numerical simulation results in Zou et al's studies 17 .During the HCPEB treatment, the Cu-Cr alloy will be heated rapidly, and some remolten liquid under the subsurface will be splashed to form the craters, accompanied by the formation of lots of small liquid droplets.When the switches are intermittented, the splashing liquid and droplets will solidify rapidly, then the remelting layer will be formed and the liquid phase separation will occur.The molten pool and liquid phase separation will repeatedly occur.As shown in Figure 5c, the craters and remelting layer occur when the switch is turned off.This refinement of Cr particles can remarkably improve the dielectric strength according to previous reports 19 .The appearance of Cr-rich spheroids is beneficial to the improvement of the dielectric strength of the Cu-Cr contact materials.Further studies are needed to understand the remelting layer in Cu-Cr alloy.

Conclusions
A remelting layer with a thickness of 3~5 μm forms on the surface of Cu-Cr alloys because of repeated rapid heating and solidifying during the HCPEB treatment.Cr-rich spheroids are observed in the remelting layer, which means that metastable liquid phase separation is involved in the microstructure evolution of rapid solidification in HCPEB-treated Cu-Cr alloy.The relatively large craters appeared in the subsurface of the specimens provide direct evidences for the numerical simulation results by other researchers.

Figure
Figure 1a illustrates the initial microstructure of CuCr25 alloy prepared by vacuum induction melting.As shown in the figure, the matrix is Cu and the undergrown dendrites are Cr phases.Figure 1b illustrates the microstructure of the master CuCr50 alloy prepared by infiltrating.It comprises coarse Cr particles distributed in the Cu matrix.Surface morphologies of the CuCr25 alloy after HCPEB treatment is shown in Figure2aand Figure2b.The microstructure of the remelting layer is significantly different from that of the master alloy (Figure1).The coarse Cr-rich dendrites cannot be seen.Various microstructure features are observed in Figure2a: the large craters as denoted by the arrow A; the cracks near the crater as indicated by the arrow B; the splashed layer as indicated by the arrow C. Figure2bindicates that the large Cr-rich spheroids (marked

Figure 2 .
Figure 2. Micrographs illustrating the microstructures of CuCr25 alloy after HCPEB treatment with 30 pulses (a) surface micrographs illustrating the craters; (b) Cr-rich spheroids illustrating the liquid phase separation; (c) cross-section micrographs illustrating the remelting layer; (d) cross-section micrographs illustrating the craters.

Figure 3 .
Figure 3. Micrographs illustrating the microstructures of CuCr50 alloy after HCPEB treatment with 50 pulses (a) surface micrographs illustrating the cracks and Cr-rich spheroids; (b) cross-section micrographs illustrating the remelting layer and craters in Cu matrix; (c) cross-section micrographs illustrating the craters in Cr particles.

Figure 4 .
Figure 4. Micrographs illustrating the microstructures of HCPEB treatment CuCr50 alloys with Cr coating with 50 pulses (a) cross-section micrographs illustrating the coating of Cr; (b) surface micrographs illustrating the Cr-rich spheroids; (c) cross-section micrographs illustrating the remelting layer; (d) cross-section micrographs illustrating the craters in Cr particles.