Influence of P Content on Microstructure and Texture Evolution of the Oxygen-Free Copper

Abstract

The present work aims to systematically investigate the influence of P content on the microstructure and texture evolution of oxygen-free copper during intermediate annealing and final cold rolling. The microstructure and texture evolution were studied by electron backscattered diffraction and transmission electron microscopy. With the addition of P, the grains refined and a large fraction of low angle grain boundaries (LAGBs) emerged after intermediate annealing. The texture transformed from pure metal type for pure Cu to the α-fiber texture which included brass and Goss texture as P was added. The recrystallization temperature increased with the addition of P, and refined grains after the final cold rolling process. The addition of elemental P would reduce the stacking fault energy, and then influence the transformation of the deformation and recrystallization texture of the copper. Accompanied by the evolution of the deformation texture, the recrystallization cubic texture {001}<100> was suppressed and a strong {236}<385> brass recrystallization texture emerged with the addition of elemental P after the intermediate annealing and subsequent final cold rolling process.

1. Introduction

Rolled copper foils are widely used in applications in the high-end electronics industry, flexible circuit boards and lithium-ion batteries due to their excellent electrical conductivity and high strength [1,2,3,4,5,6]. Most of the previous studies on high performance rolled copper and copper alloys mainly focus on the effect of impurities on the microstructure and mechanical properties. Jakani et al. [7] reported that 10 ppm sulfur added to pure copper increased the recrystallization temperature by about 60 °C. Yu et al. [8] found that with the thickness changing from millimeter to microns of the rolled copper, the specimen showed a processing softening phenomenon and showed abnormal ductility. Youssef et al. [9] prepared nanostructured Cu-1%Nb by an in situ consolidation mechanical alloying technique. Addition of 1% Nb to Cu increased the true ultimate tensile strength from 835 MPa to 1308 MPa which was attributed to the smaller grain size and solid solution hardening. Even a small amount of phosphorus (50–60 ppm) added to the pure copper made dramatic improvements in the creep strength and the creep ductility as found by Sandström [10,11].

The addition of alloy elements significantly affects the strength of the recrystallization texture and mechanical properties in high to medium stacking fault energy (SFE) materials [12,13,14,15,16]. Ray and Bhattacharjee et al. [17,18] showed that the addition of W and Mo developed a much sharper cube texture compared with pure Ni. Furthermore, the results indicated that the W and Mo enhanced the cube texture intensity in Ni by decreasing the volume fraction of the rotated cube grains. The Cu–Pd alloy with lower to medium SFE generated wider stacking faults during deformation, making the transformation of perfect dislocations into partial ones easier, and then gave rise to the annealing hardening [19].

The addition of alloying elements not only affects the microstructure and mechanical properties of copper but also the crystallographic texture during the deforming and annealing process [20,21,22]. Moreover, texture could change the microstructure and tailor material properties [23]. Therefore, a fundamental understanding of the microstructural changes, deformation texture and recrystallization texture evolution induced by the addition of phosphorus in oxygen-free copper are mandatory. In the present work, the influence of small additions of phosphorus (~100 ppm) on the microstructure and texture evolution of oxygen-free copper during intermediate annealing and the subsequent final cold rolling process were systematically investigated.

2. Materials and Methods

The electrolytic copper ingot (purity ≥ 99.9999 wt%, Ag ≤ 0.07 ppm, Si ≤ 0.005 ppm, p ≤ 0.001 ppm, Bi ≤ 0.001 ppm, Pb ≤ 0.001 ppm) was used as master batch that melted in the vacuum induction furnace under an Ar atmosphere. CuP was added to the graphite crucible when the copper melt at 1180 °C in order to obtain different P-content specimens. The processing steps of specimens are shown in Figure 1. The composition of ingots was analyzed by chemical analysis and is listed in

Table 1. Chemical compositions of the specimens.

Analyzed Composition	Pure Cu	P23	P52	P100
P content/(ppm)	0.001	23	52	100
Cu	Bal.	Bal.	Bal.	Bal.

. The ingots which had an initial outer diameter of 70 mm were hot extrusion down to sheets with a final thickness of 10 mm in one step at 900 °C, and then subjected to two passes of cold rolling down to 2 mm. The as-rolled specimens were intermediate annealed with the temperature increased to 500 °C for 5 min using a tube type furnace under an Ar + 5% H₂ mixed atmosphere and finished with cold rolling to the thickness of 0.3 mm.

Microstructure characterization and micro-texture of the intermediate annealed and finish rolled specimens were carried out using a field emitting scanning electron microscope (FE-SEM) equipped with electron back scattered diffraction (EBSD) analysis, operated at an acceleration voltage of 20 kV with a scan step size of 5 μm in the investigation. Specimens for EBSD were mechanically polished and subsequently electrolytically polished at 15 V for 10 s in the electrolyte solution at room temperature. The electrolyte solution was a mixture of 200 mL phosphoric acid, 200 mL ethanol and 500 mL distilled water. Computations were made using Channel 5 software to analyze the orientation maps, grain sizes, twin boundaries and micro-texture evolution of the specimens. A grain was defined as a single oriented region divided by high angle grain boundaries. If two neighboring pixels had a misorientation of less than or equal to 15°, they were considered part of the same grain and the grains with grain boundaries misorientation greater than 2° and less than 15° were defined as sub-grains, which were divided by low-angle grain boundaries. Transmission electron microscope (TEM) observations were performed on the rolling plane sections by using JEOL JEM-2100F TEM (JEOL, Japan) operated at an accelerating voltage of 200 kV. The TEM specimens were firstly prepared as cylinders (diameter: 3 mm) which were cut into thin discs (500 μm) with a diamond saw. The discs were then ground down to 100 μm using SiC papers. Electron transparency was achieved by twin-jet polishing in the MTP-1A system with a voltage of 20 V at 10 °C. The electrolyte consisted of phosphoric acid, ethanol, and deionized water, with a volume ratio of 1:1:1.

3. Results and Discussion

In order to investigate the effect of P content on the morphology of the oxygen-free Cu specimen after intermediate annealing, the microstructure of the samples intermediately annealed at 500 °C for 5 min were studied with an optical microscope (OM) as depicted in Figure 2. In order to get the full recrystallization structure in a short time, we took all specimens annealed at 500 °C for 5 min to avoid the incomplete recrystallization and uneven grain. The microstructure of a pure Cu specimen comprised coarse grains and some annealing twins. With the addition of elemental P, the grain size decreased obviously, and there was still a lot of fine annealing twins in the Cu specimens after intermediate annealing, as shown in Figure 2b–d. For the semiquantitative analysis of the effect of P addition on the grain refinement of the Cu specimen, the average grain size was calculated by the following equation [24]:

$$d=\left(n\times L\times {10}^3\right)/\left(Z\times N\right)$$

(1)

where d is the average grain diameter, n is the number of lines, L is the measurement line length, Z is the total number of grains on the line, and N is the magnification of the metallographic image. The average grain sizes of pure Cu, P23, P52 and P100 were 324.7 μm, 163.3 μm, 149.6 μm and 137.9 μm, respectively. The results revealed that the grains of the Cu specimen could be significantly refined by the addition of elemental P.

To further characterize the influence of elemental P addition on the recrystallized microstructure and recrystallization texture development in intermediate annealed copper specimens, we carried out the EBSD measurement. The rolling direction is defined as RD, the normal direction is defined as ND and the transverse direction is defined as TD. Figure 3 shows the crystallographic orientation maps, corresponding grain boundary maps and misorientation histograms in the specimen RD of the intermediate annealed copper specimens with diverse additions of elemental P. As shown in Figure 3a, there was a great deal of {100} and {112} oriented grain growth; meanwhile, plenty of flat annealed twin boundaries appeared in the pure Cu specimen after the intermediate annealing process. It clearly revealed that extensively {110} oriented fine grains appeared with the addition of P as shown in Figure 3b–d. Combined with the grain boundary maps and misorientation histograms, it seems that a high angle grain boundary (HAGB) structure with a slight fraction of low angle grain boundaries (LAGBs) and Σ3 twin boundaries (Σ3 TBs) were created in the pure Cu specimen. A large fraction of boundaries appeared to be LAGBs in the microstructure of fine grains, which means grain refinement was significant as P was added in. The average grain sizes in pure Cu, P23, P52 and P100 were calculated to be 281.29, 124.40, 116.36 and 112.66 μm, respectively. These results further revealed that the addition of elemental P could influence the texture evolution, and what is more, it could effectively refine the grain size of the oxygen-free copper.

Figure 4 shows the orientation distribution function (ODF) sections of φ₂ = 0°, φ₂ = 45° and φ₂ = 65° of the intermediate annealed copper specimens with diverse contents of P. Some copper texture {112}<111> and S texture {123}<634> existed in the pure Cu specimen as shown in Figure 4a. Meanwhile, the strong recrystallization cubic texture {001}<100> formed in the specimen which indicated the recrystallization took place during the intermediate annealing process. Although the copper texture and unstable S texture still existed when P was added, the recrystallization cubic texture almost vanished, and a distinct Goss texture {110}<001> and brass texture {110}<112> were found. The cubic texture was suppressed with the addition of P, and the main texture types evolved from β-fiber to α-fiber type, the strength of α-fiber type increased. With the P content increased, the preferred orientation of the grains gradually transformed to brass texture and Goss texture. Moreover, the overall texture strength gradually enhanced with the increasing addition of P.

To confirm the microstructure transformation dependence on the variational content of P, we carried out the morphology TEM observation. Figure 5 shows the bright field TEM micrograph of the intermediate annealed pure copper and P23 specimens. As shown in Figure 5a, after the large plastic deformation and intermediate annealing process, there was a high density of dislocations disorderedly arranged in the crystal and the intertwined dislocation cluster formed in the pure Cu. Flat annealing twin boundaries emerged in the pure Cu specimen and the width of twin space (d_twin) was about 0.775 μm. It can be seen from Figure 5b that the dislocation density distinctively decreased and the d_twin was about 0.185 and 0.238 μm, respectively.

As an important parameter in materials, stacking fault energy (SFE) has a great influence on the generation of twins [25]. Kumar and San et al. [26,27] showed that the deformation mechanism of FCC structure materials can be transformed, dominated by a slip into that dominated by twinning with the reduction of SFE. Compared to the pure Cu specimen, more twinning crystal structures occurred; moreover, the thicknesses of twins decreased in the P23 specimen. It may have been caused by the reduction of SFE with addition of P. This further confirms that the addition of P could effectively increase the twin content and refine grains, which is consistent with the above results.

Figure 6 shows the orientation maps and corresponding grain size distributions of the final cold rolled specimens. A transformation of morphology from partially equiaxed type to the elongated banded structure with {001} oriented grains along RD happened after the final cold rolling, and the content of twins decreased observably in the pure Cu. The deformed coarse grains mainly possessed the {001} orientation, but the other grains had various orientations. The existence of large-scale microstructural heterogeneities such as deformed bands in the material is the dominant factor for the formation of largescale recrystallized regions [28]. As shown in Figure 6a, the recrystallization took place in the elongated deformed bands and plenty of fine recrystallized grains dispersed around the deformed structure were present in the final cold rolled pure Cu.

With the addition of P, abundant equiaxed grains and twins still remained after final cold rolling, and only a small amount of fine recrystallized grains appeared. It may be noted that the deformation strain might lead to the formation of defects. In the pure Cu with a high dislocation energy of FCC structure, sufficient local dislocation density differences will be accumulated during the deformation process, resulting in the formation of large angle grain boundaries and discontinuous dynamic recrystallization [29]. Added P atoms could pin dislocation, prevent the dynamic recovery during deformation, improve the storage energy, accelerate the recovery during intermediate annealing and delay the occurrence of recrystallization.

The recrystallization phenomenon was delayed by adding P to the pure Cu, while partial recrystallization still happened in the severe stress concentration location. There were still a number of fine recrystallized grains existing along the grain boundaries, as shown in Figure 6b–d. Along with the microstructure transformation, the addition of P could also affect the texture evolution. After the cold rolling process, there was not only recrystallization texture but also deformed texture coexistence in the specimens, as shown in Figure 6. After adding P, some {110} oriented, {112} oriented coarse equiaxed grains and twinning structures were retained, a number of fine recrystallized grains generated, while the {001} oriented elongated structure disappeared. As shown in Figure 6a, heterogeneous grain size was distributed in the pure Cu specimen. Although the pure Cu specimen had a large fraction of fine recrystallized grains, a few coarse deformed {001} oriented grains existed; inhomogeneous grain size distribution was not uniform and the average grain size was calculated to be 30.32 μm. With the vanishing of the coarse deformed structure, a number of Σ3 twin boundaries existed, the microstructure became homogeneous, and the average grain sizes of P23, P52 and P100 were calculated to be 21.01, 17.31 and 17.61 μm, respectively.

Figure 7 shows the ODF of sections of φ₂ = 0°, φ₂ = 25°, φ₂ = 45° and φ₂ = 65° of the final cold rolling copper specimens with diverse additions of elemental P. Strong cubic texture {001}<100> formed due to the deformed cube bands after the final cold rolling process as mentioned in Figure 6a. The texture component was transformed, accompanied by the evolution of the microstructure as elemental P was added. As shown in Figure 7, the P23 specimen had a weaker cubic recrystallization texture {001}<100> with the addition of P. The texture component transformed from {001}<100> cube texture to strong {110}<112> brass texture and {110}<001> Goss texture, and a sharp {236}<385> brass recrystallization texture appeared. Moreover, after adding elemental P, there was a substantial drop in the overall texture intensity from 31.8 to 12.0, and then reduced further with the increasing content of P. As shown in Figure 7d, the overall texture intensity decreased to 7.2 after adding 100 ppm P to the oxygen-free copper. This result revealed that not only the overall texture intensity but also the preferred crystal orientation of the oxygen-free copper could be changed by the addition of P.

Combined with Figure 8, the influence of the addition of elemental P on the texture evolution in the final cold rolling of copper specimens was further quantitatively described. During the textural components’ statistical analysis, all the tolerance angles of each texture component were 5°. The results indicated that the component of {001}<100> cube texture significantly decreased from 26.4 to 3.64%, whereas the {236}<385> brass recrystallization texture component increased from 5.13 to 32.8% with the addition of P. What is more, the {112}<111> copper texture and {123}<634> S texture component increased, and the {110}<001> Goss texture formed.

It is well known that the SFE plays an important role as an important intrinsic parameter of the material during deformation, and the texture evolution is strongly dependent on the SFE of the material [30,31,32,33]. After intermediate annealing and the subsequent finishing cold rolling process, a strong recrystallization cube texture formed due to the medium SFE (γ_SFE = 78 mJ∙m⁻²) of the FCC structure of pure Cu [15], while even a small amount addition of P still could decrease the SFE of the pure copper [34], thus changing the texture type of the specimens. As discussed above, due to the reduction of SFE, the overall texture intensity decreased gradually with the increasing content of P. The main rolling texture transformed from the β-fiber texture which ran from the copper orientation {112}<111> to the brass orientation {110}<112> through the unstable S orientation {123}<634>, and then gradually evolved to the α-fiber texture which includes brass and Goss texture. Furthermore, accompanied by the evolution of the deformation texture, the recrystallization cubic texture was suppressed with the addition of P. As the recrystallization cubic texture component decreased, a strong {236}<385> brass recrystallization texture emerged in the specimens with the addition of P. This result further illustrated that the addition of P could change the texture evolution of the specimens.

4. Conclusions

The influence of P content on microstructure and texture evolution of oxygen-free copper during the intermediate annealing and final cold rolling process were analyzed. Based on the obtained results, the following conclusions can be drawn:

(1)

The average grain size decreased from 281.29 to 112.66 μm with the increased content of P, accompanied by a large fraction of LAGBs after the intermediate annealing process. The addition of P could effectively refine grain size and change the microstructure of the oxygen-free copper.

(2)

During the intermediate annealing process, some {112}<111> copper texture, {123}<634> S texture and a sharp recrystallization {001}<100> cubic texture formed in the pure Cu specimen. With the P content increased, the recrystallization cubic texture almost vanished, and distinct {110}<001> Goss texture and {110}<112> brass textures were found.

(3)

After the final cold rolling process, the grains were refined with the addition of P. The pure metal-type texture consisted of copper type, S and brass type gradually evolved to the α-fiber texture which includes brass and Goss texture with the increase in P. Moreover, accompanied by the evolution of the deformation texture, the recrystallization cubic texture was suppressed and a strong {236}<385> brass recrystallization texture emerged. Due to the reduction of SFE with the addition of P, it could effectively not only refine the grain size but also change the texture evolution of the specimens.