Effect of Trace Bismuth on Deformation Behavior of Ultrahigh-Purity Copper during Hot Compression

Abstract

The effect of trace Bi impurities on the flow stress, microstructure evolution, and dynamic recrystallization (DRX) of the ultrahigh-purity copper was systematically investigated by a hot compression test at 600 °C. The results show that the peak stress of the ultrahigh-purity copper gradually decreases with increasing Bi content. Trace Bi impurities can refine the microstructure of ultrahigh-purity copper. However, the refinement effect of 50 wt ppm Bi is much more significant than that of 140 wt ppm Bi during the hot deformation. This effect is ascribed to the higher concentration of Bi at GBs, which induces severe GB cracks that reduces the driving force for the nucleation of DRX grains. In addition, the introduction of Bi inhibits the DRX of the ultrahigh-purity copper and transforms its DRX process from the discontinuous dynamic recrystallization (DDRX) to the coexistence of DDRX and continuous dynamic recrystallization (CDRX) mechanisms.

1. Introduction

Copper (Cu) and its alloys possess excellent electrical and thermal properties, which have been widely used in aerospace, electronic communication, and transportation [1,2,3]. As the common impurities in Cu and Cu-based materials, Bi-induced intergranular embrittlement during a hot working process were extensively reported by numerous studies [4,5,6,7,8,9,10,11,12,13,14]. Such an embrittlement is related to the segregation of Bi at grain boundaries (GBs), which commonly results in GB decohesion and thus the occurrence of GB cracks when subjected to hot deformation [15,16,17].

Joshi and Stein [18] first observed the adsorption of Bi at GBs in Cu using an Auger electron spectrometer (AES). On this basis, Powell et al. [17,19] studied the existence form of Bi at GBs. The results showed that Bi was isolated in a single atomic layer or sub-single atomic layer, with the segregation width less than 1 nm. In addition, with increasing the Bi content, the enrichment concentration of Bi at GBs would continue to increase, ultimately leading to the formation of film-like precipitates on the GBs [20]. Our recent study revealed that a trace Bi addition led to an apparent ductility reduction in the ultrahigh-purity copper between room temperature (RT) and 900 °C. In particular, the sharp decrease occurs at 450–900 °C with the obvious characteristic of intergranular embrittlement, which resulted from the GB segregation of Bi [21,22]. However, the effect of trace Bi impurities on the hot deformation behavior of the ultrahigh-purity copper has been rarely reported. The variations of microstructure evolution and DRX caused by Bi segregation are still unclear.

In this work, the Cu–Bi samples with Bi concentrations of 50 and 140 wt ppm from our previous study [22] were selected as the research objects. The effect of trace Bi impurities on the deformation behavior of the ultrahigh-purity copper, including the microstructure evolutions and the DRX during the hot compression was systematically investigated. The hot compression temperature was set at 600 °C, within the brittle temperature range. First, the change in flow stress of the ultrahigh-purity copper due to trace Bi addition was analyzed. Second, the evolutions of grain size and misorientation distribution caused by Bi addition were investigated. Finally, the variation in the DRX mechanism after trace Bi was added is discussed in detail.

2. Materials and Methods

Three copper ingots with different Bi concentrations were produced by vacuum induction melting, which was carried out at a vacuum pressure of 6.6 × 10⁻⁴ Pa under pure Ar atmosphere protection. The ultrahigh-purity electrolytic copper with a purity of 99.99999% (7N) was selected as the smelting base material, purchased from the Henan GuoXi Ultrapure New Materials Co., Ltd. (Pingdingshan, China). The Bi impurities were added in the form of a Cu–40 wt% Bi mixed powder with a purity of 99.99%. The ingots, which have a weight of 3 kg and an initial outer diameter of 70 mm, were cooled to room temperature. Then, the casting risers were cut off, and the surfaces were turned. Subsequently, these ingots were hot-extruded down to rods with a final outer diameter of 20 mm in one step at 900 °C. The chemical compositions were determined by GB/T 5121.27-2008 [23]. The results of three independent specimens were averaged, and the concentrations of Bi were 0.001, 50, and 140 wt ppm, respectively, as reported in our previous study [22].

Cylindrical specimens with a diameter of 8 mm and height of 12 mm were prepared by electrical discharge machining (EDM) from the as-extruded rods. The compression axes were paralleled to the extrusion direction (ED). Before compression, the samples were annealed at 450 °C/1 h + 650 °C/1 h + 950 °C/0.5 h in a tube furnace with flowing Ar gas. Compression tests were carried out using a Gleeble-1500 thermal-mechanical simulation tester at 600 °C with a strain rate of 10⁻² s⁻¹. The deformation ratios were 20%, 40%, and 60%. Water quenching was applied to preserve the high-temperature structures.

Samples subjected to deformation were cut along the compression axis diameter by EDM. Electron backscatter diffraction (EBSD) was performed on OXFORD NordlysMax2 (Oxford Instruments, Oxford, UK) to investigate the evolutions of grain size and misorientation distribution, as well as the dynamic recrystallization (DRX) during the hot compression. The relative orientation between the sample coordinate system and the scanning electron microscope (SEM) system coordinate system is shown in Figure 1. The compression direction (ED) of the sample was parallel to RD direction in the EBSD coordinate system, and the ND plane was the observation plane. The EBSD samples were mechanically grinded and then electrolytically polished at a voltage of 15 V for 30 s. The electrolyte consisted of phosphoric acid and ethanol, with a volume ratio of 1:1. During the EBSD test, the scanning step size was determined according to the grain size of the sample, which was between 1 and 3 µm. The CHANNEL 5 software was used for data analysis. A lower limit boundary-misorientation cutoff of 2° was used, and a 15° criterion was employed to differentiate low-angle GBs (LAGBs) and high-angle GBs (HAGBs).

3. Results and Discussion

3.1. Initial Microstructure

Figure 2 shows the EBSD characterization results of the A1, B2, and B3 coppers before hot compression, including the orientation maps and corresponding misorientation distribution maps. It is observed that after the high-temperature solid solution treatment, the difference in grain size among the three coppers is not significant, as presented in Figure 2a–c. In addition, these coppers are mainly composed of HAGBs, with the volume fractions of 91.1%, 96.2%, and 91.3%, respectively, as shown in Figure 2d–f. Noted that the content of HAGBs in B2 copper is slightly higher than that of B3 copper, which is due to the smaller hindering effect of Bi on the GB migration and subgrain rotation during the solid solution treatment, caused by the lower Bi concentration. The high proportion of HAGBs in the three coppers indicates that a large number of dislocations generated during hot extrusion process are eliminated through the recovery or recrystallization, resulting in a more uniform microstructure. Thus, the investigated three coppers have similar initial states before hot deformation.

3.2. True Stress–Strain Curves

Figure 3 shows the true stress–strain curves and the corresponding peak stresses of the A1, B2, and B3 coppers during hot compression at 600 °C. It is obvious that with increasing the strain, the flow stress of 7N pure copper (A1) rapidly reaches its peak value and then decreases when the strain is approximately 0.1, indicating the main softening mechanism of DRX. In a later stage of deformation, the flow stress exhibits a slight increase, which is likely to relate with twinning. The interaction between twins can lead to a stress concentration, thereby increasing the flow stress. In contrast, the flow stresses for both Bi-containing coppers show a slow decreasing trend after reaching their peak values, implying that the softening mechanism is also DRX. However, the strains corresponded to DRX are greater than that of 7N pure copper, which suggests that the addition of trace Bi impurities delays the DRX process. In Figure 3b, the peak stress gradually decreases from 74.37 MPa to 62.13 MPa with increasing Bi content. Such an anomalous change is relevant to the segregation of Bi at GBs.

3.3. Microstructure Evolution

Figure 4 shows the microstructure evolutions of the A1, B2, and B3 coppers during hot compression at 600 °C, and the corresponding average grain sizes under different deformation amounts are presented in Figure 5. Before hot compression, the grain sizes of the A1, B2, and B3 coppers are 128.5 µm, 126.3 µm, and 130.7 µm, while those of the grain sizes are 174.4 μm, 125.4 μm, and 123.7 μm, respectively, at a deformation of 20%. Compared with the initial state, the grain size of 7N pure copper coarsens, while no significant changes have occurred in the grain sizes of B2 and B3 copper. This is due to the segregation of Bi atoms toward GBs reducing the GB migration rate during hot deformation. In addition, GB cracks are observed in B3 copper, as shown in Figure 4g, indicating that the concentration of Bi atoms at GBs reached the critical value for promoting the intergranular fracture.

With the deformation amount increasing to 40%, the average grain sizes of A1 and B2 coppers decrease by 14.7% and 11.1%, respectively, compared with the 20% deformation amount, whereas the grain size of B3 copper does not change significantly. As can be seen from Figure 4e, the fine DRX grains mainly nucleate at GBs, where the obvious cracks begin to form in B2 copper. This is due to the segregation of Bi atoms at GBs, reducing the GB adhesion [22]. The cracks in B3 copper spread further along GBs and the DRX grains mainly located near the GB cracks, as presented in Figure 4h. It can be considered that the grain refinement of B2 copper is due to the occurrence of DRX, while the non-refinement of B3 copper is because the severe GB cracks caused by Bi segregation reduces the driving force for the nucleation and growth of DRX grains.

When increasing the deformation amount to 60%, the average grain sizes of A1, B2, and B3 coppers are 146.3 μm, 55.8 μm, and 120.6 μm, respectively, as shown in Figure 5. There is still no apparent grain refinement of A1 and B3 coppers compared with the 40% deformation amount, while a remarkable decrease in B2 copper is observed in Figure 4f. It was reported that the segregation of Bi at GBs destroyed the balance between Cu atoms and distorted the lattice [24,25,26]. This leads to the increased interfacial strain energy, which can provide driving force for DRX grains nucleation. In B2 copper, the concentration of Bi at GBs is low, so the microstructure refinement is mainly ascribed to the DRX. By comparison, the higher Bi content in B3 copper causes crack formation along GBs in the early stage of deformation. As the deformation continues, these cracks gradually extend, resulting in serious intergranular cracking, as shown in Figure 4i. This greatly releases the stress concentration at the interface and hinders the nucleation of DRX grains. Therefore, the microstructure of B3 copper does not undergo obvious refinement during the entire deformation process.

The misorientation distributions of the A1, B2, and B3 coppers under different deformation amounts are shown in Figure 6, and the corresponding average angles are presented in Figure 7. In Figure 6a–c, it is observed that with the increase in reduction, the volume fractions of LAGBs in A1 copper are 83.1%, 4.1%, and 14.7%, respectively, with the average misorientation values of 11.7°, 52.1°, and 44.6°. At the initial stage of deformation, the content of LAGBs is very high, so the dislocation slip dominates the deformation process. When the deformation amount exceeds 40%, the content of TBs increases dramatically, turning the primary deformation mechanism from the dislocation slip to twinning.

After trace Bi impurities are added, the proportions of LAGBs in both Bi-containing coppers are above 40%, while the contents of TBs are less than 10% during the deformation, as shown in Figure 6d–i. It is obvious that with an increase in the deformation amount, the content of LAGBs in both Bi-containing coppers shows a trend in first increasing and then decreasing. This is because the dislocation density gradually increases when the reduction ratio is less than 40%, while the DRX process is accelerated in the later stage of deformation. The corresponding average misorientation values are less than 30°, as illustrated in Figure 7. This indicates that the dislocation slip is the major deformation mechanism for both Bi-containing coppers during the entire compression process.

3.4. Dynamic Recrystallization

In order to investigate the influence of trace Bi impurities on DRX of the 7N pure copper, the proportions of different types of grains in A1, B2, and B3 coppers at a compression deformation of 60% are statistically analyzed, and the results are shown in Figure 8. The blue column represents recrystallized grains. The yellow column represents substructured grains, which are in a state between the recrystallized and deformed grains. The red column represents deformed grains, in which the density of dislocations is high. It is obvious that the volume fraction of DRX grains in A1 copper is 20.8%, and almost no deformed grains. However, the volume fractions of DRX grains in B2 and B3 copper are 14.41% and 2.9%, respectively, and the contents of deformed grains are greater than 60%. Therefore, it can be seen that a trace Bi addition can inhibit the DRX of 7N copper, and the higher concentration of Bi impurities, the greater inhibition effect on the DRX process. This correlation occurs because the higher content of adsorbed Bi atoms at GBs can significantly release the stress concentration, which impedes the nucleation and growth of DRX grains.

In addition to the DRX process, trace impurities can also affect the DRX mechanism of copper and copper-based alloys [5,27,28,29,30]. Figure 9 shows the formation characteristics of DRX grains in 7N copper at the initial deformation stage. As observed in Figure 9a,b, DRX grains mainly nucleate and grow at GBs, which conform to the typical discontinuous dynamic recrystallization (DDRX) mechanism of metals with low-stacking fault energies (SFE) [31,32]. Figure 9c shows the corresponding Taylor factor diagram, where the larger Taylor factor value represents higher deformation storage energy inside the grain [33,34,35]. Due to the high deformation temperature, a sufficient driving force is provided for the growth of DRX grains, and most of the DRX grains have completed the DRX process. In addition, these DRX grains have low Taylor factor values, indicating that the dislocation density within the grains is significantly reduced, as shown in Figure 9c.

Similarly, the DDRX mechanism also exists in Bi-containing copper, as shown in Figure 10. In Figure 10a, several fine DRX grains nucleate at the interface between parent grains A and B. The orientation difference between the two grains is approximately 50°, as presented in Figure 10b. Figure 10c shows the corresponding inverse pole figure (IPF) of these grains, from which the orientations of DRX grains are quite different from that of the parent phases.

In addition to the DDRX mechanism, the continuous dynamic recrystallization (CDRX) mechanism is also found in Bi-containing copper, as shown in Figure 11. In Figure 11a, the 1# and 2# DRX grains nucleate and grow in situ within the original grains A and B, respectively. The corresponding misorientation distribution map shows that the orientation difference between grain 1# and parent grain A is about 15°, and the orientation difference between grain 2# and parent grain B is approximately 30°. It can be seen from Figure 11d,e that the rotation axis between the two DRX grains and their parent crystals is close to the [110] axis. Therefore, the formation process of CDRX grains can be inferred. First, driven by the temperature and applied stress, massive dislocations generate inside the original grains. Second, with the deformation continuous, the dislocations are gradually multiplied to form the sub-grain boundaries. Finally, the sub-crystals rotate along the [110] axis, resulting in a gradual increase in the interfacial orientation difference. Subsequently, an independent DRX grain is formed inside the original grain. Hence, one can see that the DRX mechanism of 7N copper is transformed into a coexistence mechanism of DDRX and CDRX due to the addition of trace Bi impurities.

4. Conclusions

In this work, the effect of trace Bi impurities on the flow stress, microstructure evolution and DRX of the ultrahigh-purity copper were systematically investigated. The main conclusions are as follows:

(1)

The peak stress of the ultrahigh-purity copper gradually decreases from 74.37 MPa to 62.13 MPa with increasing Bi content, which is due to the segregation of Bi at GBs.

(2)

During the hot compression process, trace Bi impurities induce severe intergranular embrittlement of the ultrahigh-purity copper. And the microstructure of B3 copper with 140 ppm Bi content does not undergo obvious refinement, which results from the inhibition of the DRX grain nucleation.

(3)

With the increase in strain, the primary deformation mechanism of ultrahigh-purity copper transforms from the dislocation slip to twinning, while the dislocation slip dominates the entire compression process for both Bi-containing coppers.

(4)

The addition of trace Bi impurities can inhibit the DRX process of ultrahigh-purity copper and transform its DRX mechanism from DDRX to the coexistence of DDRX and CDRX.