Defect analysis of the Cu-Cr-Zr-Nb alloys prepared by the non-vacuum melting process

The microstructure and properties of the Cu-Cr-Zr-Nb alloys prepared by non-vacuum melting and vacuum melting were compared and analyzed in this paper. The results showed that, compared with vacuum melting, the alloy prepared by non-vacuum melting had a more uneven microstructure and a coarser grain. In the meantime, the alloy prepared by non-vacuum melting stored less energy and had a lower proportion of HAGBs (high-angle grain boundaries), and the texture distribution along the TD and RD directions was not uniform. In terms of properties, the hardness and electrical conductivity of the Cu-Cr-Zr-Nb alloys prepared by non-vacuum melting were lower than those of the alloys prepared by vacuum melting. The electrical conductivity of the non-vacuum melting alloy was only 70.8 %IACS (International Annealed Copper Standard), and the average hardness fluctuated greatly. However, the electrical conductivity of the alloy prepared by vacuum melting could reach 81.0 %IACS, and the hardness was very uniform. Further research indicated that there were cracks and inclusions in the alloy prepared by non-vacuum melting. A large number of ZrO2 inclusions were found along the grain boundary, which demonstrated that the oxide inclusion was the primary cause of the crack formation. On this basis, three important suggestions for the preparation of the Cu-Cr-Zr-Nb alloys by non-vacuum melting were put forward to reduce the cost and improve the properties of the alloys. It provided guidance for developing high-performance Cu-Cr-Zr-Nb alloys by non-vacuum melting.


Introduction
Cu-Cr-Zr alloys are widely used for integrated circuit lead frame material, high-speed railway contact wire, heat exchange material, and thermonuclear experimental reactor material due to their high thermal and electrical conductivity as well as high strength [1][2][3][4][5][6][7][8] .However, the application of the Cu-Cr-Zr alloys still faces many challenges.In terms of traditional vacuum melting technology, it can basically meet the requirements of production.However, it has a series of drawbacks, such as expensive smelting equipment and difficulties in large-scale industrial production.Therefore, more and more researchers have developed the non-vacuum melting method to prepare this kind of alloy.
Until now, numerous studies on the preparation of the Cu-Cr-Zr alloys by the non-vacuum melting method have been reported.Tao et al. [9] reported that the burning loss of alloy elements can be significantly reduced by adding Zr as an intermediate alloy in the non-vacuum melting.Mu et al. [10] found that metal oxides such as ZrO2 and MgO can be used as crucible and furnace lining materials since these oxides have good chemical stability for Cr and Zr during non-vacuum melting of the Cu-Cr-Zr alloys.The addition of Cr and Zr can reduce dislocation energy and increase the thermal strength of the Cu-Cr-Zr alloys, as explored by Kapoor et al. [11] .Batra et al. [12] found that the addition of trace Zr could change the precipitate order of the Cu-Cr-Zr alloys.The preparation of the Cu-Cr-Zr alloys by the non-vacuum melting method also has some shortcomings, such as shrinkage cavities, porosity, and cracks [9,[13][14][15][16] .It is significant to analyze the type, morphology, and origin of defects for developing the non-vacuum melting method to prepare high-performance Cu-Cr-Zr alloys.
This paper focuses on the effects of different processes on Cu-Cr-Zr-Nb alloys.Based on the experimental results, we related the electrical conductivity and mechanical properties to careful microstructure analyses, and the root cause of defect formation was particularly discussed.

Materials and methods
In this experiment, the Cu-Cr-Zr-Nb alloys were produced by the non-vacuum melting process.The specific processes are as follows: i) The Cu-Cr-Zr-Nb alloys were melted at 1300 ℃ in an atmospheric environment, and plant ash was used as a covering agent to reduce the risk of oxidation of Cr and Zr; ii) Hot forging was carried out at 940 ℃ and the alloys were solved at 960 ℃ for 2 h; iii) The alloys were drew at room temperature with a deformation of 30 % in order to obtain plates or rods; iv) Aged treatment was at 460 ℃ for 3 h and the obtained Cu-Cr-Zr-Nb alloy was recorded as sample 1.For comparative analysis, the Cu-Cr-Zr-Nb alloys were prepared by vacuum melting at 1300 ℃, and the following processing technology was the same as the above process.The obtained Cu-Cr-Zr-Nb alloy was recorded as sample 2. Figure 1 shows the process flow diagram of the Cu-Cr-Zr-Nb alloys.The metallographic structure of the samples was observed by a DMI 3000M optical microscope (OM).The microstructural examination of the samples was carried out by using a Gemini SEM 300 scanning electron microscope (SEM).Electron backscatter diffraction (EBSD) was used to characterize the texture transformation of the alloy.OIM software was used for subsequent data processing and analysis.Hardness measurements were examined with an EM-1500L durometer, and the electrical conductivity of the samples was measured with an FD-101 eddy current conductivity tester.Five measurements were taken at the different positions on the sample, and then the average was taken.

Chemical composition analysis
By further applying chemical analysis, the chemical compositions of samples 1 and 2 are summarized in Table 1.It can be found that, compared with sample 2, the Mg content of sample 1 is obviously higher.It is probably because of the addition of excessive magnesium for deoxygenation during non-vacuum melting.It is well known that Zr is an effective addition element in the Cu-Cr alloy.The Zr element can not only improve the high-temperature performance but also enhance the effect of dispersion strengthening on the alloy.In the meantime, the electrical conductivity of the alloy might not decline much.Further analysis indicates that there is little difference in Zr content between sample 1 and sample 2, as displayed in Table 1.The addition of trace Zr may be beneficial to improving the mechanical properties of the alloy.It is already known that the coarsening of Cr precipitate makes the strengthening effect basically disappear at higher temperatures.Nevertheless, Zr can inhibit the coarsening of Cr and thus have a refinement effect on the Cr precipitated phase.This can improve the peak aging hardness of the Cu-Cr-Zr-Nb alloys to a certain extent.In the meantime, an appropriate amount of deoxidizer should be added to reduce serious oxidation and burning loss of the Zr element during the non-vacuum melting process of the Cu-Cr-Zr-Nb alloys.In the present work, a small amount of Mg was used to minimize the oxidation of the active element Zr and clean the grain boundaries.The addition of trace Nb can form stable compounds in the copper matrix and improve their high-temperature performance [17] .

Defect morphology
Figure 2 shows the typical defect morphologies of sample 1.It can be found that there are cracks, porosities, and inclusions in the Cu-Cr-Zr-Nb alloys.As can be seen from Figure 2(a), the crack of sample 1 is mainly divided into three parts: zone B in the middle and zones A and C at both ends.In all of the above areas, expanding cracks are observed, as shown in the blue dashed box in Figure 2(a).
Besides, circular pores are observed inside the Cu-Cr-Zr-Nb alloy with an average diameter of about 4 μm.The real reason is probably that the gas enters the melt and forms porosities during the non-vacuum melting, as shown by the blue arrow in Figure 2(b).In the meantime, there are also white inclusions in the pores, and some inclusions are distributed along the grain boundaries, as shown by the red arrow in Figure 2(b).The related properties of the Cu-Cr-Zr-Nb alloys, which are prepared by non-vacuum melting, could be seriously affected by the above defects.2. The result shows that, compared with sample 1, the grain size of sample 2 is refined, and the grain size of sample 2 decreases from 10.78 μm to 7.48 μm.Meanwhile, the proportion of HAGBs increases from 8.6 % to 21.2 %.HAGBs can have a corresponding strengthening effect on the alloy [18] .It can indicate that sample 2 has better mechanical properties.Figure 5 shows the kernel average misorientation (KAM) maps and the average KAM value of the Cu-Cr-Zr-Nb alloys.The higher the KAM value, the more energy is stored in the alloy [19] .It can be seen from Figure 5(c-d) that the KAM value of sample 2 (0.960) is higher than that of sample 1 (0.802).Therefore, it can be concluded that the mechanical properties of sample 2 are better than those of sample 1.

Hardness mapping
The microstructure determines the properties of the alloys.which is superior to sample 1.The average hardness of the latter is 81.6 HRB.The result indicates that there is little difference in average hardness between the two.Grain size is closely related to alloy hardness, and the relationship between grain size change and material strength follows the Hal-Petch model [20,21] : Δσgs=K/D 1/2 , where D is the grain size and k is the constant.
According to the grain sizes of sample 1 and sample 2 in Table 2, the Δσgs of the two are calculated to be 55 MPa and 66 MPa, respectively, hinting the difference of hardness values between the two.

Electrical conductivity analysis
Electrical conductivity is an essential parameter to evaluate the comprehensive properties of the Cu-Cr-Zr alloys.Figure 9 shows the electrical conductivity of the Cu-Cr-Zr-Nb alloys.It can be seen that sample 2 has an excellent electrical conductivity of 81.0 %IACS (International Annealed Copper Standard).However, the electrical conductivity of sample 1 is only 70.8 %IACS, which is obviously worse than that of sample 2.
The reason for this performance difference is that the number of Cr particles in sample 2 is higher and the distribution is more uniform, as shown in Figure 3, which reduces the electron scattering phenomenon in the alloy and shows that the alloy conductivity increases.

Discussion
According to the above results, it can be found that the difference in the properties of samples 1 and 2 was mainly due to the accumulation of inclusions and the crack defect in sample 1.The specific analysis is as follows: It can be seen from Figure 10 that a lot of coarse particles largely accumulate at grain boundaries.As a result, the cracks tend to initiate at these grain boundaries and hence propagate along the grain boundaries.Figure 11 clearly reveals that continuous crack propagation is accompanied by coarse particles.In order to further understand the crack mechanism, map scanning was conducted on the defects in sample 1, as shown in Figure 12.It can be seen that the Cu element is evenly distributed in the alloy, except for the defect location.In order to clearly observe the distribution of the elements, the distribution of the Cr element and other elements is further compared, as shown in the red circle in Figure 12(a).It is found that no other elements are observed at the location where Cr element is distributed, indicating that Cr element exists as a single substance.In addition, Zr and O elements are concentrated in a wide range of the cracks, forming coarse spherical particles, which are further analyzed in detail in the following paper.Besides, the presence of the C element is due to the introduction of graphite crucible during the non-vacuum melting.It can be clearly observed from Figure 11 that coarse particles are distributed at the crack site and accumulate in large quantities.The composition of these particles was further analyzed by EDS, as shown in Figure 13.It can be seen from Figure 13 that there are a lot of O and Zr elements in the coarse particles.Meanwhile, although the addition of Zr to the alloy is smaller than that of Cr, it can be obviously observed that the peak intensity of Zr is higher than that of Cr.This result is consistent with that of map scanning.Therefore, it can be reasonably concluded that these coarse particles are mainly ZrO2 inclusions.The main reasons are as follows: On the one hand, although plant ash has been used as a covering agent to reduce the risk of oxidation, the phenomenon of oxygen absorption still exists in the non-vacuum melting process.On the other hand, Zr and Cr elements have a strong affinity for the O element, and especially Zr has a stronger effect and tends to react with O to form the ZrO2 compound.These oxide particles are aggregated and distributed along grain boundaries.Under the action of casting stress or subsequent stress in the forming process, it will become the crack source in the Cu-Cr-Zr alloys.These factors are responsible for the worse performance of sample 1 compared with that of sample 2.

Figure 1 .
Figure 1.The process flow diagram of the Cu-Cr-Zr-Nb alloys.The metallographic structure of the samples was observed by a DMI 3000M optical microscope (OM).The microstructural examination of the samples was carried out by using a Gemini SEM 300 scanning electron microscope (SEM).Electron backscatter diffraction (EBSD) was used to characterize the texture transformation of the alloy.OIM software was used for subsequent data processing and analysis.Hardness measurements were examined with an EM-1500L durometer, and the electrical conductivity of the samples was measured with an FD-101 eddy current conductivity tester.Five measurements were taken at the different positions on the sample, and then the average was taken.

Figure 2 .
Figure 2. The defect morphologies in sample 1: (a) cracks; (b) porosities and inclusions.(The main parts of the crack are as follows: Zone A, Zone B, and Zone C.)

Figure 4
shows the inverse pole figure (IPF) and band contrast (BC) maps of the Cu-Cr-Zr-Nb alloys.Average grain size and proportion of HAGBs and LAGBs (high and low angle grain boundaries) are summarized in Table

Figure 5 .
Figure 5. KAM maps and average KAM value of the Cu-Cr-Zr-Nb alloys: (a,c) sample 1; (b,d) sample 2. Fig. 6 shows the pole figure (PF) maps of the Cu-Cr-Zr-Nb alloy.The result shows that the orientation density of the {110} plane of sample 1 shows aggregation phenomena and certain symmetry along the TD and RD directions, and the distribution of the orientation density of the other two crystal faces is relatively dispersed.Compared with sample 1, the orientation density of each crystal face of sample 2 is highly symmetrical along the TD and RD directions, and the texture strength becomes lower, showing that the alloy has excellent mechanical properties.
Figure 7 shows the average hardness values of the Cu-Cr-Zr-Nb alloys.It can be found that sample 2 displays a high average hardness (83.3 HRB), Conference of Non-Ferrous Materials Journal of Physics: Conference Series 2690 (2024) 012007

Figure 7 .
Figure 7. Hardness values of the Cu-Cr-Zr-Nb alloys.Figure 8 shows the hardness of the Cu-Cr-Zr-Nb alloys at different positions.The average hardness of sample 1 is 81.6 HRB, slightly lower than that of sample 2, which displays an average hardness of 83.3 HRB.However, the hardness of sample 1 varies from 78.5 to 83.7 HRB, with a large fluctuation.It indicates that the mechanical properties of sample 1 are unstable.It can also be found that the hardness of sample 2 fluctuates from 82.0 to 84.0 HRB and basically keeps at a high level of hardness, showing an excellent mechanical property.This is because the Cr particles in sample 2 are more evenly distributed.

Figure 8
Figure 7. Hardness values of the Cu-Cr-Zr-Nb alloys.Figure 8 shows the hardness of the Cu-Cr-Zr-Nb alloys at different positions.The average hardness of sample 1 is 81.6 HRB, slightly lower than that of sample 2, which displays an average hardness of 83.3 HRB.However, the hardness of sample 1 varies from 78.5 to 83.7 HRB, with a large fluctuation.It indicates that the mechanical properties of sample 1 are unstable.It can also be found that the hardness of sample 2 fluctuates from 82.0 to 84.0 HRB and basically keeps at a high level of hardness, showing an excellent mechanical property.This is because the Cr particles in sample 2 are more evenly distributed.

Figure 8 .
Figure 8. Hardness values of the Cu-Cr-Zr-Nb alloys at different positions.
2023 3rd International Conference of Non-Ferrous Materials Journal of Physics: Conference Series 2690 (2024) 012007

Figure 9 .
Figure 9.Comparison of conductivity of the Cu-Cr-Zr-Nb alloys.4.DiscussionAccording to the above results, it can be found that the difference in the properties of samples 1 and 2 was mainly due to the accumulation of inclusions and the crack defect in sample 1.The specific analysis is as follows: It can be seen from Figure10that a lot of coarse particles largely accumulate at grain boundaries.As a result, the cracks tend to initiate at these grain boundaries and hence propagate along the grain boundaries.Figure11clearly reveals that continuous crack propagation is accompanied by coarse particles.

Figure 10 .
Figure 10.Images of the propagation of crack through the grain boundaries in sample 1: (a) Crack tip morphology; (b) Partial enlarged image in Figure 10(a).

Figure 11 .
Figure 11.Continuous crack propagation accompanying by coarse particles in sample 1.In order to further understand the crack mechanism, map scanning was conducted on the defects in sample 1, as shown in Figure12.It can be seen that the Cu element is evenly distributed in the alloy, except for the defect location.In order to clearly observe the distribution of the elements, the distribution of the Cr element and other elements is further compared, as shown in the red circle in Figure12(a).It is found that no other elements are observed at the location where Cr element is distributed, indicating that Cr element exists as a single substance.In addition, Zr and O elements are concentrated in a wide range of the cracks, forming coarse spherical particles, which are further analyzed in detail in the following paper.Besides, the presence of the C element is due to the introduction of graphite crucible during the non-vacuum melting.

Figure 12 .
Figure 12.Map scanning analysis of defects in sample 1.It can be clearly observed from Figure11that coarse particles are distributed at the crack site and accumulate in large quantities.The composition of these particles was further analyzed by EDS, as shown in Figure13.It can be seen from Figure13that there are a lot of O and Zr elements in the coarse particles.Meanwhile, although the addition of Zr to the alloy is smaller than that of Cr, it can be obviously observed that the peak intensity of Zr is higher than that of Cr.This result is consistent with that of map scanning.Therefore, it can be reasonably concluded that these coarse particles are mainly ZrO2 inclusions.The main reasons are as follows: On the one hand, although plant ash has been used as a covering agent to reduce the risk of oxidation, the phenomenon of oxygen absorption still exists in the non-vacuum melting process.On the other hand, Zr and Cr elements have a strong affinity for the O element, and especially Zr has a stronger effect and tends to react with O to form the ZrO2 compound.These oxide particles are aggregated and distributed along grain boundaries.Under the action of casting stress or subsequent stress in the forming process, it will become the crack source in the Cu-Cr-Zr alloys.These factors are responsible for the worse performance of sample 1 compared with that of sample 2.

Figure 13 .
Figure 13.Corresponding EDS results of coarse particles in Figure 12 (a) (Yellow point A).

Table 2 .
Average grain size and Angle grain boundaries of the Cu-Cr-Zr-Nb alloys.Category Average grain size (μm) Angle grain boundaries (%)