RU2484175C1 - Cu-Cr SYSTEM ULTRAFINE COPPER ALLOY AND METHOD OF ITS PRODUCTION - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to production of ultrafine materials that feature higher strength and electric conductivity and may be used in electrical engineering for making components, conductors and terminals operated at increased temperatures and mechanical loads. Said ultrafine material comprises reinforcing phase fractions with mean disperse particle size smaller than 20 nm, while mean grain size in alloy structure is smaller than 500 nm. Note here that quantity of grains with high-angular boundaries exceeds 60%. Proposed method comprises heating, quenching, forming and ageing. Note here that quenching is performed at 1020-1050°C. Thereafter, intensive plastic deformation is performed at 20-300°C with cumulative strain number making, at last, 3. Now, ageing is carried out at 400-500°C.

EFFECT: limit strength exceeding 550 MPa, higher conductivity and thermal stability.

3 cl, 3 ex, 6 tbl, 6 dwg

Description

The present invention relates to the field of ultrafine-grained (UFG) materials with increased strength and conductivity, intended for use in the electrical industry for the manufacture of parts, conductors and electrical contacts operating at elevated temperatures and high mechanical loads.

For promising applications of copper materials in electrical contacts, it is necessary that they exhibit a combination of a high tensile strength above 550 MPa, increased thermal stability up to 500 ° C and high electrical conductivity of more than 85% of the electrical conductivity of pure copper.

High strength is necessary to ensure increased forces applied to the electrical contacts to ensure the reliability of their work in long-term operation.

The increased thermal stability of the UFG structure is important for the stability of the specified performance characteristics of parts made of copper material, for example, during possible contact heating as a result of the formation of oxide films on their surface, leading to an increase in contact resistance.

The high electrical conductivity of the material is designed to ensure the passage of increased electric current in the wires, as well as reduced electrical resistance of the contacts, which leads to a decrease in heat losses in the electrical circuit.

It is known that annealed pure copper, having an electrical resistance equal to 0.017241 μOhm · m, is a universally recognized standard of material with a high conductivity of 58 Mcm / m, for which the designation IACS (international annealed copper standard) is 100% IACS [ O.E. Osintsev, V.N. Fedorov. Copper and copper alloys. M .: Engineering. 2004. - 336 p.].

At the same time, annealed pure copper has a low strength, which lies in the range of 220-250 MPa, which significantly limits the scope of its application.

The strength properties of pure copper can be significantly increased by drawing at room temperature [K. Hanazaki, N. Shigeiri, N. Tsuji. Change in microstructures and mechanical properties during deep wire drawing of copper. Materials Science and Engineering A 527 (2010) 5699-5707; N.Takata, S.H. Lee, N. Tsuji. Ultrafine grained copper alloy sheets having both high strength and high electric conductivity. Materials Letters. 63 (2009) 1757-1760; V.Z. Spuskanyuk, A.A. Davidenko, A.N. Gangalo, L.F.Sennikova, M.A. Tikhonovsky, D.V. Spiridonov. Achieving a record level of copper wire by IPD methods. Physics and technology of high pressures. 20, 1 (2010), p.114-122], but the thermal stability of the structure and strength properties does not exceed 150 ° C [R.K. Islamgaliev, F.Chmelik, R. Kuzel. Thermal stability of submicron grained copper and nickel. Mat.Sci.Eng.A. 237 (1997), p. 43-51].

To increase the strength and thermal stability of copper materials, the principles of dispersion hardening are often used [R.K.Islamgaliev, R.Z. Valiev, A.T. Akhmedyanov, I.F. Gibadullin, N.I. Grechanyuk, V.A. Osokin, G.N. Aleshin. High strength state of dispersion-hardened copper with a submicrograin structure. Physics of metals and metal science. 2 (1993), p.145-149].

Among the dispersion-strengthened copper materials, a special place is occupied by alloys of the Cu-Cr system, which, due to the low content of alloying elements, have a high electrical conductivity of 75% IACS, and as a result of the isolation of finely dispersed Cr particles of 10-15 nm in size, they have an increased tensile strength of 500 MPa, twice the limit the strength of pure copper [C.Watanabe, R.Monzen, K.Tazaki. Mechanical properties of Cu-Cr system alloys with and without Zr and Ag. J. Mat. Sci. 43 (2008) 813-819]. This alloy, as the closest to the proposed, is selected as a prototype.

Known methods for processing copper alloys of the Cu-Cr system, characterized in that coarse-grained samples are subjected, in one case, to multiple dragging with intermediate annealing [J. Saleh, E. Fisk. Copper alloy wire and cable and method for preparing the same. Patent US6053994. Issue date 04/25/2000], and in another case, hardening and aging [O.E. Osintsev, V.N. Fedorov. Copper and copper alloys. M .: Engineering. 2004. - 336 s]. The disadvantage of these methods is the low value of tensile strength, not exceeding 415 MPa with an electrical conductivity of 82-90% IACS.

As a prototype of the selected method [C.Watanabe, R.Monzen, K.Tazaki. Mechanical properties of Cu-Cr system alloys with and without Zr and Ag. J. Mat. Sci. 43 (2008), p. 813-819], in which the cast billets of a copper alloy of the Cu-Cr system were homogenized in vacuum at a temperature of 1000 ° C for 24 hours, followed by cold rolling with a decrease in thickness by 30%. Then, the rolled strips were again heated in an argon atmosphere at a temperature of 1000 ° C for 2 hours, followed by quenching in water. After that, they were subjected to additional rolling with a decrease in thickness by 80% and aging at a temperature of 500 ° C for various holding times.

However, this method has an insufficiently high tensile strength - below 500 MPa with an electrical conductivity of up to 86% IACS. This is due to the fact that the increase in strength was achieved due to only one factor, namely, dispersion hardening, i.e. by isolating fine particles during the aging process of coarse-grained material with an average grain size of 250 microns. Then it is known that the strength can be further improved also by grinding the grain structure.

The objective of the invention is to develop a copper alloy system Cu-Cr, having a combination of high values of tensile strength of more than 550 MPa, high conductivity of at least 85% of the conductivity of pure copper and increased thermal stability to a temperature of 500 ° C.

The problem is solved by an ultrafine-grained copper alloy of the Cu-Cr system, characterized by fine particles of precipitates of the hardening phase with a size of less than 20 nm, in which, in contrast to the prototype, the average grain size in the structure is less than 500 nm, and the number of grains with high-angle boundaries is more than 60%.

The problem is solved by the method of producing ultrafine-grained copper alloy of the Cu-Cr system, including heating, quenching, deformation and aging, in which, unlike the prototype, quenching is carried out at a temperature of 1020-1050 ° C, after which intense plastic deformation (IPD) is carried out at a temperature of 20 -300 ° C with a cumulative strain of at least 3 and subsequent aging at a temperature of 400-500 ° C.

In addition, according to the invention, intense plastic deformation is carried out by torsion (IPDK), or equal channel angular pressing (ECAP), or equal channel angular pressing according to the conformal pattern (ECAP-K).

UFG copper alloy and the method of its production allows to provide a higher level of tensile strength with increased values of electrical conductivity and thermal stability due to the following.

An increase in the strength of a copper alloy is achieved due to the UFG structure of the material obtained through various SPD methods, which are based on the use of high degrees of shear deformation under conditions of low homological temperatures and high pressures [R.Z. Valiev, I.V. Aleksandrov. Volumetric nanostructured metallic materials. - M.: IKC "Academkniga", 2007. - 398 p.]. A very small grain size provides an increase in flow stress in accordance with the well-known Hall-Petch relation [E.O. Hall. The deformation and aging of mid steel. Proceedings of the Physical Society. B64, 381 (1951), pp. 747-753], and also due to the allocation of ultrafine particles in accordance with the Orowan relation [Physical metallurgy. T.Z. Under. Ed. R.U. Kana, P.T. Haazen. M .: World. 1968. - 484 p.]. In the first case, the increase in strength occurs due to dislocations overcoming a large number of grain boundaries, which are obstacles to their movement during plastic deformation. In the second case, the hardening effect occurs as a result of dislocations overcoming the increased fields of elastic stresses formed near the precipitate particles due to differences in interplanar spacings and the type of crystal lattice between the particle and the alloy. A significant increase in strength is also achieved by the fact that it is the large-angle grain boundaries, the total share of which is not less than 60%, in comparison with small-angle and special boundaries, which make the greatest contribution to hardening [N. Krasilnikov, W. Lojkowski, Z. Pakiela, R. Valiev. Tensile strength and ductility of ultra-fine-grained nickel processed by severe plastic deformation. Mat. Sci. Eng. A 397 (2005) 330-337].

Increased conductivity values are achieved due to the aging effect during annealing, as well as the dynamic aging effect during intense plastic deformation, which lead to the decomposition of the solid solution and the release of ultrafine particles. Such aging ensures the cleaning of the matrix from alloying elements, thereby contributing to an increase in electrical conductivity, and is also an additional factor contributing to the hardening of the material and to increasing thermal stability.

The invention is illustrated by illustrations of the microstructure of the Cu-Cr copper alloy subjected to IPDC at a temperature of 20 ° C (Figure 1), IPDC at 20 ° C and additional annealing at 500 ° C (Figure 2), IPDC at 300 ° C (Fig. 3), IPDC at 300 ° C and additional annealing at 500 ° C (Figure 4), ECAP at 300 ° C (Figure 5). Figure 6 presents typical images of particles in the structure of a copper alloy Cu-Cr subjected to ECAP at a temperature of 300 ° C

The invention is implemented as follows.

Example 1

For a specific implementation of the invention, samples with a diameter of 20 mm and a thickness of 0.8 mm of a copper alloy of the Cu-0.5% Cr-0.1% Ag system were subjected to intense torsion plastic deformation (IPDK) at various temperatures of 20, 200, 250, 300, 350 , 400, 450 ° С under pressure of 6 GPa, 10 revolutions. The cumulative strain ε was calculated by the formula ε = ln (φr / h) [R.Z. Valiev, I.V. Aleksandrov. Volumetric nanostructured metallic materials. - M .: ICC "Akademkniga", 2007. - 398 p.], Φ is the rotation angle in radians, r is the radius of the sample, h is the thickness of the disk. The value of ε after treatment with the IPDK method at the parameters indicated above was 5.28.

When studying in a transmission electron microscope, it was found that as a result of the use of IPDC at a temperature of 20 ° C, the grain structure of the copper alloy was crushed to an average grain size of about 200 nm (Fig. 1a). No noticeable particles of precipitates were found in the structure of the samples subjected to IPDC at 20 ° С. The complex diffraction contrast in the bright-field and dark-field images indicated the presence of large internal stresses in the structure, which arose as a result of the use of large shear deformations at high pressures.

The analysis of diffraction patterns taken from the matrix showed that point reflections are observed on electron diffraction patterns (Fig. 1b), the interplanar spacings of which coincide with the interplanar spacings of pure copper (Table 1).

As can be seen from Table 1, the difference between the interplanar distances determined experimentally from the electron diffraction pattern (Fig. 1b) and the tabular values does not exceed 0.5%, which is a fairly good match, since such a discrepancy in the value of the interplanar distances for the same reflection in pure copper, there is between the tabular data given in the various cards of the PDF-2 file cabinet. For example, to reflect (311) pure copper, 1.093 A is given on card 01-07-4611, while 1.088 A is shown on card 01-071-4609.

Table 1 Interplanar distances (nm) corresponding to point reflexes located near the rings in figb Miller indices 111 220 311 222 331 experimental results 2.080 1.271 1.083 1.038 0.828 tabular data for pure copper * 2.084 1.276 1.088 1.042 0.828 difference between experimental and tabular data,% 0.2 0.4 0.5 0.4 0 * Card PDF-2 No. 01-071-4609

A thorough analysis of the bright-field and dark-field images of the UFG structure of the samples obtained by IPDK at a temperature of 20 ° C did not reveal noticeable particles of precipitates.

After annealing at a temperature of 500 ° C, the average grain size in the sample pretreated with IPDC at 20 ° C increased slightly, but was less than 400 nm (Fig. 2a), which indicates the preservation of the ultrafine-grained structure. On figb - bright field image, fig.2g - electron diffraction pattern. Moreover, particles of precipitates with a size of less than 10 nm were observed in the structure of the samples (Fig.2b, c). In the diffraction patterns that were taken from these particles, point reflections were found whose interplanar distances coincided with the interplanar distances of pure chromium (Fig. 2d, Table 2), while the discrepancy for distant reflections between the experimental and tabular ones did not exceed 0.9% to reflect (321).

table 2 Interplanar distances (nm) corresponding to point reflexes located near the rings in FIG. Miller indices 110 200 220 310 321 400 experimental results 2.052 1.449 1.028 0.926 0.77 0.72 tabular data for pure chrome * 2.053 1.451 1.026 0.918 0.768 0.718 difference between experimental and tabular data,% 0 0.1 0.2 0.9 0.2 0.3 * Cards PDF-2 No. 01-07-2771 and No. 00-001-1251

For comparison: discrepancies in the interplanar spacing between different PDF-2 cards (for example, 01-07-2771 and 01-073-9565) for reflecting (310) in pure chromium have the same order of 0.7% (0.918 A and 0.912 A, respectively).

It is known that, when UFG is heated, samples of various alloys, compared with coarse-grained ones, often shift to the low-temperature region of the peaks of differential scanning calorimetry associated with the release of hardening particles [R.K.Islamgaliev, N.F. Yunusova, R.Z. Valiev. The influence of equal-channel angular pressing modes on the superplasticity of aluminum alloy 1420. Physics of metals and metal science. 94, 6 (2002), pp. 88-98]. The selection of particles at lower temperatures is due to the reduced energy of grain boundary diffusion and the increased volume fraction of grain boundaries in the UFG samples [Yu.R. Kolobov. R.Z. Valiev. Grain-boundary diffusion and properties of nanostructured materials. Novosibirsk: Nauka, 2001. - 232 p.].

As a result of this, the structure of Cu – Cr copper alloy samples subjected to quenching, followed by intense plastic deformation by torsion at a temperature of 300 ° C, which is lower than the temperature at which the hardening particles are released, was also studied.

On electron microscopic images of the structure of the samples subjected to IPDC at a temperature of 300 ° C, an average grain size of 200 nm was observed (Fig. 3a), and particles with an average size of about 15 nm were clearly visible (Fig. 3b, c).

The analysis of electron diffraction patterns (Fig. 3d) indicates that the interplanar spacings belonging to point reflections from these particles coincide with the interplanar spacings of pure chromium. Moreover, the discrepancy for long-range reflections between the experimental and tabular data did not exceed 1.3% (Table 3).

Thus, the results of studies of the structure in a transmission electron microscope indicate that the use of IPDC at an elevated temperature of 300 ° C leads to the formation of an average grain size of 200 nm and additionally contributes to the release of particles of the strengthening phase with an average size not exceeding 15 nm (Fig.3b, ), due to dynamic aging, as a result of which there is a decrease in the temperature of the onset of chromium particles from the copper matrix in the Cu-Cr alloy from 500 ° C to 300 ° C.

Table 3 Interplanar distances (nm) corresponding to point reflexes located near the rings in FIG. Miller indices 110 220 321 330 420 experimental results 2.052 1.013 0.780 0.675 0.649 tabular data for pure chrome * 2.05 1.01 0.77 0.677 0.642 difference between experimental and tabular data,% 0.1 0.3 1.3 0.3 1,1 * Card PDF-2 No. 00-001-1251

The heating of the samples subjected to IPDK at an elevated temperature of 300 ° C to a temperature of 500 ° C did not lead to a significant change in the average grain size (Fig. 4a, b), which indicated their increased thermal stability.

Thus, according to the results of studies of the structure in a transmission electron microscope, it was found that processing according to the IPDC scheme at an elevated temperature of 300 ° C leads to the release of chromium from the crystal lattice of the copper matrix, which favorably affects both the increase in strength characteristics due to an increase in the contribution of dispersion hardening, so and an increase in electrical conductivity due to the cleaning of the matrix of atoms of alloying elements.

Studies performed on a Micrometer 5101 microhardness tester showed that the highest values of microhardness of about 2000 MPa are shown by samples subjected to IPDC at temperatures of 250 ° C and 300 ° C (Table 4). Whereas an increase in the temperature of IPDC above 300 ° C leads to a smooth decrease in microhardness values, due to the development of processes associated with the beginning of grain growth and enlargement of precipitate particles. Smaller microhardness values in the samples subjected to IPDC at room temperature are associated with the absence of the effect of dynamic aging.

As a result of mechanical tensile tests, it was found that the maximum values of tensile strength (temporary tensile strength) are manifested by IPDK at temperatures of 250 ° C and 300 ° C, showing 610 MPa and 590 MPa, respectively (Table 4).

The conductivity measurements performed by the eddy current method indicate that the lowest conductivity value of 32.9% IACS is observed in UFG samples obtained by the IPDK method at room temperature (Table 4). This is caused by the appearance of large microdistortions of the crystal lattice, which impede the movement of conduction electrons due to the transition of alloying elements into a solid solution in quenched samples. At IPDC temperatures of 250 ° C and higher, the conductivity is restored to values of more than 86% IACS, apparently due to a decrease in the value of internal elastic stresses and the beginning of the release of dispersed particles of the second phases during the decomposition of a supersaturated solid solution.

Thus, according to the results of the studies, we can conclude that the optimal temperatures of IPDK are 250-300 ° C, which lead to the production of UFG samples thermally stable up to a temperature of 500 ° C (Figure 4), showing a combination of high microhardness values 1960-2000 MPa, tensile strength 590-610 MPa, electrical conductivity 86-88% IACS (Table 4).

Table 4 Microhardness, ultimate strength, elongation and electrical conductivity of UFG samples obtained using the IPDK method IPDK temperature, ° C Microhardness, MPa Tensile strength σ _in , MPa Elongation, δ,% Conductivity,% IACS twenty 1770 530 fourteen 32.9 250 2000 610 fifteen 86.1 300 1960 590 13 88.4 350 1800 560 17 90.0 400 1730 510 fifteen 91.1 450 1550 370 16 89.5

Example 2

, где ϕ - угол пересечения каналов, составила 5,3.To form the UFG structure in Cu-0.5% -0.1% Ag alloy samples with a diameter of 10 mm and a length of 60 mm, another SPD method was used - equal-channel angular pressing (ECAP) on a snap with a channel intersection angle of 120 °, with the number of passes equal to 8. Taking into account the experimental data on dynamic aging observed in IPDC samples (Example 1), ECAP processing was carried out at a temperature of 300 ° C. For comparison, UMP samples were also obtained by processing with ECAP at a temperature of 20 ° C. Accumulated strain calculated by the formula $$\epsilon =2Nctg\left(\phi /2\right)/\surd 3$$

where ϕ is the angle of intersection of the channels, amounted to 5.3.

From TEM photographs of the UFG structure of the sample obtained by ECAP processing at a temperature of 300 ° C, it follows that an average grain size of about 500 nm is observed in it (Fig. 5a, b), as well as particles of precipitates of strengthening phases with a size of 5 nm (Fig. 6). Most grains (not less than 60%) have large-angle boundaries misoriented relative to neighboring grains by angles of more than 15 °. A slight increase in grain size compared with a grain size of 200 nm observed in IPDK samples obtained after ECAP at the same temperature can be explained by the lower degree of shear deformation to which ECAP samples were subjected. While the smaller size (5 nm) of the particles of the strengthening phases in ECAP samples compared with the particle size (15 nm) in IPDC samples treated at the same temperature can be associated with both factors, both with a lower degree of shear deformation and with smaller the pressures used in the ECAP method.

Additional confirmation of the fact that in the ECAP samples processed at a temperature of 300 ° C, a smaller number of particles of the hardening phases was released at their smaller size are lower microhardness and electrical conductivity compared to IPDC samples, which are close to the values observed in UFG samples obtained by processing by ECAP at a temperature of 20 ° C (table 5).

After additional aging at 500 ° C for 2 hours of ECAP, samples of the Cu-Cr system alloy demonstrated a combination of high microhardness values of 1960-2010 MPa, tensile strength of 570-620 MPa, and electrical conductivity of 85% IACS (Table 5).

Table 5 Mechanical properties and electrical conductivity of UFG samples obtained using the ECAP method Temperature and type of processing Microhardness, MPa Strength σ _B , MPa Elongation, δ,% Conductivity,% IACS ECAP 20 ° C + aging 2010 620 10.6 85.0 ECAP 300 ° C + aging 1960 570 13 85.1

Example 3

To form the UFG structure in long rods with a diameter of 10 mm, the RKUP-Conform (RKUP-K) method was used.

At the first stage, the Cu-0.5% Cr-0.1% Ag alloy preforms were heated to a temperature of 1050 ° C, followed by quenching in water.

At the second stage, the hardened billet was subjected to processing by ECAP-K at a temperature of 300 ° С with accumulated deformation ε≥6. At this stage, the microstructure is crushed in the workpiece to an average grain size of not more than 500 nm. At least 60% of the grains have large-angle boundaries misoriented relative to neighboring grains by angles of more than 15 °. Simultaneously with the formation of the UFG structure in the copper matrix, dynamic aging occurs, which leads to the release of fine particles of precipitates of the strengthening phase with a size of not more than 20 nm.

At the third stage, to obtain UFG blanks in the form of a wire, additional plastic deformation is carried out by drawing at room temperature.

In the fourth stage, the UFG of the workpiece is aged at temperatures of 400-500 ° C to increase the volume fraction of precipitation particles, which increases the tensile strength and also leads to the cleaning of the matrix from alloying elements and, accordingly, to an increase in electrical conductivity.

As a result of all these treatments, the UFG bars showed the properties shown in Table 6.

As follows from the above tables UMP, the alloy of the Cu-Cr system obtained by the methods of intense plastic deformation in the specified processing modes, has higher values of tensile strength and electrical conductivity compared to the prototype.

Table 6 Mechanical properties and electrical conductivity of UFG samples obtained using the ECAP-K method Type of processing Microhardness, MPa Tensile strength σ _V , MPa Elongation, δ,% Conductivity,% IACS ECAP-K 1990 600 12 85,2

Thus, the proposed invention provides a combination of high strength, high electrical conductivity and increased thermal stability in the UFG alloy of the Cu-Cr system.

Claims (3)

1. Ultrafine-grained copper alloy of the Cu-Cr system containing fine particles of precipitates of the hardening phase less than 20 nm in size, characterized in that the average grain size in the alloy structure is less than 500 nm, and the number of grains with high-angle boundaries is more than 60%. 2. A method of obtaining ultrafine-grained copper alloy of the Cu-Cr system, including heating, hardening, deformation and aging, characterized in that the hardening is carried out at a temperature of 1020-1050 ° C, and then carry out intensive plastic deformation at a temperature of 20-300 ° C with accumulated deformation of at least 3 and subsequent aging at a temperature of 400-500 ° C. 3. The method according to claim 2, characterized in that the intense plastic deformation is carried out by torsion or equal channel angular pressing, or equal channel angular pressing according to the "conform" scheme.

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