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Research on aging precipitation in a Cu–Cr–Zr–Mg alloy
Materials Science and Engineering A 392 (2005) 422–426
Research on aging precipitation in a Cu–Cr–Zr–Mg alloy Juan-hua Sua,b,∗ , Qi-ming Dongb , Ping Liub , He-jun Lia , Bu-xi Kangb a
College of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China b College of Materials Science and Engineering, Henan University of Science and Technology, Luoyang 471003, Henan Province, China Received 4 August 2004; accepted 27 September 2004
Abstract The effects of aging processes on the properties and microstructure of Cu–0.3Cr–0.15Zr–0.05Mg lead frame alloy were investigated. Aging precipitation phase was dealt with by transmission electronic microscope (TEM). After solid solution was treated at 920 ◦ C and aged at 470 ◦ C for 4 h, the fine precipitation of an ordered compound CrCu2 (Zr, Mg) is found in copper matrix as well as fine Cr and Cu4 Zr. Along the grain boundary, there are larger chromium. The hardness and electrical conductivity can reach 109 HV and 80% IACS, respectively. Sixty percent cold-rolled deformation prior to aging at 470 ◦ C enhances the hardness of the alloy. The coherent precipitates Cr in copper matrix and the dislocations pinned by the fine precipitates are responsible for maximum strengthening of the alloy. So the hardness 165 HV and electrical conductivity 79.2% IACS are available. © 2004 Elsevier B.V. All rights reserved. Idt: TG146 Keywords: Cu–Cr–Zr–Mg alloy; Aging; Precipitation phase; Coherency; Properties
1. Introduction Cu-base alloys are the most popular lead frames alloys used in plastic packaging application due to the high mechanical and operating properties [1–3]. Cu–Cr–Zr alloy has attracted considerable interest recently because of its superior combination of high electrical conductivity and high strength [4–8]. So, this situation has led to the application of Cu–Cr–Zr alloys to the lead frame of the integrated circuit. Cold working is often carried out between the solid solution treatment and aging to assist in the aging hardening by introducing a high density of dislocation [9,10]. Aging is a commen heat treatment for many copper alloy, with the aim of raising their strength and hardness [11,12]. The high
∗
Corresponding author. Tel.: +86 3794 221049; fax: +86 3794 222632. E-mail address: [email protected] (J.-h. Su).
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.09.041
hardness of Cu–Cr–Zr alloys is due to precipitation along the dislocation and dispersion strengthening, and the excellent electrical conductivity is attributed to the very low solubility of Cr and Zr in Cu matrix. In order to control the microstructure and improve the properties of Cu–Cr–Zr alloy, it is of great value to optimize the aging process and identify the composition of the precipitates. There has been no unanimous agreement on the precipitation phase of the alloy. In [13], the precipitates within the grains were indexed to be Hesuler phase CrCu2 (Zr, Mg). At the grain boundary, Cu4 Zr was indexed. Huang and Ma [14] found Cu51 Zr14 in matrix of Cu–Cr–Zr alloy. References [15,16] showed that three phases Cr, Cu5 Zr and Cu should exist in the system. This paper deals with the microstructure and precipitation phases of aged Cu–Cr–Zr–Mg alloy with superior combination of hardness and electrical conductivity, in order to get better understanding of the strength mechanism and the composition of the precipitates of the Cu–0.3 wt% Cr–0.15 wt% Zr–0.05 wt% Mg lead frame alloy.
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2. Experimental procedure The alloy Cu–0.3Cr–0.15Zr–0.05 Mg was produced in a vacuum induction furnace with electrolytic copper, pure chromium, zirconium and magnesium as charge materials. The billet was homogenized at 900 ◦ C for 2 h and hot-rolled to a thickness of 5 mm. The specimens cut from plate were solution-treated at 920 ◦ C for 1 h and water-quenched. The aging treatments were carried out in a tube electric resistance furnace under a fluid atmosphere of argon with temperature accuracy of ±5 ◦ C. The electrical resistivity was determined by measuring the resistance of sample in 100 mm length using ZY9987-type standard direct-current four-probe technique. The mean value of three measurements had an estimated accuracy of less than ±0.0002 . The Vickers micro-hardness was measured on a HVS-1000-type hardness tester under a 100 g load and holding for 10 s. Every sample was tested at five times with an accuracy of ±5%. The transmission electronic microscope (TEM) samples were prepared by conventional electro-polishing method using an electrolyte of HNO3 :CH2 OH = 1:3. The electron microscopy for this study was carried out using a H-800 TEM at 200 kV.
3. Results and discussion 3.1. Effects of aging process parameters on hardness and electrical conductivity Fig. 1 shows the effect of aging time on hardness of Cu–Cr–Zr–Mg alloy with and without deformation before aging at 470 ◦ C. After aging, the hardness with 60% cold rolling is much higher than that with no deformation. The dislocations resulting from the rolling deformation act as diffusion paths for solute atoms and provide nucleation site for precipitation during aging treatment and result in the precipitation hardening effect. After rolled 60% and aged at 470 ◦ C for 1 h, the peak hardness of Cu–Cr–Zr–Mg alloy attains 165 HV. Solid solution treated and aged at 470 ◦ C for 4 h, the peak hardness is 109 HV. At the peak hardness, the fuller precipitation is available and the hardening effect is
Fig. 1. Variation of hardness of Cu–Cr–Zr–Mg alloy with and without rolling before aged at 470 ◦ C vs. aging time.
Fig. 2. Conductivity of Cu–Cr–Zr–Mg alloy with and without rolling before aged at 470 ◦ C vs. aging time.
optimum. With increasing the aging time, the precipitates coarsen and lose coherency with the matrix. After the peak hardness, over-aging occurs. Fig. 2 reveals that the electrical conductivity increases with increasing the time. The longer time brings about more precipitates. The growth of precipitates reduces the contents of solute atom in matrix and results in a continuous increase in electrical conductivity during the aging. So, the conductivity in Cu–0.3Cr–0.15Zr–0.05Mg lead frame alloy remains at a higher level. After 60% deformed and aged at 470 ◦ C, at the initial stage of aging, the conductivity increases sharply. It is the precipitation along the dislocations that results in the initial sharp increase of conductivity. At 470 ◦ C aging for 4 h, the electrical conductivity is 80% IACS. After 60% deformed and aged at 470 ◦ C for 1 h, the electrical conductivity is 79.2% IACS as shown in Fig. 2. 3.2. Analysis of microstructure The hardness depends mainly on the microstructure of the materials, which in turn depends on the aging treatment. The morphology of precipitates upon aging at 470 ◦ C for 4 h is given in Fig. 3. The microstructure characteristic of Cu–Cr–Zr–Mg alloy is the fine dispersed precipitates in the
Fig. 3. Precipitation morphology of the Cu–Cr–Zr–Mg alloy aged at 470 ◦ C for 4 h.
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Cu matrix, having a size of about 10–20 nm as shown in Fig. 3. These fine precipitates together with Cu matrix give rise to peak hardness. The hardness increases following the empirical Orowan relationship. τ = k f 1/2 R−1 where τ is the increase in shear stress; k is constant; f is volume fraction of precipitates; and R denotes the diameter of precipitates. By means of the TEM analysis of the Fig. 3, the volume fraction of precipitates f is about 30%. According to the results of tensile test, the constant k is equal to 2.7 N/m. The more volume fraction of precipitates, the smaller the precipitates, the more increase of τ, the higher hardness of the alloy. Fig. 4 shows the selected area electron diffraction (SAED) pattern of Fig. 3 and the schematic drawing for SAED pattern. It was found that the electron-diffraction evidence is consistent with the crystal structure of the phase belong to the space group Fm3m, which is common to the group of inter-metallic compounds known as Hesuler alloys. For the Cu–Cr–Zr–Mg and Cu–Cr–Mg alloys, the composition of the precipitate is likely to be CrCu2 (Zr, Mg) which has a similar lattice as Fe3 Al [17]. The Hesuler alloys have a fcc crystal structure with a large unit cell containing 8Cu, 4Cr and 4Zr or Mg atoms. This unit cell can be thought of containing eight bcc subcells with each subcell having a Cu atom at its center, and the corner sites alternately occupied by Cr and either Zr or Mg atoms [18]. The bcc sublattice was used to index the electron-diffraction pattern in Fig. 4 and it is the sublattice that obeys the Nishyama–Wasserman orientation relationship with the matrix. (1 1 1)fcc //(1 1 0)bcc [0 1 1]fcc //[0 0 1]bcc [2 1 1]fcc //[1 1 0]bcc In this orientation relationship, the close-packed planes in the two phases, viz. {1 1 1}fcc and {1 1 0}bcc are parallel [8]. It could be postulated that chromium and zirconium could be enriched together, it was easy to form inter-metallic phase of chromium and zirconium. In fact, in our experiment, intermetallic phase of chromium and zirconium could be found. In addition to the CrCu2 (Zr, Mg) precipitates, Cr and Cu4 Zr precipitates are also indexed as shown in Fig. 4. In [8,9], the precipitates within the grains were indexed to be Hesuler phase CrCu2 (Zr, Mg). At the grain boundary, Cu4 Zr was indexed. In our experiment, Hesuler phase CrCu2 (Zr, Mg), Cu4 Zr and Cr are all within the grain at the same time. In [18,19], peak hardness was consistent with the phase of Hesuler alloy type. Overaging of the Cu–Cr–Zr–Mg resulted in Cu4 Zr and Cr phase. In Fig. 4, it could be assumed that the Hesuler phase CrCu2 (Zr, Mg) is being decomposed into Cu4 Zr and Cr phase. The distinction from above researches is that the fine precipitates of Cr, Cu4 Zr and ordered CrCu2 (Zr, Mg) all distribute in matrix Cu within the grain.
Fig. 4. SAED pattern of the Cu–Cr–Zr–Mg alloy aged at 470 ◦ C for 4 h.
Fig. 5 indicates that there are some larger particles along the grain boundary after aging at 470 ◦ C for 4 h. The analysis of energy dispersive X-ray spectroscope (EDS) shows the larger particle is Cr phase as shown in Fig. 6. Fig. 7 shows the micrograph of the dislocation and precipitates by 60% rolling and aging at 470 ◦ C for 1 h. The dislocations provide nucleation site for precipitation and are
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Fig. 5. Precipitates and larger particles aged at 470 ◦ C for 4 h.
Fig. 8. Precipitation morphology and SAED pattern of the Cu–Cr–Zr–Mg alloy by 60% rolling and aging at 470 ◦ C for 1 h.
Fig. 6. EDS of the larger particle.
pinned by the precipitates and result in the precipitation hardening effect from a finer size of precipitates. Fig. 8 shows the precipitates image and SAED pattern of Cu–Cr–Zr–Mg alloy by 60% rolling and aging at 470 ◦ C for 1 h. Many lobe-lobe contrasts are found. Because no extra reflection spots other than those from matrix are detected by selected electron diffraction and from its morphology and size, it is supposed that the particle should be coherent chromium particle. From SAED pattern of the precipitates shown in Fig. 8, indexing of the precipitates is bcc Cr. The result is similar to that observed in coherent particle of Cr particle in a Cu matrix [14]. The atom mismatch degree of coherent interface is larger which leads to higher elastic strain energy around the coherent interface. The increase in shear stress τ due to the coherency keeps to the following formula [20] τ = 1.84Gε f 1/2 where the G is the shear elastic modulus of matrix; ε is the function of the mismatch degree δ; and f is volume fraction of precipitates. So, the coherent strain is one of the important factors to strengthen the alloy.
4. Conclusions
Fig. 7. Micrograph of the dislocation and precipitates by 60% rolling and aging at 470 ◦ C for 1 h.
(1) After solid solution treated at 920 ◦ C for 1 h and aged at 470 ◦ C for 4 h, the dispersed fine precipitates of the Cu–Cr–Zr–Mg alloy are formed. There are there types of precipitates within the grain: CrCu2 (Zr, Mg), Cu4 Zr and Cr. Along the grain boundary, there is bigger Cr phase. The hardness and electrical conductivity can reach 109 HV and 80% IACS, respectively. (2) By 60% rolling and aging at 470 ◦ C for 1 h, the elected area electron diffraction pattern of the precipitates reveals that there is coherent phase of Cr in Cu matrix.
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The dislocations are pinned by the fine precipitates. So, the hardness 165 HV and electrical conductivity 79.2% IACS are available.
Acknowledgements This work is funded by the State “863 plan” (No. 2002AA331112) and supported by the Doctorate Foundation of Northwestern Polytechnical University.
References [1] D.M. Zhao, Q.M. Dong, P. Liu, Mater. Chem. Phys. 79 (2003) 81–86. [2] J.H. Su, Q.M. Dong, P. Liu, Trans. Nonferrous Met. Soc. China 13 (6) (2003) 1419–1423. [3] H.I. Choi, K.Y. Lee, S.L. Kwun, J. Mater. Sci. Lett. 16 (1997) 1600–1602. [4] J.W. Davis, G.M. Kalinin, J. Nucl. Mater. 258-263 (1998) 323–328. [5] J.P. Tu, Wear 249 (2002) 1021–1027.
[6] J.B. Correir, H.A. Davies, C.M. Sellars, Acta Mater. 45 (1) (1997) 177–190. [7] I.S. Batra, G.K. Dey, U.D. Kulkarni, S. Banerjee, J. Nucl. Mater. 299 (2001) 91–100. [8] D.L. Zhang, K. Mihara, S. Tsubokawa, H.G. Suzuki, Mater. Sci. Technol. 16 (2000) 357–363. [9] H.J. Ryu, H.K. Baik, J. Mater. Sci. 35 (2000) 3641–3646. [10] J.H. Su, Q.M. Dong, P. Liu, J. Mater. Sci. Technol. 19 (6) (2003) 529. [11] H. Fernee, J. Nairn, J. Mater. Sci. 36 (2001) 2711–2719. [12] S. Nagarjuna, K.K. Sharma, I. Sudhakar, D.S. Sarma, Mater. Sci. Eng. A313 (2001) 251–260. [13] W.X. Qi, J.P. Tu, Y.Z. Yang, Mater. Sci. Eng. A343 (2003) 89–96. [14] F.X. Huang, J.S. Ma, Scr. Mater. 48 (2003) 97–102. [15] K.J. Zeng, Scr. Metall. Mater. 32 (12) (1995) 2009–2014. [16] U. Holzwarth, H. Stamm, J. Nucl. Mater. 279 (2000) 31–45. [17] P. Liu, B.X. Kang, X.G. Cao, Mater. Sci. Eng. A265 (1999) 262–267. [18] N.Y. Tang, D.M.R. Taplin, G.L. Dunlop, Mater. Sci. Technol. 1 (1985) 270. [19] E. Batawi, D.G. Morris, M.A. Morris, Mater. Sci. Technol. 6 (1990) 892–899. [20] P. Liu, B.X. Kang, Acta Metall. Sin. 35 (6) (1999) 561–564, in Chinese.
Research on aging precipitation in a Cu–Cr–Zr–Mg alloy Juan-hua Sua,b,∗ , Qi-ming Dongb , Ping Liub , He-jun Lia , Bu-xi Kangb a
College of Materials Science and Engineering, Northwestern Polytechnical University, Xi’an 710072, China b College of Materials Science and Engineering, Henan University of Science and Technology, Luoyang 471003, Henan Province, China Received 4 August 2004; accepted 27 September 2004
Abstract The effects of aging processes on the properties and microstructure of Cu–0.3Cr–0.15Zr–0.05Mg lead frame alloy were investigated. Aging precipitation phase was dealt with by transmission electronic microscope (TEM). After solid solution was treated at 920 ◦ C and aged at 470 ◦ C for 4 h, the fine precipitation of an ordered compound CrCu2 (Zr, Mg) is found in copper matrix as well as fine Cr and Cu4 Zr. Along the grain boundary, there are larger chromium. The hardness and electrical conductivity can reach 109 HV and 80% IACS, respectively. Sixty percent cold-rolled deformation prior to aging at 470 ◦ C enhances the hardness of the alloy. The coherent precipitates Cr in copper matrix and the dislocations pinned by the fine precipitates are responsible for maximum strengthening of the alloy. So the hardness 165 HV and electrical conductivity 79.2% IACS are available. © 2004 Elsevier B.V. All rights reserved. Idt: TG146 Keywords: Cu–Cr–Zr–Mg alloy; Aging; Precipitation phase; Coherency; Properties
1. Introduction Cu-base alloys are the most popular lead frames alloys used in plastic packaging application due to the high mechanical and operating properties [1–3]. Cu–Cr–Zr alloy has attracted considerable interest recently because of its superior combination of high electrical conductivity and high strength [4–8]. So, this situation has led to the application of Cu–Cr–Zr alloys to the lead frame of the integrated circuit. Cold working is often carried out between the solid solution treatment and aging to assist in the aging hardening by introducing a high density of dislocation [9,10]. Aging is a commen heat treatment for many copper alloy, with the aim of raising their strength and hardness [11,12]. The high
∗
Corresponding author. Tel.: +86 3794 221049; fax: +86 3794 222632. E-mail address: [email protected] (J.-h. Su).
0921-5093/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2004.09.041
hardness of Cu–Cr–Zr alloys is due to precipitation along the dislocation and dispersion strengthening, and the excellent electrical conductivity is attributed to the very low solubility of Cr and Zr in Cu matrix. In order to control the microstructure and improve the properties of Cu–Cr–Zr alloy, it is of great value to optimize the aging process and identify the composition of the precipitates. There has been no unanimous agreement on the precipitation phase of the alloy. In [13], the precipitates within the grains were indexed to be Hesuler phase CrCu2 (Zr, Mg). At the grain boundary, Cu4 Zr was indexed. Huang and Ma [14] found Cu51 Zr14 in matrix of Cu–Cr–Zr alloy. References [15,16] showed that three phases Cr, Cu5 Zr and Cu should exist in the system. This paper deals with the microstructure and precipitation phases of aged Cu–Cr–Zr–Mg alloy with superior combination of hardness and electrical conductivity, in order to get better understanding of the strength mechanism and the composition of the precipitates of the Cu–0.3 wt% Cr–0.15 wt% Zr–0.05 wt% Mg lead frame alloy.
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2. Experimental procedure The alloy Cu–0.3Cr–0.15Zr–0.05 Mg was produced in a vacuum induction furnace with electrolytic copper, pure chromium, zirconium and magnesium as charge materials. The billet was homogenized at 900 ◦ C for 2 h and hot-rolled to a thickness of 5 mm. The specimens cut from plate were solution-treated at 920 ◦ C for 1 h and water-quenched. The aging treatments were carried out in a tube electric resistance furnace under a fluid atmosphere of argon with temperature accuracy of ±5 ◦ C. The electrical resistivity was determined by measuring the resistance of sample in 100 mm length using ZY9987-type standard direct-current four-probe technique. The mean value of three measurements had an estimated accuracy of less than ±0.0002 . The Vickers micro-hardness was measured on a HVS-1000-type hardness tester under a 100 g load and holding for 10 s. Every sample was tested at five times with an accuracy of ±5%. The transmission electronic microscope (TEM) samples were prepared by conventional electro-polishing method using an electrolyte of HNO3 :CH2 OH = 1:3. The electron microscopy for this study was carried out using a H-800 TEM at 200 kV.
3. Results and discussion 3.1. Effects of aging process parameters on hardness and electrical conductivity Fig. 1 shows the effect of aging time on hardness of Cu–Cr–Zr–Mg alloy with and without deformation before aging at 470 ◦ C. After aging, the hardness with 60% cold rolling is much higher than that with no deformation. The dislocations resulting from the rolling deformation act as diffusion paths for solute atoms and provide nucleation site for precipitation during aging treatment and result in the precipitation hardening effect. After rolled 60% and aged at 470 ◦ C for 1 h, the peak hardness of Cu–Cr–Zr–Mg alloy attains 165 HV. Solid solution treated and aged at 470 ◦ C for 4 h, the peak hardness is 109 HV. At the peak hardness, the fuller precipitation is available and the hardening effect is
Fig. 1. Variation of hardness of Cu–Cr–Zr–Mg alloy with and without rolling before aged at 470 ◦ C vs. aging time.
Fig. 2. Conductivity of Cu–Cr–Zr–Mg alloy with and without rolling before aged at 470 ◦ C vs. aging time.
optimum. With increasing the aging time, the precipitates coarsen and lose coherency with the matrix. After the peak hardness, over-aging occurs. Fig. 2 reveals that the electrical conductivity increases with increasing the time. The longer time brings about more precipitates. The growth of precipitates reduces the contents of solute atom in matrix and results in a continuous increase in electrical conductivity during the aging. So, the conductivity in Cu–0.3Cr–0.15Zr–0.05Mg lead frame alloy remains at a higher level. After 60% deformed and aged at 470 ◦ C, at the initial stage of aging, the conductivity increases sharply. It is the precipitation along the dislocations that results in the initial sharp increase of conductivity. At 470 ◦ C aging for 4 h, the electrical conductivity is 80% IACS. After 60% deformed and aged at 470 ◦ C for 1 h, the electrical conductivity is 79.2% IACS as shown in Fig. 2. 3.2. Analysis of microstructure The hardness depends mainly on the microstructure of the materials, which in turn depends on the aging treatment. The morphology of precipitates upon aging at 470 ◦ C for 4 h is given in Fig. 3. The microstructure characteristic of Cu–Cr–Zr–Mg alloy is the fine dispersed precipitates in the
Fig. 3. Precipitation morphology of the Cu–Cr–Zr–Mg alloy aged at 470 ◦ C for 4 h.
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Cu matrix, having a size of about 10–20 nm as shown in Fig. 3. These fine precipitates together with Cu matrix give rise to peak hardness. The hardness increases following the empirical Orowan relationship. τ = k f 1/2 R−1 where τ is the increase in shear stress; k is constant; f is volume fraction of precipitates; and R denotes the diameter of precipitates. By means of the TEM analysis of the Fig. 3, the volume fraction of precipitates f is about 30%. According to the results of tensile test, the constant k is equal to 2.7 N/m. The more volume fraction of precipitates, the smaller the precipitates, the more increase of τ, the higher hardness of the alloy. Fig. 4 shows the selected area electron diffraction (SAED) pattern of Fig. 3 and the schematic drawing for SAED pattern. It was found that the electron-diffraction evidence is consistent with the crystal structure of the phase belong to the space group Fm3m, which is common to the group of inter-metallic compounds known as Hesuler alloys. For the Cu–Cr–Zr–Mg and Cu–Cr–Mg alloys, the composition of the precipitate is likely to be CrCu2 (Zr, Mg) which has a similar lattice as Fe3 Al [17]. The Hesuler alloys have a fcc crystal structure with a large unit cell containing 8Cu, 4Cr and 4Zr or Mg atoms. This unit cell can be thought of containing eight bcc subcells with each subcell having a Cu atom at its center, and the corner sites alternately occupied by Cr and either Zr or Mg atoms [18]. The bcc sublattice was used to index the electron-diffraction pattern in Fig. 4 and it is the sublattice that obeys the Nishyama–Wasserman orientation relationship with the matrix. (1 1 1)fcc //(1 1 0)bcc [0 1 1]fcc //[0 0 1]bcc [2 1 1]fcc //[1 1 0]bcc In this orientation relationship, the close-packed planes in the two phases, viz. {1 1 1}fcc and {1 1 0}bcc are parallel [8]. It could be postulated that chromium and zirconium could be enriched together, it was easy to form inter-metallic phase of chromium and zirconium. In fact, in our experiment, intermetallic phase of chromium and zirconium could be found. In addition to the CrCu2 (Zr, Mg) precipitates, Cr and Cu4 Zr precipitates are also indexed as shown in Fig. 4. In [8,9], the precipitates within the grains were indexed to be Hesuler phase CrCu2 (Zr, Mg). At the grain boundary, Cu4 Zr was indexed. In our experiment, Hesuler phase CrCu2 (Zr, Mg), Cu4 Zr and Cr are all within the grain at the same time. In [18,19], peak hardness was consistent with the phase of Hesuler alloy type. Overaging of the Cu–Cr–Zr–Mg resulted in Cu4 Zr and Cr phase. In Fig. 4, it could be assumed that the Hesuler phase CrCu2 (Zr, Mg) is being decomposed into Cu4 Zr and Cr phase. The distinction from above researches is that the fine precipitates of Cr, Cu4 Zr and ordered CrCu2 (Zr, Mg) all distribute in matrix Cu within the grain.
Fig. 4. SAED pattern of the Cu–Cr–Zr–Mg alloy aged at 470 ◦ C for 4 h.
Fig. 5 indicates that there are some larger particles along the grain boundary after aging at 470 ◦ C for 4 h. The analysis of energy dispersive X-ray spectroscope (EDS) shows the larger particle is Cr phase as shown in Fig. 6. Fig. 7 shows the micrograph of the dislocation and precipitates by 60% rolling and aging at 470 ◦ C for 1 h. The dislocations provide nucleation site for precipitation and are
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Fig. 5. Precipitates and larger particles aged at 470 ◦ C for 4 h.
Fig. 8. Precipitation morphology and SAED pattern of the Cu–Cr–Zr–Mg alloy by 60% rolling and aging at 470 ◦ C for 1 h.
Fig. 6. EDS of the larger particle.
pinned by the precipitates and result in the precipitation hardening effect from a finer size of precipitates. Fig. 8 shows the precipitates image and SAED pattern of Cu–Cr–Zr–Mg alloy by 60% rolling and aging at 470 ◦ C for 1 h. Many lobe-lobe contrasts are found. Because no extra reflection spots other than those from matrix are detected by selected electron diffraction and from its morphology and size, it is supposed that the particle should be coherent chromium particle. From SAED pattern of the precipitates shown in Fig. 8, indexing of the precipitates is bcc Cr. The result is similar to that observed in coherent particle of Cr particle in a Cu matrix [14]. The atom mismatch degree of coherent interface is larger which leads to higher elastic strain energy around the coherent interface. The increase in shear stress τ due to the coherency keeps to the following formula [20] τ = 1.84Gε f 1/2 where the G is the shear elastic modulus of matrix; ε is the function of the mismatch degree δ; and f is volume fraction of precipitates. So, the coherent strain is one of the important factors to strengthen the alloy.
4. Conclusions
Fig. 7. Micrograph of the dislocation and precipitates by 60% rolling and aging at 470 ◦ C for 1 h.
(1) After solid solution treated at 920 ◦ C for 1 h and aged at 470 ◦ C for 4 h, the dispersed fine precipitates of the Cu–Cr–Zr–Mg alloy are formed. There are there types of precipitates within the grain: CrCu2 (Zr, Mg), Cu4 Zr and Cr. Along the grain boundary, there is bigger Cr phase. The hardness and electrical conductivity can reach 109 HV and 80% IACS, respectively. (2) By 60% rolling and aging at 470 ◦ C for 1 h, the elected area electron diffraction pattern of the precipitates reveals that there is coherent phase of Cr in Cu matrix.
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The dislocations are pinned by the fine precipitates. So, the hardness 165 HV and electrical conductivity 79.2% IACS are available.
Acknowledgements This work is funded by the State “863 plan” (No. 2002AA331112) and supported by the Doctorate Foundation of Northwestern Polytechnical University.
References [1] D.M. Zhao, Q.M. Dong, P. Liu, Mater. Chem. Phys. 79 (2003) 81–86. [2] J.H. Su, Q.M. Dong, P. Liu, Trans. Nonferrous Met. Soc. China 13 (6) (2003) 1419–1423. [3] H.I. Choi, K.Y. Lee, S.L. Kwun, J. Mater. Sci. Lett. 16 (1997) 1600–1602. [4] J.W. Davis, G.M. Kalinin, J. Nucl. Mater. 258-263 (1998) 323–328. [5] J.P. Tu, Wear 249 (2002) 1021–1027.
[6] J.B. Correir, H.A. Davies, C.M. Sellars, Acta Mater. 45 (1) (1997) 177–190. [7] I.S. Batra, G.K. Dey, U.D. Kulkarni, S. Banerjee, J. Nucl. Mater. 299 (2001) 91–100. [8] D.L. Zhang, K. Mihara, S. Tsubokawa, H.G. Suzuki, Mater. Sci. Technol. 16 (2000) 357–363. [9] H.J. Ryu, H.K. Baik, J. Mater. Sci. 35 (2000) 3641–3646. [10] J.H. Su, Q.M. Dong, P. Liu, J. Mater. Sci. Technol. 19 (6) (2003) 529. [11] H. Fernee, J. Nairn, J. Mater. Sci. 36 (2001) 2711–2719. [12] S. Nagarjuna, K.K. Sharma, I. Sudhakar, D.S. Sarma, Mater. Sci. Eng. A313 (2001) 251–260. [13] W.X. Qi, J.P. Tu, Y.Z. Yang, Mater. Sci. Eng. A343 (2003) 89–96. [14] F.X. Huang, J.S. Ma, Scr. Mater. 48 (2003) 97–102. [15] K.J. Zeng, Scr. Metall. Mater. 32 (12) (1995) 2009–2014. [16] U. Holzwarth, H. Stamm, J. Nucl. Mater. 279 (2000) 31–45. [17] P. Liu, B.X. Kang, X.G. Cao, Mater. Sci. Eng. A265 (1999) 262–267. [18] N.Y. Tang, D.M.R. Taplin, G.L. Dunlop, Mater. Sci. Technol. 1 (1985) 270. [19] E. Batawi, D.G. Morris, M.A. Morris, Mater. Sci. Technol. 6 (1990) 892–899. [20] P. Liu, B.X. Kang, Acta Metall. Sin. 35 (6) (1999) 561–564, in Chinese.