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Aging hardening characteristic and electrical conductivity of Cu-Cr-Zr-Mo in situ composite

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Science and Engineering of Composite Materials
From the journal Science and Engineering of Composite Materials Volume 23 Issue 6

Abstract

Cu-0.85Cr-0.5Zr-0.5Mo in situ composite was prepared by continuous casting, cold drawing, and aging treatment. By means of the analysis of microhardness, electrical conductivity, and transmission electron microscopy, the aging characteristics and microstructures of the in situ composite were investigated at different aging temperatures and aging times after cold deformation. The results show that the Cu-0.85Cr-0.5Zr-0.5Mo in situ composite has an excellent combination of the microhardness and electrical conductivity aged at 500°C for 4 h, and the microhardness and electrical conductivity reach 171 HV and 81.3% IACS, respectively. Small dispersed particles could strongly pin dislocations and stabilize the substructure from deformation. The main strengthening mechanisms for the in situ composite alloy are the work hardening and the secondary phase or interface strengthening. It was suggested that cold deformation before aging treatment can accelerate the precipitation of the secondary phase and improve the comprehensive properties of the Cu-Cr-Zr-Mo in situ composite.

Keywords: aging treatment; in situ composite; microstructures; precipitation strengthening

1 Introduction

Copper matrix composite is widely used in many applications, such as trolley wires, electrodes for resistance welding, and third generation of lead frame materials, because of its high strength, high conductivity, and corrosion resistance [1], [2], [3], [4], [5]. The high electrical conductivity of the composite is due to the very low solubility of Cr, Zr, Y, V, Ni, or Mo in the copper matrix, whereas the excellent strength is attributed to precipitation and particle-dispersion strengthening. Thermal aging treatment is normally used to obtain the high strength and simultaneously the high electrical conductivity of the copper matrix composite [6], [7], [8], [9], [10]. It is well known that, during prime aging, the supersaturated solid solution decomposes as well as finely and homogeneously dispersed precipitates, which strengthens the alloy without degrading the electrical conductivity of the composite [11], [12], [13], [14]. For example, fine grain structure and fine dispersed precipitate of the rapid solidification and aging account for the increase in hardness and conductivity of the Cu-Cr-Sn-Zn composite [15]. It has been proven experimentally that the microstructure and properties of Cu-Cr-Zr or Cu-Cr-Zr-Mg composite were under the influence of many factors, such as the content of copper, alloy element addition, degree of deformation, solidification condition, and thermomechanical processing [16], [17].

This paper aims to develop a kind of high strength and high conductivity in situ composite and to understand the aging hardening characteristic of the copper matrix composite. The Cu-Cr-Zr-Mo in situ composite was selected as the research object. Electrical resistivity measurement, microhardness measurement, and transmission electron microscopy (TEM) were performed to characterize the microstructures and properties of the in situ composite during the aging process.

2 Materials and methods

The 99.95% Cu, 99.99% Cr, 99.99% Zr, and 99.99% Mo in purity were used to prepare the Cu-0.85Cr-0.5Zr-0.5Mo in situ composite. The alloy ingots were prepared by continuous casting equipment of RT100 made by Rautomead Ltd. (Dundee, UK). The ingots were solidified rapidly through pouring the alloy melt into a water-cooled copper mould. Subsequently, the alloy ingots of 20 mm in diameter were cold forged and drawn to wires of 4 mm in final diameter. The aging treatments were carried out using a tube electric resistance furnace (Kejing Star Technology Company, Shenzhen, China) under an atmosphere of nitrogen with a temperature accuracy of ±5°C. The samples with different aging temperatures and aging times during the aging process were taken to form the in situ composite microstructure, which keeps the basic conductive capacity of the copper matrix phase and obtains the strengthening effect of the secondary phase.

The microhardness was determined using HMV-FA2 micro-Vickers hardness tester (Shimadzu Co., Ltd., Kyoto, Japan) with a load of 1.961 N and a holding time of 10 s, and each sample was measured five times to obtain the value. The electrical conductivity was determined by measuring the alloy samples using FD101 metal conductivity tester (Taili Instrument Company, Shenzhen, China) with an accuracy of ±0.1% IACS, and every sample was tested five times. The tensile strength measurements were performed on a Shimadzu AG-X material testing machine (Shimadzu Co., Ltd., Kyoto, Japan) at room temperature with a velocity of 0.1 mm/s. TEM samples were sliced from the aged samples and further reduced to approximately 25 μm by chemical polishing. TEM examinations were carried out using H-800 TEM (Hitachi Group, Kyoto, Japan) operating at 200 kV.

3 Results and discussion

3.1 Microstructures of the composite

TEM images of the Cu-Cr-Zr-Mo composite at a typical processing state are shown in Figure 1. The results show that the matrix of the composite is a substantially solid solution in the solution state, but there are still some small particles that are not solid solved completely. After aging treatment, in addition to the retention of the subdislocation structure, there are some nanoscale precipitates existing in the copper matrix, as the alloy element content is the excess of equilibrium solid solubility in the alloy. The precipitates with subspheroidal shape mainly form in the grain. Because the solid solubility of Cr, Mo, and Zr in copper is <0.01% at room temperature, the precipitates are intermetallic compounds particles that may have an orientation relationship with the copper matrix.

Figure 1: TEM images of the Cu-Cr-Zr-Mo in situ composite solution at 900°C for 3 h (A) and aging at 500°C for 2 h (B).
Figure 1:

TEM images of the Cu-Cr-Zr-Mo in situ composite solution at 900°C for 3 h (A) and aging at 500°C for 2 h (B).

3.2 Effect of aging on the microhardness and electrical conductivity of the composite

The effect of aging temperatures for varying times between 1 and 7 h on the microhardness is shown in Figure 2. It can be seen that the microhardness of the Cu-Cr-Zr-Mo composite increases rapidly at the initial stage with the increase of aging time and then gradually increases until it reaches a peak, and then gradually decreases, because of the dispersion of the precipitation phase and coherence along with the copper matrix. The increase of the precipitate volume fraction during subsequent aging treatment is consistent with the increase in the hardness of the composite sample, and it implies that, during aging treatment, precipitates form and contribute effectively in the strengthening of the copper matrix composite. From the aging curves in the interval from 400°C to 600°C, it can be seen that the higher the aging temperature is, the lower the peak microhardness will be, and the aging time to the peak microhardness decreases, whereas the shape of aging curve changes significantly with the aging temperature. The higher the aging temperature is, the more rapidly the secondary phase precipitates. A peak microhardness of 171 HV is observed after aging at 500°C for 4 h, beyond which it decreases with the increase in the aging time. In addition, the higher the aging temperature is, the greater growth inclination of the precipitate is.

Figure 2: Curves of the microhardness with aging time and aging temperature.
Figure 2:

Curves of the microhardness with aging time and aging temperature.

Figure 3 shows the effect of aging at various temperatures for varying times between 1 and 7 h on the electrical conductivity of the Cu-Cr-Zr-Mo composite. It indicates that the electrical conductivity of the composite increases with the increase of aging time and temperature. At the early stage of the aging process, the electrical conductivity of the composite increases rapidly owing to the high saturation, the precipitation force of the secondary phase is great, and the precipitation speed is fast and then tends to be stable; the higher the aging temperature is, the quicker the increase of the electrical conductivity at the early stage of the aging process will be. For instance, upon aging at 500°C for 4 h, the value of the electrical conductivity is 81.3% IACS, but it is only 77.2% IACS aging at 400°C. A solute in solid solution with copper has a much more powerful effect on decreasing the electrical conductivity than when it is present partly or wholly as a secondary phase. The increase in the electrical conductivity of the composite is attributed to the removal of Cr, Zr, or Mo from the copper solid solution to form a secondary phase. The longer the aging time is, the less the supersaturated vacancies and thus the slower the precipitation process will be.

Figure 3: Curves of the electrical conductivity with aging time and aging temperature.
Figure 3:

Curves of the electrical conductivity with aging time and aging temperature.

When the specimens are aged for 4 h, the electrical conductivity tends to keep constant. This is because the solute atoms precipitate out from the supersaturated solid solution. The scattering effect on electrons from the solute atom becomes less, which leads to the increase of the conductivity greatly at the first stage of aging. The precipitation reduces the contents of solute atoms in the matrix and leads to a continuous increase in the electrical conductivity during aging. Therefore, the electrical conductivity improves continuously with transformation. Finally, the electric conductivity tends to be stable because of the decrement in the kinetics of the precipitation.

From the electrical conductivity curves in the interval from 400°C to 500°C, it can be seen that the higher the aging temperature is, the more the electrical conductivity of the composite increases. As a result, the Cu-Cr-Zr-Mo composite can attain a combination of the microhardness and electrical conductivity with aging at 500°C for 4 h, and the microhardness and electrical conductivity are approximately 171 HV and 81.3% IACS, respectively.

3.3 Properties of the composite after cold deformation and aging

The cold deformation before aging can greatly increase the number of defects, such as dislocation and vacancy, leading to the aberration of lattice and the improvement of free energy, which is beneficial to the nucleation and growth of the secondary phase. The curves showing the electrical conductivity and microhardness of the composite aging at 400°C after 40% deformation with aging time are presented in Tables 1 and 2, respectively. It can be seen that the change tendency of electrical conductivity, microhardness and tensile strength is almost the same as the one directly aging after solution treatment. Due to the cold deformation before aging, the processes of precipitation are accelerated, making it possible that the amplitude of the electrical conductivity and microhardness is larger at the early stage of aging.

Table 1

Properties of the composite by aging at 400°C without cold deformation.

Aging time (h)0123456
Electrical conductivity (%IACS)64.268.374.376.677.278.078.5
Microhardness (HV)76104114121129139127
Tensile strength (MPa)588593617639651669658
Table 2

Properties of the composite by aging at 400°C with 40% cold deformation.

Aging time (h)0123456
Electrical conductivity (%IACS)59.363.577.178.779.680.180.2
Microhardness (HV)84113126144135126119
Tensile strength (MPa)610624644679670651641

The TEM image of the composite aging at 400°C for 30 min after 40% deformation is shown in Figure 4. It can be seen clearly that the high dislocation density still remained in the copper matrix and the secondary phase is dispersed, hardening the composite to some degree. The microstructure essentially consists of a dislocation network and fine dispersed precipitates. The dislocation is the thermodynamically unstable defect that has higher energy and accelerates the nucleation and growth of the secondary phase. The dislocations act as the heterogeneous sites, and the volume fraction of the secondary phase increases. As a result, the microhardness of the Cu-Cr-Zr-Mo composite with 40% cold deformation increases considerably and is higher than that without cold deformation. The electrical conductivity increases rapidly due to the enhanced removal of Cr, Zr, or Mo from the copper matrix as well as the increased volume fraction of the secondary phase.

Figure 4: Morphology of the precipitates and dislocation in the copper matrix.
Figure 4:

Morphology of the precipitates and dislocation in the copper matrix.

4 Conclusions

  1. The microhardness of the Cu-0.85Cr-0.5Zr-0.5Mo in situ composite increases quickly at the initial stage of aging and then decreases with increasing time, and the electrical conductivity increases with the increase of aging time and temperature. The best combination of hardness and conductivity is achieved at 500°C for 4 h, and the values of the microhardness and electrical conductivity are 171 HV and 81.3% IACS, respectively.

  2. The precipitation of the secondary phase can be accelerated with cold deformation before aging. The Cu-Cr-Zr-Mo in situ composite exhibits high strength and high conductivity only after sufficient solid solution, aging treatment, and plastic deforming.

  3. The precipitation of the alloy element from the supersaturated copper matrix results in the increase of the electrical conductivity and microhardness. The cold deformation can accelerate the precipitation of Cr, Zr, or Mo from the copper matrix, because the dislocations act as the heterogeneous and nucleation sites.

Corresponding author: Song Wang, State Key Laboratory of Advanced Technologies for Comprehensive Utilization of Platinum Metals, Kunming Institute of Precious Metals, Kunming, Yunnan 650106, P.R. China, e-mail:

Funding source: National Natural Science Foundation of China

Award Identifier / Grant number: u0837601

Award Identifier / Grant number: 51164015

Funding statement: This work was supported by the National Natural Science Foundation of China (No. u0837601 and 51164015) and the Innovative Team Foundation of Kunming City (No. 2012-01-01-A-R-07-0005). We are also grateful to Professor Wang in the Center for Materials Testing of Kunming University of Science and Technology, where we carried out the TEM and tensile strength tests.

Acknowledgments

This work was supported by the National Natural Science Foundation of China (No. u0837601 and 51164015) and the Innovative Team Foundation of Kunming City (No. 2012-01-01-A-R-07-0005). We are also grateful to Professor Wang in the Center for Materials Testing of Kunming University of Science and Technology, where we carried out the TEM and tensile strength tests.

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Received: 2014-12-8
Accepted: 2015-2-13
Published Online: 2015-5-9
Published in Print: 2016-11-1

©2016 Walter de Gruyter GmbH, Berlin/Boston

This article is distributed under the terms of the Creative Commons Attribution Non-Commercial License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

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https://doi.org/10.1515/secm-2013-0313
Keywords for this article
aging treatment; in situ composite; microstructures; precipitation strengthening
Creative Commons
BY-NC-ND 3.0

Articles in the same Issue

  1. Frontmatter
  2. Original articles
  3. Effect of polyarylene ether nitrile on the curing behaviors and properties of bisphthalonitrile
  4. Consolidation and swell characteristics of dispersive soils stabilized with lime and natural zeolite
  5. Aging hardening characteristic and electrical conductivity of Cu-Cr-Zr-Mo in situ composite
  6. Modeling cement hydration by connecting a nucleation and growth mechanism with a diffusion mechanism. Part II: Portland cement paste hydration
  7. Unsaturated flow characteristics in dual-scale fibre fabrics at one-dimensional constant flow rate
  8. Crushing analysis and crashworthiness optimization design of reinforced regular hexagon honeycomb sandwich panel
  9. Effects on the microstructure and mechanical properties of Sn-0.7Cu lead-free solder with the addition of a small amount of magnesium
  10. Investigation of chip seal performance under cold climate conditions
  11. 2D macro-mechanical FE simulations for machining unidirectional FRP composite: the influence of damage models
  12. The grinding surface characteristics and evaluation of particle-reinforced aluminum silicon carbide
  13. Influence of tire cord layers and arrangement direction on mechanical properties and vibration isolation of chloroprene rubber composite materials reinforced with polyester tire cord
  14. Material parameter identification in functionally graded structures using isoparametric graded finite element model
  15. Improved impact response of hygrothermally conditioned carbon/epoxy woven composites
  16. An experimental study on the effect of length and orientation of embedded FBG sensors on the signal properties under fatigue loading
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