Ultrafine grained copper alloy sheets having both high strength and high electric conductivity
Introduction
It is nowadays known that severe plastic deformation (SPD) of metallic materials results in ultrafine grained (UFG) microstructures with mean grain sizes smaller than 1 µm [1], [2]. The accumulative roll bonding (ARB) process [1], [2], [3] is a kind of SPD process using rolling deformation. The UFG materials perform very high strength which is about 3–4 times higher than that of the same material with conventional grain sizes of 10–100 µm [4]. Not only mechanical properties but also electric and thermal conductivities are important for Cu alloys from a viewpoint of their practical application. Although it is known that the SPD processed Cu alloys exhibit high strength [5], [6], [7], there have been a few reports on the electric conductivity of the SPD/UFG Cu alloys [8]. Extremely high density of lattice defects, especially grain boundaries, are introduced into the SPD processed materials, so that it is important to know the effect of the lattice defects on the electronic property of Cu alloys. In this study, the microstructure of Cu alloys ARB processed by various cycles was quantitatively characterized by electron back scattering diffraction (EBSD) technique for clarifying the formation process of the UFG microstructures during the ARB. Concerning Cu alloys, microstructure evolution during SPD processes is still unclear, in comparison with other materials like Al alloys [9]. The mechanical properties and electric conductivity of the ARB processed Cu alloys were also investigated. The correlation between the microstructural parameters and mechanical and electrical properties of the SPD/UFG Cu alloys is discussed.
Section snippets
Experimental
In the present study, three kinds of Cu alloys, i.e. an oxygen free Cu (OFC; 99.99% purity), a deoxidized low phosphorous Cu (DLP; Cu–0.02wt.%P–0.017wt.%Pb) and a commercial Cu alloy containing iron (PMC90; Cu–1.0wt.%Fe–0.017wt.%Pb) all having fully recrystallized microstructures were used as the starting materials. The Cu-alloy sheets with the thickness of 1 mm were provided for the ARB process. The principle and the detailed processing procedures of the ARB have been reported previously [2].
Results and discussion
Fig. 1 shows the grain boundary maps obtained from EBSD measurements of the ARB processed DLP and OFC alloys, respectively. These images were obtained at the quarter thickness location of the ARB processed sheets. In those figures, black lines indicate high angle boundaries whose misorientation is larger than 15°, while grey lines indicate low angle boundaries with misorientation ranging from 2° to 15°. The boundaries having misorientation smaller than 2° were removed in order to cut off the
Conclusions
The ultrafine grained microstructure, mechanical properties and electric conductivity of the Cu alloys severely deformed by ARB were systematically investigated. It was found that fabricating UFG microstructures with grain size of approximately 200 nm can significantly increase the strength of bulky materials without a loss of their electric conductivity.
Acknowledgement
The authors would like to thank the financial support by the Grant-in-Aid for Scientific Research on the Priority Area “Giant Straining Process for Advanced Materials Containing Ultra-High Density Lattice Defects,” both through the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.
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