Effects of Small Addition of Ti on Strength and Microstructure of a Cu-Ni-Si Alloy
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I.
INTRODUCTION
CU-NI-SI alloys with compositions corresponding to approximately Cu-2 mass pct(2 at. pct)Ni-0.5 mass pct(1 at. pct)Si exhibit a significant age-hardening effect.[1,2] Typically, such alloys are aged at about 723 K (450 C) after quenching from a given solution treatment temperature, and tensile strengths of 600 to 800 MPa are achieved, depending on the aging time and whether the alloy is cold worked before the aging treatment. The precipitates responsible for the strengthening effect have been identified as d-Ni2Si.[1,2] The Cu-Ni-Si system alloys are widely used in applications, such as semiconductor lead-frames, because of their high strength and good electrical conductivity. The requirement of high strength has become increasingly important with the automation of the semiconductor packaging process. One straightforward method of increasing the strength of Cu-Ni-Si alloys is to increase their Ni and Si contents. Several attempts have been made to improve the strength of Cu-Ni-Si alloys by increasing their Ni and Si contents and/or adding alloying elements such as Mn, Mg, and Sn.[1,3-5] However, these measures generally reduce the electrical conductivity of the alloys. Recently, Lee et al. employed thermodynamic calculations to demonstrate that the addition of small amount of Ti to Cu-Ni-Si alloys enhanced the precipitation of the d precipitates and considerably accelerated CHIHIRO WATANABE, Associate Professor, and RYOICHI MONZEN, Professor, are with the Division of Mechanical Science and Engineering, Kanazawa University, Kakuma-machi, Kanazawa, Ishikawa 920-1192, Japan. Contact e-mail: [email protected]. ac.jp SATOSHI TAKESHITA, Graduate Student, formerly with the Division of Mechanical Science and Engineering, Kanazawa University, is now with the Toyota Industries Corporation, 2-1 Toyodacho, Kariya, Aichi 448-8671, Japan. Manuscript submitted October 2, 2014. METALLURGICAL AND MATERIALS TRANSACTIONS A
the aging response.[6] According to their calculated Cu-Ni2Si quasi binary phase diagram, the addition of Ti is expected to improve both the strength and electrical conductivity of the alloy. However, the expected improvement in strength has not been experimentally achieved with the addition of 0.12Ti (all compositions in this paper are expressed in mass pct) to a Cu-3Ni-1Si alloy or 0.24Ti to a Cu-6Ni-1.29Si alloy because adding Ti to the alloys leads to the agglomeration of coarse d precipitates as well as the accelerated formation of discontinuous precipitation cells upon aging.[6] In this study, the effect of small addition of Ti on the strength of a Cu-2Ni-0.5Si alloy was investigated metallographically. We have found that the addition of 0.04Ti to the alloy enhances the strength of the alloy without decreasing its electrical conductivity. In addition, the origin of the improvement of strength is also discussed.
II.
EXPERIMENTAL
Ingots of Cu-2Ni-0.5Si, Cu-2Ni-0.5Si-0.04Ti, and Cu2Ni-0.5Si-0.2Ti alloys were prepared by melting under an argon atmosphere. The chemical compositions of the alloy
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