Effect of Solute Clusters on Stress Relaxation Behavior in Cu-Ni-P Alloys
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INTRODUCTION
COPPER alloy sheets are widely used for electrical terminals in automotive applications. Terminals are manufactured by pressing out copper sheets of the correct shape and then bending them into box shapes. One of the most important performance requirements of the terminals is that the contact pressure at the point of spring contact be maintained over long periods. Stress relaxation leads to a decrease in the spring contact pressure with increasing time. The main accelerating factor for the stress relaxation is temperature, making it a particularly important material property for connectors used in high-temperature environments, such as inside the engine space in automotive applications. It is well known that the stress relaxation resistance of copper alloys can be improved markedly by lowtemperature annealing below the recrystallization temperature after cold rolling.[1] Stress relaxation behavior is also seen to vary significantly with slight changes in the alloy composition. Recently, Nishijima and his co-workers[2] have reported that copper with both Ni and P additions has an improved stress relaxation performance over Cu-Ni and Cu-P alloys. In addition, they have shown that the low-temperature annealing of YASUHIRO ARUGA, Researcher, formerly with the Department of Materials, University of Oxford, Oxford, OX1 3PH, United Kingdom, is with the Materials Research Laboratory, Kobe Steel, Ltd., Kobe 651-2271, Japan. Contact e-mail: aruga.yasuhiro@kobelco. com DAVID W. SAXEY and EMMANUELLE A. MARQUIS, Postdoctoral Researchers, and ALFRED CEREZO and GEORGE D.W. SMITH, Professors, are with the Department of Materials, University of Oxford. HISAO SHISHIDO, Researcher, is with the Materials Research Laboratory, Kobe Steel, Ltd. YUYA SUMINO, Researcher, is with the Copper Sheet Plant CHOFU Works, Aluminum & Copper Company, Kobe Steel, Ltd., Shimonoseki 752-0953, Japan. Manuscript submitted April 16, 2009. Article published online September 24, 2009 2888—VOLUME 40A, DECEMBER 2009
Cu-1.4Ni-0.07P (wt pct) at 300 C for 30 seconds improves the stress relaxation resistance remarkably. Below 500 C, the intermetallic compound consisting of Ni and P is more favorable than copper phosphide.[3] Some researchers have already confirmed the presence of equilibrium nickel phosphides, formed after annealing at 450 C[3,4] or 400 C[5] for 12 hours or more, using transmission electron microscopy (TEM) and selected area electron diffraction pattern analysis. However, no one has yet been able to detect a change in microstructure after annealing at lower temperatures, such as at 300 C for less than 1 minute. Consequently, there has been no clear description of the microstructural effects associated with these changes in the stress relaxation behavior. In the present work, the ultrafine structures in Cu-P and Cu-Ni-P alloys are observed by means of TEM and a three-dimensional atom probe (3DAP). The mechanism of stress relaxation behavior in these alloys in relation to the dislocation density and solute clusters is discussed.
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