Spontaneous deformation during aging under stress in a copper-beryllium alloy
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INTRODUCTION
IT is known that Cu-Be alloys exhibit an excellent hardenability through aging after solutionizing. The aged Cu-Be alloys have the highest hardness, tensile strength, and proof stress among copper-based alloys and possess high electrical and thermal conductivities. Therefore, the Cu-Be alloys are widely used for electric connectors, mechanical parts, and injection molds for plastics. The authors[1] have found a newly revealed phenomenon of the Cu-Be alloy which is similar to the shape-memory effect with respect to the reversible shape change with a large deformation and its restoration. The large spontaneous deformation revealed in the CuBe alloy is caused by short-time aging under stress and by subsequent prolonged aging under a stress-free condition. Up to now, however, the relationship between the aging under stress and the spontaneous deformation behavior has not yet been clarified. The aim of this study is to investigate the effect of the initial stress and/or strain introduced prior to aging and the subsequent aging behavior on the spontaneous deformation, in connection with microstructural evolution. II. EXPERIMENTAL PROCEDURE Cold-rolled plates of a Cu-Be alloy (Japanese Industrial Standard No. C1720), which contain 1.9 mass pct beryllium, 0.25 mass pct cobalt, and other trump elements, were used in this investigation. Rectangular testpieces with a size of 80 mm in length, 10 mm in width, and 2.5 mm in thickness were cut out from the plates, as shown in Figure 1. The cutout testpieces were solutionized at 1073 K for 2 hours. Before the aging treatment, the testpiece was bent by H. ERA, Associate Professor, and K.KISHITAKE, Professor, are with the Department of Materials Science and Engineering, Kyushu Institute of Technology, Kitakyushu, 804-0015, Japan. K. NAITO, formerly Graduate Student, Department of Materials Science and Engineering, Kyushu Institute of Technology, is with Nippon Gaishi Co. Ltd., Handa, Aichi, 4750825, Japan. Manuscript submitted March 28, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS A
loading at the free end of the testpiece using a screw fabricated in a hand-made cantilever jig, as shown in Figure 1. The strains induced by the load were measured using strain gages attached at points A, B, and C. Figure 2 shows strains at points A, B, and C as a function of bending deflection. The strains at points B and C increase linearly with an increasing bending deflection and are rather small compared to the strain at point A. On the other hand, the strain at the neck of the cantilever (point A) increases with an increasing bending deflection, showing a larger tangent, then deviates from the linearity at Hb 5 4 mm, i.e., « 5 2 3 1023, which corresponds to the proof stress of the as-solutionized alloy. Therefore, when the strain at the neck of the cantilever, which is denoted «A , is less than 2 3 1023, the testpiece is under the elastic condition. The solutionized testpiece was aged at 523, 633, or 773 K, giving a constant bending deflection at the free end of the cantilever usin
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