Fatigue of Cu-1 Pct Cd
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I.
INTRODUCTION
THE alloy
Cu-I pct has been used for many years in applications in which high electrical conductivity has to be combined with increased mechanical strength. The efficacy of the 1 wt pct Cd addition is due to the relatively low valency of Cd combined with a large effect of Cd on the lattice parameter of Cu, which gives strong dislocation pinning without greatly increasing the resistivity. 1The result of strong pinning is that Cu-1 pet Cd shows a sharp yield point, strain aging, and high work hardening at large strains. 2.3 These features of the tensile behavior are probably related to the good creep and fatigue properties shown by the alloy) It is thus of interest to examine further the fatigue properties of this alloy. One notable feature of the present results is that fatigue failure is almost entirely intergranular at low stresses and at long fatigue lives. Apart from this, the superior fatigue properties of the alloy, relative to those of Cu, are due to the strong interaction between Cd atoms and dislocations.
II.
then annealed in argon. The annealed specimens were finally electropolished in orthophosphoric acid solution for three minutes, care being taken to prevent etching. Specimens of different grain sizes were obtained by annealing at temperatures between 460 and 900 ~ The average grain diameter was measured by the linear intercept method, counting at least 1000 grains and counting twin boundaries as grain boundaries. The properties of the annealed material are shown in Table I. The development of slip bands and cracks on the surface was studied in the scanning electron microscope after various stages in selected fatigue tests. To observe persistent slip bands (PSBs) the specimens were first electropolished and then loaded at the designated fatigue load for a small number of cycles. The surface strain markings produced by the initial monotonic loading were then removed by electropolishing. PSBs were then developed by subsequent fatigue cycling and examined periodically after accumulating fixed number of cycles up to the fatigue life. Subsurface observations of the fatigue cracks were made after final fracture by longitudinal taper sectioning of nickel-plated specimens.
EXPERIMENTAL M E T H O D
All the fatigue tests were carried out at room temperatures on specimens prepared from commercial grade Cu-1 pct Cd and on high conductivity Cu in the form of rod and thin wire specimens. Wire specimens were tested only in the cold worked condition. The fatigue tests on the rod were done in rotating bend at 100 Hz using small waisted specimens, maximum diameter 6.4 mm, minimum 4 mm, under constant load conditions. The fatigue tests on wires were done using fixed pure bending in conjunction with rotation. The wire testing machine was constructed along the lines of the apparatus described by Geminov and Kopyev. 5 The material in rod form (composition 0.98 pct Cd, 0.005 pet Sn, 0.002 pct Zn) was obtained from commercial hot rolled 12.7 mm bar by cold drawing. The fatigue specimens were mechanically
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