Mechanical behaviour and the evolution of the dislocation structure of copper polycrystal deformed under fatigue–tension and tension–fatigue sequential strain paths

Abstract

Two sequences of tension–fatigue and fatigue–tension tests were performed on copper polycrystal sheet, with a mean grain size of 32 μm. For the angle between the two successive loading directions, two typical values (0 and 45°) have been chosen. The effect of strain path change on subsequent initial work hardening rate and saturation stress in tension–fatigue, as well as the effect of strain path change on subsequent yield and flow behaviour in fatigue–tension have been investigated. The strain rate for the tension tests was 5×10 −3 s −1, while the fatigue tests were performed under constant plastic strain amplitude control with different values of amplitudes ( ε pl=6×10 −4, 1.5×10 −3, 3×10 −3). Slip morphology and dislocation microstructure were observed by optical and transmission electron microscopy (TEM) after mechanical tests. Under these conditions, in the case of fatigue–tension, it was found that fatigues prestraining influences the subsequent yield and flow behaviour in tension. However, the subsequent mechanical behaviour of samples seems only to be affected by the magnitude of strain path change (namely, the angle between the two successive loading directions), and not by the value of the plastic strain amplitude of the preceding fatigue tests. In the case of tension–fatigue, the strain amount of preloading in tension obviously affects the initial cyclic hardening rate, while it has almost no effect on the saturation stress of subsequent fatigue tests, irrespective of the value of the angle between the two successive loading directions. The occurrence of microbands in the saturation fatigue dislocation structures of samples prestrained in tension implies that fatigue is a more effective loading mode than tension, in causing intense glide on the activated slip systems. The correlation between mechanical properties and microstructural observations is discussed.