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.

Introduction

In recent decades, much research has been done on the mechanical behaviour and the substructural changes in metal polycrystals (especially copper, a typical material, which shows wavy slip characteristics) strained under plastic deformation with strain path change [1], [2], [3], [4], [5], [6]. It was found that the mechanical behaviour during subsequent loading appears to be only slightly affected by the type of initial loading mode. It is mainly dependent on the magnitude of the strain path change, for example, a parameter α, defined by the cosine of the angles between the two vectors that represent the successive strain tensors, has been proposed [7]. In most cases, the yield stress upon reloading (back extrapolated stress) is larger than the stress reached at a given equivalent strain for the same material deformed along the same load path without preloading. The subsequent strain hardening exhibits a transient stage with lower values just after the reloading yield stress.

Microscopic substructures developed during sequences of double loading are not only affected by the sequential strain mode and the magnitude of the strain path change, but are also affected by the grain sizes [2]. In large grain size (for example 250 μm) specimens, microbands appear inside many grains just after yielding, these microbands are aligned with the trace of {111} planes, corresponding to the principal active slip planes in tension. In smaller grain size specimens (20 μm, for example), by contrast, no trace of localised deformations was noted, the dislocation structure evolves in a more or less continuous manner by a partial dissolution of the prestrain substructure. The different microstructural behaviours can be interpreted on the basis of strain accommodation principles. In large grain size specimens, the strain accommodation between adjacent grains is constricted in the vicinity of grain boundaries and grain boundary triple junctions. Thus, most of the volume of grain behaves like a single crystal, which is the reason why the strain distribution is not homogenous at the level of the grain size. Smaller grains are mainly influenced by their surroundings, three or more non-coplanar systems can be equivalently activated, so that their behaviour agrees with the Taylor approach to multiple slip [8], a more homogenous intragranular deformation is observed in this case.

The mechanical behaviour and the microstructural development of cyclically deformed copper polycrystals have also been studied in considerable detail and many results have been obtained [9], [10], [11], [12]. The Taylor and Sachs factors, as well as the exponent law can be applied for the cyclic stress strain (CSS) curves depending on the loading condition as well as on the grain size. No plateau region occurred, which is a common phenomenon in the CSS response of single crystals oriented for single slip and some special double and multiple slip [13]. The dislocation patterns are mainly cell and parallel wall structures, due to the activation of multiple slip systems to meet the need for strain compatibilities in the vicinity of grain boundaries. Persistent Slip Bands (PSBs), which prevail in cyclically deformed monocrystals, can also be found in some grains because of the different strain values accommodated by different grains. Nevertheless, the PSB ladder structure does not lead to the occurrence of a plateau region in the CSS curves, as in single crystals, in which PSBs and the presence of a plateau have a one-to-one correlation as indicated in Winter's double phase model [14].

While it is easy to imagine that sequential loading interfered with cyclic loading, which is highly relevant to technical application, as far as the authors know, until now, little work has been done on polycrystalline materials deformed under strain path change, with fatigue as one of the loading modes [15]. The loading modes for strain path changes are mainly rolling, shearing and tension. This paper concentrate on studying the macroscopic properties and the microstructural development of copper polycrystals with a relatively small grain size (32 μm) deformed in sequential strain paths of fatigue–tension and tension–fatigue. The following will be emphasised in the paper, the effects of preloading in tension on the initial cyclic hardening behaviours and CSS response of subsequent fatigue; the effects of preloading in fatigue on the yield stress and flow behaviour of subsequent tension; the evolution of the dislocation microstructures after reloading.