Effects Of Stress Amplitude And Temperature On Fatigue And Damage Behaviour Of Ultrafine-grained Copper

In recent years, there has been a growing interest in ultrafine-grained (UFG) materials having some unique mechanical properties, which are produced by severe plastic deformation (SPD), especially by equal channel angular (ECA) pressing. However, knowledge of high-and low-cycle fatigue deformation mechanisms of such UFG materials, as well as of their high-temperature fatigue behaviour, still remains incomplete and less. These studies are actually of practical significance for their further development and engineering application. In the present work, UFG copper produced by ECAP was adopted as the target materials to examine the effects of stress amplitude and testing temperature on their macroscopic cyclic deformation response behaviour, surface deformation and damage features as well as the corresponding microstructural changes. At a constant temperature of 25℃, the applied stress amplitudes were selected within the range of 100 MPa to 200 MPa, to ensure that fatigue tests are performed within both low-cycle fatigue (LCF) and high-cycle fatigue (HCF) regimes. Under a constant stress amplitude of 200 MPa, the testing temperatures were selected within the range of 25℃to 300℃, according to the relevant recrystallization temperature (i.e.215℃), to understand their cyclic instabilities at temperatures, which are around the recrystallization temperature.It is found that the fatigue and damage behaviours of UFG copper are strongly dependent upon the applied stress amplitude and testing temperature. The details are presented as follows.At different stress amplitudes (i.e., HCF regime, LCF regime and in-between regime), cyclic softening occurs in each case, and this phenomenon takes place more significantly with increasing stress amplitude. On cyclic stressing in these three regimes, the plastic component of the total strain amplitude would, to different extents, contribute to the fatigue deformation, leading to the following markedly distinctive features:(1) In the HCF regime, the macroscopic shear bands (SBs) consisting of many short single shear bands form on the surface, and cracks form separately along the single shear bands. The material finally fractures in the form of a cleavage brittle mode. The corresponding microstructural changes are embodied by grain coarsening, and no typical dislocation arrangements are visible in coarsened grains, except for some single dislocations or dislocation tangles; (2) In the in-between regime, the dense SB clusters locally form on the surface, and many micro-cracks form collectively within SB clusters. Some dimples with very small sizes form at the cleavage steps on the fracture surface, exhibiting a micro-plasticity. Remarkable grain coarsening takes place, and some developing dislocation arrangements, such as thin walls and loose cell structures, form in a few of coarsened grains. (3) In the LCF regime, the surface is fully occupied by the large-scale SBs, and cracks and voids form along them. A more ductile mixed-mode fracture surface featuring dimples and cleavage planes is found. Different dislocation structures, such as dislocation walls, cells and PSB ladder-like walls can be found to form in many coarsened grains.At all temperatures within the range of 25℃to 300℃, cyclic softening occurs in each case, and this phenomenon takes place even earlier and more significantly with increasing temperature. An exponential decay of fatigue life with temperature is found. Large-scale SBs formed at room temperature, whereas finer and discontinuous SBs are found to become the dominant feature with increasing temperature, resulting from the reduction in quantity and volume fraction of grain boundaries (GBs). As the temperature is above recrystallization, SBs disappear almost completely, and dislocation slip deformation within grains becomes the major plastic deformation mode, causing the nucleation of cracks along slip bands in grains or along GBs, in contrast to the nucleation along SBs at temperatures below recrystallization. The fracture surface shows an enhanced plasticity with increasing temperature, i.e., changes from mixed fracture of dimples and cleavage planes at room temperature to a pure dimple fracture at 300℃. In addition, the increase in temperature gives rise to the general change of microstructures from dislocation walls to well-defined cells. A certain quantity of annealing twins emerge in the final microstructures of fatigued UFG copper at comparatively high temperatures, and the quantity of twins would increase with increasing temperature.