| 其他摘要 | As one of the most important concerns of industrial components, the fatigue property is affected by various factors, such as the microstructure, mechanics and the environment. For the polycrystalline materials, grain boundaries (GBs) usually play an important role and affect its fatigue behaviors significantly. GBs could strengthen materials by blocking the motion of dislocations; but the stress concentration caused by the dislocations pile-up in the vicinity of GBs result in the initial fatigue crack easily. Twin boundary (TB), a special kind of coherent internal interface, can also strengthen materials by blocking the motion of dislocations in a manner similar to that of GBs. Our studies have indicated that a high density of nano-scale twin lamellae can provide a high strength without significantly compromising ductility in Cu samples, which is fundamentally different from that of GB strengthening. So far, most studies of the TB-related fatigue and cracking behaviors are concentrated on the twins with a thickness of few or tens micrometers. However, the information about the TB-related fatigue behavior in the nanometer scale is rare.
In this work, the high-purity Cu samples with high density of nano-scale coherent TBs were synthesized by means of the pulsed electro-deposition (PED). The fatigue endurance limit and the fatigue crack propagation characteristics of the nano-twinned Cu (nt-Cu) samples were studied as a function of the lamella thickness through stress-controlled fatigue tests. Possible mechanistic origins of fatigue damage and crack propagation of nt-Cu were explored. The main results are summarized as followed:
1. Pure Cu thin films with the similar grain size but different concentration of twin dneisties, i.e. different twin lamella thicknesses, were synthesized by the PED technique. The microstructures of the as-deposited Cu samples are uniform. The grains are roughly equiaxial with an average size of 500 nm and separated by clear grain boundaries. Inside each grain, there are a high density of internal coherent twin boundaries, whose thicknesses range from tens to a hundred nanometers. Statistics indicate that the average twin lamella thicknesses () for the two kinds of Cu samples are 85 and 32 nm, respectively. The microstructure of the as-deposited Cu samples indicates that the ultrafine-sized grains are subdivided further into the nano-scale twin/matrix/twin lamellar structure.
2. Tensile tests show that both the strength and the ductility of nt-Cu samples increase with decreasing the at room temperature. Cyclic tension—tension tests under constant stress amplitude control at room temperature indicate that when the decreases from 85 nm to 32 nm, both the total fatigue life and fatigue endurance limit increase. The beneficial effect of nano-twin structure on the fatigue behavior is contributed from the twin boundary blocking the motion of dislocations, which could strengthen materials and inhibit the crack initiation. In addition, the improvement in the plastic deformation capacity, resulted from the twin lamellae refinement, should also contribute to the extension of fatigue life by retarding the crack propagation.
3. Two different crack nucleation mechanisms were observed in the nano-twin Cu samples with the different twin lamella thicknesses. At large nmonly limited plastic deformation can be accommodated by TB-related dislocation activities, and the deformation is mainly assisted by the shear banding. Therefore a dominant shear band cracking was observed. For the Cu with a thin (32 nm), the high density of TBs can afford amount of rooms for the storage of dislocations and facilitate the plenty of plastic deformation. In this case, the cracking would be formed preferentially along TBs because of the stress concentrations.
4. The dependence of twin lamella thickness on the fatigue crack propagation rate was also explored on the nano-twin Cu samples. The fatigue crack growth rate is much lower in the nt-Cu samples with a thinner than that in the nt-Cu with a large . The decreased crack growth rate with decreasing the twin thickness is fundamentally different from the trend observed in the nanocrystalline metals, where grain refinement usually leads to an increase in the crack growth rate. The possible reason may originate from the 2-dimension feature of the TB. With decreasing the TB would be a strong barrier to the crack advancement in the direction vertical to TBs, which may change crack path frequently and reduce the effective driving force for the crack propagation. In addition, the closure effect due to crack face asperities can also contribute to the reduction of driving force for the crack propagation. |
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