Study On Mechanical Behavior And Relevant Mechanisms Of Nano-crystalline Cu, Ni And Ni-Fe Alloy

When conventional coarse-grained polycrystalline metals deform plastically, dislocation nets produced by dislocation interactions on cross-slip planes would limit motion of dislocations and result in strain hardening. Upon unloading, dislocation structures remain stable, thus coarse-grained metals exhibit uauslly elastic unloading behavior. However, for nanocrystalline metals, dislocation sliding occurs on limited slip plans and does not form dislocation nets, dislocatons are in high stress state. Upon holding or unloading, dislocation activity does not cease and thus produce significant holding or unloading plastic deformation effect. Due to different initial deformed structures and stress state, dislocation activity and grain boundary deformation process of nanocrystalline metals during holding or unloading would differ considerably that occurred in loading stage. Also, due to the these highly mobile dislocation activity and grain boudary-mediated process, the nanocrystalline metals often exhibit high strength and low elongation. How to enhance the elongation of nanocrystalline metals has been a difficult issue in the front of the scientists. In the coarse-grained polycrystalline, the extremely high ductility arises from the interactions of dislocations on adequate slipping systems or planes. It is naturally anticipated that the nano-structured metals can obtain high elongation without sacrificing high strength with increasing the grain size appropriately. Additionally, the previous studies showed that the elongation of ultrafine-grained/nanocrystalline metals have depended strongly on the variation of the strain rate. Indeed, we really hope that ultrafine-grained/nanocrystalline metals can obtain more stable elongation, i.e., having no sensitive with the strain rate. Study on the elongation stability of the ultrafine-grained and nanocrystalline metals could help us comprehend their fracture process. Studying plastic behavior of nanocrystalline metals and relevant dislocation and grain boundery mechanisms can enhance our understanding for plastic deformation nature and also provide the basis for reasonable application of nanocrystalline metals.In this paper, the series compression cycling tests, tensile test, nanoindentation tests will be performed to examine the unloading plastic deformation behaviors of nanocrystalline Cu and their relationship to deformation rate and loading/unloading mode, tensile strenght and elongation of ultrafine-grained and nanocrystalline Cu and their relationship to loading strain rates, nanoindentation creep behavior of nanocrystalline Cu, Ni and Ni-Fe alloy and their relationship to loading strain rate and stacking fault energy, and nanoindentation creep behavior of nanocrystalline Cu in the incremental unloading-holding test. The results of the paper are shown as following:1. The experimental finding of significant plastic deformation that emerges in the unloading regime in the compressive cyclic test of 25 nm nanocrystalline Cu at room temperature have been studied. The magnitude of plastic strain produced during unloading depends strongly on loading and unloading rates. This plastic unloading behavior arises from the rapid absorption of dislocations accumulated during loading, which was quantitatively interpreted by performing the incremental unloading tests and developing a relationship between the dislocation density and the loading and unloading rates based on the models of the statistical absorption of dislocations by grain boundaries and the dislocation emission from grain boundary ledges. Concurrently, the evolution of deformation structures during the cyclic deformation was also analyzed in terms of the interactions of gliding dislocation-twin boundaries.2. Bulk ultrafine-grained and nanocrystalline Cu were synthesized by the electric brush-plating technique using bath without any organic additive. The tensile tests results of the 59 nm Cu and 110 nm Cu at room temperature compared with the previous result, i.e., the tensile tests of 200 nm Cu have been studied. The tensile strength and elongation of ultrafine-grained and nanocrystalline Cu depend strongly on grain sizes and loading strain rates. The deformation mechanism of these three ultrafine-grained and nanocrystalline Cu are dislocation motion at high strain rates and are grain boudaries-mediated process with the dislocation activity at low strain rates, which induce much higher strength with increasing the loading strain rates. For 59 nm Cu, the stress concentration and shear bands formation at high strain rate and stress relaxtion at low strain rate could induce the elongation has a decreaing tendency with increasing strain rate. For 110 nm Cu, the combined effect of the enhanced deformation compatibility at high strain rates and shear stress generated by grain boundaries at low strain rates could lead to the almostly unchanged elongation with the variation of the strain rates. For 200 nm Cu, the above effect will be much strong that results in the increasing elongation with increasing the strain rates.3. Creep behaviors of nanocrystalline Cu, Ni-20(wt.%)Fe and Ni with comparable grain sizes examined by nanoindentation tests have been studied. It was showed that the creep behaviors depend on loading strain rate and stacking fault energy. The very high initial creep rates of three nanocrystalline materials are attributed to the rapid absorptions of the stored dislocations in the loading regime and the newly nucleated dislocations in the holding regime. With increasing holding time, the dominate creep deformation mechanism will be the grain boudaries sliding or diffusion. The higher creep strain rates at high loading strain rate are attributed to the more stored disloaction in the loading regime after dislocations emission from grain boundaries. Besides that, the nanocrystalline with low stacking fault energy could cause the higher density of the stored dislocations via the enhanced interactions of dislocations with twins and stacking faults.4. Creep behaviors of nanocrystalline Cu in the incremental unloading-holding test examined by nanoindentation tests have been studied. It is showed that creep behaviors depends on the applied stress and internal microstructure evolution. As the effective stress is high enough, the higher creep strain rate are attributed to the rapid absorptions of the stored dislocations in the loading regime and creep deformation mechansim is the disloaction motion. With increasing holding time, the dominate creep deformation mechanism will be the grain boudaries sliding or diffusion; As the effective stress is small or almost close to the zero value, a competition between the disloaction recovery motion and grain boudaries-mediated process will occur. As the internal stress is dominated stress, the residual disloactions have reverse propagtion toward the originl grain boudaries inducing the negative creep behavior.