| Due to its high yield strength and fracture strength, high hardness, and excellent resistance to corruption, the research on the synthesis and properties of nanocrystalline (nc) materials has been a major topic all around the world. Lots of work has been done on the research of mechanical behavior of nc materials. However, because of its difficulties in synthesizing such materials, the truly nc materials with super metallurgical properties, large grain size and ideal micro-structure has not yet been synthesized, thus pose a limitation to our understanding of the nature of the plastic deformation of nc metal. It has been suggested that grain boundary (GB) process and dislocation activities are the two main mechanisms controlling mechanical properties of nc materials through both experiments and modeling simulation. It is an essential problem to investigate the effects of mechanical activation and thermal activation caused by the changes in grain size, temperature and strain rate on the two mechanism mentioned above.Our efforts have been focused on the issues mentioned above and relevant conclusions are presented as follow:1. A new electrical brush-plating technique for synthesizing bulk nc-Cu with high metallurgical qualities and truly nc structure was developed. The stylus is made of a stainless steel (AISI304) wrapped by cotton and polypropylene fabric, the bath contents CuSO4·5H2O, NH4NO3, C6H8O7·H2O and a small amount of additives. During the brush-plating operation, [Cu(NH 3)6]2+ complex ions are reduced under ultra-voltage into Cu deposition. The formation of nc-Cu deposition takes form of 2D nucleation and then 3D growth. The deposition structure of nc-Cu is affected by substrate surface state. A good substrate surface state can lead to a high quality of deposition structure. The additives can control the grain size of nc-Cu deposition and restrain the formations of crystallite clusters and coarse-grains. The friction between stylus and substrate increases nucleation rate and cleanses the surface. By appropriate controls of the parameters such as voltage, temperature, stylus velocity and bath flux, a bulk nc-Cu with high metallurgical quality can be obtained by the brush-plating technique.2. Surface morphologies, microstructure of the brush-plated nc-Cu were characterized by means of XRD, FESEM, TEM, etc. It was demonstrated that the brush-plated nc-Cu is characterized with equiaxed grains separated by predominant high-angle GBs and no detectable porosities or voids. The mean grain size of the brush-plated nc-Cu is about 24nm and the crystalline (root-mean-square) micro-strain is about 0.28%. The brush-plated nc-Cu has a smooth deposition surface. All these demonstrate that the brush-plated nc-Cu is a truly nc material.3. The nanoindentation results revealed that, the indentation load required to impose a displacement of 2000nm increases from 191.08mN to 237.31mN and the hardness increases from 1.80 GPa to 2.545 GPa when the strain rate increases from 2.5×10-3 s-1 to 5.0×10-2 s-1. The load-displacement curves of the brush-plated nc-Cu show a distinct and experimental detectable effect of indentaion strain rates. The calculated m value is 0.11, which is the highest m of Cu obtained through nanoindentation, the corresponding flow stress activation volume is 5.33b3, which is the smallest activation volume of Cu obtained through nanoindentation. The load-displacement curves show a unique phenomenon at high strain rate conditions, during the holding period of 10 seconds at the maximum load, the load remains constant while the displacement keeps increasing until unloading process begins, we call it"plateau", the width of the plateau increases from 33nm to 121nm when the strain rate increases from 2.5×10-3 s-1 to 5.0×10-2 s-1.4. The room temperature compressive test revealed that, in the strain rate tested, the strength at 2% plastic strain increases from 664MPa to 1516MPa, respectively, with the latter is the highest strength value of all Cu so far. A pronounced strain rate sensitivity with an m=0.084 at low strain rates (lower than 3×10?2s?1) and an m=0.036 at high strain rates (larger than 3×10?2s?1) has also been observed. The flow stress activation volumesν?of the two corresponding strain rate regions are 6.3b3 and 9.25b3, respectively. At high strain rates, a strain rate-dependent flow softening was observed, local adiabatic thermal softening role is supposed to be responsible for this phenomenon.5. According to our analysis, the thermally activated GB sliding at lower strain rates and the reduction in its contribution with an increase in strain rate should be responsible for the significant change inν? or m with strain rate observed for our brush-plated nc-Cu. As strain rate increases, the GB sliding would become more and more difficult. A higher applied stress is thus needed for GB diffusion to proceed until a much higher strain rate at which the dislocation activities can be effectively activated at a much higher stress level. The relatively increasedν? at high strain rate arises from the increased length scale of emit and propagating dislocations as compared with that of GB atomic activities. This transition in plastic deformation mechanism with strain rate or the changes in m andν? values is a result of the competition between the above two mechanisms.
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