| Interests of scientists in the world on the nanocrystalline materials are focused on im-proving and optimizing their mechanical properties and revealing the strain rate sensitivity and mechanical fracture mechanism of these materials. As the grain size is decreased, an in-creasing fraction of atoms can be ascribed to the grain boundaries. Nanocrystalline materials exhibit various unique chemical, physical or mechanical properties such as increased strength/hardness, reduced elastic modulus and ductility in comparison with conventional polycrystalline materials. For the technical problems of materials preparing, nano-metal with high metallurgical quality, large-size and ideal micro-structure can not be prepared, which limit the understanding to the nature of plastic deformation of nano-metal. On the other hand, studies of experimental work, theoretical and simulation have shown the grain boundary processing and dislocation activities are two important mechanisms for controlling the me-chanical properties of nano-metals. To explore effect of mechanical activation and thermal activation caused by changing grain size, strain rate and other factors to these two mecha-nisms is the substance of the deformation mechanisms of nano-metallic. Therefore, it is es-sential to fabricate high-quality bulk nano-materials and their alloys using a new type of preparation process and undertake extensive studies on their microstructures and mechanical properties.It was found that the irrationality process equipment, process recipes, process conditions and other factors seriously affected the preparation of nano-metals and alloy microstructure of nano-metals and alloys, which result a lot of difficulties to the study of properties and de-formation mechanisms of nano-mechanical. In this paper, a new electrodeposition technique used to make nanocrystalline materials was developed by changing and optimizing techno-logical conditions and technological parameters based on conventional electrodeposition technique, and the electrodeposited equipment is produced by us. A bull nanocrystalline Ni-Fe alloys and nanocrystalline Cu were prepared by this electrodeposition technique. Mi-crostructure and mechanical performance of these nanocrystalline materials were observed and analyzed by SEM, TEM, XRD and MTS.Based on the above points, the following research work was conducted in this disserta-tion and the conclusions are presented as the following:1) High-quality nanocrystalline Ni-Fe alloy was successfully fabricated through devel-oping composition of bath, adjustment technological conditions and technological parame-ters of electrodeposition, using the self-made electrodeposition equipment with cycle of fil-tering, mixing, temperature control and cathodic mobile. And a detailed discussion about the effect of the bath composition, pH, bath temperature, cathode current density, additives con-tent, and add cycle to the surface of micro-morphology, Vickers microhardness and micro-structure of nanocrystalline Ni-Fe alloy was given. The main ingredients were NiSO4.6H2O,FeSO4.7H2O,H3BO3,NaCl,Na3C6H5O7·2H2O,wetting agent,brightener A and brightener B. To corresponding, the optimum conditions were given as the following: Cathode current density is 5-7A/dm2; value of pH is 3.4-3.6; temperature is 60-62℃, cycle of filtration and speed of cathode mobile is 18 times / minute, journey is 120mm. Nanocrystalline Ni-Fe alloys synthesized using the above-mentioned ingredients and process conditions have uni-form composition, dense crystallization, shiny surface, no nodulation and pock marking, the highest microhardness could be more than 600HV. Analyzing the reaction mechanism of electroplating deposition and process of deposition, we can see that electrodeposition of nanocrystalline Ni-Fe alloys belong to an unusual co-deposition theory.2) The components ,structure and microstructure of this nanocrystalline Ni-Fe alloys can be observed and analyzed by EDS, XRD, SEM,TEM and AFM. It was demonstrated that the percents of Fe is 20.78wt.% in this Ni-Fe alloy ,there is no detectable porosities or voids in this material, the grain sizes of this material are fine, the orientation of the crystal-lography is uniform. To statistic about hundreds of grain size, it can be obtained that the range of gain size is 10-100nm with the average grain size of about 28±5nm. Root-mean-square roughness of nanocrystalline Ni-Fe alloys has no brightener is 5.519nm, an average roughness is 4.417nm. While root-mean-square roughness of the one added brightener is 4.333nm, an average roughness is 3.392nm.3) The effect of the cathodic current density to the structure, microstructure and room temperature tensile mechanical properties of nanocrystalline Ni-Fe alloys (component is Ni-20wt%Fe) was studied. The results shown that high-quality electrodeposition nanocrys-talline Ni-Fe alloys isγ-phase solid solution with face-centered cubic structure, No other phase exist in the XRD spectra of the samples synthesized under different current density. The grain sizes of nanocrystalline Ni-Fe alloys are fine, the orientation of the crystallography is uniform, there is no detectable porosities or voids in this material. When the current den-sity for the preparation of nanocrystalline Ni-Fe alloys is 7A/dm2, the average grain size is about 22nm, the maximum tensile strength is 1922Mpa, the largest fracture strain is 10.8%. When the current density for the preparation of nanocrystalline Ni-Fe alloys is 3A/dm2, the average grain size is about 33nm, the maximum tensile strength is 1792Mpa, the largest fracture strain is 8.5%. Observing the fracture surface, under different current density it was found that nanocrystalline Ni-Fe alloys showed the fracture characteristics of ductile fracture, at the side of the largest fracture strain, necking phenomenon and shear zones can be clearly observed.4) Comparing tensile mechanical properties and fracture behavior of nanocrystalline Ni-Fe alloys (Ni-20wt% Fe) and nanocrystalline Ni with the similar average grain size, dur-ing the strain rate increases from 1×10-5s-1 to 1s-1, the result showed that the 0.2 percent yield strength of nanocrystalline Ni-Fe alloys increases from 1134MPa to 1368MPa, the ul-timate tensile strength increases from 1762MPa to 1939MPa, the changing range of breaking strain is 8.5% ~ 9.3% , the changing range of homogeneous strain is 6.3% to 7%, increasing basically with the strain rate increasing, but the fracture strain and the homogeneous strain have little change. At the same range of strain rate (1×10-5s-1~1s-1), with increasing of strain rate, the ultimate tensile strength of nanocrystalline Ni have an increase trend, but the frac-ture strain and the homogeneous strain decrease rapidly. When the plastic strain (εP) is 0.2%, the strain rate sensitivity index (m) of the nanocrystalline Ni-Fe alloys m = 0.017, for nanocrystalline Ni, m = 0.031. When the plastic strain (εP) is 2.0%, the strain rate sensitivity index (m) of the nanocrystalline Ni-Fe alloys m = 0.010, for nanocrystalline Ni, m = 0.024. To the plastic strain of 0.2% and 2.0%, the nanocrystalline Ni-Fe alloys showed smaller ten-sile strain rate sensitivity than the nanocrystalline Ni. Comparing fracture surface morphol-ogy of nanocrystalline Ni-Fe alloys and nanocrystalline Ni, it was found that there are micr-hunches of tens to hundreds of nanometer on the surface of nanocrystalline Ni-Fe al-loys. At the department of maximum strain, it was found a large number of weakening shear zone and the existence of obvious necking. These result showed that the addition of Fe atoms has changed the original crystal structure of Ni, leading to dislocation activity, atomic diffu-sion and grain boundary slipping may be the main mechanism for the facture. which could stabilize the ability of continuous deformation result in relatively good and almost invariable ductility.5) Systematic cycling and the incremental unloading test was carried out on pulse elec-trical brush-plating nanocrystalline Cu to analysis dislocation emission-absorption process in complexity path. Little inertia strain(Δεip)indicated grain boundary sliding play a major accommodate role on the plastic deformation during loading at the low strain rates. At high strain rates, with the reduction of unloading stress rate, the significant increment of inertial strain (Δεip) confirm existence of thermal activation in the process of dislocations migration. The results showed that high-energy dislocation depinning is the main control mechanism in nc Cu, the strengthening of nc metals arises from the long-range stress field built-up due to the increased difficulty for the disloca-tion depinning in addition to the high internal stress built-up during loading.
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