Synthesis And Compressive Properties Of Bulk Nanocrystalline Cu By Electrical Brush-plating

The mechanical properties of nanocrystalline (nc) metals have been a main research topic in materials science communities, which involve quantificational descriptions of macro-mechanical behavior and qualitative analyses of micro-deformation mechanisms of nc metals. However, due to synthesis problems, bulk nc metals with high metallurgical qualities and truly nc structures cannot be prepared so far. This leads to considerable limits to substantial understanding of plastic deformation mechanisms of nc metals. On the other hands, it has been suggested that grain boundary (GB) process and dislocation activities are the two main mechanisms controlling mechanical properties of nc metals. It is a center problem of plastic deformation mechanism investigations to reveal the effects of both mechanical and thermal roles caused due to changes in grain size, temperature and stain rate.Based on the above two points, the following research work was conducted in this doctoral dissertation and the resultant conclusions are presented as the following: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 by 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(NH₃)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 use of additives can control the grain size of nc-Cu deposition and restrain the formations of crystallite clusters and coarser grains. The friction between stylus and substrate increases nucleation rate and has a cleanse role. By the 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, compositions, and density of the brush-plated nc-Cu were characterized by using XRD, SEM, FESEM, TEM, etc. It was demonstrated that the brush-plated nc-Cu has equiaxed grains separated by predominant high-angle GBs and no detectable porosities or voids with the mean grain size of about 26nm and the crystalline (root-mean-square) micro-strain of 0.28%. The brush-plated nc-Cu has a smooth deposition surface, a good coverage.3. The room temperature compressive creep test revealed a Coble creep at lower stress level and the subsequent two power-law creeps with the stress exponents of n =1/m=4.41 and 11.24, respectively, with the latter being consistent with the m of the first stage of the compressive deformation. The three creeps do not show the presence of the threshold stress. The room temperature compressive test revealed also a pronounced strain rate sensitivity with an m=0.084 at low strain rates and an m=0.036 at high strain rates. respectively. The strain rate jump test revealed a decrease of m from 0.18 to 0.018 .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. At high strain rates, a strain rate-dependent flow softening was observed. The deformation mechanism analysis showed that the plastic deformation of the brush-plated nc-Cu is controlled by the dominant GB sliding at low strain rates and by the dominant dislocation activity at high strain rates. Such a transition in the deformation mechanism arises from the suppression of the strain rate on thermal activation role. The reduced contribution of the GB deformation and the increased contribution of the dislocation deformation lead to a large strength increment. The local adiabatic thermal softening role is responsible for the enhanced flow softening at high strain rates.