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

The mechanical properties of nanocrystalline (nc) metals have been amain research topic in materials science communities, which involve quanti-ficational descriptions of macro-mechanical behavior and qualitative analysesof micro-deformation mechanisms of nc metals. However, due to synthesisproblems, bulk nc metals with high metallurgical qualities and truly nc struc-tures cannot be prepared so far. This leads to considerable limits to substan-tial understanding of plastic deformation mechanisms of nc metals. On theother hands, it has been suggested that grain boundary (GB) process and dis-location activities are the two main mechanisms controlling mechanicalproperties of nc metals. It is a center problem of plastic deformation mecha-nism investigations to reveal the effects of both mechanical and thermal rolescaused due to changes in grain size, temperature and stain rate.Based on the above two points, the following research work was con-ducted in this doctoral dissertation and the resultant conclusions are presentedas the following:1. A new electrical brush-plating technique for synthesizing bulk nc-Cuwith high metallurgical qualities and truly nc structure was developed.The stylus is made by a stainless steel (AISI304) wrapped by cottonand polypropylene fabric, the bath contents CuSO₄·5H₂O, NH₄NO₃,C6H8O7·H2O and a small amount of additives. During the brush-platingoperation, complex ions are reduced under ultra-voltageinto Cu deposition. The formation of nc-Cu deposition takes form of2D nucleation and then 3D growth. The deposition structure of nc-Cu isaffected by substrate surface state. A good substrate surface state canlead to a high quality of deposition structure. The use of additives cancontrol the grain size of nc-Cu deposition and restrain the formations ofcrystallite clusters and coarser grains. The friction between stylus andsubstrate increases nucleation rate and has a cleanse role. By the ap-propriate controls of the parameters such as voltage, temperature, stylusvelocity and bath flux, a bulk nc-Cu with high metallurgical quality canbe obtained by the brush-plating technique.2+[Cu(NH3)6]2. Surface morphologies, microstructure, compositions, and density of thebrush-plated nc-Cu were characterized by using XRD, SEM, FESEM,TEM, etc. It was demonstrated that the brush-plated nc-Cu has equi-axed grains separated by predominant high-angle GBs and no detect-able porosities or voids with the mean grain size of about 26nm and thecrystalline (root-mean-square) micro-strain of 0.28%. The brush-platednc-Cu has a smooth deposition surface, a good coverage, a density of8.93g/cm3 and a purity of 99.73wt%.3. The room temperature tensile creep test revealed a Coble creep at lowerstress level and a subsequent power-law creep with a stress exponent of=4.11 and both do not show the presence of the thresholdstress. The maximum stress for the Coble creep corresponds to the ten-sile yield strength at the lowest strain rate. The observed creep fractureat high creep stress (325MPa) is due to the high creep strain and theabsence of the strain hardening of the creep deformation. The roomtemperature tensile test revealed a pronounced strain rate sensitivity ofthe brush-plated nc-Cu with an =0.104, which is the highest m of allnc metals so far. In the strain rate tested, the ultimate tensile strengthn =1/mmincreases from 296MPa to 865MPa, the elongation to failure decreasefrom 5.6% to 3.4%. The lower tensile ductility of the brush-platednc-Cu is due to predominant high-angle GBs. The deformation mecha-nism analysis showed that the plastic deformation of the brush-platednc-Cu is controlled by the dominant GB sliding and also includes thecontribution of the dislocation interactions.4. The room temperature compressive creep test revealed a Coble creep atlower stress level and the subsequent two power-law creeps with thestress exponents of =4.41 and 11.24, respectively, with thelatter being consistent with the m of the first stage of the compressivedeformation. The three creeps do not show the presence of the thresh-old stress. The room temperature compressive test revealed also a pro-nounced strain rate sensitivity with an m=0.084 at low strain rates andan m=0.036 at high strain rates. The flow stress activation volumesn =1/mν ?of the two corresponding regions are 4.6b3 and 33b3, respectively.The strain rate jump test revealed a decrease of m from 0.18 to 0.018and an increase of ν ? from 4.6b3 to 33b3. In the strain rate tested, thestrength at 2% plastic strain increases from 664MPa to 1516MPa, re-spectively, with the latter is the highest strength value of all Cu so far.At high strain rates, a strain rate-dependent flow softening was ob-served. The deformation mechanism analysis showed that the plasticdeformation of the brush-plated nc-Cu is controlled by the dominantGB sliding at low strain rates and by the dominant dislocation activityat high strain rates. Such a transition in the deformation mechanismarises from the suppression of the strain rate on thermal activation role.The reduced contribution of the GB deformation and the increased con-tribution of the dislocation deformation lead to a large strength incre-ment. The local adiabatic thermal softening role is responsible for theenhanced flow softening at high strain rates.