Study On Processing Parameters And Tensile Properties Of Electrodeposited Nanocrystalline Copper

The landmark paper by Gleiter redirected a significant portion of the global research efforts in materials science. Nanocrystalline materials have been the subject of widespread over the past couple of decades with significant advancement in their understanding especially in the last few years. As the grain size is decreased, an increasing fraction of atoms can be ascribed to the grain boundaries. Nanocrystalline materials may exhibit increased strength/hardness, improved toughness, reduced elastic modulus and ductility, enhanced diffusivity, higher specific heat, enhanced thermal expansion coefficient, and superior soft magnetic properties in comparison with conventional polycrystalline materials. Nanostructured materials provide us not only with an excellent opportunity to study the nature of solid interfaces and to extend our understanding of the structure-property relationship in solid materials down to the nanometer regime, but also present an attractive potential for technological applications with their novel properties.The electrodeposition technique has significant advantages over other methods for synthesizing nanocrystalline materials: (1) potential of synthesizing large variety of nanograin materials-pure metals, alloys and composite systems with grain sizes as small as 20 nm, (2) low investment, (3) high production rates, (4) few size and shape limitations, and (5) high probability of transferring this technology to existing electroplating and electroforming industries. The technique can yield porosity-free finished products that do not require subsequent consolidation processing. I did some research and got results as follow.1. In the experiment of direct current electrodeposition, effects of pH value of the electrolytes on electrodeposited nanocrystalline copper were investigated. Results show that with the increase of pH value of the electrolytes, the electrodepositing speed rate decreases and microstructure of the electrodeposits changes. When pH value equals to 8.0, the electrodeposited Cu has distinct textures in (111) direction and the grain size is over 100nm. As pH value ascends, prior growing facet in (220) direction of the as-deposited Cu increases and the grain size descends. When pH value equals to 9.0, the average grain size of the nanocrystalline Cu is 48 nm. It can be found that the optimal region of the pH value is from 8.8 to 9.0 in this experiment.2. Effects of the addition of SeO2 on the surface, microstructure and microhardness of direct-current-electrodeposited Cu were investigated by means of XRD and SEM et al. Results indicate that the surface of the as-electrodeposited Cu becomes smooth and compact, and the prior growing degree of the (111) facet of nanocrystalline Cu increases with 0.02g/L of the additive in the electrolyte. Meanwhile, the grain size of nanocrystalline Cu decreases to 28nm, and the microhardness increases to HV277, which is about 5 times higher than that of the coarse-grained copper.3. Influence of current density on the electrodepositing speed, current efficiency, microstructure and microhardness of nanocrystalline Cu deposition were investigated by means of SEM, TEM and XRD et al. Results indicate that the electrodepositing speed increases remarkably and the current efficiency decreases by the increment of current density. As the current density is up to 3.2A/ dm2, porosity-free deposited Cu with a mirror surface can be obtained, which has a strong texture on (111) growth orientation. Meanwhile, the average grain size of as-deposited Cu decreases to 24nm, and the microhardness increases to HV297, which is about 5 times higher than that of the coarse-grained Cu.4. Fully dense nanocrystalline Cu with an average grain size of 56 nm was synthesized by a direct-current electrodeposition technique. Tensile tests performed at room temperature indicated that both the strength and the ductility of the nc Cu increased by the increment of the strain rates, especially a pronounced strain rate dependence of tensile ductility was observed. As the strain rate was raised from 1.04×10-5 to 1.04 s-1, the fracture strain increased from 24.1% to 42.8%, with the ultimate tensile strength increased from 255 MPa to 343 MPa. This phenomenon might be attributed to two sides. First, the strain hardening behavior increased with increasing strain rate, resulting in an enhanced uniform elongation. Second, the collective grain-rotations were revealed when the nc Cu necked at higher strain rate, which contributed to the increase of strain after instability.