Synthesis And Mechanical Property Of Nano-sized Growth Twins Cu By Electrochemical Depositing Technique

Nano-science reveals a tempting prospect.Nano-technology could provide some new products with specific functions and devices for mankind as a technology. Therefore, the research of preparation of nano-materials becomes the hot spots in the modern time. Electrochemical Preparation of electrodeposited nanocrystalline materials Electrodeposition is the typical technology of the electrochemical preparation of nanocrystalline materials. However, the great efforts between the electroplating technology and the basic theory are restricted by the complexity and uncertainty of the experiment extremely. The interest people on the mechanical properties of nano-structured materials is due to the unique mechanical properties which could be presented or indicated by the materials that is from the preparation of gas condensation method. Recently ,the addressing hot spot is how to master its unique mechanical propertie.,including the core issue that lack the understanding of deformation and fracture mechanism of the nano-crystalline materials.Obviously, it need to invest more efforts in the theory to guide crystal experiments and the direction of optimizing performance specified.So far as now, the work is very limited in this area.According a number of past reported in the literature, it is analyzed a important deformation mechanism of nano-crystals which is called mechanical twins and there is few theoretical studies on exploring this possibility.Therefore, this paper uses the method of direct current deposition and studies repeatedly on the influences of process conditions to the deposition morphology, microstructure and other effects. Discussed on the changes of parameters to the impact of the sediment layers and finalized the feasibility of an experimental process conditions, it is focused on analyzingnd discussing in detail the impact of the structure of nano-twins on the tensile properties of nano-crystalline copper and the main parameters on the microstructure characterization of structural features in the electrodeposition process.Through the above experimental study and theoretical analysis, several conclusions are presented as follow:1. After repeatedly contrast and microscopic tissue analysis, the final process conditions is that:Electrolyte composition : CuSO4·5H2O which provides the Cu2+ in the deposition process is the main salt of the electrolyte.The content of CuSO4 is 150 ~ 220g / L.It will lead to a small range of allowing working current density when the content of copper is too low and will crystallize because of the limitation of solubility when the content is too high.NH2CH2CH2NH2Cu2+ is the main complexant of Cu2+. It is helpful to reduce the grain size of sediment in improving the level of NH2CH2CH2NH2Cu2+. However, excessive levels can lead to lower deposition rate. N(CH2COOH)3 not only has the role of 2-rank complexation, but also promotes the anodic dissolution and improves the dispersion capacity of the electrolyte. (NH4)2SO4 can improve the conductivity of the electrolyte and increase the current density and electro-deposition nucleation rate. While NH4+ and Cu2+ can also form a stable complex. All the pharmaceutical used is AR reagents.Supply current density: During the process of electrochemical deposition, we can get the nanocrystalline Cu deposits that the current density is from 1.5 A/dm2 to 3.2A/dm2. This paper selected the current density at 2.5 A/dm2 in order to ensure a stabe process. Compared with the DC deposition, pulse electrodeposition could reduce the grain size of sediment further. However, it may be prone to produce the defects such as micropore. Though the grain size would increase slightly, we can eliminate the micropores by the means of optimizating the process parameters.Additive: The rare earth oxides called SeO2 is the main additive in the process. When the content is over 3mg/L, it was smooth, and the reunion is almost disappeared.while the grain size is less than 100nm.PH Value: The process choosed the pH value at 8.4. The increase of pH of the electrolyte could make a decline in electro-deposition rate and an increase in the degree of preferred orientation at {220} crystal plane. But it may lead to sediment quality with too high PH.2. The growth twins are emergencing gradually with the decrease of current density and the increase of additive content. There is no growth twins when the current density is 3.2A/dm2and the content of additive is 1.5mg/L while there are a large of growth twins of the {111}/[112] type in the condition of 1.5mg/L when the current density is in the same . The lamellar thickness is distributed in the 20nm to 340nm and the mean is 88nm.The length of twin boundary is from 60nm to 2.4μm and the average length is 684nm. However, it also can produce many growth twins when the content of additive doesn't change and the current density decreases to 2.5A/dm2. The thickness of nano-twin lamellae is distributed in 10nm to 250nm and the average thickness is 79nm. The length of twin boundary is in the range of 45nm ~ 875nm, 282nm average. So either reducing the current density or increasing the content of additive can produce the growth twins and reducing the current density also can decrease the average length of the twin boundary and appears obviously the cross-twin simultaneously. It could be further divided the lamellar structure into the fine small parallelogram block. The current density reduced to 2.5A/dm2 and the content of additive increased to 3mg/L. The lamellar thickness is in the range of 10nm ~ 185nm with an average thickness of 61nm.The distribution of the length of twin boundary is in the 25nm ~ 430nm.The average length is 148nm. Thus reducing the current could make the thickness of twins lamellae and the length of twin boundary decreases. With reducing the current density and increasing the content of additives further (1.5A/dm2, 4mg/L),the lamellae structure has disappeared and were almost cut into the structure of parallelogram box.In the meantime, the film thickness of twins is the same as the length of twin boundary almost. The average lamellae thickness is 44nm.The average length of twin boundary is 81nm.3. Surface morphologies, microstructure of the electrodeposited nc Cu were characterized by means of XRD( D/max 2500PC), TEM( H-800), SEM (JSM-5600),etc. The grains contain a high density of growth twins, which separate grains into nano-meter thick lamellar structures. The average length is 90 nm. Because the mean lamella thickness is much smaller than the grain size, the dislocation will be blocded by the twin boundary inevitably before facing the grain boundary. Twin boundary, like grain boundary, is very effect to block dislocation motion. So it is about four times higher than the yield stress determined by Hall-Patch relation with the same grain size 2μm. Twin boundary by being divided into nano-lamellar structure, the dislocation is hardly intersected in the structure of nano-lamellar which is divided by the twin boundary. As long as the flow stress can be maintained within a certain range, the dislocations would penetrate the twin boundary. Accordingly there is rarely an behavior of strain-hardening in most region of the plastic deformation of nano-twin Cu. The strain rate sensitivity (m=0.016) is higher than the similar grain size counterpart without growth twin but the volume activation is a process of decline which is from 84 b3 at 1% plastic strain down to 69 b3 at 9% plastic strain and far less than the coarse-grained Cu. It is because that twin boundaries cut down the dislocation motion domain, and the mean length of dislocation must be shortened. The decreased activation volume with strain and the dimples allover the fracture surface together proves that the dislocation motion dominates the deformation of the Cu sample. Thus it can be seen , like the grain boundary, the twin boundary could the dislocation motion so that it could increases the yield strength of the materials. The difference is that the d dislocation motion also can cross through the twin boundary to avoid the phenomenon of strain hardening. This is the twin boundary with a special circulation role. It is also proved that the dislocation motion is the dominant force of the deformation in this nano-twin Cu.