| Nanocrystalline (NC) materials have been the subject of widespread research in the fields of material science due to their special structures and properties. Among various synthesis methods, electrodeposition has significant advantages such as simple operation, low investment, and high probability to obtain NC materials with bulk sides and full density. However, the processes of electrodepositing NC Cu are not fully developed so far, and toxic cyanide electrolytes are often used in these processes.Compared with their coarse-grained counterparts, NC materials usually have a remarkable increase in stress and hardness, while exhibiting limited plastic strain and ductility. Theoretical analysis has suggested that dislocation nucleation and accumulation become impossible inside the grains when the grain size is less than 100 nm. In fact, dislocations are emitted from one grain boundary (GB) segment, traverse the small grains and tend to disappear into the opposing GBs. As the grain size is further decreased below a critical level, the deformation mechanisms of NC materials may change from dislocation activities to GB sliding and/or diffusion. Special deformation mechanisms such as shear localization, twinning deformation and grain coalescence may also take place. On the other hand, numerous experimental results have shown that external factors such as synthesis methods, impurity contents, loading applications and specimen sizes also have important effects. In a word, there is no consensus of opinions on the mechanical properties and deformation mechanisms of NC materials until now.In this paper, a nontoxic alkaline electrolyte for producing NC Cu is developed, which is mainly composed of CuSO4·5H2O, NH2CH2CH2NH2, (NH4)2SO4 and N(CH2COOH)3. By modulating the processing parameters of bath-electrodeposition, bulk NC Cu with high purity and density can be obtained. Moreover, its grain sizes can be optionally controlled in the range of 33 ~ 104 nm. Three types of NC Cu together with a nanostructured Cu were produced from this electrolyte. In order to decrease the grain size and increase the thickness of the deposition sheets, another NC Cu was produced by an electric blush-plating technique. Detecting equipments including XRD, TEM, SEM and MTS were employed to study the microstructures and mechanical behaviors of the produced NC/nanostructured Cu. The main results were shown as follows:1. SeO2 is the main additive in bath-electrodeposition electrolyte, which has pronounced effects in producing NC Cu. A smooth and porosity-free deposition surface with the grain size less than 100 nm can be obtained when 0.02 g/L SeO2 is added. By controlling the current density in the range of 1.5 ~ 3.2 A/dm2, the grain sizes of the deposited Cu can be made into nanometer regime. 2.5 A/dm2 is usually selected in direct-current electrodeposition in order to stabilize the process. A higher pH value in the electrolyte leads to a decrease in depositing rates and grain sizes, while the preferential orientation along the {200} plane increases. The grain size can be suppressed to ~10 nm by pulse electrodeposition. However, pores and other defects often emerge at this situation.2. NC Cu with an average grain size of 33 nm and a {200} texture was produced by pulse electrodeposition. A high yield stress of ~624 MPa was obtained, which was comparable to the highest yield stresses for NC Cu in the literature. However, the elongation to failure was less than 5.5%, and it decreased at higher strain rates. The absence of hardening mechanism might be responsible for the fracture in the early stage of plastic deformation. An enhanced strain rate sensitivity ( m =0.029) and a brittle intergranular fracture were also revealed. The deformation mechanism of the NC Cu was dominated by dislocation activities at GBs. Meanwhile, GB sliding and diffusion might be involved at lower strain rates, resulting in a measurable stain after instability.3. NC Cu with an average grain size of 90 nm was produced by direct-current electrodeposition, which exhibited a much improved combin- ation of stress and plasticity in tensile tests at room temperature. An increased stress with a decreased plastic strain was obtained at higher strain rates. The strain rate sensitivity was measured to be ~0.015, and it was little affected by increasing the plastic strain. The activation volume decreased as the uniform deformation proceeded. The persistence of work hardening at large strains and strain rate sensitivity might be responsible for the enhanced ductility of the NC Cu.4. NC Cu with an average grain size of 200 nm and a majority of grain clusters was synthesized by direct-current electrodeposition. Low-angle GBs were predominant within the clusters. An abnormal strain rate effect on plastic strain was observed in this NC Cu, in which the elongation to failure increased significantly at higher strain rates. Two reasons might be responsible for this phenomenon. First, the strain hardening behavior increased with increasing the strain rate, resulting an enhanced uniform elongation. Second, shear localization process was found in the necking stage. At higher strain rate, the relaxation of internal stress took place due to the fragment of local shear units, thus an increased strain after instability was obtained. Fracture morphologies of the NC Cu were also found to be strongly strain rate dependence.5. Nanostructured Cu with a broad grain size distribution from 20 to 600 nm was designed and synthesized. Microstructural analysis revealed that the total number fraction of the grains less than 100 nm was ~42.8%, much larger than their volume fraction, 5.7%. The average grain size of the nanostructured Cu was ~310 nm. Tensile tests at room temperature indicated a relative low work hardening rate, while an improved stress and plasticity was still obtained. By the increment of the strain rate, the post-uniform strain increased and the yield stress was little affected. It was proved that preferable mechanical properties could be obtained in materials with broad grain size distributions.6. Bulk NC Cu with a thickness of ~9.0 mm was synthesized by pulse electric blush-plating. TEM and XRD analysis at different locations along the deposition growth direction revealed no detectable differences in its microstructures. The NC Cu was characterized by an average grain size of 25 nm and predominant high-angle GBs. In iso-rate compressive tests, an ultrahigh stress with a certain flow softening was obtained. A distinct transition in plastic deformation mechanism took place at strain rate of 1.0×10-2 s-1. At higher strain rates, the deformation mechanism was dominated by dislocation activities. In contrast, GB diffusion and thermally-activated GB sliding were enhanced when the strain rate was decreased below 1.0×10-2 s-1. This transition was validated by performing rate-jump compressive tests with different methods. The ultrafine grains in critical value range, uniform grain size distribution and high-angle GBs of the NC Cu might be responsible for the transition in plastic deformation mechanism. Shear bands could not be observed on the specimen surfaces after compression. The extended direction of the microcracks was approximately parallel to the compressive axis, and the fracture mode belonged to brittle fracture. |
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