| The developments of the microelectromechanical systems (MEMS), nanoelectromechanical systems (NEMS), integrated circuits, and micro- and nanoscale devices demand to synthesize nanostructrued (NS) materials with high performances. The functionality and overall reliability of these systems and devices are closely tied to the mechanical properties of the NS materials from which they are constructed. Therefore, it is essential to fabricate novel NS materials by simple methods and undertake extensive studies on their microstructures and mechanical properties. NS materials usually are reported to have high strength and hardness, elevated strain rate sensitivity, low temperature superplasticity and limited plastic strain at room temperature. At present, interests on the NS materials are focused on improving and optimizing their mechanical properties and revealing the strain rate sensitivity and mechanical mechanism of these materials.NS materials, which can be defined as materials with crystal size less than 100 nm in dimension, are synthesized by either "top-down" or "bottom-up" processes. The "top-down" methods for processing NS materials involve starting with a bulk solid and building nanostructure by structure decomposition. The typical processes of "top-down" are mechanical milling and sever plastic deformation. The "bottom-up" approach, such as inert gas condensation, electrodepostion, crystallization of amorphous phases, starts with atoms, ions, or molecules as "building blocks" and assemble bulk materials from them. Compared with other fabrication methods for bulk NS materials, the advantage of electrodeposition are low cost, industrial applicability, simple operation, and versatility. Therefore, many researchers pay more attention on the electrodeposition and use the electrodeposited NS materials as the model materials to study the real mechanical mechanism in nanoscale.In this work, based on the fact that the content of electroplating additive-1,4-butenediol has an obvious effect on the microstructure of electrodeposits, a surfactant-assistant electrodeposition technique was proposed and used to fabricate NS Ni and Ni-Co alloys. Microstructures and mechanical properties were extensively studied by scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction, MTS tensile testing system, etc. The main results are shown as follows:1. Electrodeposited NS Ni has an average grain size of 40 nm with a wide grain size distribution of 5-80 nm. There are some grain clusters with sizes of about 150-250 nm in the microstructure of electrodeposited NS Ni revealed by TEM observations. XRD analysis indicates the NS Ni possesses a strongly (200) preferred texture. The room temperature tensile tests performed at strain rates of 1.04×10-4-1.04 s-1 show that the NS Ni exhibits obvious necking behaviors and enhanced plastic strains of about 5.4-8.0%. The average ultimate tensile strength (UTS) is about 1200 MPa. The other reported NS or nanocrystalline (nc) Ni usually had limited plastic strains less than 3%. The strain rate sensitivity, m value, and the average activation volume, V, of our electrodeposited Ni are about 0.012 and 27b3, respectively, which deduce that the dislocation movements should be responsible for the plastic deformation. Furthermore, The surface relief morphologies, like the superplasticity deformation, are observed on the deformation region surfaces by SEM. In addition, the XRD detections on the necking regions shown that the original (200) texture is weaken with increasing the strain, which means that the grains or grain clusters rotation would occur during the plasticity of the NS Ni.2. Bulk electrodeposited nc Ni-1.7%Co alloy has an average grain size of about 25 nm and a narrow grain size distribution of 10-55 nm. XRD analysis reveals that the Ni-1.7%Co alloy possesses the single f.c.c phase structure and a strong (200) texture. Tensile tests at RT show that the nc Ni-1.7%Co alloy has a good combination of high UTS of about 1600 MPa and enhanced plastic strains of～7% over a wide strain rate range of 1.04×10-6-1.04 s-1. The good ductility may be due to the increase in the strain hardening rate by adding the alloying element, Co, which would lead to a reduction in the stacking fault energy. The strain rate sensitivity, m value, and the average activation volume, V, of the electrodeposited Ni-1.7%Co alloy are about 0.016 and 18b3, respectively, which reveals that the dislocations might be emitted from the grain boundary (GB) sources (triple points, facets, steps, and jogs), and this process of GB-defect-assisted dislocation generation and dislocation motion should be responsible for the strength and plasticity of the NS Ni-1.7%Co alloy. However, when the Ni-1.7%Co alloy deformed at a low strain rate of 1.04×10-6 s-1, atoms or grain boundary diffusion would occur, which results that the flow stress exhibits serration character with increasing the strains. The plastic strain of～7.4% was obtain in nc Ni-1.7%Co alloy when it deformed at the strain rate of 1.04×10-6 s-1.3. Electrodeposited nc Ni-8.6%Co alloy possesses the single f.c.c phase structure and has an average grain size of 13 nm with uniform tiny nano-grains. Tensile tests show that the tensile behaviors of the Ni-8.6%Co alloy are strongly depended on the strain rate. Significantly, a ductile-brittle-ductile transition in the fracture mechanism through raising the strain rate from 1.04×10-5 to 1.04 s-1 is found in the tensile tests of the NS alloy. Furthermore, ultrahigh tensile strength of 2.0-2.4 GPa was obtained at room temperature in this nc alloy. For the 13 nm grained alloy, no or less dislocations and GB atoms would be activated at room temperature, which leads to a brittle fracture with very limited plastic strain (<1%) at the strain rates of 2.08×10-3-4.17×10-2 s-1. However, a gradual brittle-ductile transition occurs as the strain rates decreased from 2.08×10-3 to 1.04×10-5 s-1, where the lower strain rates allow the GB atoms diffuse easily, which would relax the stress concentrations and hence enhance the ductility. In addition, another brittle-ductile transition occurs at high strain rates of 4.17×10-2 to 1.04 s-1, where the stress-assisted activation of GBs would be responsible for the improved plasticity. The strain rate sensitivity, m value, and the average activation volume, V, of the electrodeposited Ni-8.6%Co alloy are about 0.025 and 7b3, respectively. The ductile-brittle-ductile transition in the fracture mechanism through raising the strain rate from 1.04×l0-5 to 1.04 s-1 of 13 nm grain sized Ni-8.6%Co alloy gives the experimental evidences that the strain rates do have influence on the mechanical mechanism of nc metals.4. The relationship between strain rate sensitivity m and activation volume V for face-centered cubic (fcc) metals is proposed based on the bow-out model of single dislocation from its source, which gives a reasonable prediction of the enhanced strain rate sensitivity occurred in NS and ultrafine grained Ni and Cu. The relationship is shown as following:where T is the absolute temperature, kB is the Boltzmann constant, G is theshear modul, b is the Burgers vector, and a is the constant of 0.36. The observed strain rate sensitivity of 0.02-0.04 for nc Cu and 0.01-0.02 for nc Ni is suggested as a result of the reduction of dislocation length to about 10-100b. It has no direct relation to the GB deformation mechanism.5. The layered NS Ni with the modulated hardness on the cross-section is fabricated by adjusting the content and supplement period of the electroplating additive-1,4-butenediol during the electrodeposition process. TEM observations show that the layers consists of the nano-sized crystalline grains (with an average grain size of 18 nm) and the ultra-fine crystalline grains (with the grain size ranging of 100-500 nm) alternately. The layers of high and low hardness values correspond to the small and large grains layers, respectively. The mechanical properties of the layered NS Ni are located between that of the 18 nm grain sized Ni and that of the ultra-fine grained Ni (with an average grain size of 350 nm), where the strength was about 1130 MPa and the plastic strain was about 5.7%. It is believed that the hard layers in the layered NS Ni impart high strength and the soft layers stabilize the plasticity. However, it is found that the mechanical properties of the layered NS Ni composed of orderly mixture of large and small grains is worse than that of the reported bimodal grain sized Ni composed of randomly mixture of large and small grains. The reason for this might be that the incompatible strains between hard and soft layers lead to the early fracture of the layered Ni. Improved mechanical properties of the layered NS Ni would be expected by adjusting and optimizing the distributions of soft layer and hard layer. |
