Microstructures And Properties Characteristic Of Electrodeposited Nanocrystalline Ni And Ni-Co Alloys

Nanocrystalline (NC) materials have attracted worldwide attention due to their unique properties and potential applications in many technological areas. In past decades, interests on NC materials were forced on preparing ideal materials with excellent properties in order to satisfy many advanced demands. Therefore, a better understanding of the relationship between microstructures of NC materials and their properties is necessary. The corrosion property is important for both practical utility and fundamental physicochemical of the NC materials. But different synthesis methods give rise to difference in microstructure of NC materials, it is difficult to summarize and predict their corrosion properties. Additionally, the NC materials often showed high strength while having disappointing ductility at room temperature (RT), which limits their practical utility as structure materials. As a result, various strategies for improving the ductility of NC materials without sacrificing their superior strength have been proposed in recent years. However, the ductility of the NC materials with critical grain size (10-20 nm) is still a problem and need to be substantially resolved. Furthermore, most studies about mechanical behaviors of NC materials were carried out mainly using computer simulation techniques. Experiments are needed to confirm the theoretical predictions for the NC materials. Thus, optimizing the properties of NC materials and understanding their essential behaviors become significant for actual application and scientific study.Based on the fact that content of grain refiner during electrodeposition has an obvious effect on the microstructures of deposits, a surfactant assistant deposition technique is proposed. Nanostructure (NS) Ni with different grain size, grain size gradient (GSG) NS Ni coating and NC Ni-Co alloy with different Co content were fabricated by controlling the content of additives and Co element in the electrolyte. Microstructures and compositions of these materials were extensively studied by X-ray diffractmeter (XRD), inductively coupled plasma atomic emission spectrometry (ICP-AES), transmission electron microscope (TEM), scanning electron microscope (SEM) etc. The main results are shown as follows:1. Electrodeposition Ni with different grain sizes (16 nm-2μm) were fabricated by adjust the content of saccharin in the electrolyte, the thickness is 350μm. Effect of grain size on electrochemical corrosion behavior of these Ni deposits in different corrosion media was characterized using potentiodynamic polarization tests, electrochemical impedance spectroscopy (EIS), X-ray photoelectron spectroscopy (XPS) and immersion corrosion tests. Results showed that the NC Ni exhibited improved corrosion resistance with grain size decrease in the NaOH and NaCl solutions. But in H2SO4 solution, higher grain boundary density accelerated corrosion since no passive process and the corrosion resistance of NC Ni decreased with refining grain size. The distinct experimental results of NC Ni in corrosion behavior can be reasonably explained by the positive or negative effect of high-density grain boundaries in different corrosion media.2. A GSG NS Ni coating on steel sheet was prepared by gradually increasing the grain refiner (saccharin) concentration during direct-current electrodeposition. The XRD analysis indicated that preferred orientation of the deposit changes to (111) from (200) crystal orientation with increasing the saccharin concentration. The grain size varies from 22 nm on surface to about 586 nm near the coating-substrate interface observed by transmission electron microscopy. Accompanying the gradual grain refining, a hardness gradient variation from 1.9 GPa up to 6.0 GPa on the outer layer of the deposition was observed. The bend test and electrochemical measurement reveal that the GSG NS Ni coating shows excellent adhesion, good ductility and exhibited more noble anticorrosion properties compared with the uniform grain size NS Ni coating.3. By changing the rate of Ni2+/Co2+ in the electrolyte,Ni-Co alloys with different Co contents were prepared: Ni-13%Co, Ni-26.2%Co, Ni-49.2%Co, Ni-59%Co and Ni-66.7%Co alloys. XRD and TEM analysis indicate the phase structures and grain sizes of Ni-Co alloys gradually changed with increase Co content. When the Co content in the deposit below 50 wt.%, there are only face-centered cubic (fcc) phase in the Ni-Co alloys and the average grain sizes decrease rapidly with increase Co content. Keep increasing the Co content to 66.7 wt.% (above 63 wt.%), the Ni-Co alloy show a mixture of the fcc and hexagonal close-packed (hcp) phases, but the grain size remained approximately constant. HRTEM observation indicates there are three kinds of grains in the Ni-66.7%Co alloy: single fcc equiaxed grains, fcc twins and hcp twins. The average size of nanoscale twins are 10-20 nm.4. Uniaxial tensile tests performing at RT have dominanted the high strength and evidently enhanced ductility of Ni-Co alloy. It is clear that the NC Ni-Co alloys work hardened largely stronger than the 20 nm Ni did during the plastic deformation and enhanced with Co content increase. All Ni-Co alloys show evident ductile feature with deep dimples Abstract structure. The average sizes of the dimples are several times larger than their grain size, which indicates that the fracture mechanism operates involving collective grain activity. Ni-49.2%Co and Ni-66.7%Co show the highest strength and largest ductility during tensile test, respectively. The grain refinement, solid-solution hardening, decrease of stacking fault energy (SFE) and cooperative deformation of the two phases, which are caused by the alloying of Co element, should be responsible for the excellent mechanical performance.5. The Ni-49.2%Co alloy has a narrow grain size distribution (5-30 nm) according to TEM image and the average grain size is 15 nm. XRD pattern indicates that single fcc phase is observed in the Ni-Co alloy. Tensile tests at RT for strain rates ranging from 1.04×10-5 to 1.04 s-1 show the Ni-49.2%Co alloy has a high ultimate tensile strength (σUTS) of 2250 MPa and enhanced elongation to failure (δETF) of 13%. The strain rate sensitivity (m) and the experimental activation volume (V) of the alloy are 0.021 and 12b3, respectively. Correspondingly, Ni-66.7%Co alloy possesses a mixture structure of fcc and hcp phase. The average grain size is 16 nm. Ni-66.7%Co exhibited anσUTS of 2080 MPa, together withδETF of 15% in tensile tests. The m and V of the Ni-66.7%Co alloy are 0.025 and 16b3, respectively. Based on the relationship between the m and V, the dislocation motion should be responsible for the plastic deformation of the above Ni-Co alloys.6. The high strength of Ni-Co alloys is attributed to the grain refinement and solid-solution hardening effects. The addition of Co element decreases the SFE of NC Ni, which improves the strain hardening ability and thus enhances the ductility. Cooperative deformation of the two phases effectively releases the stress during deformation, which contributed to the sustained high strain hardening of the dual-phase (DP) Ni-66.7%Co alloy. Analysis of deeply dimpled fracture morphology of the DP alloy revealed that this morphology is more closely related to the ductility than to the grain size. Present study found that alloying may be a new approach to achieving the coexistence of good ductility and high strength in NC materials. Improved mechanical properties of the NC materials would be expected by adjusting and optimizing their microstructure.