Superplasticity And Superplastic Forming Of Nanocrystalline Ni And ZrO₂/Ni Composite Prepared By Electrodeposition

Bulk nanocrystalline materials refer to the polycrystalline materials with the grain sizes of 1¹00nm, which possess unique physical and mechanical properties and are among the most interesting topic in the materials research. Pulse electrodeposition is the most promising method for preparing nanocrystalline metals, and it can produces nanocrystalline materials with high density and free of porosity. Recently nanocrystalline materials produced by electrodeposition are used as perfect models for superplasticity, however, because nanocrystalline pure metal is lack in the pinning effect on the grain boundaries during deformation, the grain growth becomes rapidly, resulting the reduced superplasticity. Nano reinforced particles are helpful for superplasticity since they enhance the thermal and structural stability of nanocrystalline materials.Nanocrystalline Ni and ZrO₂/Ni nanocomposite were produced by pulse electrodeposition. The different mechanical properties and microstructural evolution were investigated. Superplastic bulging and micro deepdrawing were also performed to study the mechanical properties under the complex stress.Nanocrystalline Ni and ZrO₂/Ni nanocomposite were produced by electrodeposition. Effects of depositional factors, such as pulse current density and C7H5NO3 density, on microstructures and brittleness were investigated. The optimal deposition parameters were determined by orthogonal experiment and the average grain sizes of obtained nanocrystalline Ni and ZrO₂/Ni nanocomposite were 70nm and 45nm respectively. Most grains remained in equiaxed shape, which are suitable for superplasticity.Tensile test and nanoindentaion test were used to study the mechanical properties of nanocrystalline Ni and ZrO₂/Ni nanocomposite at room temperature. Strengthening effects of grain size and reinforced particles were also analyzed. Superplasticity of nanocrystalline Ni and ZrO₂/Ni nanocomposite were investigated by tensile tests at temperatures of 370℃～500℃and strain rates of 8.33×10-4s-1～5×10-2s-1, and effects of temperature and strain rate were concluded. Nanocrystalline Ni reveals low temperature superplasticty, while ZrO₂/Ni nanocomposite exhibits low temperature and high strain rate superplasticity.Fracture surface and surface characteristics of nanocrystalline Ni and ZrO₂/Ni nanocomposite during superplastic tensile test were analyzed by SEM, and microstructural evolution during deformation was measured by TEM. Based on the observed results, the fracture behavior and grain motion were explained, and the accommodation processes of dislocation, twin and Ni-S segregation were also discussed. Grain growth during superplastic deformation was mainly attributed to the effect of temperature. Comparing with nanocrystalline Ni, the grains of ZrO₂/Ni nanocomposite grew slowly, which is the key reason for the enhanced superplasticity of nanocomposite.FEM simulated the strain rate distribution and thickness distribution during superplastic bulging at 450℃and strain rates of 1.67×10-3s-1～1.67×10-2s-1. The optimal strain rate was decided by comparison of FEM results. The Pressure-Time curve was also simulated by FEM, and the high value of strain rate sensitivity exponent m was used to improve the uneven thickness distribution. Hemisphere samples of ZrO₂/Ni nanocomposite were produced by superplastic bulging with die diameters of 1mm,2mm and 5mm. Evolutions of the microstructure and cavity were observed by TEM, and the formation reasons were explained. The microforming behavior was influenced by temperature obviously. The micro drawn samples were successfully produced at 450℃and punch displacement rates of 1mm/min, 5mm/min and 10mm/min, indicating the good microforming ability.