Room Temperature Plasticity And Constitutive Relationship Model Of Nanocrystalline Ni Foil Assisted By Pulse Current

In recent years, nanocrystalline materials have drawn the attention of worldwide researchers for their unique physical and chemical properties. Nanocrystalline metals prepared by pulse electrodeposition are widely applied to the manufacture of minisize workpieces due to the merits of even grain size distribution and low porosity.However, the applications of nanocrystalline materials are greatly restricted because of the poor room temperature plasticity. In roder to improve plasticity, by using nanocrystalline Ni foils prepared by pulse electrodeposition as raw materials and introducing high density pulse current into the deformation process at room temperature, microstructure evolution rules and deformation mechanism of the nanocrystalline Ni foils assisted by pulse current were investigated systematically in this paper and a neural network model of the constitutive relation was also developed.The effects of factors such as pulse current density and electrodeposition temperature on coating hardness were investigated. The nanocrystalline Ni foils with dense surface as well as high performance were successfully obtained after optimum technological parameters of electrodeposition had been determined by orthogonal tests. Mechanical properties of nanocrystalline Ni foils were studied through roomtemperature uniaxial tensile test and the research showed that the room temperature plasticity was significantly improved by electropulsing treatment, the elongation tended to augment nonlinearly as peak current density increased and the tensile strength remained at a high level though declined slightly as a whole. Modification of surface morphology of the tensile fracture and microstructure change around the fracture of nanocrystalline Ni foils were observed by means of SEM and TEM respectively. Fracture characteristics of nanocrystalline Ni foils by pulse current with different densities and effect of pulse current on twinning and dislocation etc were analyzed. It was found that pulse current was conducive to improving plasticity bypromoting grain rotation and grain boundary sliding, formation of the twinning and dislocation sliding in bigger grains to some extent.Using size factor, current density and strain as the input parameters, flow stress as the output parameter, data obtained from the room-temperature uniaxial tensile test as samples, a BP neural network model of the constitutive relation of nanocrystalline Ni foils was developed. By utilizing back-propagation learning rule of the BP neural network, the model was trained and tested. Comparing predictive values of the model with experimental values, it indicated that the integrated error was no more than 6 % and the correlation coefficient was close to 1. These results verified the reliability of the model and laid the theoretic foundation for subsequent microforming technology of nanocrystalline materials.