Synthesis Of Electrodeposition Nanocrystalline Ni With Broad Microhardness Distribution And The Diffusion Behavior Of S In Ni

In recent years, advanced nanostructured metal materials have gained considerable interest all over the world. Because of the special structure, nanocrystalline metal materials exhibit some outstanding properties compared with the traditional coarse-grain materials, such as the exceptionally high strength. However, they are intrinsically brittle which decreased the practical application values of them. Recently, some articles have found that as the grain size with broad or gradient distribution, the extraordinary intrinsic plasticity of the structures could offers their potential for use as advanced coatings of bulk materials. This would be the best routes to improve the integrated mechanical properties of the nanostructured metal materials.There are many routes to prepare nanocrystalline materials such as electrodeposition, pulsed laser deposition, chemical vapour deposition and thermal spray. Among them, direct current electrodeposition is a convenient and economical method to produce dense nanocrystalline, pure metal or composite materials. As a consequence, the influence of the current density and saccharin concentration on the the deposition velocity, grain size, orientation texture, microhardness and internal stress of the nanocrystalline nickel synthesized by direct current electrodeposition was systematically analyzed in this paper. We also researched the relationship between the thermal stability and the grain sizes of the samples. The results would be obtained as follows:(1). For low current densities ranging from0.5A/dm2to1.5A/dm2, nanocrystalline nickel with broad microhardness distribution ranging from415Hv to603Hv could be synthesized by adjusting saccharin concentration.(2). As the saccharin concentration was1.0g/L, with the increasing of the current density from0.5to4.1A/dm2, the deposition velocity increased, the preferred orientation of the specimens is constanand, the grain size decreased first to2.2A/dm2and then increased.(3). As the current density was0.5A/dm2, with the increasing of the saccharin concentration from0.4g/L to5.0g/L, the deposition velocity increased, the preferred orientation in the deposits existed and changed progressively from a (111) orientation texture to a (200) orientation texture, and the grain size decreased first and then kept constant as2.0g/L. The internal stresses of the samples decreased to nearly OMPa as saccharin concentration increased to1.2g/LThe results of DSC curves show that when grain size was10nm, the samples initially grew abnormally at317℃, and with the increasing of the annealing temperature, the preferred orientation in the deposits changed progressively from a (200) orientation texture to a (111),(200) double orientation texture, the grain size increased as400℃. Moreover, thermal stability decreased sharply. While when the grain size ranged from28nm to77nm, the orientation texture is constant and the grain size increased a little, the samples were stable near to bulk nickel.Due to the obvious effect of the impurity on the mechanical propersities of the nanocrystalline nickel, a systematic investigation regarding the structure stability and other properties of the impurity in bulk Ni was carried out via first-principles methods. All the structures with N, O, P, S as the solution atom in bulk Ni were consistent with the requirement of thermal stability. The order of the structure formation from difficult to to easy is Ni31N<Ni31O<Ni31S<Ni31P. The order of the structure stability from weak to strong is Ni31S<Ni31P<Ni31N<Ni31O. The system became less stable after S insertion into Ni and in substitutional site was most stable, while the tetrahedral interstitial site was least stable. Under the applied tensile stress, the structure of the Ni-S system became more stable and S diffusion accelerated, further improved the segregation of S.