Donggang Li

Evolution of morphology in electrodeposited nanocrystalline Co–Ni films by in-situ high magnetic field application

The effect of high magnetic fields up to 12 T, applied during electrodeposition process, on the morphology of nanocrystalline CoNi films has been investigated. The magneto-induced dramatic modifications in the morphology were observed by using field-emission scanning electronic microscopy and atomic force microscopy. Along with the increase of magnetic flux density (B), the grain size and the surface roughness of the films increased to reach a maximum value at a field of 9 T. Meanwhile, higher magnetic flux density could improve cobalt atomic percentage in the film due to the impacts of magnetohydrodynamic effect. However, at a high field of 12 T, the paramagnetic force played a predominant role in a decrease of mass transport, resulting in minimum grain size and roughness, even smaller than that of the ordinarily (B=0 T) sample.

Enhancement of the Kirkendall effect in Cu–Ni diffusion couples induced by high magnetic fields

The shift of the Kirkendall marker at the interface of Cu–Ni diffusion couples was examined under the influence of high magnetic fields up to 12 T. The shift distance of the marker increased remarkably with increasing magnetic flux density (B) in the case of the direction of diffusion parallel to B. However, if the direction of the magnetic field was perpendicular to that of diffusion, there was a negligible effect of the magnetic field on diffusion behavior. A calculation of intrinsic diffusivities of both components showed that the interdiffusion rates (∥B) are increased by high magnetic fields. These effects can be attributed to an increase in the chemical potential gradient induced by magnetic free energy in a high magnetic field.

Composition, concentration and configuration dependence of the icosahedral transformations in Cu-based bimetallic clusters

The dependence of icosahedral transformations on the composition, concentration and configuration in Cu-based bimetallic clusters was studied by using molecular dynamics with the embedded atom method. The results show that the transformation is strongly related to the release of excess energy and can be controlled by tuning the composition, concentration and configuration. The transformation can be easily induced by the addition of Co while it is difficult in the case of Ni. The transformation temperature Ttrans decreases with the increase in Co concentration and increases with the increase in Ni concentration. The transformation can be controlled by fabricating different configurations and distributions of the composition.

Effects of high magnetic fields on solidified structures of Mn-90.4 wt% Sb hypoeutectic alloy

Abstract Mn-90.4 wt% Sb alloy specimens were solidified under both uniform magnetic field and magnetic field gradient conditions. The solidification behavior was examined to elucidate the effects of high magnetic fields on the solidified structure evolution of this hypoeutectic alloy. The macrostructures on the longitudinal section of the alloys were investigated by optical microscopy and x-ray diffraction (XRD). The volume fraction of primary MnSb phases and the interrod spacing of the eutectic were measured by metallographic analysis. It was found that the segregation of the primary MnSb particles at the certain regions of the specimens occurred under the influence of high magnetic field gradients. The MnSb phases obtained under magnetic fields were oriented with their (h0 l) planes along the direction of the magnetic field. Both the volume fraction of primary MnSb phases and the interrod spacing of the eutectic were decreased upon the application of the high magnetic fields.

Control of the Alloying Element Distribution in Al-Alloys by High Magnetic Fields

The distribution and solidified structure of alloying elements are important for the quality and the properties of alloys. In the present study, the solidification behavior of aluminum-rich alloys is studied under various high magnetic field conditions, and the influences of uniform and gradient magnetic fields with different intensity and direction on the distribution and the morphology of solute elements of Al-Cu and Al-Mg alloys are investigated. It is found that because of the differences of the electromagnetic force (Lorentz and magnetization forces) acting on Cu element and Mg element with different physical properties in the matrix, the regularities of distribution for Cu element and Mg element are opposite just in the intracrystalline and intergranular under high uniform magnetic field condition, and not only the content but the distributions of Cu and Mg elements are obviously different under high gradient magnetic field conditions as well. It can be concluded that high magnetic field has different effect on the solute distribution in alloys with different physical properties such as density, susceptibility, conductivity, etc. And the experimental results indicate that it is possible to control the terminal solubility and morphology of the solute elements in alloys by high magnetic fields.

The Coupled Magnetic Field Effects on the Microstructure Evolution and Magnetic Properties of As-Deposited and Post- Annealed Nano-Scaled Co-Based Films — Part II

Superimposed external magnetic fields during electrodeposition process offers the possi‐ bility to tailor the microstructure and properties of the obtained films in a very efficient, contactless, and easily controllable way, which is caused by so-called magnetohydrody‐ namic (MHD) effect. On the other hand, the non-equilibrium state of as-electrodeposited nanocrystalline films provides a strong thermodynamic potential for microstructural transformation. This means that the beneficial effect of magneto-electrodeposition on a nanocrystalline film can be completely consumed by thermal exposure at a relatively low temperature. Magnetic field annealing has been confirmed to be useful for tailoring the microstructure of as-deposited nanocrystalline films for their widespread uses. The particular interest of this book chapter, “Growth of Co-based magnetic thin films by magnetic fields (MF) assisted electrodeposition and heat treatment,” is the finding that the microstructure and magnetic properties of nanocrystalline Co-based alloys and ox‐ ides like CoX (X = Cu, Ni, NiP, FeO.) are improved by imposition of MF during elabora‐ tion process or post-annealing process. According to the previous study, the targeted scientific activities pay more attention to develop alloys and oxides in nano-scale using pulsed electrodeposition assisted by high magnetic field (HMF). (Note: Since the instanta‐ neous current density during pulse electrodeposition is higher than that during direct current plating, the microstructure of the nano-scale electrodeposits can be more easily controlled by perturbing the desorption/adsorption processes occurring in the pulse elec‐ trodeposition process). During the experiment, high magnetic field is an in situ method for the control of electro‐ deposition process. The obtained material is then annealed or oxidized after elaboration under HMF. Comparative studies are performed concerning the electrodeposition proc‐ ess in a high magnetic field, by changing the magnetic field parameters, such as magnetic

Reactive Diffusion at the Liquid Al/Solid Cu Interface in a High Magnetic Field

The kinetics of the reactive diffusion at the liquid Al/solid Cu interface was investigated at T = 973 K, 1023 K, and 1073 K in a high magnetic field of 11.5 T. During the annealing process, three stable compounds (δ, ξ2, and η2) layers were formed at the interface of the couples, and a power function relationship between the mean thickness of the diffusion layers and the annealing time kept stable. Without magnetic field, the exponent of the power function for each compound layer was higher than 0.5, but it was close to or even smaller than 0.5 with a magnetic field. Compared with the field-free environment, the migration of the liquid/solid interface due to interdiffusion decreased in the presence of a magnetic field. A considerable decrease in the effective diffusion coefficient under a magnetic field provided a likely explanation for the experimental results.

Alignment of Primary Al3Ni Phases in Hypereutectic Al-Ni Alloys with Various Compositions under High Magnetic Field Gradients

The microstructural changes of the primary Al3Ni phases in hypereutectic Al-Ni alloys solidified under various high magnetic field gradients were investigated. It was found that the application of a magnetic field gradient induced an aligned structure of the primary Al3Ni phases similar to those in a high uniform magnetic field. However, the high magnetic field gradient showed more obvious effect on the alignment of the primary Al3Ni phases than the uniform magnetic field, although this effect strongly depended on the alloy composition.

Effects of High Magnetic Field Postannealing on Microstructure and Properties of Pulse Electrodeposited Co-Ni-P Films

The influence of high magnetic field annealing on the morphology, microstructure, and properties of pulsed-electrodeposited Co-Ni-P films was investigated. The as-deposited film with a rough surface changed into uniform nanocrystalline during the magnetic field annealing process. In particular, the formation of intestine-like appearance with spherical clusters vanishing is favored from a moderate magnetic field strength of 6 T, due to the polarized effects. Meantime, the diffraction peak (111) of α (fcc) phase shifts to the right direction, which is attributed to the fact that more Co atoms from phosphide phase are incorporated into the Ni lattice, in comparison with the case of annealing under 0 T and 12 T magnetic fields. The mechanical and magnetic properties of the films reach relative optimum values at  T. The evolution of magneto-induced modification in the Co-Ni-P morphology, structure, and properties can be explained by the polarized effect and the diffusion-acceleration effect under a high magnetic field.

Diffusion Behavior and Interfacial Reaction of Heterogeneous Metal Systems Controlled by High Magnetic Fields

The interdiffusion behaviors and interfacial reaction in solid Cu/solid Ni, liquid Bi/solid Bi0.4Sb0.6, liquid Al/solid Cu, and gas Al/solid Cu diffusion couples have been experimentally studied under a high magnetic field of up to 12T. The effects of magnetic flux density and the direction of magnetic field (B) on the evolution of interfacial microstructure (including interfacial migration, phase composition and thickness of diffusion layers) have been examined systematically. We found that (1) The shift distance of Kirkendall marker and the inter-diffusion coefficient in solid Cu/solid Ni diffusion couples increased with increasing magnetic flux density in case of the direction of diffusion parallel to B; (2) The migration of the Bi/Bi0.4Sb0.6 interface due to the self-diffusion of the liquid metal into the solid alloy decreased markedly with the increase of magnetic flux density; (3) High magnetic fields exerted a non-monotonic influence on the thickness of diffusion layers during the reactive diffusion process between liquid Al and solid Cu; (4) The application of a high magnetic field during chemical reactive-diffusion process of gas Al/solid Cu system induced a significant change in the final products.