Study On The Diffusion Behavior And Interfacial Reaction Of Heterogeneous Metal Systems Controlled By High Magnetic Fields

Due to its unique properties of non-contact, directionality, marked magnetizability and magnetic force, a high magnetic field as one kind of extreme conditions can exert an obvious influence on the processing of many metallic materials, such as solid-state phase transformation, grain boundary migration, recrystallization, solidification, and chemical reaction. These processes mentioned above have a closely relationship with the diffusion behavior of atoms. Diffusion is an important way for mass transport process, being a key factor for the control of the microstructure and properties of materials, and playing a basic role during the process of metallurgical physical chemistry. Therefore, a detailed discussion of the effects of high magnetic field on the diffusion behavior is the precondition and foundation for the control of microstructures using a high magnetic field.The interdiffusion behaviors and interfacial reaction in solid Cu/solid Ni, liquid Bi/solid Bi0.4Sb0.6, liquid Al(Zn)/solid Cu, and gas Al/solid Cu diffusion couples have been experimentally studied under high magnetic field of up to 12T. The effects of magnetic flux density(B), direction and gradient on the structural evolution of diffusion layers have been examined systematically. Based on the distance of interfacial migration and the thickness of diffusion layers, we have obtained the effective diffusion coefficient, the diffusion constant and the diffusion activation energy under various high magnetic field conditions as viewed from the kinetics and thermodynamics of atom diffusion, respectively. Theoretical analyses in terms of the magnetic free energy and external force induced by the magnetic field have been made to explain the experimental results. Moreover, the influence regularity and mechanism of high magnetic fields on the diffusion behaviors at heterogeneous interface have been discussed primarily. Following are the main results:(1) The shift distance of Kirkendall marker and the thickness of diffusion layers in the solid Cu/solid Ni diffusion couples increased with increasing magnetic flux density in case of the direction of diffusion parallel to B. But the effects of high magnetic field on the shift distance intended to get saturation when B was over a critical threshold. An analysis of the diffusion coefficients by means of the Matano plane and Darken equations indicated that the interdiffusion between Cu and Ni was accelerated by the high magnetic fields. On the other hand, in case of the direction of diffusion perpendicular to B, the shift distance of Kirkendall marker almost kept invariant under the application of varying magnetic fields. In addition, the interdiffusion behavior was retarded markedly by a "negative" magnetic field gradient.(2) The kinetic of interface migration in the liquid Bi/solid Bio.4Sbo.6 diffusion couple appeared to follow a parabolic relationship whether with or without a high magnetic field. It was found that the migration of the interface due to interdiffusion decreased markedly with the increasing strength of magnetic-field, even becoming 1200μm smaller at 11.5T than without a magnetic field. From the above result, we concluded that the interdiffusion behavior between the liquid metal and the solid alloy was strongly retarded by the application of high magnetic fields. This was relevant to the increasing of the viscosity in the liquid metal under a high magnetic field, as in turn caused the diffusion activation energy to increase. Moreover, the retarding effect of magnetic field gradient on diffusion was found to be more significant than that of a uniform magnetic field.(3) Reactive diffusion experiments at the liquid Al(Zn)/solid Cu interface were experimentally investigated under a high magnetic field. Although there was no noticeable influence of the magnetic field on the microstructure and phase composition of diffusion layers, the thickness of diffusion layers exhibited a decrease with a superposed peak centred on a field of 8.8T(Al/Cu) and 8T (Zn/Cu). The difference of the position of peak value might be induced by the different physical propertied between Al and Zn. In addition, the mean thickness of the diffusion layers (parallel to B) was found to be always greater than that of the diffusion layers (perpendicular to B) under the applied magnetic fields. These phenomena should be attributed to the effects of magnetic fields suppressing natural convection and inducing thermo-electromagnetic convection at the liquid/solid interface. The theoretical derivation of diffusion equation under high magnetic fields indicated that the effective diffusion coefficient went through a non-monotonic variation with magnetic flux densities, as agreed well with the experimental results. Thermodynamic analysis for the growth of diffusion layers shown that a high magnetic field of 11.5T enhanced the activation energy for atom diffusion, which resulted in the decrease of diffusion coefficient comparing with that without a magnetic field. A negative or positive field gradient could retard the diffusion of liquid Al(Zn) in solid Cu, as was similar with the experimental result obtained in the liquid Bi/solid Bi0.4Sb0.6 diffusion couple.(4) The application of a high magnetic field during pack aluminization process at a temperature of 1173K for 4.5h induced a significant change in the final products at the surface of substrate Cu. Experimental studies demonstrated that the coatings consisted of two layers namelyα+γ2 and a with B ranging from 0 to 8.8T, whereas only theαlayer with Kirkendall voids was observed in case of B≥10T. With the magnetic flux density increasing, the total thickness of coating first increased to a maximum at a field of 6.6T, and then decreased with further increasing magnetic flux density. These results may be attributed to the contradictory effects of high magnetic fields on chemical reaction and on diffusion。The studies in this paper might be helpful to understand the metallurgical phenomena related to diffusion in high magnetic fields, and provide foundation for the control of diffusion and reaction behaviors at a heterogeneous interface using high magnetic fields.