| Enormous amounts of magnetization energy can be transferred to a material at atomic scale without any contact when the material is placed in a high magnetic field. As a result, the thermodynamic state is changed obviously and thus the arrangement, matching, migration and other behaviors of atoms and molecules of the material are affected. Therefore, the high magnetic field can result in an enormous and profound influence on the microstructure and mechanical properties of materials. Diffusion is one of the most important fundamental scientific problems in material science. Many processes and phenomena are closely related to the diffusion. Consequently, it is beneficial to investigate the elements diffusion under high magnetic field for exploring the physical nature of new phenomena in high magnetic field and revealing intrinsic mechanisms of processes. It would lay a theoretical foundation for preparing new materials and developing new technologies.Optical microscope (OM), scanning electron microscope (SEM), electron probe microanalysis (EPMA), transmission electron microscope (TEM) and other methods were employed to investigate the elements diffusion in Ni-Cu and Ni-Al system under high magnetic field. The formation and growth of diffusion-induced recrystallization (DIR) regions in Cu-Ni-Cu diffusion couples were also studied. In addition, the mechanism of the atom diffusion retarded by the applied magnetic field was discussed. On the basis of the above law, the effects of high magnetic fields on the microstructure and property of Ni-based K52 after aging treatment were studied deeply. The primary conclusions are as follows:During the atomic interdiffusion in Ni-Cu system under high magnetic field, it is found that the interdiffusion coefficients increase with the increasing molar fraction of Ni atoms in Ni-Cu solid solution when the diffusion couples annealed with and without magnetic field. It is noted that all the interdiffusion coefficients and the penetration distance of Ni atoms in Cu phase under magnetic field are smaller than those without magnetic field. It indicates that the magnetic field retards the atomic interdiffusion in Ni-Cu system. This retardation is achieved through reducing the frequency factors but not changing the interdiffusion activation energies.The formation and growth of DIR regions in Ni-Cu diffusion couples were investigated. The results demonstrate that the DIR regions are formed at the Ni/Cu interface in all diffusion couples with and without magnetic field. The thickness and size of recrystallized grains of DIR regions increase with increasing annealing temperature and time, respectively. However, when the couples were annealed under magnetic field, the size of recrystallized grains is smaller than those without magnetic field. Meanwhile, the growth of DIR regions are retarded obviously by applying the magnetic field and such retardation action is strengthened with increasing magnetic induction intensity, but the growth of DIR regions is independent of the magnetic field direction. As for the retardation action, it can be attributed to the following two aspects. First, when a magnetic field is applied, the magnetic Gibbs free energy of the band alloyed region at the front end of DIR region near the migrating boundary which composition is less than 57at%Cu will increase. As a result, it will go against the growth of DIR region and this band would become a barrier layer to inhibit the growth of the whole DIR region towards the Ni phase. Second, the magnetic field retards the atomic interdiffusion in Ni-Cu system and recrystallization process.The effects of high magnetic field on the constituent and growth of intermetallic phases formed at the interface of Ni-Al diffusion couples were studied. A preliminary explanation was made on the mechanism of the atom diffusion retarded by applying high magnetic field based on the ambipolar diffusion theory. The results demonstrate that the diffusion zones compose of Ni2Al3 and NiAI3 when the couples were annealed with and without magnetic field. It means that the magnetic field has not modified the phase constituent of the couples. As for the growth of intermetallic phases, it is found that they always obey the parabolic law with and without magnetic field. It suggests that their growth was controlled by volume diffusion. It is noted that the growth of both intermetallic phases under the magnetic field is retarded through decreasing the frequency factors but not changing the activation energies. Besides, the growth of intermetallic phases is related to the magnetic field direction.The shape of the coarseγ'particles changes from spherical to cuboidal when the alloy is subjected to aging treatment under high magnetic field. The tendency of shape change increases with increasing magnetic induction intensity. It can be attributed to the following two factors. First, the magnetic field might increase the lattice misfit between the y matrix andγ'precipitates. Second, the competition between the elastic strain energy and interfacial energy is also an important reason for the shape change of the coarseγ'particles. With the increase of the magnetic induction intensity, the number of nano-precipitates within the secondary y'precipitates remarkably decreases. When a 10T magnetic field was applied, these nano-precipitates disappeared completely. The hardness of the alloy decreases when it is subjected to aging treatment under high magnetic field. |
