Effects of Magnetic-Field on Elemental Process for Microstructural Development of Iron-Based Polycrystalline Materials

Since the bulk properties such as mechanical and functional properties of polycrystalline materials are strongly affected by microstructure, the control and optimization of the microstructure are essential to obtain desirable properties and to develop high performance polycrystalline materials. The application of external fields such as stress, electric and magnetic fields have been noticed as a useful technique of controlling microstructure of structural and functional materials. However a magnetic field has never been applied to industrial technologies in the materials processing due to the difficulty of generating strong magnetic field enough to affect microstructural evolution of materials. For this reason, there were only a few reports of the observation of microstructural changes under a magnetic field during annealing, which were motivated not by industrial application, but by the academic interests. Then strategies for controlling microstructures and properties of materials by the application of a magnetic field have been suggested particularly since the helium-free superconducting magnet was developed in 1990’s. Nowadays, the application of a strong magnetic field enough to affect the microstructural evolution of materials can be obtained without difficulty; the generators which can apply more than 10 T magnetic field are available. Although there have been reported many interesting metallurgical phenomena under a magnetic field, most of those mechanisms of magnetic field effects have not yet been revealed. The application based on the fundamental research infrastructure is essential to extensively apply magnetic processing to the technologies for materials development. Therefore, to conduct the systematic studies of EPM, the fundamental studies are required to clarify the elemental processes for microstructural development under a magnetic field. The aim of current work is to obtain comprehensive knowledge of the mechanism of magnetic field effects on microstructural evolution in iron-based materials in order to conduct the structural and functional controls by the application of strong magnetic fields. The effects of strong magnetic fields on fundamental metallurgical phenomena (diffusion, grain nucleation and grain growth) were investigated to obtain comprehensive knowledge of the mechanism of the effects of magnetic field on microstructural evolution in iron-based materials. In addition, the fundamental physical properties of grain boundaries of iron closely concerning with the grain nucleation and growth were also investigated. The following results and conclusions were obtained in each chapter. In Chapter 2, some experiments were conducted to investigate the effect of a magnetic field on the carbon diffusion and its solid solubility in pure iron. Measurements of carbon diffusion in iron under a magnetic field and the field gradient were conducted with the explosive welded diffusion couples of pure