|2. Brief description of our research activity|
- Fabrication of nanocrystalline ferrite by heavy deformation:Nanocrystalline materials are polycrystals with a crystalline size less than 100 nm (typically 5-50 nm, 1nm = 10-9m). This type of material has found to possess novel properties quite different from conventional polycrystalline materials with coarse crystals (typically several tens of micrometers). Intensive researches on the production, structure characterization and properties of nanocrystalline materials have been carried out since 1984 and a variety of nanocrystalline materials have been produced by inert gas condensation, chemical vapor deposition, physical vapor deposition, mechanical alloying, etc.
In our laboratory, efforts have been made to produce nanocrystalline Fe-C alloys through heavy deformation process. Since Fe-C alloys, known as steels, are the most popular alloys applied all over the world, the significance of producing steels with nanocrystalline structure is great to either academic research and practical application. In the present stage, processes such as ball milling, weight drop test, cold rolling, etc., are being carried out on pure iron, martensite, ferrite and spheroidite. We have succeeded in producing nanocrystalline ferrite through ball milling in the above Fe-C alloys. The detailed microstructure characterization and mechanical properties of the obtained nanocrystalline ferrite are under investigation.
- Production and characterization of bulk cementite:Cementite (Fe3C, theta phase) is one of the most important phases in steels, which plays a critical role in the mechanical properties of steels. It has been claimed to be metastable at all temperatures with respect to graphite and its saturated solution in iron. It has a complicated crystal structure (a=0.45248 nm, b=0.50896 nm and c=0.67443 nm), which can be understood as a regular stacking of a prism of Fe atoms containing a carbon atom in the center position. It is harder than ferrite and is quite brittle. However, detailed study in its properties was obstructed by the difficulty to produce bulk specimen of cementite. Most of the measurements up to date were performed in the cementite either extracted from cast iron or steels or cementite films (2.5-3.0 micrometer thick) produced by PVD method. Larger discrepancy exists therefore due to the large difference in the individual specimens.
Recently, by combining mechanical alloying (MA) with spark plasma sintering (SPS), bulk cementite has been successfully fabricated, with a diameter of 15mm and a relative density of >95%. This is the first time in the world that a cementite was fabricated. We have performed the characterization of microstructure and property measurements (hardness, compression, Young's modulus, magnetic, thermal expansion, specific heat, thermoelectric, electric resistance, etc.) of bulk cementite. Now we are trying to produce alloyed cementite with various alloying additions (Cr, Mn, Mo, V, Ti, Ni, Si, W, etc.). Both experimental and theoretical approaches are being carried out. It has been found that additions of one of Cr, Mn, Mo and V can stabilize cementite to a much higher temperature. The microstructure and properties of alloyed cementite have also been studied. The latest result was the microstructure observation of unalloyed and alloyed cementite, in which we have observed the grain boundaries of cementite first time in the world. The more detailed investigation is being carried out.
- Thermoelectric Materials:The thermoelectric effect, discovered in 1823 by Seebeck, is findingmore applications than ubiquitous thermocouples. FIG.1 is a schematicsof an electric generator using a thermoelectric semiconductor. It can beused to extract electric power from waste heat by keeping the p-n junctionside of the U-shaped devices hot and the other side cold. We have beenfocusing on the development of metallic silicide thermoelectric materials,using MA and SPS. By fine tuning the microstructure, especially its grainsize, and by appropriate doping, it is possible to maximize the efficiencyof the power generation.