Control of degradation of materials properties in recycling by grain boundary engineering

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Today, recycled materials are universally growing in importance because of savings of material resources, energy. However, harmful impurities like Sn and Cu are inevitably contained in scrap steel, and segregate to grain boundaries due to their low solid solubility in iron. Their segregation severely degrades mechanical properties of recycled steel. The motivation of this project was to develop methodology for controlling materials degradation on the basis of grain boundary engineering. The chief results obtained are follows.1. Controlling grain boundary segregation and segregation embrittlement by application of magnetic field : It was found that magnetic annealing of Fe-Sn alloy at 973K in ferromagnetic temperature region could suppress Sn segregation to grain boundaries, resulting in a significant increase of fracture toughness. The higher magnetic field strength, the more increases fracture toughness of Fe-Sn alloys.2. Magnetic property of magnetically annealed Fe-Sn alloy : VSM measurements revealed that coercive force for magnetically annealed Fe-0.8at%Sn alloy decreased with increasing magnetic field strength : i.e., softness of this alloy can be enhanced by magnetic annealing.3. Introduction of insensitive microstructure to grain boundary segregation : We studied materials processing to introduce insensitive microstructure to grain boundary segregation, and found that a rapid solidification was useful to increase a fraction of special boundaries. The fraction of special boundaries was approximately 45% in a rapidly solidified sample, whereas it was 20% in a thermo-mechanically treated one. Three-point bend tests of these samples revealed that the former could show a good plasticity but the latter fractured without showing any plastic deformation.