Bulk amorphous FC20 (Fe-C-Si) alloys with small amounts of B and their crystallized structure and mechanical properties

The addition of a small amount (0.4 mass%) of B to a commercial FC20 cast iron was found to cause the formation of an amorphous phase in melt-spun ribbon and cast cylinders with a diameter of up to 0.5 mm. The structure of a melt-spun B-free FC20 alloy consisted of alpha-Fe, gamma-Fe and Fe3C. The effectiveness of additional B is presumably due to the generation of attractive bonding nature among the constituent elements. The amorphous alloy ribbon exhibits a high tensile strength of 3480 MPa and good bending ductility. The annealing causes the formation of an amorphous phase containing alpha-Fe particles with a size of about 30 nm. The mixed phase alloy exhibits an improved tensile strength of 3800 MPa without detriment to good ductility. With further increasing temperature, the mixed amorphous and alpha-Fe structure changes to alpha-Fe + Fe3C + graphite through the metastable structure of alpha-Fe + Fe3C. The structure after annealing for 900 s at 1200 K has fine grain sizes of about 0.5 mu m for alpha-Fe, 0.3 mu m for Fe3C and 1 mu m for graphite. The graphite-containing alloy exhibits high tensile strength of 1200-2000 MPa and large elongation of 5-13%. The high tensile strength and good ductility were also obtained for the 0.5 mm cylinder annealed at 1200 K. The good mechanical properties are due to the combination of fine subdivision of crack initiation sites by the homogeneous dispersion of small graphite particles and the dispersion strengthening of Fe3C particles against the deformation of the alpha-Fe phase. The synthesis of the finely mixed alpha-Fe + Fe3C + graphite alloys having good mechanical properties by crystallization of the new amorphous alloy in the melt-spun ribbon and cast cylinder forms is encouraging for the future development of a new Fe-based high-strength and high-ductility material. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.