Bulk Amorphous FC20 (Fe–C–Si) Cast Iron with Small Addition of B

Abstract

The addition of a small amount (1.5 at%) of B to a commercial FC20 alloy was found to cause the formation of an amorphous phase in a melt-spun ribbon as well as in a cast cylindrical ingot with a diameter of 0.5 mm. The melt-spun B-free FC20 alloy is composed of α-Fe, γ-Fe and Fe 3 C. The additional B has great effectiveness on the increase in the glass-forming ability presumably because of the generation of attractive bonding nature among the constituent elements. The melt-spun amorphous alloy exhibits a high tensile strength of 3480 MPa and good bending ductility. The crystallization takes place by the following process; amorphous (Am)→Am'+α-Fe→Am+α-Fe+Fe 3 C→α-Fe+Fe 3 C →α-Fe+Fe 3 C+graphite. The final structure obtained by annealing for 900 s at 1200 K has fine grain sizes of about 0.5 μm for α-Fe, 0.3 μm for Fe 3 C and 1 μm for graphite, The graphite-containing alloy ribbon also exhibits high tensile strength of 1200 to 2000 MPa and large elongation of 5 to 13% which exceed largely those (200 to 300 MPa and approximately zero %) for FC20 cast iron, depending on annealing condition. The cast amorphous cylinder with a diameter of 0.5 mm annealed for 3.6 ks at 1200 K also exhibits high tensile strength of 1530 MPa, large elongation of 9% and good bending ductility. The good mechanical properties are presumably due to the combination of fine subdivision of crack initiation sites caused by the homogeneous dispersion of small graphite particles and the dispersion strengthening of fine Fe 3 C particles against the deformation of α-Fe phase. The synthesis of the mixed bulk alloy of α-Fe, Fe 3 C and graphite phases having good mechanical properties by crystallization of the new amorphous alloy is promising for the future development of the FC20 type alloy as a new type of high-strength and high-ductility material.