Formation and mechanical properties of bulk amorphous FC20 (Fe–C–Si) cast iron with small addition of B

Abstract

By the addition of a small amount (1.5 at.%) of B to a commercial FC20 alloy, an amorphous phase was formed in a melt-spun ribbon as well as in a cast rod of 0.5 mm in diameter. The melt-spun B-free FC20 alloy is composed of α-Fe, γ-Fe and Fe 3C. The additional B has great effect on the increase in the glass-forming ability (GFA). The melt-spun amorphous alloy exhibits a high tensile strength ( σ f) of 3480 MPa. The crystallization takes place by the process; amorphous (Am)→Am′+α-Fe→Am″+α-Fe+Fe 3C→α-Fe+Fe 3C→α-Fe+Fe 3C+graphite. The structure after annealing for 900 s at 1200 K has fine grain sizes of about 0.5 μm for α-Fe, 0.3 μm for Fe 3C and 1 μm for graphite. The graphite-containing alloy ribbon exhibits high σ f of 1200–2000 MPa and large elongation ε f of 5–13% which exceed largely those (200–300 MPa and ∼0%) for FC20 cast iron. The cast alloy rod of 0.5 mm in diameter annealed for 3.6 ks at 1200 K exhibits high σ f 1530 MPa and large ε f of 9%. The good mechanical properties are presumably due to the combination of subdivision of crack initiation sites caused by the homogeneous dispersion of fine graphite particles and the dispersion strengthening of fine Fe 3C particles. The synthesis of the bulk alloy rods 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.