Microstructures and superplastic behavior of eutectic Fe-C and Ni-Cr white cast irons produced by rapid solidification

Abstract

Superplastic behavior of two commercial grade white cast irons, eutectic Fe-C and Ni-Cr white cast irons, was investigated at intermediate temperatures (650 to 750 °C). For this purpose, rapidly solidified powders of the cast irons were fully consolidated by compaction and rolling at about 650 °C. The volume fractions of cementite in the eutectic cast iron and in the Ni-Cr cast iron were 64 pct and 51 pct, respectively, and both cast irons consisted of fine equiaxed grains of cementite (1 to 2 μm) and ferrite (0.5 to 2 μm). The cast iron compacts exhibited high strain-rate sensitivity (strain-rate-sensitivity exponent of 0.35 to 0.46) and high tensile ductility (total elongation of 150 pct to 210 pct) at strain rates of 10-4 to 10-3 s-1 and at 650 °C to 750 °C. Microstructure evaluations were made by TEM, SEM, and optical microscopy methods. The equiaxed grains in the as-compacted samples remained unchanged even after large tensile deformation. It is concluded that grain boundary sliding (e.g., along cementite grain boundaries in the case of the eutectic cast iron) is the principal mode of plastic deformation in both cast irons during superplastic testing conditions.