Evolution of Microstructure in Directionally Solidified Cast Iron Treated with Cerium and Magnesium

Abstract

Cast iron is a eutectic alloy consisting primarily of iron, carbon, and silicon. Depending on the composition and solidification conditions, the carbon can solidify either as cementite or graphite with a variety of morphologies. Because the microstructure directly influences the mechanical properties and the performance of the macroscopic part, a deeper understanding of the solidification behavior of the material is required. In this work, the solidification velocity and composition were independently and systematically varied in a well-controlled environment. Near-eutectic samples with varying amounts of Mg, Ce, and Si were directionally solidified with velocities from 10 to 30 $\frac{{\mu {\text{m}}}}{{\text{s}}}$ in a Bridgman furnace. White cast iron was used as a starting material, which effectively suppressed flotation of graphite, so that a uniform composition and microstructure along the length of the samples was achieved. A range of morphologies were observed, including spheroidal austenite–graphite colonies, austenite–cementite eutectic, nodular graphite, and two distinct types of austenite dendrites. These structures were qualitatively and quantitatively assessed, and compared with a recently published schema describing the solid/liquid interface behavior of cast iron as a function of cooling rate and nodularizing elements.