ERI: Understanding Austenite Refinement Mechanism in Nickel-Alloyed Ductile Cast Irons

This Engineering Research Initiation (ERI) grant enables fundamental understanding of refining austenite grain structure in nickel-alloyed ductile irons. Compared to steel and non-ferrous alloys, ductile iron castings are easy to manufacture by established metal casting processes. Ductile irons are widely used to produce structural components in automotive, agriculture, mining, construction, municipal water/sewer, and defense industries and are easily recyclable. This research is aimed at providing an economical method to improve mechanical properties of the as-cast ductile iron via refining its grain structure without the need of large amounts of expensive alloying element additions or energy-intensive heat treatment. This award supports fundamental research to provide the needed knowledge for the development of a robust in-situ grain refinement process for austenitic ductile iron. The refined grain structure improves the strength and toughness of the ductile iron, which leads to the development of higher strength-to-mass ratio castings and thinner section and lighter weight components. The benefits of lightweight ductile iron cast parts are reduced energy consumption and carbon emission during production, service, and recycling, which benefits the U.S. economy and society. This research involves several disciplines including metallurgy, materials science, and manufacturing. The multi-disciplinary approach helps broaden participation of women and underrepresented students in research and positively impacts engineering education. Refinement of austenite in ductile iron can result in a finer final microstructure improving its mechanical properties. However, the approach to achieve in-situ austenite grain refinement during the melting and solidification process and the associated mechanisms are not well understood. A nickel-alloyed ductile iron is designed with thermodynamic equilibrium calculations to retain austenite structure during solidification. Four elemental additions, cerium, titanium, aluminum, and bismuth are selected. The selected alloying additions are hypothesized to refine the austenite microstructure either by promoting heterogeneous nucleation of austenite grains, or by impeding austenite grain growth. To determine the grain refinement mechanism, partially solidified iron is quenched to capture early formation of austenite on nucleation sites and retain growth front of austenite grain in contact with liquid. Samples are analyzed using electron back scattered diffraction, scanning electron microscopy, energy dispersive X-ray, transmission electron microscopy, and selected area electron diffraction to understand the austenite grain refinement mechanism. The project evaluates processing-structure-property relationships in the grain refined nickel-alloyed ductile iron in the as-cast state. This foundational research is generalizable and could be applied to understand grain refinement mechanisms in other cast irons such as gray iron, compacted graphite iron, and white iron. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.