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

这项工程研究启动（ERI）赠款使人们对镍合金球墨铸铁的细化奥氏体晶粒结构有了基本的了解。与钢和非铁合金相比，球墨铸铁铸件很容易通过既定的金属铸造工艺制造。球墨铸铁广泛用于生产汽车、农业、采矿、建筑、市政供水/下水道和国防工业的结构部件，并且易于回收。本研究旨在提供一种经济的方法来改善铸态球墨铸铁的力学性能，通过细化其晶粒组织，而不需要大量昂贵的合金元素添加或能源密集型热处理。该奖项支持基础研究，为开发稳健的奥氏体球墨铸铁原位晶粒细化工艺提供所需的知识。细化的晶粒组织提高了球墨铸铁的强度和韧性，从而发展出高强度质量比的铸件和更薄的断面和更轻的重量部件。轻量化球墨铸铁件的优点是在生产、服务和回收过程中减少能源消耗和碳排放，有利于美国经济和社会。这项研究涉及多个学科，包括冶金学、材料科学和制造。多学科方法有助于扩大妇女和代表性不足的学生参与研究，并对工程教育产生积极影响。球墨铸铁中奥氏体的细化可以产生更精细的最终组织，从而改善其力学性能。然而，在熔炼和凝固过程中实现原位奥氏体晶粒细化的方法及其相关机制尚不清楚。采用热力学平衡计算设计了一种镍合金球墨铸铁，使其在凝固过程中保持奥氏体结构。四种元素的添加，铈，钛，铝和铋被选择。假设所选择的合金添加物通过促进奥氏体晶粒的非均相形核或阻碍奥氏体晶粒长大来细化奥氏体组织。为了确定晶粒细化机制，对部分凝固的铁进行淬火，以捕捉奥氏体在成核部位的早期形成，并保留与液体接触的奥氏体晶粒的生长前缘。利用电子背散射衍射、扫描电镜、能量色散x射线、透射电镜和选择区域电子衍射对样品进行分析，了解奥氏体晶粒细化机理。本项目对铸态晶粒细化镍合金球墨铸铁的加工-组织-性能关系进行了评价。这一基础研究具有推广意义，可用于了解灰铸铁、致密石墨铸铁和白口铸铁等其他铸铁的晶粒细化机制。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.