CAREER: Understanding the origins of pearlite discontinuities in eutectoid microstructures: Modeling & Experiments

第一部分：非技术概述钢有许多不同的类型和几种分类的方法。对它们进行分组的一种方法是根据人们在显微镜下仔细观察时看到的结构类型。研究金属的科学家把这种只有在极高倍率下才能看到的结构称为“微结构”。“珠光体”或“珠光体微结构”是一种具有层状组织的钢，具有坚硬而坚固的结构，通常用于铁路、吊桥支撑电缆和切割工具等需要高强度的应用。然而，在许多其他应用中，钢在成形或加工成复杂形状之前需要软化。虽然钢的使用和研究已经有很多代人了，但了解不同的工艺条件如何具体影响钢的微观组织在某种程度上是有限的，特别是在商业钢中，复杂的化学成分使理解原子水平上发生的事情变得困难。这项集计算、实验和表征技术于一体的职业奖项，将允许进行一项有组织的研究，考察珠光体钢层状微结构的断裂以及产生断裂的原子相互作用。具体地说，这项研究将对只含有四种元素(铁、碳、锰或硅)中的三种的模型钢进行加热和冷却实验，并结合计算机模拟和先进的2D、3D和4D(3D通过时间)成像来研究材料的制造方式与它们所具有的微观结构之间的关系。建立这些关系将使钢铁部件制造中的组织水平控制成为可能，而这是目前炼钢领域缺少的一点知识。该项目的外展部分解决了弥合材料研究与全国材料科学与工程(MSE)年度招生短缺之间巨大差距的迫切需要。为了追求这一目标，该项目鼓励各级学生通过以学生为中心的交流，结合主动学习，在MSE领域追求教育和职业生涯。这种方法可以帮助解决人口代表性不足的许多社区特定需求，将被纳入为高中、本科生和研究生量身定做的外展计划。第2部分：技术总结由片状珠光体组成的钢显微组织因其层状组织而具有高抗拉强度、良好的韧性和硬度。然而，为了提高这种微观组织的成形性，钢必须软化，这通常是通过促进珠光体球化或非合作演化的退火热处理来实现的。添加合金元素，如锰和硅，也会影响珠光体球化，导致片层不连续和改善延展性。不幸的是，目前对珠光体不连续产生的机制以及它们是如何受到合金成分、加工温度、先前的奥氏体晶尺寸和位错密度的影响的了解还不够深入。虽然理解加工过程中的多元扩散和珠光体组织演变是具有挑战性的，但对最终决定力学性能的组织施加更大的控制是必要的步骤。由于不能单独通过实验或数值计算来预测相形态的演变，因此需要开发一种将相场模型与实验、表征和显微组织量化相结合的综合方法，以更好地理解工艺与微结构的关系。为了整合和利用现有的高保真工具，这项职业奖检验了这样一个假设，即钢组织中珠光体片层的不连续生长是由非合作机制造成的。为了验证这一假设，三元Fe-C-Mn和Fe-C-Si钢的计算和退火研究将得到先进的表征技术的补充，如X射线计算机断层扫描、利用能谱的分析透射电子显微镜和电子背散射衍射。虽然使用空间相关函数对模拟的和表征的显微组织进行比较将有助于对珠光体组织中导致片层不连续的机制有一个基本的了解，但通过对所获得的数据集进行主成分分析来推断加工-显微组织之间的联系。该项目的更广泛影响存在于两个部分。第一个是开发和自由传播用于钢中片层不连续的综合实验、计算和四维表征协议。第二个项目利用该项目的技术研究来部署以学生为中心的教育和推广计划，使用在四个活动部分实现的主动学习：(I)本科生和高中的“偶像”计划，(Ii)本科生研究，(Iii)本科生和研究生课程的开发，以及(Iv)远程阶段实地工作坊。该奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

PART 1: NON-TECHNICAL SUMMARY There are many different types of steels and several ways to categorize them. One way to group them is according to the types of structures one sees when looking closely at them under a microscope. Scientists who study metals call these structures that are only visible at extremely high magnification, "microstructures". "Pearlite" or "Pearlitic microstructures" are a type of steel that has a layered microstructure that is hard and strong and is commonly used in applications that require high strength such as railroads, drawbridge support cables, and cutting tools. However, in many other applications, steel needs to be softened before they can be formed or machined into complex shapes. Although steels have been used and studied for many generations, understanding how different processing conditions specifically impact steel microstructure is somewhat limited, particularly in commercial steels where complex chemical compositions make understanding what is occurring on an atomic level difficult. This CAREER award, which integrates computational, experimental, and characterization techniques, will allow for an organized study examining breaks in the layered microstructures of Pearlitic steels and the atomic interactions that produce them. Specifically, this study will use heating and cooling experiments on model steels containing only three of four elements (Fe, Carbon and Manganese or Silicon) in conjunction with computer simulations and advanced 2D, 3D and 4D (3D through time) imaging to investigate the relationship between how materials are made and what microstructures they come to possess. Establishing these relationships will enable microstructure level control in the manufacturing of steel components, which is currently a missing bit of knowledge in steel-making. The outreach component of this project addresses the pressing need to bridge the vast gap between materials research and national annual enrollment shortages in Materials Science and Engineering (MSE). In pursuit of this goal, this project encourages students at all levels to pursue education and careers in MSE fields through student-centric communication that incorporates active learning. This approach, which can help address many community-specific needs for underrepresented populations will be incorporated into a tailored outreach plan for high-school, undergraduate and graduate students.PART 2: TECHNICAL SUMMARYSteel microstructures consisting of lamellar pearlite are known to possess high tensile strength, excellent toughness, and hardness due to their layered microstructure. However, to enhance the formability of such microstructures, steel must be softened, which is typically accomplished via annealing heat treatments that facilitate the spheroidization or non-cooperative evolution of pearlite. The addition of alloying elements, such as Manganese and Silicon can also impact pearlite spheroidization, leading to lamellar discontinuities and improved ductility. Unfortunately, current understanding of the mechanisms by which pearlitic discontinuities arise and how they are influenced by alloy composition, processing temperature, prior austenite grain size, and dislocation densities leave much to be desired. While comprehending multi-component diffusion and pearlitic microstructural evolution during processing is challenging, it is a necessary step to exert greater control on microstructure which ultimately determines mechanical properties. Since the evolution of phase morphologies cannot be predicted through experiments or numerical calculations in isolation, the development of an integrated approach, that combines phase-field models with experiments, characterization, and microstructure quantification is required to better understand processing-microstructure relations. To integrate and leverage existing hi-fidelity tools, this CAREER award tests the hypothesis that discontinuous growth of pearlitic lamellae in steel microstructures occur by a non-cooperative mechanism. To examine this hypothesis, computational and annealing studies of ternary Fe-C-Mn and Fe-C-Si steels will be complemented by advanced characterization techniques such as X-ray Computed Tomography, Analytical Transmission Electron Microscopy leveraging Energy Dispersive Spectroscopy, and Electron Backscatter Diffraction. While a comparison of simulated and characterized microstructures using spatial correlation functions will facilitate a basic understanding of the mechanisms that induce lamellar discontinuities in pearlitic microstructures, processing-microstructure linkages will be deduced through Principal Component Analyses of the obtained datasets. The broader impacts of this project exist in two parts. The first is the development and free dissemination of an integrated experimental, computational and four-dimensional characterization protocol for lamellar discontinuities in steels. The second leverages the technical research of this project to deploy student-centric education and outreach programs using active learning realized in four segments of activity: (i) undergraduate and high-school “Idol” programs, (ii) undergraduate research, (iii) undergraduate and graduate course development, and (iv) remote phase-field workshops.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.