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

Arizona State University, Scottsdale AZ

Investigators

Abstract

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 students will be incorporated into a tailored outreach plan for high-school, undergraduate and graduate students. PART 2: TECHNICAL SUMMARY Steel 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.