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ATOMISTIC SIMULATION AND FIRST PRINCIPLES CALCULATION OF INTERFACIAL ENERGETICS OF GALVANIZED ADVANCED HIGH STRENGTH STEELS
AdvisorLi, Bin B.L.
Materials Science and Engineering
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The 3rd generation (3G) advanced high strength steels (AHSSs) have drawn significant attention in recent years because applications of AHSSs will enable vehicle mass reduction with safety standards met, which can reduce the emissions of greenhouse gases of vehicles. However, the increased concentration of alloying elements that are contributors to the improved strength, i.e., Si, Mn, in AHSSs raises concerns of how galvanizing processes for these steels will be affected. Galvanizing is a cost-effective processing method widely used in industry to improve the corrosion resistance of steels. For the galvanizing processes of AHSSs, Si and Mn can be selectively oxidized during annealing and their oxides are difficult to be reduced. The oxides will affect the subsequent hot-dipping of the steel into liquid Zn bath to form a Zn coating. During hot-dipping, complex metallurgical reactions happen. As a result, a Zn coating consisting of inhibition layer, Fe-Zn intermetallics and Zn overlay forms. The oxides formed during annealing will influence the metallurgical reactions and may lead to galvanizing defects such as pinholes and bare spots. To investigate the interfacial structures between the steel substrate and the Zn overlay, extensive experimental studies have been conducted, by using post-mortem scanning electron microscopy (SEM), transmission electron microscopy (TEM) observation, along with other materials characterization technologies. However, the size of the interface zone is typically on the order of 100 nm, and complex interfacial reactions occur in a short time period (seconds). Thus, it is challenging to fully understand the physics of galvanizing solely by conducting experimental studies. In recent years, computational studies have been extensively used in material science and engineering. However, to handle the complex interfacial reactions involving multiple elements and phases, an interatomic potential with high fidelity is needed. Current research on galvanizing coating interface is restricted to calculations at the electronic scale. Mesoscale method such as phase field modeling was also restricted due to the lack of physical properties of surface and interfaces in galvanizing. In this work, based on the reported Fe-Mn-Si-C Modified Embedded Atom Method (MEAM) potential describing low alloy steels, we develop a seven-element second nearest neighbor (2NN) MEAM potential to describe multiple phases present at the interface between the steel substrate and the Zn overlay. The complex crystal structure and mechanical properties of the inhibition layer and the Fe-Zn intermetallics are well captured. The geometry of the oxide phases, MnO and alpha-quartz SiO2 can also be well described. A MATLAB toolkit is developed for potential development. This toolkit is used to reduce the property errors between MEAM prediction and the experimental values, like most potential development work does. Three criteria are proposed and used to avoid physically inadmissible solution space. Through potential development, it is found that satisfying proper criteria is more important than reducing property errors. After the MEAM potential is developed, surface and interface properties are investigated. Molecular dynamics (MD) simulations are conducted to calculate surface energy and interfacial energy between steel substrate, inhibition layer, Fe-Zn intermetallics and oxides. The calculation results show a negative interfacial energy between Fe substrate and the inhibition layer, a manifestation of the high affinity between Fe and Al. Negative interfacial energy is also found in Fe-FeZn intermetallics interface but its value is higher than that between Fe and the inhibition layer. Negative interfacial energy is found for Fe-SiO2, and positive interfacial energy for Fe-MnO, which may explain why SiO2 is detrimental to galvanizing coating process while MnO is less detrimental. Positive interfacial energy is found for Fe3Al8-oxides interface, but negative interfacial energy is found for interface between FeZn intermetallics and oxides, which seems to disagree with experimental results. Work of adhesion (WOA) is then calculated based on the obtained surface and interfacial energy to characterize the strength of the interface. The calculation results agree with the results obtained by other methods. Importantly, the results presented in this thesis provide a comprehensive database for the surface and interface properties of phases in galvanizing coating, which are not available now to the community.