Multiscale study of the pearlitic microstructure in carbon steels: atomistic investigation and continuum modeling of iron and iron-carbide interfaces
Date
2018
Authors
Guziewski, Matthew, author
Weinberger, Christopher, advisor
Heyliger, Paul, committee member
Kota, Arun, committee member
Ma, Kaka, committee member
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Abstract
While the behavior of carbon steel has been studied extensively for decades, there are still many questions regarding its microstructures. As such, classical atomistics is utilized to obtain further insight into the energetics, structure, and mechanical response of the various interfaces between iron and iron-carbides. Simulations were constructed for the commonly reported orientation relationships between ferrite and cementite within pearlite: the Bagaryatskii, the Isaichev, and the Pitsch-Petch, as well as their associated near orientations. Dislocation arrays are found to form for all orientation relationships, with their spacing and direction a function of lattice mismatch. Within each orientation relationship, different interfacial chemistries are found to produce identical dislocation spacings and line directions, but differing interfacial energies. This chemistry component to the interfacial energy is characterized and it is determined that in addition to the lattice mismatch, there are two structural factors within the cementite terminating plane that affect the energetics: the presence of like site iron pairs and proximity of carbon atoms to the interface. Additionally, an alternate method for determining the interfacial energy of systems in which there are multiple chemical potentials for a single element is developed and implemented, an approach which is likely valid for other similar systems. Atomistics finds the Isaichev orientation relationship to be the most favorable, while the "near" orientation relationships are found to be at least as energetically favorable as their parent orientation relationships. A continuum model based on O-lattice theory and anisotropic continuum theory is also applied to the atomistic results, yielding interfacial energy approximations that match well with those from atomistics and allowing for the characterization of the Burgers vectors, which are found to lie in high symmetry directions of the ferrite on the interface plane. The continuum model also allowed for the analysis of the system with changing lattice and elastic constants. This revealed that while most of the orientations had relatively small variation in their energetics with these changes, the Isaichev orientation was in fact very sensitive to variations in the lattice constants. The use of temperature dependent values for lattice and elastic constants suggested that while the Isaichev is most favorable at low tempertaures, other orientations may become more favorable at high temperatures. This combined atomistic/continuum approach was also applied to the austenite-cementite system and used to compare the proposed habit planes of both the Pitsch and Thompson-Howell orientation relationships. This analysis found the two orientation relationships to be unique, a point of previous contention, with the Pitsch the more favorable. Atomistic modeling was further used to investigate the mechanical response to compressive and tensile straining of the pearlitic orientation relationships. A range of interlamellar spacings and ferrite to cementite ratios are considered, and values for important mechanical properties including elastic modulus, yield stress, flow stress, and ductility are determined. Mechanical properties are shown to be largely dependent on only the volume ratios of the cementite and ferrite, with the interlamellar spacing having an increasing role as it reaches smaller values. Slip systems and Schmid factors are determined for a variety of loading states in both the transverse and longitudinal directions and were used to fit to simple elasto-plastic models. Transverse loading is observed to follow simple 1-D composite theory, while longitudinal loading requires the consideration of the strain compatibility of the interface. Orientation, and specifically the alignment of slip planes in the ferrite and cementite, was also determined to play a role in the mechanical response. Alignment of favorable slip planes in the cementite, notably the {100}θ and {110}θ, with high symmetry directions in the ferrite was found to greatly enhance the ductility of the system in longitudinal loading, as well as allow for lower flow stresses in transverse loading.
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Subject
dislocations
plastic deformation
pearlite
atomistics