Gibbons, Sean Lee (2016-03). Austenite-Grain Refinement and Epsilon-Carbide Precipitation Optimization in Ultra High Strength Steel Alloy ES-1. Doctoral Dissertation.
Thesis
In this work, two thermo-mechanical treatments of Eglin Steel (ES-1) are designed that leverage computational tools and focus on microstructural grain refinement and optimal epsilon-carbide (Fe2:4C) precipitation. More specifically, ES-1 is presented as a candidate for Integrated Computational Materials Science and Engineering (ICME) optimization, highlighting key areas of desired improvement (e.g., tensile strength, hardness, and toughness). The two primary areas addressed throughout this work are optimal epsilon-carbide precipitation strengthening and parent austenite grain refinement. The methods used to assess and improve both parameters are outlined. Initially, the current state of ES-1 is outlined in terms of processing, microstructure, properties, and performance. It is then shown that the epsilon-carbide ground state crystal structure can be determined through a combination of first principle calculations and cluster expansions, which allows for the calculation and comparison of the ground state free energy surfaces of the various crystal structures proposed in literature. The ground state configuration of the most representative crystal structure is then used as the basis for Monte Carlo simulations to determine the finite temperature free energy surface. It is further shown that the epsilon-carbide free energy information can be coupled with existing thermodynamic and kinetic databases in order to develop a thermodynamically self-consistent phase field model (PFM) for the precipitation of epsilon-carbide. The PFM prognostic equations are outlined and methods for parallelization and evaluation are discussed. The model is then used to simulate the precipitation of epsilon-carbide at 470 K and the results are compared against experimental findings, showing good qualitative agreement. It is also shown that the precipitation of epsilon-carbide can be potentially optimized by using the PFM method in conjunction with derived system thermodynamics to develop epsilon-carbide precipitation process maps. Leveraging existing kinetic and thermodynamic databases, the remaining carbide precipitation sequences (i.e., Fe3C, M23C6, M6C, and VC) are mapped, creating a robust precipitation map of ES-1. Selected tempering regimes are experimentally assessed and used for verification of the epsilon-carbide process map. Furthermore, CALPHAD software is used to assess the thermodynamic stability of austenite in ES-1 in order to determine equal channel angular pressing (ECAP) procedures to refine parent austenite grain sizes, prior to transforming to martensite. Ultimately, this work shows that through the application of a proper thermo-mechanical treatment that the properties and performance of ES-1 and similar alloys can be significantly improved and/or tailored.
