The Influence of Alloying and Temperature on the Stacking-fault Energy of Iron-based Alloys

Hojjat Gholizadeh Noush Abadi

    Research output: ThesisDoctoral Thesis

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    Abstract

    The mechanical properties of steels are influenced by their plastic deformations. In austenitic steels, plastic deformations may occur through different mechanisms including dislocation gliding, twinning (twinning-induced plasticity, TWIP), and phase transformation (transformation-induced plasticity, TRIP). The stacking-fault energy (SFE) governs the activation of these mechanisms; therefore it is a crucial parameter for understanding the plastic deformations. The aim of this thesis is to calculate the SFE in austenitic steels. In order to investigate the influence of interstitial carbon on the SFE in austenitic carbon steels, we calculate the γ-curve which contains the SFE. Explicit faults are simulated in pure iron, Fe24C, and Fe3C, corresponding to 0, 0.89, and 6.67 wt.% of carbon, respectively. Our first-principles calculations are performed using the all-electron full-potential linearized augmented planewave (FP-LAPW) method implemented in the WIEN2k code. Our results demonstrate a strong dependence of the behavior of the γ-curve on (i) the carbon content, and also on (ii) the position of the interstitial carbon with respect to the fault plane. In agreement with the earlier experimental and theoretical works, we find that the SFE increases with carbon content. Moreover, our results show that the increase rate is not constant, but it is smaller at high concentrations. Finally, we expand the γ-curve to evaluate the entire γ-surface. In order to investigate the temperature dependence of the SFE in stainless steels, we calculate it for the random alloy Fe0.716Cr0.200Ni0.084 over the temperature range of 298–1273 K (25–1000 °C). The SFE is calculated using the axial next-nearest-neighbor Ising (ANNNI) model. The random alloy and the paramagnetic state are taken into account, respectively, using the coherent-potential approximation (CPA) and the disordered local moments (DLM) approach, as implemented in the exact muffin-tin orbitals (EMTO) code. The lattice parameter at different temperatures is provided using the thermal lattice expansion data measured by X-ray diffraction (XRD). The temperature dependence of the local magnetic moments is evaluated by accounting for the fluctuations in the size of magnetic moments. The influence of different approximations and contributions, i.e., the electronic entropy, the thermal expansion, the frozen-core approximation, and the exchange–correlation functional, are intensively investigated. Our results demonstrate that the SFE increases with temperature due to an increase in the lattice volume and in the local magnetic moments. We find that the temperature dependence of the SFE is mainly influenced by the lattice expansion. The thermal excitations of magnetic moments exhibit a rather small influence on the temperature dependence of the SFE. We also find that the choice of the exchange–correlation functional significantly influences the SFE. Good agreement with experimental data can be achieved using the generalized-gradient approximation (GGA).
    Translated title of the contributionÜber den Einfluss von Legierungselementen und der Temperatur auf die Stapelfehlerenergie von Eisen-basierten Legierungen
    Original languageEnglish
    QualificationDr.mont.
    Supervisors/Advisors
    • Puschnig, Peter, Assessor A (internal)
    • Draxl, Claudia, Supervisor (internal)
    Publication statusPublished - 2013

    Bibliographical note

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    Keywords

    • stacking fault
    • stacking-fault energy
    • gamma-curve
    • gamma-surface
    • γ-curve
    • γ-surface
    • carbon steel
    • stainless steel
    • iron-based alloy
    • temperature
    • interstitial
    • substitutional
    • density-functional theory
    • annni model
    • disordered alloy
    • paramagnetic state
    • WIEN2k
    • EMTO

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