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Multiscale Modeling of Creep Behavior of Nickel-Based Superalloys

Tolooei Eshlaghi, Golsa | 2023

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 56723 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Khoei, Amir Reza
  7. Abstract:
  8. Nowadays, single-crystal nickel-based superalloys are widely used in the manufacture of gas turbine blades in the aerospace industry due to their high resistance to creep, fatigue and corrosion at high temperatures. The superior behavior of these materials at elevated temperatures is a result of their two-phase microstructure, which includes the matrix phase (γ) of nickel and the precipitate phase (γ') of Ni3Al intermetallic compounds with a high volume fraction. The aim of this thesis is to develop computational modeling tools to study the creep deformation of single-crystal Ni-based superalloys. At high temperatures, the creep deformation of Ni-based superalloys is determined by the atomic processes of diffusion, which govern numerous mechanisms of creep deformation and deformation recovery, such as the climb of dislocations, diffusion of vacancies, precipitate rafting and micro-twinning. In addition, the alloying elements, morphology of the two-phase microstructure and interphase boundaries play a crucial role in defining the creep behavior of these materials. Investigating the effects of these factors requires a precise model of material behavior and an understanding of the properties and atomic structure of dislocation cores. Hence, due to the limitations of continuum approaches in the investigation of discrete deformation mechanisms, the novel approach of this thesis is to employ the molecular dynamics (MD) modeling method to analyze the atomic processes of creep deformation at high temperatures. In atomistic simulations of creep deformation, temperature, stress, crystal orientation of the interphase boundaries, creep loading direction, and rhenium alloying element are considered. Based on the modeling results, creep deformation mechanisms at low temperatures include precipitate shearing and micro-twinning, and at high temperatures include dislocation accumulation in channels, dislocation climb and rafting. For each model, a creep deformation map is presented based on the power law creep equation as a function of stress, temperature, creep activation energy and stress exponent. It is observed that an increase in stress and temperature increases the creep strain and the minimum strain rate, and that rhenium atoms act as a hardening factor in the tertiary creep. Molecular dynamics approach is also used to study the anisotropy of mechanical properties and to explore the effects of temperature, stress, strain rate, and the presence of alloying elements on misfit dislocations in the γ/γ' phase interfaces and energy features of stacking faults in γ and γ' phases. It is found that increasing the temperature and decreasing the strain rate decreases the yield stress and elastic modulus and accelerates the misfit dislocation network damage. On the other hand, considering the high computational cost of the MD method in simulating creep at the grain scale, in order to study the creep deformation at intermediate temperatures and high stress, a phenomenological model in the form of a constitutive law in the crystal plasticity finite element method (CPFEM) is used in a hierarchical framework. This model considers the γ/γ' microstructure as a representative volume element (RVE) in the material points of the finite element method. Based on the obtained creep curves, it is observed that the increase in temperature and stress increases the creep strain and steady-state strain rate
  9. Keywords:
  10. Single Crystals ; Nickel-Base Superalloy ; Molecular Dynamic Modeling ; Finite Element Method ; Phenomenological Property ; Crystal Plasticity ; Hierarchical Approach ; Creep Behavior ; Multiscale Modeling

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