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An Investigation of Alloying Elements Effect on Creep Behavior of Ni-Based Super Alloys Using Molecular Dynamics

Youzi, Mehrdad | 2022

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  1. Type of Document: M.Sc. Thesis
  2. Language: Farsi
  3. Document No: 54760 (09)
  4. University: Sharif University of Technology
  5. Department: Civil Engineering
  6. Advisor(s): Khoei, Amir Reza
  7. Abstract:
  8. The aim of this study is to investigate the role of the temperature, stress, strain rate, and rhenium (Re) on the γ\/γ^' interfacial misfit dislocation network and mechanical response of Ni-based single crystal superalloys. The interfacial network plays an important role in the superior creep behavior of Ni-based superalloys. Therefore, a comprehensive understanding of this topic would help us to go through the impact of various factors on the mechanical and creep properties at high temperatures to achieve an optimal design. Due to the mismatch between the two phases, a dislocation network forms after aging at high temperatures to alleviate the stress field. The (100), (110), and (111) phase interface models are generated to further study the properties of the superalloy by applying molecular dynamics simulations. It is noted that the strength and stability of the network are diminished as the thermal condition intensifies owing to the dispersed atomic potential energy at the interface. By applying a constant strain rate of 2×〖10〗^8 (s^(-1) ) at 0 K, the (100) and (111) phase interface models lose the co-coordinating role of maintaining the dynamic equilibrium. Hence, dislocations pile-up in the damaged area, and the network is no longer able to fortify the interface. For the (110) phase interface model, the dominant deformational mechanism is precipitate shearing. As temperature increases, the elastic modulus, initial mismatch stress, and yield strength decrease. Moreover, the yield strength of material increases as the strain rate increases. The pinning effect of Re atoms is surveyed by replacing 3.293at%, and 5at% of matrix Ni atoms with Re at 1600 K. The dislocation hampering property of Re is more perceptible when enough dislocations in the γ phase are moving at elevated temperatures. In addition, Re manages to soothe the stress field at the interface and does not affect the network morphology. Finally, an investigation of the creep behavior of the superalloy is provided. It is observed that the escalated damage to the interfacial network due to the increased temperature leads to the domination of the softening mechanisms (cross-slip and dislocation climb) on the deformation and shortens the steady-state creep. Also, as the constant stress increases, the network will be damaged earlier resulting in escalated creep strain rate. It has been observed that the creep in (100) crystallographic direction is more sensitive to stress than temperature since increasing the stress significantly shortens the dynamic equilibrium between softening and hardening mechanisms. Moreover, Re atoms act as an extra hardening factor to improve the tertiary creep. Rhenium atoms are capable of increasing the equilibrium duration between softening and hardening. Hence, they will defer the exponentially increase in strain rate after secondary creep
  9. Keywords:
  10. Dislocation Pileup ; Dislocation Climb ; Nickel-Base Superalloy ; Creep ; Interfacial Misfit Dislocation Network ; Lattice Mismatch

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