Date of Award

2021

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy in Materials Science and Engineering (PhD)

Administrative Home Department

Department of Materials Science and Engineering

Advisor 1

Yu Wang

Committee Member 1

Stephen Kampe

Committee Member 2

Ranjit Pati

Committee Member 3

Douglas J. Swenson

Abstract

The atomic-scale Monte Carlo simulation study focuses on the intrinsic behaviors of a defect-free crystal that undergoes a cubic-to-tetragonal martensitic transformation. The quasi-spin variable associated with the lattice sites characterizes the local unit cells of the orientation variants of the ground-state martensite phase, which interact with each other through long-range elastic interactions. It is shown that the diffuse scattering in the premartensitic austenite state results from the spatial correlation of the atomic-scale heterogeneous lattice displacements and manifests the displacement short-range ordering. The effects of temperature, elastic anisotropy, and shear modulus softening on the diffuse scattering and displacement short-range ordering are investigated. It is found that elastic softening in the shear modulus ��# = (��&& − ��&()/2 promotes 110 | 110 displacement plane waves that stabilize the cubic austenite phase through increased entropy, decreasing the martensitic transformation temperature. The simulated diffuse scattering is compared and agrees with the complementary synchrotron X-ray single-crystal diffuse scattering experiment.

DFT-based Monte Carlo has been used to determine the lattice site occupancies of solute atoms to study the disorder-order transitions in the Fe- 25at%Ga alloy. Both the chemical energy and the elastic energy from the Ga atomic misfit have been considered. The parameters used in the simulation are determined from First Principle. The effective atomic radius misfit has been determined indirectly. Up to third-nearest-neighbor x chemical bond energies and elastic energies have been considered. The energetic and kinetic properties of different types of the DO3 antiphase domain boundaries (APDBs) have been analyzed. Four BCC-based phases: A2, B2, DO3, and B32 during the A2→DO3 transition have been analyzed. Through separating the chemical disorder and the static atomic displacement, it is found that the atomic displacement has more effects on APDBs/B2 clusters than DO3 from the long range order and the Ga-Gapair is not sensitive to the temperature from short range order.

Available for download on Saturday, December 31, 2022

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