Date of Award

12-2008

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Physics and Astronomy

Advisor

Daw, Murray S

Committee Member

Manson , Joseph R

Committee Member

Sosolik , Chad E

Committee Member

Brittain , Sean

Abstract

With the recent advancements in computer technologies and the improvements in simulation algorithms, along with the theories, has allowed scientists to carry out diffusion calculations more efficiently, especially for the cases where either the diffusion itself takes longer, or there is no available isotope to carry out the diffusion experiments such as tracer diffusion.
In this work, we present a systematic approach to diffusion in intermetallic alloys such as weakly clustered Cu — Ni and weakly ordered Au — Ag. We use Accelerated Molecular Dynamics (AMD) combined with the Embedded Atom Method (EAM) to find the necessary saddlepoint energies. With this technique, we calculate the tracer diffusivity coefficients for Cu — Ni (temperature range 700 — 1300K) and for Au — Ag (temperature range 800 — 1300K) as a function of composition and temperature. We assume that the vacancy-assisted diffusion mechanism is governing the whole process. In our calculations we keep the vacancy concentration fixed. We observe that the results are in agreement with Arrhenius behavior as discussed in detail in Chapter 4: Results. However closer to critical temperature, the results are overwhelmed by statistical fluctuations.
During the simulations at low temperatures, we sometimes find that the vacancy spends a large number of steps moving locally without accomplishing significant displacements or accumulating much simulated time. To overcome this 'sandtrap' problem, we develop a systematic approach which is discussed in Chapter 5: Sandtrap Limitation and How to Overcome it in detail.
We also analyze the motion and the 'width' of the Anti-Phase Boundary (APB) perpendicular to its slip plane for Ni3Al using the same approach mentioned above for various temperatures (1000, 1200 and 1500K) under no driving force. We create the APB in the center of the sample and observe its motion. In addition, we create the APB farther away from the center and fix the atoms at the end of each side. This puts a bias on the motion of the APB by limiting the number of possible escapes in a certain direction, thus 'forcing' it to move towards the center. We observe the motion of the APB and the results are discussed in detail in Chapter 4: Results.

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