On the Structure, Energy, and Segregation Behavior of Grain Boundaries in Metallic Systems

Author

Yasir MahmoodFollow

Date of Award

8-2024

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

Committee Chair/Advisor

Fadi Abdeljawad

Committee Member

Murray S. Daw

Committee Member

Enrique Martinez Saez

Committee Member

Gang Li

Abstract

Nearly all structural metallic systems are multi-component polycrystalline aggregates; their microstructures are composed of crystalline grains that are internally joined at grain boundaries (GBs). This thesis focuses on GB structure, energy, and chemistry, as these greatly influence many material properties and processes, including boundary dynamics during processing treatments or under operating conditions.

Using atomistic simulations, we examine the impact of metastable GB structures on solute segregation. A wide range of GB geometries and metastable structures are used in our study. The Al-Mg alloy is used because it is of interest for light weighting. The atomistic simulation results are used to develop a Gaussian Process machine-learning model relating the GB segregation energy to the local atomic environment.

In another study, we employ experimentally obtained GB concentration data in a model Pt-Au alloy to parameterize a GB solute drag model with the goal of quantifying the dependence of dynamic solute drag on GB character. In the context of GB dynamics, solute drag results when segregated alloying elements exert a resistive force on migrating boundaries, hindering their motion. GB solute drag plays an important role in stabilizing nanocrystalline alloys, which undergo rapid GB migration and grain coarsening even at low temperatures. Our analysis of Pt-Au alloys shows that the GB geometry greatly influences solute drag values, which in turn affects grain coarsening rates in these alloys.

Finally, we direct our attention to classical atomistic simulations of GBs. State-of-art atomistic techniques employ the Embedded Atom Method (EAM). However, it is not clear from the published literature how the particular fit of EAM interatomic potentials influences computed GB properties. Using a simple parameterization of EAM and a diverse set of GBs, we perform atomistic simulations and theoretical analysis to quantify the impact of EAM fitting parameters on computed GB energies in pure FCC metals. It is shown that variations in computed GB energy due to EAM parameters can be much larger than those variations due to the GB geometry. This highlights the need to consider sensitivity to details of empirical potentials when performing quantitative studies of GB physics. The atomistic simulation data are used to derive a simple fit of the GB energy in the EAM parameter space, which can be used to obtain boundary energies in real FCC metals by selecting the corresponding regions within the EAM parameter space.

Recommended Citation

Mahmood, Yasir, "On the Structure, Energy, and Segregation Behavior of Grain Boundaries in Metallic Systems" (2024). All Dissertations. 3695.
https://open.clemson.edu/all_dissertations/3695