Thesis Defense: Nutth Tuchinda

Topic

Polycrystalline Grain Boundary Solute Segregation at Finite Sizes and Temperatures

Abstract

Engineering defect chemistry allows novel pathways to control bulk material behavior, especially for high-defect-density materials like nanocrystalline alloys. Solute segregation at grain boundaries can be employed as a design tool to stabilize such systems against coarsening. However, the understanding and availability of grain boundary data are still limited for polycrystals that contain a profuse spectrum of grain boundary sites, especially for the effects of grain size and temperature on grain boundary site spectra. This thesis aims to fill the literature gap by providing a framework for studying grain size dependences (when grain boundary and triple junction volume fractions become finite) and solute segregation at finite temperatures where vibrational entropy becomes non-negligible.
 
The thesis first demonstrates extractions of triple junction segregation spectra which show characteristics of the defect type vis-à-vis grain boundaries. The developed size-dependent isotherms suggest that triple junctions can show a strong contrast of local solute content compared to adjacent grain boundaries. Although the bulk size effect can be negligible beyond approximately 20 nm in grain size due to the low junction volume fraction depending on the system chemistry and grain geometry. An analysis from an Al-based hybrid quantum mechanical-molecular mechanical dilute segregation energy dataset suggests a periodic chemical trend with a cluster of solute elements preferring triple junctions. Site environment analysis of the solute elements that prefer triple junctions also suggests both chemical and elastic contributions that influence the spectrality of solute segregation and thus the energetic contrast observed.
 
Aside from grain junctions, temperature can play a major role in the stability of nanocrystals. This thesis demonstrates estimations of dilute site-wise vibrational segregation entropy spectra in polycrystals within harmonic approximations. The majority of the systems calculated show a positive correlation between site-wise energy and entropy, in agreement with the past literature for simplified boundaries. The framework is extended in combination with data sciences to create a dilute segregation energy-entropy spectral database, allowing multiscale translation from site-wise segregation energy-entropy spectra to bulk average McLean segregation free energies often reported in the literature. The outcome suggests that some of the average segregation entropies reported could overestimate vibrational effects in grain boundary segregation due to the interplay between configurational and vibrational entropy in spectral grain boundary solute segregation. It is hoped that the method and database presented in this thesis can serve as a pathway toward finite temperature grain boundary genome, allowing full exploitation of grain boundary defects in polycrystalline materials.

Thesis Advisor

Christopher A. Schuh, POSCO Professorship of Materials Science and Engineering, MacVicar Fellow, MIT