A phase field study of compositional patterning in irradiated binary immiscible alloys

Li, Qun

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https://hdl.handle.net/2142/113858 Copy

Description

Title

A phase field study of compositional patterning in irradiated binary immiscible alloys

Author(s)

Li, Qun

Issue Date

2021-12-02

Director of Research (if dissertation) or Advisor (if thesis)

Bellon, Pascal

Doctoral Committee Chair(s)

Bellon, Pascal

Committee Member(s)

• Averback, Robert S
• Zuo, Jian-Min
• Aluru, Narayana R

Department of Study

Materials Science & Engineerng

Discipline

Materials Science & Engr

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Compositiona patterning
• driven alloys

Abstract

It has been shown by modeling and experiments that irradiation can drive immiscible alloys into steady-state compositional patterning (CP) of finite size and fixed composition. In this thesis, we employ phase field (PF) modeling to study idealized case of CP in an irradiated immiscible binary alloy, using a kinetic model to describe two competing processes: a thermally activated one promoting macroscopic phase separation (MPS) and a forced one resulting in finite-range random mixing. A series of approximate kinetic models are introduced, referred to as order-n extended Cahn-Hilliard (ECH) model, making it possible to obtain explicit, real-space effective potentials in irradiated alloys valid across the three steady states, and build steady-state diagrams for each order-n ECH model. In the CP regime, a new definition for the effective interfacial energy is proposed as the interfacial energy becomes an extensive property. The order-n ECH model is extended to study systems of non-equiatomic concentrations, the single phase shows a hexagonal pattern of disk-like precipitates, which are in a weak segregation regime when the system operates close to the steady-state phase boundary with a solid solution, and strong segregation regime when the system operates close to the steady-state phase boundary with macroscopic phase separation. Extended Cahn-Hilliard models were also developed to include interface anisotropy due to the directionality of atomic interactions in equilibrium systems with cubic symmetry. The resulting interface energies, calculated from phase field simulations, are in qualitative agreement with values reported in the literature from Monte-Carlo simulations. The above irradiation studies were confined to intragranular CP. The second part of the present work considers patterning in the more general case of a polycrystal. The coupled evolution of grain interiors and grain boundaries is investigated by a multi-order parameter phase field model, specifically adding non-conserved order parameters to distinguish different grains. Solute concentration and grains are coupled through a Gibbs free energy functional consisting of chemical free energy terms defined uniquely in GBs and bulk grain. Simulation results show that CP can take place at GBs provided that they exhibit appropriate positive equilibrium segregation energies. In addition, a global patterning involving simultaneous self-organization in the bulk and at GBs is observed but only for specific alloy thermodynamics and compositions. For rationalizing these self-organization reactions, a local-γ hypothesis is introduced and evaluated, based on an a posteriori knowledge of the average solute compositions at GBs and in the grain interiors.

Graduation Semester

2021-12

Type of Resource

Thesis

Permalink

http://hdl.handle.net/2142/113858

Copyright and License Information

Copyright 2021 Qun Li

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