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Title:Investigation of the mechanical properties and phase stability of Ti-transition metal alloys using first-principle calculations
Author(s):Kim, Hyojung
Director of Research:Trinkle, Dallas R.
Doctoral Committee Chair(s):Trinkle, Dallas R.
Doctoral Committee Member(s):Bellon, Pascal; Krogstad, Jessica; Schleife, Andre
Department / Program:Materials Science & Engineerng
Discipline:Materials Science & Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):density functional theory
mechanical properties
Abstract:The ternary Ti-X-B (X=Mo/V/Fe/Nb) alloy system has three distinct material classes depending on the fraction of monoboride phase in the Ti-matrix, which makes them useful for broad applications such as aerospace and biomedical fields. Based on the design principle of searching for Ti-monoboride ceramic phases with higher Young’s modulus, higher Pugh’s ratio, and lower stacking fault energies compared to TiB, we compute the mechanical properties of monoborides with different compositions (X11−xX2x)B using density functional theory. Among all (X10.5X20.5)B, we find that mixed (Ti0.5Mo0.5)B and mixed (Ti0.5V0.5)B as promising candidates for metallic boride compositions for Ti-based alloys and bulk ceramics. Among these two ternary mixed monoborides, we focus on β-stabilizer Mo and study toughness of BCC phase Ti-Mo alloys to understand the effect of solutes on the plastic deformation. We calculate elastic constants, stacking fault energies and dislocation core geometries of non-dilute BCC Ti-Mo random alloys. Studies of dislocation core structures have relied on a variety of coupling techniques to manage the far-field strain fields; one very successful approach for pure materials has been the flexible boundary condition (FBC) method based on the lattice Green function. However, applying FBC to compute the dislocation core structures in systems with multiple components adds more complexity due to the initial atomic forces in the far-field where the atomic positions are not updated within the FBC framework. We describe a methodology to compute the screw dislocation core geometry of non-dilute BCC TiMo random alloy efficiently by combining first-principles calculations with an optimized Ti-Mo Gaussian Approximation Potential (GAP) specifically designed for use with flexible boundary conditions. We illustrate our algorithm to determine the sizes of LGF buffer and relaxation buffer in dislocation geometry using our Ti-Mo GAP model. We also use the GAP model for relaxation of the initial dislocation geometry and computing the force constants for the dislocation of BCC Ti0.5Mo0.5 random alloy to construct the lattice Green function. The relaxed dislocation screw core structure of the BCC Ti-Mo random alloy are discussed. Finally, we demonstrate the DFT database to build our Ti-Mo GAP model and show the validation and predictions for energy, force, elastic constant, and force constants of Ti-Mo alloys.
Issue Date:2019-08-16
Rights Information:Copyright 2019 Hyojung Kim
Date Available in IDEALS:2020-03-02
Date Deposited:2019-12

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