Abstract
Defects are ubiquitous in materials and play a crucial role in determining the mechanical properties of materials, as they are the main carriers of plastic deformation. To understand the relationships between defects and plastic deformation, we conduct the computational simulations of dislocation in Ti and grain boundaries within a multiscale framework.In Chapter 1, we systematically study the dislocations in 𝛼-Ti by a multiscale approach based on molecular dynamics (MD) with a state-of-art deep-learning (DP) interatomic potential. It is the first time to use interatomic potential to reproduce the screw dislocation core structures as compared to the density function theory (DFT) simulations, which proves the reliability of our developed DP interatomic potential.
We further study the dynamics of an ⟨𝑎⟩ screw dislocation in 𝛼-Ti statistically at finite temperatures with MD simulations in Chapter 2. Kinetic parameters for the core dissociation transitions and Peierls barriers for dislocation glide as a function of temperature are extracted by linear regression from the MD simulations, used as input to kinetic Monte Carlo (kMC) simulations, which help to predict the dislocation glide behavior in 𝛼-Ti with a much affordable computational source. The random walk dis- location trajectories from kMC agree well with those predicted by MD. Under applied driving force, dislocations glide via a locking-unlocking mechanism. We also find that some dislocations glide in directions that are not parallel with the core dissociation direction. The MD/kMC multiscale method proposed is applicable to dislocation motion in simple and complex materials (not only screw dislocations in Ti) as a function of temperature and stress state.
In Chapter 3, through our developed Monte Carlo (MC) simulations, we demonstrate that grain boundaries (GBs) with disconnections which have dislocation character undergo a finite-temperature topological phase transition of the Kosterlitz-Thouless (KT) type. This transition corresponds to the screening of the long-range elastic interaction between disconnections. Notably, we also observe that the KT transition and roughening transition cannot be both observed in GBs within a single disconnection mode, but possible in those with multiple disconnection modes. Our findings underscore the pivotal role of disconnections, as topological defects in crystals, in determining the occurrence of the KT transition. Utilizing kMC simulations, we additionally discover that KT phase transitions manifest in the jump of activation energy, specifically featured by GB mobility and shear viscosity. This work significantly contributes to advancing our understanding of GB migration, crystal evolutions, and the brittle-to-ductile transition (BDT).
| Date of Award | 6 Aug 2024 |
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| Original language | English |
| Awarding Institution |
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| Supervisor | Jian HAN (Supervisor) & David Joseph SROLOVITZ (Co-supervisor) |