Mesoscale Interface-Mediated Plasticity Model and its Application to Study the Mechanical Properties on Polycrystals

Abstract

Dislocation-interface interactions play a crucial role in determining the mechanical properties of polycrystalline materials. These interactions involve dislocation absorption, emission, reflection, and interface sliding. In this study, we derive a mesoscale interface boundary condition based on bi-crystallography and Burgers vector reaction/conservation. Our proposed interface boundary condition is established upon Burgers vector reaction kinetics and is applicable to any type of interfaces in crystalline materials with any number of slip systems. We apply this approach to predict slip transfer across crystalline interfaces under various stress states and compare the results with widely used empirical methods. Importantly, our results demonstrate the direct applicability of the proposed interface boundary condition to many existing dislocation plasticity simulation methods.

The second major focus of this thesis is to demonstrate the unification of this boundary condition with different plasticity simulation approaches, such as the crystal plasticity finite element method, continuum dislocation dynamics, and discrete dislocation dynamics methods. First, we present a comprehensive review of our mesoscale interface boundary condition, emphasizing Burgers vector conservation and kinetic dislocation reaction processes. We then outline the theory behind integrating the proposed interface boundary condition with different simulation methods in detail. To validate our interface boundary condition, we conduct numerical simulations using both the crystal plasticity finite element method and a two-dimensional continuum dislocation dynamics model. Our results highlight that our compact and physically realistic interface boundary condition can be seamlessly integrated into multiscale simulation methods, yielding novel results that align with experimental observations.

As is shown by many well-known experiments, interfaces and grain boundaries significantly influence the mechanical properties of materials, particularly at micro- or nano-scales. The distinct interactions between dislocations and grain boundaries can lead to diverse plastic deformation characteristics. In this thesis, we employ a two-dimensional continuum dislocation dynamics model to investigate the mechanical properties of materials. To accurately capture the physical interactions between lattice dislocations and interfaces/grain boundaries, we incorporate our mesoscale interface boundary condition. We consider various numerical cases, including single slip systems, multiple slip systems, and the presence of included particles. Our interface boundary condition enables the simulation of dislocation emission, reflection, and transmission at the interface, allowing for a comprehensive understanding of their effects on mechanical performance. Moreover, the accumulation of dislocations near the interface can induce local stress concentrations, potentially leading to brittle fracture. Consequently, materials experience a competition between ductile and brittle fracture modes during the loading process. Finally, we investigate the phenomenon of the strain hardening rate up-turn under different reaction constants, providing a mechanistic explanation based on grain boundary strengthening.

Date of Award	20 May 2024
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Jian HAN (Supervisor), David Joseph SROLOVITZ (Supervisor) & Zhaoxuan WU (Supervisor)