|M.Sc Student||Danino Bar David|
|Subject||Relations between Material Properties and Barriers for|
Twin Wall Motion in Ferroic Materials
|Department||Department of Mechanical Engineering||Supervisors||Professor Dan Mordehai|
|Professor Doron Shilo|
|Full Thesis text|
Ferroic materials, including ferroelectrics, ferromagnetics, ferroelastic minerals, and shape memory alloys, typically exhibit a micro/nano-structure that contains twins or domains separated by twin walls, due to their non-cubic crystalline structure. The motion of twin walls produces large deformations and governs the mechanical, electrical, and physical properties of these materials. The lattice barrier for twin wall motion is an important property that determines all the aforementioned properties. Despite that, numerical and experimental evaluations of the lattice barrier are scarce and are limited to specific materials.
The Landau-Ginzburg model is a widely accepted phenomenological phase-field model used to describe the material properties of the twin walls. However, the Landau-Ginzburg model is incapable of describing energy barriers for motion due to the lack of atomistic description. Atomistic simulations have been capable of describing the energy barrier for twin wall motion, but due to their use of an empirical potential calibrated for a given material, these simulations are limited to certain materials and cannot provide general insights into the behaviors of other materials. In addition, presently, there are almost no empirical potentials describing ferroic materials.
In this work, we present a general methodology for studying the relations between the lattice barrier for twin wall motion and measurable material properties. An atomistic model system is constructed, with a single twin wall separating crystals of different orientations. We propose a new interatomic potential based on the continuum Landau-Ginzburg model and parameterize it to meet with the lattice structure of the model system and the twin wall width. After validating that the atomistic simulations are in accordance with the predictions of the Landau-Ginzburg model, a minimum energy path technique (the Nudged Elastic Bands method) is employed to calculate the energy barriers for the motion of twin walls of different widths under different externally-applied shear strains. These results are united to an expression relating the energy barriers with material properties and the external loading. The energy barrier functions extend the Landau-Ginzburg model and allow treating the motion of twin walls as a thermally activated process.