Computational studies of deformation in HCP metals and defects in a lead-free ferroelectric ceramic

Plastic deformation of nanocrystalline Mg and Ti is examined using molecular dynamics (MD) simulation. Slip, twinning, and GB processes are observed in textured (2D) columnar and random (3D) microstructures. The deformation simulations of Mg reproduce various twinning modes: tensile {101¯2} < 101¯1 > twins, and compressive {101¯1} <101¯2>, {112¯2¯} <112¯3¯> and {101¯3} <303¯2> twins. Two pyramidal <c+a> slip modes are manifested in strained structures: first-order {101¯1} <1¯1¯23> and second-order {112¯2} <112¯3>. Nucleation processes and mechanisms of dislocations and twins are identified (e.g. slip-assisted twin nucleation mechanism of {112¯2} <112¯3>. Nucleation processes and mechanisms of dislocations and twins are identified (e.g. slip-assisted twin nucleation mechanism of {112¯2} <112¯3> twin, initiation of {101¯2} <101¯1> twins by migration at GB). The crossover of initiation process between slip and twinning are found in 2D textures. The strongest grain size in 3D fully dense Mg occurs at 24nm. Single prismatic <a> dislocations and their interactions directly result in formation of <c+a> and <c> dislocations. Among the empirical potentials for Ti, the Henning MEAM potential displays slip and twinning processes most consistent with experiments.;The cation arrangements of Na and Bi in Na0.5 Bi0.5TiO 3 are investigated using density functional theory (DFT). The structure with alternative stacking of Na and Bi layers in the perovskite axis has the lowest energy of the cation arrangements. The R3c structure, known tp be the room temperature phase of Na0.5 Bi0.5TiO3 has a higher energies than structures with random cation arrangements. The cation-layered structure is revealed to have a P1 phase by distortion of octahedra in its perovskite. To analyze the structure distortion, elements (Ti-Bi and Ti-La) causing the second-order Jahn-Teller effect are chosen and, compared in structure and charge density. The combination of lone pairs and the d 0 transition metal can deform octahedra of a perovskite cell more severely than that of d0 transition metals. The cation-layered structure of Na0.5 Bi0.5TiO3 may be influenced by the Jahn-Teller effect, thereby having the lowest energy.