Wonsuk Ko

Phase transitions in nickel-titanium shape-memory alloys are investigated by means of atomistic simulations. A second nearest-neighbor modified embedded-atom method interatomic potential for the binary nickel-titanium system is determined by improving the unary descriptions of pure nickel and pure titanium, especially regarding the physical properties at finite temperatures. The resulting potential reproduces accurately the hexagonal-close-packed to body-centered-cubic phase transition in Ti and

Molecular dynamics simulations are performed to investigate temperature- and stress-induced phase transformations in nanocrystalline nickel-titanium shape-memory alloys. Our results provide detailed insights into the origins of the experimentally reported characteristics of phase transformations at the nanoscale, such as the decrease of the transformation temperature with grain size and the disappearance of the plateau in the stress-strain response. The relevant atomic scale processes, such as n

A new interatomic potential for the pure tin (Sn) system is developed on the basis of the second-nearest-neighbor modified embedded-atom-method formalism. The potential parameters were optimized based on the force-matching method utilizing the density functional theory (DFT) database of energies and forces of atomic configurations under various conditions. The developed potential significantly improves the reproducibility of many fundamental physical properties compared to previously reported mo

An interatomic potential for pure phosphorus, an element that has van der Waals, covalent and metallic bonding character, simultaneously, has been developed for the purpose of application to metal-phosphorus systems. As a simplification, the van der Waals interaction, which is less important in metal-phosphorus systems, was omitted in the parameterization process and potential formulation. On the basis of the second-nearest-neighbor modified embedded-atom method (2NN MEAM) interatomic potential

First-principles density functional theory (DFT) calculations were performed to investigate the effect of ternary alloying on the hydrogenation properties of the TiFe system. Al, Be, Co, Cr, Cu, Mn and Ni were selected as substitutional elements for Fe sites, in the light of their reported enhancement of activation, kinetic and thermodynamic properties. The use of special quasi-random structures to account for disordering of solute elements in the sub-lattice allowed a quantitative assessment of

Martensitic transformations in nanoscaled shape-memory alloys exhibit characteristic features absent for the bulk counterparts. Detailed understanding is required for applications in micro- and nanoelectromechanical systems, and experimental limitations render atomistic simulation an important complementary approach. Using a recently developed, accurate potential we investigate the phase transformation in freestanding Ni-Ti shape-memory nanoparticles with molecular-dynamics simulations. The resu

Atomistic simulations based on interatomic potentials have frequently failed to correctly reproduce the brittle fracture of materials, showing an unrealistic blunting. We analyse the origin of the unrealistic blunting during atomistic simulations by modified embedded-atom method (MEAM) potentials for experimentally well-known brittle materials such as bcc tungsten and diamond silicon. The radial cut-off which has been thought to give no influence on MEAM calculations is found to have a decisive