Sardar Sikandar Hayat

Effect of Crystal Defects on the Melting Temperature of Ni and Al

The molecular dynamics simulation technique has been applied to study the effects of temperature on Ni and Al with point and planar defects. For this purpose a well established program dyn86 containing the DYNAMO subroutine has been used. Semi-empirical potentials based on the embedded atom method (EAM) have been employed to calculate the lattice parameter and energy per atom in order to determine the melting point. The effects of point defects, including self, substitution, and interstitial, on the melting point have been investigated. The twin formation energy of some low index (111), (112), (113), and (114) twin-interfaces and their effects on the melting point of Ni and Al are also studied. It is observed that the presence of defects (point or planar) lowers the melting point of metals, as compared to the simulated melting point of a defect-free crystal.

THERMAL DIFFUSION DYNAMIC BEHAVIOR OF TWO-DIMENSIONAL Ag-SMALL CLUSTERS ON Ag(1 1 1) SURFACE

In this paper, the thermal diffusion behavior of small two-dimensional Ag-islands on Ag(1 1 1) surface has been explored using molecular dynamics (MD) simulations. The approach is based on semi-empirical potentials. The key microscopic processes responsible for the diffusion of Ag1−5 adislands on Ag(1 1 1) surface are identified. The hopping and zigzag concerted motion along with rotation are observed for Ag one-atom to three-atom islands while single-atom and multi-atom processes are revealed for Ag four-atom and five-atom islands, during the diffusion on Ag(1 1 1) surface. The same increasing/decreasing trend in the diffusion coefficient and effective energy barrier is observed in both the self learning kinetic Monte Carlo (SLKMC) and MD calculations, for the temperature range of 300–700 K. An increase in the value of effective energy barrier is noticed with corresponding increase in the number of atoms in Ag-adislands. A reasonable linear fit is observed for the diffusion coefficient for studied temperatures (300, 500 and 700 K). For the observed diffusion mechanisms, our findings are in good agreement with ab initio density-functional theory (DFT) calculations for Al/Al(1 1 1) while the energy barrier values are in same range as the experimental values for Cu/Ag(1 1 1) and the theoretical values using ab initio DFT supplemented with embedded-atom method for Ag/Ag(1 1 1).

Interaction of point defects with twin boundaries in Au: A molecular dynamics study

The molecular dynamics simulation technique with many-body and semi-empirical potentials (based on the embedded atom method potentials) has been used to calculate the interactions of point defects with (1 1 1), (1 1 3), and (1 2 0) twin boundaries in Au at different temperatures. The interactions of single-, di-, and tri-vacancies (at on- and off-mirror sites) with the twin interfaces at 300 K are calculated. All vacancy clusters are favorable at the on-mirror arrangement near the (1 1 3) twin boundary. Single- and di-vacancies are more favorable at the on-mirror sites near the (1 1 1) twin boundary, while they are favorable at the off-mirror sites near the (1 2 0) twin boundary. Almost all vacancy clusters energetically prefer to lie in planes closest to the interface rather than away from it, except for tri-vacancies near the (1 2 0) interface at the off-mirror site and for 3.3 and 3.4 vacancy clusters at both sites near the (1 1 1) interface, which are favorable away from the interface. The interaction energy is high at high temperatures.

A study of dynamical evolution of small two-dimensional Copper islands’ diffusion on Ag(111) surface and observed surface effects

We study the diffusion of two-dimensional Cun(1 ≤ n ≤ 9) islands on Ag(111) surface using molecular dynamics (MD) simulations. The work is the extension of calculations of monomer and dimer Hayat et al. [Phys. Rev. B 82 (2010) 085411] and trimer results Shah et al. [Phys. Lett. A 378 (2014) 1732]. Simulations carried out at three different temperatures — 300, 500, and 700 K — show the concerted motion to be dominant for the smaller islands (2- to 4-atoms), while the shape-changing multiple-atom processes are responsible for the diffusion of larger islands. Arrhenius plots of the diffusion coefficients reveal that the effective energy barrier is less than 260 ± 5 meV for the largest island size of Cu/Ag(111). There is a scaling of the effective energy barrier with size to some extent, but most notably it remains constant for islands with 4- to 6-atoms. The diffusion coefficient increases within a factor of 10 at the three temperatures 300, 500, and 700 K. The observed anharmonic features of the Cun adislands (breakage and pop–up) at Ag(111) surface as well as the surface anharmonicity of the Ag-substrate (fissures, dislocations, vacancy generation, and atomic exchange), are also presented. These findings can serve as an input for kinetic Monte Carlo (KMC) simulations. For the smaller sized islands the variation in the effective energy barrier with the island size is in good agreement with the experimental findings.

Effect of Density and Pointed Corner Degree of Pore on Local Stress in Welded Structures: Defect in Marine Structures

The process of assembly by welding, in marine structures, led to the creation of microstructural heterogeneities zones. Welded structures generally contain defects such as lack of penetration, slag inclusion, porosity, and misalignment. Generally these zones can be discontinuities geometrical. At the origins of stress concentration, these regions are favourable sites where fatigue cracks can initiate and propagate. In this study, the 3D finite element method is used to analyze the distribution of stress (strain) generated by the presence in the weld nugget of a pore formed during the welding process. This analysis was made in the matrix-pore interface. The effects of density and pointed corner degree of pore on the stress levels were also analyzed.

VACANCY GENERATION AND ADSORPTION OF Cu ATOM AT Ag(1 1 1) SURFACE DURING DIFFUSION OF Cu-TRIMER

In this paper, the vacancy generation at (1 1 1) surface of silver (Ag) and the adsorption of copper (Cu) atom in the vacant site at Ag(1 1 1) surface has been studied using molecular dynamics (MD) simulation technique. The run of micro-canonical constant energy (NVE) ensemble results the generation of vacancy at 700 K in the presence of Cu-trimer, and its migration is observed toward the island as a result of self-diffusion of the surface layer atom. The energy of vacancy generation at Ag surface is found to be Ev = 1.078 eV, in the presence of Cu-trimer. The popped-up Ag adatom combines with Cu-trimer, making a mixed-tetramer (Cu3–Ag island). The adsorption energy of the Cu-atom among the mixed-tetramer island into the substitutional site is Es = ~ -0.813 eV, leaving a mixed-trimer (Cu2–Ag island) at Ag(1 1 1). Moreover, the presence of the island creates some anharmonic effects like dislocations and fissures at the surface. These dislocations and fissures show a trend of attraction to each other and also migrate toward the island, during the time evolution of the system.