S. A. Ahmad

Molecular dynamics study of the mechanical characteristics of Ni/Cu bilayer using nanoindentation

In the present work, a three-dimensional molecular dynamics simulation is carried out to perform the nanoindentation experiment on Ni single crystal. The substrate indenter system is modeled using hybrid interatomic potentials including the many-body potential embedded atom method (EAM), and two-body morse potential. To simulate the indentation process, a spherical indenter (diameter = 80 A, 1 A=0.1 nm) is chosen. The results show that the mechanical behaviour of a monolithic Ni is not affected by crystalline orientation. To elucidate the effect of a heterogeneous interface, three bilayer interface systems are constructed, namely Ni(100)/Cu(111), Ni(110)/Cu(111), and Ni(111)/Cu(111). The simulations along these systems clearly describe that mechanical behaviour directly depends on the lattice mismatch. The interface with the smaller mismatch between the specified crystal planes is proved to be harder and vice versa. To describe the relationship between film thickness and interface effect, we choose various values of film thickness ranging from 20 A to 50 A to perform the nanoindentation experiment. It is observed that the interface is significant only for the relatively small thickness of film and the separation between interface and the indenter tip. It is shown that with the increase in film thickness, the mechanical behaviour of the film shifts more toward that of monolithic material.

Dynamic characteristics of nanoindentation in Ni: A molecular dynamics simulation study

In this work, three-dimensional molecular dynamics simulation is carried out to elucidate the nanoindentation behaviour of single crystal Ni. The substrate indenter system is modelled using hybrid interatomic potentials including the manybody potential (embedded atom method) and two-body Morse potential. The spherical indenter is chosen, and the simulation is performed for different loading rates from 10 m/s to 200 m/s. Results show that the maximum indentation load and hardness of the system increase with the increase of velocity. The effect of indenter size on the nanoindentation response is also analysed. It is found that the maximum indentation load is higher for the large indenter whereas the hardness is higher for the smaller indenter. Dynamic nanoindentation is carried out to investigate the behaviour of Ni substrate to multiple loading-unloading cycles. It is observed from the results that the increase in the number of loading unloading cycles reduces the maximum load and hardness of the Ni substrate. This is attributed to the decrease in recovery force due to defects and dislocations produced after each indentation cycle.

Mechanical behavior of Cu—Zr bulk metallic glasses (BMGs): A molecular dynamics approach

In the present work, three-dimensional molecular dynamics simulation is carried out to elucidate the nanoindentation behaviors of CuZr Bulk metallic glasses (BMGs). The substrate indenter system is modeled using hybrid interatomic potentials including both many-body Finnis Sinclair (FS) and two-body Morse potentials. A spherical rigid indenter (diameter = 60 ?(1 ? = 10?10 m)) is employed to simulate the indentation process. Three samples of BMGs including Cu25Zr75, Cu50Zr50, and Cu75Zr25 are designed and the metallic glasses are formed by rapid cooling from the melt state at about 2000 K. The radial distribution functions are analyzed to reveal the dynamical evolution of the structure of the atoms with different compositions and different cooling rates. The mechanical behavior can be well understood in terms of load-depth curves and Hardness-depth curves during the nanoindentation process. Our results indicate a positive linear relationship between the hardness and the Cu concentration of the BMG sample. To reveal the importance of cooling rate provided during the processing of BMGs, we investigate the indentation behaviors of Cu50Zr50 at three different quenching rates. Nanoindentation results and radial distribution function (RDF) curves at room temperature indicate that a sample can be made harder and more stable by slowing down the quenching rate.

Theoretical investigations of half-metallic ferromagnetism in new Half–Heusler YCrSb and YMnSb alloys using first-principle calculations

Structural, electronic, and magnetic properties of new predicted half-Heusler YCrSb and YMnSb compounds within the ordered MgAgAs C1b-type structure are investigated by employing first-principal calculations based on density functional theory. Through the calculated total energies of three possible atomic placements, we find the most stable structures regarding YCrSb and YMnSb materials, where Y, Cr(Mn), and Sb atoms occupy the (0.5, 0.5, 0.5), (0.25, 0.25, 0.25), and (0, 0, 0) positions, respectively. Furthermore, structural properties are explored for the non-magnetic and ferromagnetic and anti-ferromagnetic states and it is found that both materials prefer ferromagnetic states. The electronic band structure shows that YCrSb has a direct band gap of 0.78 eV while YMnSb has an indirect band gap of 0.40 eV in the majority spin channel. Our findings show that YCrSb and YMnSb materials exhibit half-metallic characteristics at their optimized lattice constants of 6.67 A and 6.56 A, respectively. The half-metallicities associated with YCrSb and YMnSb are found to be robust under large in-plane strains which make them potential contenders for spintronic applications.

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.

Molecular dynamics simulation of nanoscale surface diffusion of heterogeneous adatoms clusters

Molecular dynamics simulation employing the embedded atom method potential is utilized to investigate nanoscale surface diffusion mechanisms of binary heterogeneous adatoms clusters at 300 K, 500 K, and 700 K. Surface diffusion of heterogeneous adatoms clusters can be vital for the binary island growth on the surface and can be useful for the formation of alloy-based thin film surface through atomic exchange process. The results of the diffusion process show that at 300 K, the diffusion of small adatoms clusters shows hopping, sliding, and shear motion; whereas for large adatoms clusters (hexamer and above), the diffusion is negligible. At 500 K, small adatoms clusters, i.e., dimer, show almost all possible diffusion mechanisms including the atomic exchange process; however no such exchange is observed for adatoms clusters greater than dimer. At 700 K, the exchange mechanism dominates for all types of clusters, where Zr adatoms show maximum tendency and Ag adatoms show minimum or no tendency toward the exchange process. Separation and recombination of one or more adatoms are also observed at 500 K and 700 K. The Ag adatoms also occupy pop-up positions over the adatoms clusters for short intervals. At 700 K, the vacancies are also generated in the vicinity of the adatoms cluster, vacancy formation, filling, and shifting can be observed from the results.

Structural, electronic, and optical properties of ZnO1−xSex alloys using first-principles calculations

The structural, electronic, and optical properties of binary ZnO, ZnSe compounds, and their ternary ZnO1−xSex alloys are computed using the accurate full potential linearized augmented plane wave plus local orbital (FP-LAPW + lo) method in the rocksalt (B1) and zincblende (B3) crystallographic phases. The electronic band structures, fundamental energy band gaps, and densities of states for ZnO1−xSex are evaluated in the range 0 ≤ x ≤ 1 using Wu—Cohen (WC) generalized gradient approximation (GGA) for the exchange—correlation potential. Our calculated results of lattice parameters and bulk modulus reveal a nonlinear variation for pseudo-binary and their ternary alloys in both phases and show a considerable deviation from Vegards law. It is observed that the predicted lattice parameter and bulk modulus are in good agreement with the available experimental and theoretical data. We establish that the composition dependence of band gap is semi-metallic in B1 phase, while a direct band gap is observed in B3 phase. The calculated density of states is described by taking into account the contribution of Zn 3d, O 2p, and Se 4s, and the optical properties are studied in terms of dielectric functions, refractive index, reflectivity, and energy loss function for the B3 phase and are compared with the available experimental data.

First-principles calculation of the structural, electronic, and magnetic properties of cubic perovskite RbXF3 (X = Mn,V,Co, Fe)

First-principles calculations by means of the full-potential linearized augmented plane wave method using the generalized gradient approximation with correlation effect correction (GGA+U) within the framework of spin polarized density functional theory (DFT+U) are used to study the structural, electronic, and magnetic properties of cubic perovskite compounds RbXF3 (X = Mn, V, Co, and Fe). It is found that the calculated structural parameters, i.e., lattice constant, bulk modulus, and its pressure derivative are in good agreement with the previous results. Our results reveal that the strong spin polarization of the 3d states of the X atoms is the origin of ferromagnetism in RbXF3. Cohesive energies and the magnetic moments of RbXF3 have also been calculated. The calculated electronic properties show the half-metallic nature of RbCoF3 and RbFeF3, making these materials suitable for spintronic applications.

Ab initio study of structural, electronic and optical properties of ternary CdO1−xSex alloys using special quasi-random structures

The structural, electronic, and optical properties of binary CdO, CdSe, and their ternary CdO1−xSex alloys (0 ≤ x ≤ 1) in the rock salt and zinc blend phases have been studied by the special quasi-random structure (SQS) method. All the calculations are performed using full-potential linearized augmented plane wave plus local orbitals (FP-LAPW+lo) method within the framework of density function theory (DFT). We use Wu—Cohen (WC) generalized gradient approximation (GGA) to calculate structural parameters, whereas both Wu—Cohen and Engel—Vosko (EV) GGA have been applied to calculate electronic structure of the materials. Our predicted results of lattice constant and bulk modulus show only a slight deviation from Vegards law for the whole concentrations. The obtained band structure indicates that for the rock-salt phase, the ternary alloys present semi-metallic behavior, while for the zinc blend phase, semiconductor behavior with direct bandgap is observed with decreasing order of x except for CdSe. Finally, by incorporating the basic optical properties, we discuss the dielectric function, refractive index, optical reflectivity, the absorption coefficient, and optical conductivity in terms of incident photon energy up to 14 eV. The calculated results of both binaries are in agreement with existing experimental and theoretical values.

Effect of Twin Interfaces on Surface Adsorption for Thin Film Formation

A molecular dynamics (MD) study of thin film formation was performed to understand the adsorption phenomena of Cu nanoparticles on Ag substrate using the EAM potential. The purpose of the present study was to investigate the effects of twin interfaces on surface adsorption for thin film formation. The (110) substrate with higher surface energy shows higher adsorption as compared to the (111) substrate which has the lowest surface energy. To investigate the adsorption of Cu nanoparticles the atomic radial distribution function, spreading index, and coalescence index were calculated. The results indicate that the presence of a twin in the substrate significantly increases the adsorption process. The adsorption phenomena were found to be highly temperature dependent. The adsorption was slightly higher when the nanoparticles were exactly on both sides of the twin interface (BSTI) as compared to being on the twin interface (OTI).