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.

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 in- dentation process, a spherical indenter （diameter = 80A, 1A=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.

Structural, electronic, and optical properties of ZnO_(1-x)Se_x alloys using first-principles calculations

The structural, electronic, and optical properties of binary ZnO, ZnSe compounds, and their ternary ZnOl_xSex alloys are computed using the accurate full potential linearized augmented plane wave plus local orbital （FP-LAPW ＋ lo） method in the rocksalt （B 1） 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 Vegard＇s 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.

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 Clb-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 and 6.56 ,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 l） twin boundary, while they are favorable at the oft-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.

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 nanoinden- tation 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 A（1 A = 10 ^-10 m）） is employed to simulate the indentation process. Three samples of BMGs including Cu25Zr75, CusoZr50, 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 CusoZr50 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.

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.

Influence of grazing exclosure on vegetation biomass and soil quality

This study investigated the influence of sixteen years exclosure from unmanaged grazing on aboveground vegetation biomass,soil organic matter(SOM),soil aggregation and nitrogen(N)mineralization in arid shrubland of Baluchistan,Pakistan.Sampling was carried out from three sites along the chronosequence of secondary succession.One site was located at open-for-grazing area(grazed site)and the other two sites were located in the area that is protected since 1998.One of the protected site is more remote from grazing(protected site 1)where the land is less disturbed and has thick vegetation than the other protected site(protected site 2).Results showed a significant difference for aboveground vegetation biomass across sites and was in the order of protected site 1>protected site 2>grazed site.Soil organic matter was 53% and 46% higher in protected sites than grazed site.Aggregates larger than 2 mm size were not detected in soil from grazed site but represented 4.5% and 3% of the sample soil profile at the protected site 1 and protected site 2,respectively.Rate of N mineralization was lower in soils at the grazed site as compared to soils at the protected sites.Soil moisture contents were significantly lower at grazed site and showed a strong positive correlation with aboveground vegetation biomass.This study demonstrates that unmanaged grazing severely affected aboveground vegetation biomass,soil organic matter,large-sized soil aggregates,nitrogen mineralization and soil moisture contents.Short term exclosure from grazing(~16 years)can enhance aboveground vegetation biomass and soil quality in terms of soil organic matter accumulation,soil aggregation,retention of soil moisture and nitrogen mineralization in this arid rangeland.

First-principles prediction of magnetically ordered half-metals above room temperature

Half-metallicity(HM)offers great potential for engineering spintronic applications,yet only few magnetic materials present metallicity in just one spin channel.In addition,most HM systems become magnetically disordered at temperatures well below ambient conditions,which further hinders the development of spin-based electronic devices.Here,we use first-principles methods based on density functional theory(DFT)to investigate the electronic,magnetic,structural,mixing,and vibrational properties of 90 XYZ half-Heusler(HH)alloys(X=Li,Na,K,Rb,Cs;Y=V,Nb,Ta;Z=Si,Ge,Sn,S,Se,Te).We disclose a total of 28 new HH compounds that are ferromagnetic,vibrationally stable,and HM,with semiconductor band gaps in the range of 1-4 eV and HM band gaps of 0.2-0.8 eV.By performing Monte Carlo simulations of a spin Heisenberg model fitted to DFT energies,we estimate the Curie temperature,T_(C),of each HM compound.We find that 17 HH HM remain magnetically ordered at and above room temperature,namely,300≤T_(C)≤450 K,with total magnetic moments of 2 and 4 mB.A further materials sieve based on zero-temperature mixing energies let us to conclude 5 overall promising HH HM that remain magnetically ordered at and above room temperature:NaVSi,RbVTe,CsVS,CsVSe,and RbNbTe.We also predict 2 semiconductor materials that are ferromagnetic at ambient conditions:LiVSi and LiVGe.