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The effects of doping concentration and doping models of yttrium (Y) on mechanical behaviors are researched by using molecular dynamics and the second nearest-neighbor modified embedded-atom method (2NN-MEAM) formalism for nano Mg-Y alloy systems. The results confirm the effect of Y-doping on the mechanical properties and the polycrystalline structure of Mg-Y alloys. When the content of Y atoms is about 7-8%, the models of pristine doping and grain boundaries (GBs) doping have the highest Young's modulus, and the strength also increases obviously. Doping Y atoms at GBs can enhance the stability of GBs and significantly improve the elastic modulus and strength of nano Mg alloys. And the smaller the grain size is, the more obvious the strengthening effect is. As the grain size decreases to the critical value, Young's modulus of the nano Mg-Y alloys decreases, and the strengthening effect weakens. These results have important guiding significance for the design and improvement of mechanical properties of Mg-Y alloys and will provide references for preparing high strength nano Mg alloys materials by controlling impurity atoms doping. [ABSTRACT FROM AUTHOR]

Study on the dynamic process of CL-20 crystal under impact is critical for the safe utilization of energetic materials under extreme conditions. Herein, the mechanical and structural evolution of CL-20 under the impact of a diamond ball is investigated by using molecular dynamics simulation. The considerations are given to the effect of different impact velocity, impact direction and impact angle. It is found that a high impact velocity results in a large indentation depth and force, as well as more significant energy transition and the formation of a large number of molecular fragments. Moreover, CL-20 exhibits weak anisotropy along different impact directions due to the crystalline distribution anisotropy. Furthermore, the mechanical response of CL-20 is angle-dependent, which is caused by the discrepancy in local molecular re-arrangement. These results may enhance the understanding of the mechanical behavior of CL-20 and promote its wide applications. [Display omitted] • The mechanical and structural evolution of CL-20 crystal under the impact was simulated by using molecular dynamics. • The weak anisotropy of CL-20 along different impact directions was presented. • The effect of different impact velocity, direction and angle on CL-20 was considered. • The mechanical response of CL-20 under different impact angles showed angle-dependence. [ABSTRACT FROM AUTHOR]

The effects of doping ratio of calcium (Ca) on mechanical behaviors are investigated using molecular dynamics (MD) and the second nearest-neighbor modified embedded-atom method (2NN-MEAM) formalism for nanocrystalline (NC) Mg-Ca alloys system. Research results indicate that mechanical behaviors of Mg-Ca alloys are independent of lower strain rate (under 1.0 × 109 s−1). In addition, we observe that Ca doping can affect the mechanical properties of the Mg-Ca alloys, and the optimal 2.0 at% of Ca atoms, which has excellent plasticity, is revealed. When the doping ratio is lower than critical atomic percent (CAT) of Mg2Ca, Young's modulus and yield stress decrease increasing at% of substitutional Ca. The pyramidal dislocations are observed frequently at more active grain boundary (GB) with higher Ca doping ratios. In contrast, with doping ratio above CAT, Mg2Ca reinforcement dominates brittleness Mg/Mg2Ca nanocomposites to obtain high strength. By calculating, a significant increase of strength is discovered when at% of Mg2Ca is above 18.85 (5.34 at% Ca). Intergranular fractures are more likely to nucleate and propagate along weaker Mg/Mg2Ca interfaces. These results are instrumental in design and improving the mechanical properties of Mg-Ca alloys. [ABSTRACT FROM AUTHOR]

The plastic deformability of brittle ceramics, e.g. , Al 2 O 3 , at a small scale can expand their potential structural applications. In this work, we propose to investigate the nanomechanics and plasticity of α -Al 2 O 3 using molecular dynamics (MD) simulations at room temperature. First, nanoindentation MD simulations are performed with single crystalline α -Al 2 O 3. Four crystallographic orientations are investigated, m 1 1 ̄ 00 , a 2 1 ̄ 1 ̄ 0 , R 1 ̄ 012 and c 0001 , including a detailed analysis of Al 2 O 3 dislocation-based mechanisms. The results show that the O atoms undergo a phase transformation, changing from the hexagonal close-packed structure to face-centered cubic and body-centered cubic structures. Second, during nanoindentation, we focus on pop-in events and the transformation point from elastic to inelastic response during loading forces. The results are discussed in the context of recent transmission electron microscope experiments, possibly opening new doors towards ceramic bulk material processes. • Molecular dynamics simulation was used to studied deformation mechanisms of α -Al 2 O 3 single crystals during nanoindentation. • The orientation along the c plane exhibited the strongest deformation resistance. • It shows the pop-in events and the transition point from elastic to inelastic response during indentation based on dislocation nucleation. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations using a three-body potential show that the melting and mechanical behaviors of silicon nanowires are strongly dependent on their cross-section area. For the wire with a small cross-section area, rearrangements of surface atoms greatly affect thermal stability in a relatively low temperature regime. For these wires with a relatively large area, while some surface atoms adjust their positions, most of the interior atoms hold their tetrahedra packing patterns. At a high temperature, the accumulation of structural disorder can quickly extend into the entire wire, which resembles the melting of the bulk phase. By applying the uniaxial tensile, these silicon nanowires present the typical mechanical behavior of plastic materials. The atomic local stress in the necking region is apparently larger than that outside of the necking region. As the cross-section area becomes large, both the yield strength and tensile strength increase. With the increasing temperature, the elasticity decreases significantly.

[Display omitted] • This is the only large-scale molecular model of carbon cathodes for aluminum smelting that is consistent with both experimental characterization results, such as XRD, XPS and HRTEM, as well as common sense of chemistry. • It is the first time to apply the ReaxFF MD simulation to explore the atomic scale mechanism of sodium penetration and its relationship with mechanical behaviors, so as to elucidate the origin of electrode strength from a new perspective. • An effective theoretical framework has been established, which can well quantify and predict the structural failure behaviors involving sodium expansion and creep deformation in carbon electrodes. Sodium penetration of carbon electrodes leads to large deformation and concurrent changes in their mechanical properties. Taking the industrial carbon cathode for aluminum smelting as an example, this work constructed a complex molecular model of the carbon cathode (over 5,0000 atoms). Then, the large-scale ReaxFF simulations were applied to correlate the microstructural changes with the associated macroscopic response, revealing the atomic-scale mechanisms of sodium penetration and its relation to mechanical behaviors including sodium expansion and creep deformation. In particular, we provided a detailed knowledge involving the microstructural origin of mechanical deterioration and the molecular basis of cathode strength. This study might be beneficial to improve the understanding of the structure-property relationship, and provide theoretical support for the optimal design of carbon electrode materials against failure. [ABSTRACT FROM AUTHOR]

• Shock responses of MG-SMA nanocomposite are investigated via MD simulation. • The interface reflects compressive/refraction waves, depending on the wave direction. • The B2 CuZr SMAs have stronger shear resistance than the Cu50Zr50 MG. • The B2 CuZr SMAs exhibit different plastic mechanisms at various shock intensities. It has been extensively reported that metallic glass-shape memory alloy (MG-SMA) composites possess superior ductility and strength. Here, shock responses of bilayered MG-SMA (Cu 50 Zr 50 -B2 CuZr) nanocomposites are investigated by atomistic simulations. The results show that the MG/SMA interface can reflect compression/rarefaction waves, depending on the shock direction. Higher shear stress of the SMA means it has stronger shear resistance than the MG. At the velocity of 0.8 km/s, a B2 to BCT phase transformation is shown in the SMA regions, which is more obvious in the monocrystalline than that in the nanocrystalline regions because the grain boundary compression and grain rotation assimilate substantial plastic strain. At the velocity of 1.6 km/s, a two-stage phase transformation of B2 to BCT to amorphous structure is captured in the monocrystalline SMA. But for the nanocrystalline SMAs, the B2 phase is rapidly amorphized due to the promoting effects of the grain boundary. [ABSTRACT FROM AUTHOR]

Phase separation phenomena in high-entropy alloys (HEAs) have attracted much attention since their discovery, but little attention has been given to the dynamics of the deformation mechanism of this kind of HEA during uniaxial tension, which limits their widespread and practical utility. In this work, molecular dynamics simulation was used to study the effect of phase separation on the mechanical properties of an HEA under uniaxial tensile loading. Moreover, the associated deformation behavior of the Co–Cr–Cu–Fe–Ni HEA was investigated at the nanoscale. Models with Cu-rich grain boundaries or grains were constructed. The results showed that Cu-rich grain boundaries or grains lowered the strength of the Co–Cr–Cu–Fe–Ni HEA, and Cu-rich grain boundaries significantly reduced ductility. This change of mechanical properties was closely associated with a deformation behavior. Furthermore, the deformation behavior was affected by the critical resolved shear stress of Cu-rich and Cu-depleted regions and the uneven stress distribution caused by phase separation. In addition, dislocation slipping and grain boundary sliding were the main mechanisms of plastic deformation in the Co–Cr–Cu–Fe–Ni HEA. [ABSTRACT FROM AUTHOR]

Molecular dynamics simulations using a three-body potential show that the melting and mechanical behaviors of silicon nanowires are strongly dependent on their cross-section area. For the wire with a small cross-section area, rearrangements of surface atoms greatly affect thermal stability in a relatively low temperature regime. For these wires with a relatively large area, while some surface atoms adjust their positions, most of the interior atoms hold their tetrahedra packing patterns. At a high temperature, the accumulation of structural disorder can quickly extend into the entire wire, which resembles the melting of the bulk phase. By applying the uniaxial tensile, these silicon nanowires present the typical mechanical behavior of plastic materials. The atomic local stress in the necking region is apparently larger than that outside of the necking region. As the cross-section area becomes large, both the yield strength and tensile strength increase. With the increasing temperature, the elasticity decreases significantly. [ABSTRACT FROM AUTHOR]

Nanoscale mechanical behavior of kaolinite as a fundamental failure mechanism has been investigated under uniaxial tension and compression using Molecular Dynamics (MD) simulation methods. External deformation has been applied on kaolinite with a strain rate of 5 × 10−7fs−1 for tensile and compressive tests in the directions parallel (x -, y -direction)/perpendicular (z -direction) to clay mineral layers. Results showed that better mechanical performance was presented in the directions parallel to clay mineral layers than the other direction due to its continuous lattice in this plane. However, the elastic modulus of kaolinite in the z -direction was almost half of that in the other directions, which nearly equals the overall elastic modulus of kaolinite with a value of about 72.6 GPa. Compression in the x - and y -directions resulted in the separation of clay mineral layers then bending toward the octahedral sheet till crack. Compression in the z -direction resulted in slippage of clay mineral layers at the first fracture then resistance till the second fracture at the strain of about 0.2. Tension would cause cracks in the direction perpendicular to the strain direction, which may be a cleavage fracture or cracks in clay mineral sheets. Different failure modes under tension and compression were originated from the layered structure of kaolinite. • Compression along the direction perpendicular to kaolinite layers can lead to double fractures with slippage of layers. • To damage kaolinite layers needs more energy than to separate them. • Elastic modulus of kaolinite in the three directions is expressed as the average of tensile and compressive results. • The overall elastic modulus of kaolinite is dominated by that in the direction perpendicular to clay mineral layers. [ABSTRACT FROM AUTHOR]

This paper is aimed at investigating the mechanical properties and deformation mechanisms of nanocrystalline copper under nanoindentation. The Voronoi tessellation method is adopted to generate nanocrystalline structures with stochastic grain orientations that mimic those in experiments. Grain-size effect is studied and discussed via molecular dynamics simulations. The results reveal the inversion of Hall-Petch relation about hardness at a grain size of 15.1 nm, which agrees well with previous works in tension. Below the critical grain size, grain boundary sliding, grain growth and grain rotation are easily observed. Grain boundary motion is the dominant deformation with smaller grain size below 15.1 nm while dislocation motion dominates above the critical value. It is noteworthy that the elastic recovery in indentation direction, increases with larger grain size and monocrystalline copper behaves with the strongest elastic recovery. The study further reveals the deformation mechanism of nanocrystalline copper under nanoindentation and accelerates the functional applications of nanocrystalline materials. Material science; computational material. • Nanocrystalline structure is generated by Voronoi tessellation method. • Nanoindentation behaviors of nanocrystalline copper are investigated. • The critical grain size for the inversion of Hall-Petch relation is about 15.1 nm. • Grain boundary sliding, grain rotation and grain growth are easily observed. • The depth elastic recovery is sensitive to grain size. [ABSTRACT FROM AUTHOR]

Regular black phosphorus (BP) sheets possess strongly anisotropic properties due to the unique puckered atomistic configuration, making such BP mechanically very weak in the armchair direction. The present work aims to address this issue by proposing an angle-ply double-layer black phosphorus (DLBP) structure in which two individual atomic layers with different orientation angles are stacked up. The molecular dynamics simulations based on Stillinger-Weber potential show that the in-plane mechanical properties of such a DLBP structure, e.g., Young's modulus and tensile strength are significantly influenced by the stacking angle of each layer. The property anisotropy of DLBP decreases as the stacking angle difference δ between two layers increases and becomes isotropic when δ = 90°. This work also shed insight into mechanisms of angle-ply layers underlying the mechanical behaviors of DLBP at the nanoscale, suggesting that the anisotropic material properties can be effectively controlled and tuned through the appropriately selected stacking angles. [ABSTRACT FROM AUTHOR]