Atomistic Simulations Of Mechanical Properties For Silicon-Carbide And Copper-Zirconium Nano-materials

The state of the art on the research and applications for nanomaterials, especially for their mechanical behavior is presented. The study highlights and applications on the dynamic behaviors of ceramic materials and pseudo-elastic effect of shape memory materials are comprehensively reviewed. The molecular dynamics method for simulating mechanical properties of solid matters is explained thoroughly and systematically. The characteristics of stress waves in different crystallographic directions of (3-SiC, toughening mechanisms of differentβ-SiC materials and structures, as well as the stress-induced martensitic phase transformations in nanowires of Cu and Cu-Zr alloy are investigated with molecular dynamics simulations. The corresponding atomistic mechanisms are discussed.The methodology of molecular dynamics is elucidated in details from the point of view of simulating the thermo-mechanical behavior of solid materials. Some key issues, such as the control of temperature and pressure, and the inter-atomic potential functions adopted in the present course are addressed.Orientation dependence of planar wave propagation on crystallographic direction inβ-SiC material is studied with molecular dynamics method. Simulations were implemented under impact loadings for four main crystallographic directions i.e., [100], [110], [111] and [112] respectively. The dispersion of stress states for different cases above increases as the impact velocity rising, which imply anisotropic characteristic of shock wave propagation forβ-SiC material. The Hugoniot relations between the shock wave velocity and the impact velocity is obtained. It is found that the shock velocity fall into a plateau beyond a threshold value of impact velocity. The shock velocity of the plateaus is dependent on the shock directions, while [111] and [112] can be regarded as an equivalent direction as they almost reach a same plateau. Comparison between the atomic stress from molecular dynamics and the stress from Rankine-Hugoniot jump conditions are also performed. The results are consistent with each other very well. Especially for [100] direction, part of cubic lattice atoms transforms into amorphous state when the impact velocity exceeds the critical value of 4.91 km/s. The size of the amorphous region is well proportional to the particle velocity.Several possible toughening mechanisms forβ-SiC material are investigated with molecular dynamics. For nanocrystalline P-SiC with small-sized grains, the fracture strain can be enhanced by breaking grains into amorphous clusters. For P-SiC nano-laminates, amorphous layer with smaller thickness can improve the yield strain and show behavior similar with plastic flow. All [111] P-SiC nanowires coated with amorphous layers of different thickness show ductile behavior under low temperature, while the toughness almost has no changes under room temperature.The stress-induced martensitic phase transformation and relevant pseudo-elastic effects in copper nanowires under loops of uniaxial tensile loading and unloading are studied with molecular dynamics. A combination of thermal and surface stress leads to structural reorientation in metallic cooper nanowire, from<100> structure to <110>/{111} rhombic structure in relaxation process. If uniaxial tensile load is applied, the reoriented<110>/{111} structure can transformed back to<100> structure. Stress and strain both can return back to zero if unloading is performed before elastic limit reached. The loading curve and the unloading curve form a hysteresis loop, which exhibits good pseudo-elastic effects for copper nanowires.The pseudoelastic effects induced by martensitic phase transformation from body-centered cubic (B2) to body-centered tetragonal (BCT) lattice in Cu-Zr nanowires are investigated with molecular dynamics method. The phase transformation occurs through nucleation and propagation of {100} twin boundary, which differs from the {101} twin boundary for B2 Ni-Al nanowires. Inverse martensitic phase transformation occurring in the unloading procedure will lead to full recovery of a pseudo-elastic strain up to 40%, which far exceeds the pseudo-elastic strain of 5%-10% for bulk shape memory alloys. The obtained results imply that Cu-Zr nanowires may be a kind of excellent functional components for nano-electromechanical systems. The martensitic phase transformation in Cu-Zr intermetallic nanowires reveals significant size effect. In the initial state of uniaxial loading, the long wires undergo a martensitic inter-phase transformation, from body-centered cubic phase to unstable body-centered hexahedral phase. As increasing the loading, the long wire will complete the martensitic phase transformation, from body-centered hexahedral inter-phase to body-centered tetragonal phase. A mid-peak which independent from the first and the last peak is observed in the simulation and the magnitude of the mid-peak is less than that of the two peaks. The relation between the mid-peak and the martensitic inter-phase transformation is verified by the details of atomistic configurations.