Microstructure in polycrystalline materials, either coarse-grained or nano-crystalline, is characterized by the topological structure of grain boundary networks which are composed of an array of complex geometric entities with different dimensions such as grain volume, grain boundary plane, triple junction line, and vertex point. The ensemble of these entities gives rise to statistical properties represented by their distribution functions, means, variances, and correlation functions. Moreover, contrast to Gibbs’ description, on atomic scales these entities are no longer mathematically abstract geometric objects such as simple plane, line or point; rather they possess finite thickness and volumes, as well as certain specific atomic structures and chemistry. While some of these entities can be measured from experiment, a large number of them still remain inaccessible, that includes identification of the full range of topological properties and the structure characterization on atomic scales. In this article, we present algorithms and numerical methods to characterize systematically these entities in grain boundary networks in polycrystalline samples which are either from serial sectioning of real polycrystals or from digital microstructures generated using inverse Monte Carlo methods. The rendered microstructures are represented by the topological and geometric properties such as the grain volume, grain boundary area, triple junction length, and their statistical properties. Most importantly we give the atomic coordinates and label the type of the topological entities to which each atom belongs in the polycrystalline and nano-crystalline materials. Such quantitative characterization, unavailable before, enables detailed and rigorous treatment of microstructures in a wide range of modeling applications including both atomistic simulation and continuum modeling.

In traditional view, atomic packing is random in glasses made of metallic elements with non-directional interactions as the glass-forming liquid needs to be excited to remain in liquid state before being cooled sufficiently fast to a glass. Locally ordered packing however is possible if certain conditions are favorable, such as a strong bonding between elements, or low configuration energy of a cluster of atoms as suggested by Frank. In alloy systems made of different metallic elements, we show that Frank’s criterion alone does not necessarily lead to certain specific local ordered packing or cluster formation such as icosahedral packing. In this context, we revisit the issue of atomic packing and cluster formation, and show that an alloy system with fairly random liquid configuration could be sufficient to produce a variety of noticeable locally ordered packing with low energy, albeit largely statistical in nature. Therefore, we emphasize the importance of the system parameters such as the atomic size, alloy concentration, and interaction potential in their collective contribution to local atomic packing.

A systematic study of the surface structure and properties of NiZr model metallic glasses is reported using atomistic simulations. It is found that at low temperatures below the glass transition temperatures, the surface retains the amorphous structure and the surface energy γ is significantly lower (∼50%) than that of the corresponding crystalline alloy constituents. The variation of alloy concentration has little effect on γ; but increase in cooling rate and annealing temperature can lead to large decrease in γ. At elevated temperatures, γ increases with temperature and surface melting occurs at a temperature about 30% below Tg. At all temperatures up to Tg, the surface remains atomically smooth.

Mechanical response of amorphous metal Ni40Zr60 under applied tensile loading is investigated using large-scale atomistic simulations. To obtain intrinsic properties, homogeneous samples with atomically smooth surface are used while samples with deliberately introduced surface notches of varying depths and root radii are used to test extrinsic effects. It is found that the notch-free samples show strength close to the theoretical fracture strength and extremely large ductility. Apparent strain hardening and strain rate sensitivity are also observed in these studies. It is argued that the free volume generation and localized shear displacement are responsible for the mechanical properties. Therefore, the presence of surface imperfections can greatly reduce the strength and ductility. The results suggest that to retain and improve the intrinsic mechanical properties of amorphous metals, surface treatment may be needed, which has been practiced in oxide glass industry for centuries but received little attention in metallic glass community.

Atomic mechanisms are investigated for solid-state amorphization using a diffusion couple made of metallic glass Cu46Zr54 and single-crystal Al. Our extensive molecular dynamics simulation reveals that amorphization occurs in the crystalline metal at the interface via a series of highly coordinated and complex atomic motion involving all elements in both the glassy phase and the crystal. Chemical mixing occurs through asymmetric interdiffusion of more Cu and Zr in the glass phase into the crystal than Al into the glass. The faster diffuser Cu is found to hop into the Al lattice position, whereas Zr trails behind and provides a supporting role by pulling Al atoms off the lattice position. This highly choreographed atomic motion creates cooperative diffusion and mixing at the interface region that causes large lattice distortion and eventually leads to the collapse of the crystalline phase when critical amounts of Cu and Zr are fused into Al. Extension of this atomic mechanism to a more general setting is discussed, particularly in the context of elastic instability.The top and side views of the atomic configurations of the advancing interface at (a) 0ps to (m) 6000ps in the diffusion couple with (100) Al plane orientation parallel to the interface. The crystallinity of each atom is shown by the static structure factor (inside the frame); and only the amorphous phase are shown in the rest of the figures. The scale bar shows the distance (Å) of the nucleating and growing amorphous phase along the z-axis from the selected starting time (0 ps). Each figure contains a top view and a side view and the color represents the advancing distance of the interface along the z-coordinate. The positions of the corresponding side views in each top view are marked by the dashed lines. The top view is taken as the upper boundary of the interface where amorphization transition is occurring. The dash lines point out the sections of the upper edge of the side view.