The role of liquid–liquid transition in glass formation of CuZr alloys

Nanoscale-to-Mesoscale Heterogeneity and Percolating Favored Clusters Govern Ultrastability of Metallic Glasses

Comprehending and controlling the stability of glasses is one of the most challenging issues in glass science. Here we explore the microscopic origin of the ultrastability of a Cu-Zr-Al metallic glass (MG). It is revealed that the ultrastable window (0.7-0.8 T_g) of MGs correlates with the enhanced degree of nanoscale-to-mesoscale structural/mechanical heterogeneity and the connection of stability-favored clusters. On one side, the increased fraction of stability-favored clusters promotes the formation of a stable percolating network through a critical percolation transition, which is essential to form ultrastable MG. On the other side, the enhanced heterogeneity arising from an increased distribution in local clusters may promote synergistically a more efficient and frustrated packing of amorphous structure, contributing to the ultrastability. The present work sheds new light on the stability of MGs and provides a step toward next-generation MGs with superior stability and performances.

Kinematic Viscosity ofMulticomponent FeCuNbSiB-BasedMelts

The work investigated the temperature dependences of the kinematic viscosity for multicomponent melts of nanocrystalline soft magnetic alloys. It is shown that there is a linear relationship between the reduced activation energy of viscous flow E_a·(RT)⁻¹ and the pre-exponential factor ν₀. This ratio is universal for all quantities, the temperature dependence of which is expressed by the Arrhenius equation. It is shown that the activation energy of a viscous flow is linearly related to the cluster size on a natural logarithmic scale, and the melt viscosity increases with decreasing cluster size. The change in the Arrhenius plot in the anomalous zone on the temperature dependence of viscosity can be interpreted as a liquid–liquid structure transition, which begins with the disintegration of clusters and ends with the formation of a new cluster structure.

Atomic-level crystallization in selective laser melting fabricated Zr-based metallic glasses

As a promising additive manufacturing technique, selective laser melting (SLM) provides the possibility of fabricating metallic glassy components free of the constraints of geometrical complexity and dimensions. However, unexpected crystallization greatly affects the microstructure and degrades the mechanical performance of SLM-fabricated metallic glasses (MGs). To clarify the crystallization mechanism and the effect of laser processing on the crystallization, we investigate the atomic-level crystallization in the SLM Zr₉₀Cu₁₀ MG by using molecular dynamics simulations. The results show that crystallization highly related to scan speed lies in the atomic-level cluster changes. Lower scan speed leads to a dramatically increased fraction of the BCC crystal phase, accompanied by the nucleation of a few HCP and FCC crystal phases. As scan speed increases, more icosahedron-like clusters are formed, leading to the formation of the MG, while the nucleation of the crystal phase is suppressed. The suppression of crystallization is further attributed to a higher average temperature variation rate induced by higher scan speed, which reduces the relaxation time, preventing the nucleation and growth of crystal phases. This work contributes to the understanding of the crystallization in MGs during the SLM process at the atomic level, providing guidance to suppress the crystallization in the SLM process of desired metallic glassy components.

Crystallization behavior of a confined CuZr metallic liquid film with a sandwich-like structure

Despite the fact that its crystal state is thermodynamically stable, Cu₆₄Zr₃₆ alloy is prone to form metastable glass at a high cooling rate. However, the confinement can induce nano-crystallization with a novel sandwich-like hierarchical structure consisting of pure Cu layers, pure Zr layers and mixed layers by conducting molecular dynamics simulations. The liquid-to-crystal transition temperature and interatomic repulsion softness display abnormal oscillations, instead of monotonous variation, as the wall-wall separation increases. When the confinement size is 10 Å and 12 Å, the transition temperature reaches a maximum, resulting from the pending new sandwich layer. The atomic movement and dynamical heterogeneity are demonstrated to play a vital role in the abnormal oscillation behavior of physical properties of the nano confined metallic glass. The sandwich-like structure can alter the Cu-Zr bond fraction, which eventually influences the liquid-to-crystal transition temperature and interatomic repulsion softness. Our findings provide a deep insight into the hierarchical nanostructures and its liquid-to-crystal transition characteristics under confinement at the atomic level.

Topologically close-packed characteristic of amorphous tantalum

The structural evolution of tantalum (Ta) during rapid cooling was investigated by molecular dynamics simulation, in terms of the system energy, the pair distribution function and the largest standard cluster analysis. It was found that the critical cooling rate for vitrification was about R ≥ 0.25 K ps-1, two orders lower than other metals (such as Au, Ag, Al, Zr and Zn) and that the meta-stable σ phase (β-Ta) not only appears on the pathway from liquid to the stable body-centred cubic crystal, but is also easily obtained at room temperature as a long-lived metastable phase with some probability. The most interesting point is that the liquid, amorphous and β-Ta phases share a nontrivial structural homology; the intrinsic topologically close-packed (TCP) structures in liquids are inherited and developed in different ways, resulting in amorphous or crystalline solids, respectively. With highly local packing fractions and geometrical incompatibility with the global close-packed (such as hcp, fcc and bcc) crystals, TCP structures inevitably result in structural heterogeneity and favour vitrification. As a superset of icosahedrons, TCP structures are ubiquitous in metallic melts, and just before the onset of crystallization reach their maximal number, which is much bigger in Ta than in other poor-GFA metals; so we argue that the strong forming ability of TCP local structures significantly enhances the glass forming ability of pure metals. These findings open up a new perspective that could have a profound impact on the research into metallic glasses.