Atomic rearrangements in metallic glass: Their nucleation and self-organization

The atomic rearrangement in metallic glass (MG) is a dynamics process. However, this problem has always been investigated within the quasi-static limit of the material. As such, the physical mechanisms of how the local rearrangements nucleate and coalesce into shear bands have not been fully understood. This study is to clarify the issue with the aid of a systematic molecular dynamics analysis. The present study unveils that the underlying mechanism of plastic deformation is through the rearrangement of atoms, characterized by the sudden surge in kinetic energy or strain rate of a local region, and that the shear banding is a stress-driven dynamic coalescence of these rearranging clusters, propagating in the speed of a shear wave. The ratio of the internal strain rate in forming a cluster to the applied strain rate is a measure of the severity of the local atomic rearrangement. The larger the severity, the easier the shear banding forms. The additional kinetic energy associated with the atom rearrangement in clustering is due to the descending of the potential energy after crossing the energy barrier. As temperature increases, the thermal vibration energy becomes larger than the barrier height, leading to thermal activations in MG and hence giving rise to a homogeneous deformation.     

Oxygen vacancy diffusion in alumina: New atomistic simulation methods applied to an old problem

Understanding diffusion in alumina is a long-standing challenge in ceramic science. The present article applies a novel combination of metadynamics and kinetic Monte Carlo simulation approaches to the investigation of oxygen vacancy diffusion in alumina. Three classes of diffusive jumps with different activation energies were identified, the resulting diffusion coefficient being best fitted by an Arrhenius equation having a pre-exponential factor of 7.88 × 10⁻² m² s⁻¹ and an activation energy of 510.83 kJ mol⁻¹. This activation energy is very close to values for the most pure aluminas studied experimentally (activation energy 531 kJ mol⁻¹). The good agreement indicates that the dominating atomic-scale diffusion mechanism in alumina is vacancy diffusion.     

Quantitative measure of local solidity/liquidity in metallic glasses

Metallic glasses are viscoelastic materials that contain a wide spectrum of local configurations that range from more “solid-like” to more “liquid-like” varieties. To quantitatively characterize the local solidity (or liquidity) in metallic glasses, here we present a robust method and a universal materials parameter. The new parameter, solidity (liquidity) index, is based on the evaluation of bond orientational anisotropy. The usefulness of the proposed index in assessing the varying degrees of solidity (or liquidity) is demonstrated in metallic glass models produced via molecular dynamics simulations. The spectrum of local solidity is also analyzed in terms of its dependence on local structural features, particularly on the basis of different types of Voronoi polyhedra and atomic volume.     

Formation of chemical short range order and its influences on the dynamic/mechanical heterogeneity in amorphous Zr–Cu–Ag alloys: A molecular dynamics study

The chemical short range order of metallic glasses is expected to be correlated with their mechanical properties. In this article, classic molecular dynamics simulations of amorphous Zr₄₅Cu₄₅Ag₁₀ alloys were carried out to reveal such links in metallic glasses. Our calculations of Warren–Cowley parameter indicate the growth of chemical short range order during supercooling process, which also depends on the effective cooling rates. The chemical short range ordering is related to the energetic stability of the system. Based on the chemical preference or avoidance for different bonds, the model is separated into Cu-rich regions and Ag-rich regions. Simulated structural relaxation and shear loading process were performed to study how chemical bonds affect the distribution of dynamic and mechanical heterogeneity in our systems. The Cu-rich regions exhibit slower dynamics and higher shear resistance, whereas Ag-rich regions have faster dynamics and prefer to be plastically deformed.     

Anatomy of nanomaterial deformation: Grain boundary sliding,plasticity and cavitation in nanocrystalline Ni

The deformation of nanocrystalline metals is a complex process that involves a cascade of plastic events, including dislocation motion, grain boundary activity and cavitation. These mechanisms act simultaneously and synergistically during fracture, masking their individual roles and often resulting in a wide range of failure modes in the same material. Using large-scale molecular dynamics simulations, we dissect the size-dependent deformation of nanocrystalline Ni nanowires for a range of diameters spanning a few nanometers to the bulk. By analyzing the localization of von Mises shear strain and stress triaxiality, we identify the key nanostructural features, the role of each elementary process and the dominant deformation mechanism as a function of sample diameter. Our atomic level analysis not only provides a fundamental understanding of the deformation of nanocrystalline Ni, but also demonstrates that large-scale simulations can be an essential complement for modern in situ electron microscopy/atom-probe tomography.     

Instrumented indentation of a Pd-based bulk metallic glass: Constant loading-rate test vs constant strain-rate test

Most of the previous nanoindentation experiments on bulk metallic glasses (BMGs) were made under a constant ‘loading rate,’ although ‘strain rate’ is a more useful parameter than loading rate to analyze the inhomogeneous plasticity in the BMG according to the classic free-volume theory. Here, we explore the strain-rate dependency of plastic characteristics in a Pd-based BMG through nanoindentation tests under a variety of constant strain rates (0.01–0.25 s⁻¹). The results are compared with those from nanoindentations under various constant loading rates (0.05–5 mN/s) and discussed in terms of the influences of strain rate on the plastic flow characteristics in the BMG.     

Local structural fluctuation in Pd–Ni–P bulk metallic glasses examined using nanobeam electron diffraction

Local structural fluctuation in relation to the medium range order (MRO) in a Pd–Ni–P bulk metallic glass was examined using nanobeam electron diffraction (NBED) technique. We found diffraction spots with strong intensities in the NBED patterns taken from any observation sites in the specimen. This indicates a presence of MRO regions densely formed in the specimen. The diffraction spots in NBED were normally dispersed around positions corresponding to the first halo-diffraction ring in selected area diffraction. A low-temperature annealing led to form a nanocrystalline microstructure consisting of unidentified phosphides. In the course of annealing, the MRO structures deduced from the NBED patterns have no structural similarity to the phosphides found in the primary crystallization stage. The MRO structure changes into a similar structure with the primary crystals just before the crystallization. A discussion is made for the MRO structure and its relation to the glass stability and also to the phosphide nucleation in the primary crystallization.     

Thermal stability, microstructure and crystallization kinetics of melt-spun Zr-Ti-Cu-Ni metallic glass

Melt-spun Zr_63.33Ti_8.89Cu_15.45Ni_12.33 glassy ribbons display a double-step devitrification behavior characterized by the precipitation of a metastable quasicrystalline phase in the first stage of the crystallization process, followed by the formation of crystalline phases in the following crystallization event. Investigation of the crystallization kinetics reveal that the initial part of the glass-to-quasicrystalline transformation (x ≤ 55 vol.%) occurs by diffusion controlled growth with an increasing nucleation rate (Avrami exponent n ≥ 2.5), whereas the later stage of the transformation (x > 55 vol.%) is dominated by the growth of the formed nuclei rather than by the generation of new nuclei (2.0 ≤ n ≤ 2.5). The activation energy for quasicrystallization is 360 kJ/mol, which is comparable to the values reported for other quasicrystal-forming Zr-based metallic glasses.     

Effect of ball-milling and shot-peening on Zr55Al10Ni5Cu30 alloys

Effect of ball-milling and shot-peening on a metallic glass Zr₅₅Al₁₀Ni₅Cu₃₀, which possesses a large supercooled liquid region, has been investigated by means of differential scanning calorimetry, x-ray diffractometry and transmission electron microscopy. Metallic glassy ribbons, powders and plates were prepared by melt-spinning, gas-atomizing and mold-clamp casting techniques, respectively. No structural changes were observed in both the ribbon and powder specimens by ball-milling for around 100 h; however, the powder specimens were crystallized by Fe contamination when they were ball-milled for 540 h. No structural evolution was also observed when the plate specimens were subjected to shot-peening, while crystallized plate specimens were easily amorphized by mild and short period shot-peening. These results imply high phase stability of the Zr₅₅Al₁₀Ni₅Cu₃₀ metallic glass against deformation.     

The role of magnetic element Fe in pronounced slow β-relaxation in metallic glasses

Most metallic glasses (MGs) exhibit weak evidence of slow β-relaxation in their dynamic mechanical spectroscopy. In contrast to other MGs, we find a series of Fe-based MGs with unusual distinct β-relaxation peaks in their relaxation spectra. The investigation on the β-relaxation behavior of Fe-based MGs suggest that the magnetic element Fe plays the critical role in the pronounced β-relaxation behaviors in the Fe-based metallic glasses, which could provide deep insight into the mechanism of β-relaxation dynamic mode in metallic glasses.     

Crystallization Behavior of Metallic GlassCo_（65．1）Fe_（4．7）Ni_（4．6）Si_（10．2）B_（15．4）

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