Search

The liquid structures of a series of Mg-Zn alloys have been investigated in the present work by means of ab initio molecular dynamic simulation. Strong chemical short-range orders are observed for the eutectic alloy Mg 70 Zn 30 , i.e. the best glass former of Mg-Zn binary alloys, which is consistent with the experiment results. The pair analysis indicates the extraordinary abundance of icosahedral clusters in this eutectic liquid alloy. Furthermore, the self-diffusion coefficients of Mg and Zn reach minimum for the Mg 70 Zn 30 alloy. The entire results offer an interpretation for the better glass forming ability of the Mg 70 Zn 30 alloy. [ABSTRACT FROM AUTHOR]

The local structure of Al86Ni(8-x)Y6Agx(X = 0,1,2) molten and glassy alloy was investigated byabinitiomolecular dynamics simulation. It transpired that the Al86Ni(8-x)Y6Agxalloy can be considered as a combination of Ni- and Y-centred Al clusters with Ag as ‘glue atoms’. First and second Ag–Ag coordination was scarcely found in Al86Ni7Y6Ag1, but a medium-range order between 7.5 and 9 Å was observed. The enhanced glass-forming ability and thermal stability of the alloy compared with Al86Ni8Y6can be attributed to the medium-range order of the Ag–Ag correlation. A second Ag–Ag coordination occurs in Al86Ni6Y6Ag2and results in a decrease in glass-forming ability. The inter-atomic distances between all constituting elements increase during cooling. This increase is ascribed to a change in distribution of clusters around Al atoms towards clusters with higher coordination number around Al. [ABSTRACT FROM PUBLISHER]

Abstract: Structural evolution of the Fe78Si9B13 alloy during rapid quenching was investigated by ab initio molecular dynamics simulation. The second peak splitting has been perceived even at 1473K in the partial pair correlation functions though not in the total pair correlation function. The (0,3,6,0) polyhedra are abundant in the liquid state while the distorted (0,3,6,0) polyhedra are the featured local structure around B atoms in the amorphous state. The diffusion coefficients of the three elements are evaluated to understand the dynamics of quenching. From 1173 to 873K the three coefficients are coherent. We think this temperature range corresponds to the supercooled liquid region, and 873K serves as the glass transition temperature. [Copyright &y& Elsevier]

Abstract: The structure of Fe80Si10B10 alloy melt was investigated by ab initio molecular dynamic simulation from the local atomic environments. It presents that Fe80Si10B10 alloy can be considered as the combination of B-centered prism-like clusters and bcc-like Fe-Si solid solution. The poor glass-forming ability of the alloy has been investigated and can be attributed to the bcc-like environment around Si atoms and the relative high content of pure Fe clusters. [Copyright &y& Elsevier]

• A general role of rare earth elements in dynamic characteristic of series of FeB-based bulk-glass-forming liquids has been uncovered. • A negative correlation between the degree of dynamic transition F in melts and the glass-forming ability has been established in FeB-based bulk-glass-forming alloys. • The mechanism underlying the indistinct dynamic transition and the improved glass-forming ability of alloys induced by rare earth elements has been discussed. We investigated the viscosities of several series of FeB-based bulk-glass-forming melts with rare earth element additions. The viscosity discontinuity generally exists in the melts. With the rare earth element addition, such dynamic transition becomes indistinct. By the concept of superheated liquid fragility, the addition of rare earth elements significantly reduces the degree of the dynamic transition F from larger than 1 to less than 1. A generally negative correlation between F and GFA has been demonstrated. By conducting the ab initio molecular dynamics simulations, the competition between the formations of square antiprism structures and the stable rare-earth-contained <0, 3, 6, 0> polyhedra around B atoms have been found to determine the F value, as well as the GFA of FeB-based liquids. This work demonstrates the mechanism underlying which the glass-forming ability of alloys is greatly improved by rare earth elements, and provides a novel route to obtain bulk-size Fe-based glasses. [ABSTRACT FROM AUTHOR]

• The relaxation brittleness and non-brittleness was related to icosahedral cluster. • Zr 60 Cu 30 Al 10 alloy also showed brittleness with increasing relaxation temperature. • Large cluster assembly was revealed by electron microscopy 3D reconstruction. Relationship between relaxation embrittlement and atomic cluster structure was explored by simulation and experimental methods for two amorphous alloys: Zr 60 Cu 30 Al 10 and Zr 50 Cu 40 Al 10 (referred to as Zr 60 and Zr 50 , respectively). Simulation results from large-scale molecular dynamics revealed that the Zr 50 alloy exhibited a higher content of icosahedral clusters, which increased more obvious during the quenching process compared to the Zr 60 alloy. As icosahedral clusters exhibit brittleness similar to quasicrystalline structures, we predicted that the Zr 50 alloy would have a higher glass transition temperature T g and more pronounced relaxation-induced brittleness than the Zr 60 alloy based on this observation. To validate the predictions, experimental results confirmed the occurrence of relaxation brittleness in Zr 50 and relaxation non-brittleness in Zr 60 , consistent with the XRD analysis and electron microscope-based three-dimensional reconstruction in terms of icosahedral clusters. The agreement between the simulation calculations and the experimental data provides a reference for understanding the mechanism of relaxation embrittlement in amorphous alloys. [ABSTRACT FROM AUTHOR]

Bond breaking related with plastic deformation in a deformed metallic glass Zr 50 Cu 50 is investigated by molecular dynamics simulations. The results show that the spatial distributions of broken bonds are closely correlated with local shear strains, and the clustering behaviors of atoms with broken bonds (flow defects) are characterized with different stages of plastic deformation. The average distance among those clusters of flow defects decreases as the strains increase, which follows the curvature quadrupole model for flow defects in ideal amorphous solids. For the first time, the features of bond breaking processes are quantitatively measured with the chemical composition, bond length and orientation, bond pairs, local five-fold symmetry and quasi-nearest atoms, whose threshold values are important factors that could characterize the flow defects in metallic glasses under plastic deformation. The shape, orientation and energetics of flow defects quantitatively characterized by the bond breaking analysis thus facilitate our understanding on the deformation mechanisms in metallic glasses. Image 1 • Geometry, orientation and energetics of flow defects are revealed. • Chemical and structural factors are quantitatively measured. • A mechanistic model for flow defects is proposed. [ABSTRACT FROM AUTHOR]

Athermal quasi-static compressive deformation for a model metallic glass is investigated by molecular dynamics simulations. A new structural parameter, quasi-nearest atom (QNA), is used to characterize local atomic packing of initial structure. We find that regions with higher nonaffine displacement have more atoms with more QNA. The coarse grained number of QNAs exhibits significant spatial heterogeneities and these structural fluctuations show close correlation with the nonaffine displacement. The correlation between the coarse grained number of QNA and nonaffine displacement does not decrease as the strain increases but show the maximum at a certain strain, indicating that strain localized regions are activated individually. Therefore, our results provide a direct evidence of the correlation between atomic structure and deformation in metallic glass. [ABSTRACT FROM AUTHOR]

The liquid structure of seven representative Fe–Si–B alloys has been investigated by ab initio molecular dynamics simulation focusing on the role of clusters in terms of glass-forming ability (GFA) and crystallization. It is demonstrated that the type of primary phase precipitated from amorphous state under heat treatment is determined by the relative fraction and role of various clusters in melt. The alloy melt shows higher stability and resultantly larger GFA when there is no dominant cluster or several clusters coexist, which explains the different GFAs and crystallization processes at various ratios of Si and B in the Fe–Si–B system. The close correlation among clusters, crystalline phase and GFA is also studied. [ABSTRACT FROM AUTHOR]

Abstract: The atomic and electronic structures of several well-selected Al70LM20EM10 (LM=Fe, Co and Ni, EM=Ti, Zr, Hf, V and Cr) melts were investigated systemically by the first-principles method. Structural parameters of liquid Al70LM20EM10, such as partial pair correlation functions and coordination numbers, reveal manifest change trends with LM and EM elements, and are closely correlated with the variations of their glass-forming ability as observed experimentally. LM elements (Fe, Co and Ni) have a strong interaction with Al atoms, while the LM–Al bond strength increases slightly from Fe to Ni. As EM elements added into liquid Al–LM alloys, the local structures around EM atoms change from Ti to Cr regularly that the Zr and Hf intend to be distributed uniformly in the Al–LM matrix while V and Cr like to be self-coordinated. [ABSTRACT FROM AUTHOR]

The correlation between local structures and dynamics of the central Al atoms of the icosahedral clusters in the Al 90 Sm 10 super-cooled liquid was investigated by classic molecular dynamics simulations. Three typical order parameters (i.e. the chemical composition, cluster penetration, and quasi-nearest atom) of the Al-centered icosahedral clusters were calculated and we demonstrated that the correlation between each of these parameters and dynamic propensities of the central Al atoms of the icosahedral clusters is weak. Then a simple combined parameter including these three parameters was proposed and it is found that there is a strong correlation between this new parameter and dynamic propensities of the central Al atoms of the icosahedral clusters. The present observations imply that the dynamic property of icosahedral clusters was mainly determined by multiple structural features of local structures. Our results are helpful to understand the relationship between atomic structures and dynamics in super-cooled liquids. [ABSTRACT FROM AUTHOR]