Investigating medium range order in Mg-Al binary metallic glasses: Molecular dynamics approach (original) (raw)
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Using molecular dynamics simulations with the embedded atomic method (EAM) to describe interatomic interactions , structural and mechanical properties of Mg-Al metallic glasses (MG) have been investigated for different compositions. The atomic structure is characterized using various techniques such as the Radial Distribution Function (RDF), the Voronoi Tessellation Analysis (VTA) and coordination number (CN) distribution. The results confirmed that the Mg-Al glass formation is accompanied by a splitting of the RDF second peak upon cooling process. The glass transition temperature is determined using different method involving a new suggested way consisting of the cross-over between low and high coordination numbers curves during cooling process. This last technique gives approximate results that converge to those given by classical methods. On the other hand, we applied a strain-rate of 10 10 s −1 at 300 K and showed that with the increase of Al composition, the maximal stress incr...
Journal of Alloys and Compounds , 2021
Molecular dynamics (MD) simulations based on the embedded atomic method (EAM) are used to study the annealing effect on elastic and structural behavior for different cooling rates of binary Mg Al 25 75 metallic glasses (MGs). Firstly, we examine the cooling rates effect (K s 5 10 / 11 × , K s 10 / , 12 K s 5 10 / 12 < > < > clusters and the degree of the five-fold symmetry increase as a function of the increase of annealing time.
Using molecular dynamics simulations, we have studied the atomic correlations characterizing the second peak in the radial distribution function (RDF) of metallic glasses and liquids. The analysis was conducted from the perspective of different connection schemes of atomic packing motifs, based on the number of shared atoms between two linked coordination polyhedra. The results demonstrate that the cluster connections by face-sharing, specifically with three common atoms, are most favored when transitioning from the liquid to glassy state, and exhibit the stiffest elastic response during shear deformation. These properties of the connections and the resultant atomic correlations are generally the same for different types of packing motifs in different alloys. Splitting of the second RDF peak was observed for the inherent structure of the equilibrium liquid, originating solely from cluster connections; this trait can then be inherited in the metallic glass formed via subsequent quenching of the parent liquid through the glass transition, in the absence of any additional type of local structural order. Increasing ordering and cluster connection during cooling, however, may tune the position and intensity of the split peaks.
Journal of Non-Crystalline Solids, 2021
To clarify the origin of structural heterogeneity in metallic glasses (MGs), liquid structures upon cooling of Zr-CoAl alloys were simulated. Being different from Zr 10 Co 85 Al 5 alloy, the first peak of pair distribution functions (PDFs) for Zr 65 Co 30 Al 5 alloy exhibits a splitting behavior, which is attributed to the competition between Co-and Zr-centered clusters. The coordination numbers and chemical short-range order (SRO) analysis show that the chemical structural heterogeneity also governs the formation of Zr 65 Co 30 Al 5 MG. Bond pair analysis reveals the prominence of bond pairs with five-fold symmetry for both MGs, but bond pairs in Zr 65 Co 30 Al 5 MG exhibit noticeable six-fold symmetric stability. Voronoi polyhedron analysis further confirms the formation of strong chemical SROs in Zr 65 Co 30 Al 5 MG. Dynamic mechanical experiments (DMA) prove the existence of structural heterogeneity, while Zr 65 Co 30 Al 5 MG with more pronounced (chemical) structural heterogeneity displays larger ductility. The present work may provide a new insight into the origin of structural heterogeneity in MGs.
When a liquid is cooled well below its melting temperature at a rate that exceeds the critical cooling rate Rc, the crystalline state is bypassed and a metastable, amorphous glassy state forms instead. Rc (or the corresponding critical casting thickness dc) characterizes the glass-forming ability (GFA) of each material. While silica is an excellent glass-former with small Rc < 10−2 K/s, pure metals and most alloys are typically poor glass-formers with large Rc > 1010 K/s. Only in the past thirty years have bulk metallic glasses (BMGs) been identified with Rc approaching that for silica. Recent simulations have shown that simple, hard-sphere models are able to identify the atomic size ratio and number fraction regime where BMGs exist with critical cooling rates more than 13 orders of magnitude smaller than those for pure metals. However, there are a number of other features of interatomic potentials beyond hard-core interactions. How do these other features affect the glass-forming ability of BMGs? In this manuscript, we perform molecular dynamics simulations to determine how variations in the softness and non-additivity of the repulsive core and form of the interatomic pair potential at intermediate distances affect the GFA of binary alloys. These variations in the interatomic pair potential allow us to introduce geometric frustration and change the crystal phases that compete with glass formation. We also investigate the effect of tuning the strength of the many-body interactions from zero to the full embedded atom model on the GFA for pure metals. We then employ the full embedded atom model for binary BMGs and show that hard-core interactions play the dominant role in setting the GFA of alloys, while other features of the interatomic potential only change the GFA by one to two orders of magnitude. Despite their perturbative effect, understanding the detailed form of the intermetallic potential is important for designing BMGs with cm or greater casting thickness.
Mechanical responses of Mg-based bulk metallic glasses
Intermetallics, 2010
In this study, the mechanical responses of the bonded interface method derived Mg 58 Cu 28.5 Gd 11 Ag 2.5 bulk metallic glasses (BMG) under the micro-/nano-indentation are investigated. A modified expanding cavity model is developed to analyze the morphological observations of shear band by Vickers indentation. Results indicate that the radius ratio of any two adjacent shear band circles is approximately constant. The ratio of the shear band deformation zone to the contact radius induced by indentation is also a constant, which depends on the constraint factor (the ratio of hardness to yield strength) of the material. The features predicted by the present model are consistent with the results obtained from the indentation experiments, indicating the validity of the expanding cavity model for describing the mechanical behaviors of the BMG materials. The cross-sectional transmission electron microscopy (XTEM) observations reveal that the terraced shear bands are formed on the interface of the bonded Mg-based BMG. The microstructures for the Vickers indentation-induced terraced shear bands are also discussed.
Collective dynamics of Bulk Metallic Glasses Studied by Molecular Dynamics Simulations
LIST OF FIGURES 5.5 Atomic structure of a LJ system at 1K showing 80% of A atoms (orange) and 20% of B atoms (green). The disordered structure of the system and the density uctuations are revealed.. .. .. 5.6 Velocity Autocorrelation Function of Lennard-Jones binary system of 1098500 atoms at the equilibrium and quenching rate η 2. 5.7 Vibrational Density of States calculated from the velocity autocorrelation function in a Lennar-Jones binary system of 1098500 atoms at the equilibrium and quenching rate η 2. .. .. .. .. . 5.8 Boson Peak observed in of Lennar-Jones binary system of 1098500 atoms at the equilibrium and quenching rate η 2. .. .. .. .. . 5.9 Debye level versus quenching rate determined in simulation boxes of dierent sizes. The size of the box, namely 1 • 10 4 , 1 • 10 5 , 1 • 10 6 , 2 • 10 6 and 3 • 10 6 atoms, is
Structural Investigation of the Metallic Glasses Mg85.5Cu14.5 and Mg70Zn30
Zeitschrift für Naturforschung A, 1983
The total structure factors as well as the total pair correlation functions for amorphous Mg85.5Cu14.5 (by neutron and X-ray diffraction) and for amorphous Mg70Zn30 (by X-ray diffraction) were determined. Both alloys show similar chemical short range order effects. From the total pair correlation function of the Mg85.5Cu14.5 glass, partial coordination numbers and atomic distances could be extracted. Comparison with the structure of crystalline Mg2Cu suggests that the short range order around the copper atoms is similar in the amorphous and the crystalline phase. The densities of both amorphous alloys were measured yielding negative excess volumina.
Metals and Materials International, 2013
Atomic size differences between constituting elements and the heat of mixing are key factors in designing a metallic glass system. In this study, the effects of atomic size differences and the heat of mixing on the glass-forming ability and the local structure of metallic glasses were studied via molecular dynamic simulations of an ideal system known as the Lennard-Jones embedded-atom method model. The atomic size difference and the heat of mixing of the system were varied by means of the Lennard-Jones parameters. The glass transition behavior was characterized based on the chemical short-range order and by a Voronoi analysis. Our simulations lead to optimized windows of atomic size differences and heat of mixing parameters for metallic glass-forming of the model system. Both a greater negative heat of mixing and a larger atomic size difference are necessary for the enhancement of the glass-forming ability.