Icosahedral Short-range Order Research Articles

Glassy NiTi produced with different cooling times: Structural investigation using molecular dynamics simulations

In this research we investigate the structure of glassy NiTi using molecular dynamics simulations. The final configurations are obtained by decreasing the temperature of liquid NiTi rapidly from 2500 K to 300 K, with various cooling times from 0.1 ns to 2.0 ns. We found that the height of the peak values for structural factors slightly increases with an increase in cooling time. From the analysis of local atomic packing using the bond-angle method, we also find that the count of hexagonal close packed (HCP)-like structure drops at the longer cooling time, a finding which is contrary to the trend for body-centered cubic (BCC)-like and icosahedral short-range order (ISRO) structures. It is also observed that face-centered cubic (FCC)-like structure is insensitive to change in cooling time.

Pressure dependence of the structure of liquid NiTi: a molecular dynamics study

We evaluate the structure of liquid NiTi under various pressures from 0 GPa to 40 GPa in the atomic level using molecular dynamics simulations. The structure factor and radial distribution function are used to investigate the general structural change of the system. Further identification of the local structures is examined by the bond-angle method and bond-angle distribution analysis. From our results, we found that the count of the local structure of fcc, hcp, bcc, and icosahedral short-range order monotonically increase when the pressures increase. We also observed in our results that the size of the local cluster grows as the pressure increases, and the long-range connectivity of the quasi-crystal is achieved at high pressure.

Heat Flow in Earth's Core From Invariant Electrical Resistivity of Fe‐Si on the Melting Boundary to 9 GPa: Do Light Elements Matter?

AbstractThe electrical resistivity and thermal conductivity of liquid Fe alloys are the least constrained parameters in Earth's outer core (OC). These parameters are important as they modulate energy budget available for the geodynamo and affect the spatiotemporal evolution of the core. We report results of electrical resistivity measurements on solid and liquid Fe‐4.5 wt%Si from 3–9 GPa using a large volume multianvil press. The internally modified 18/11 octahedron cell was used to maintain the geometry of the liquid sample and to delay the onset of contamination. Electrical resistivity of solid Fe‐4.5Si decreases steadily with pressure and is very sensitive to increasing temperature‐driven onset of phase transitions. Along the melting boundary and within error, electrical resistivity remains constant and assumes the same value of 120 μΩcm as observed in pure liquid Fe. The results are interpreted in the context of icosahedral short‐range order (ISRO) structures that exhibit higher concentration with increasing pressure. Along the melting line, the increasing concentration of ISROs reduces charge carrier mean free path and prevents decrease of electrical resistivity with increasing pressure as seen in liquid metals, Cu, Ag, and Au, which do not have ISROs. In light of recent developments in understanding of the structure and dynamics of liquid transition metals and alloys, we postulate that our findings are applicable to other Fe alloys in the OC with small light element (C, S, and O) content. We calculated an adiabatic heat flow of 9.4–12 TW at the OC‐CMB interface, which admits thermal convection.

Maximum supercooling studies in Ti39.5Zr39.5Ni21, Ti40Zr30Ni30, and Zr80Pt20 liquids-Connecting liquid structure and the nucleation barrier.

Almost three quarters of a century ago, Charles Frank proposed that the deep supercooling observed in metallic liquids is due to icosahedral short-range order (ISRO), which is incompatible with the long-range order of crystal phases. Some evidence in support of this hypothesis had been published previously. However, those studies were based on a small population of maximum supercooling measurements before the onset of crystallization. Here, the results of a systematic statistical study of several hundred maximum supercooling measurements on Ti39.5Zr39.5Ni21, Ti40Zr30Ni30, and Zr80Pt20 liquids are presented. Previous X-Ray and neutron scattering studies have shown that the structures of these liquid alloys contain significant amounts of ISRO. The results presented here show a small work of critical cluster formation (W* = 31-40 kBT) from the analysis of the supercooling data for the Ti39.5Zr39.5Ni21 liquid, which crystallizes to a metastable icosahedral quasicrystal. A much larger value (W* = 54-79 kBT and W* = 60-99 kBT) was obtained for the Ti40Zr30Ni30 and Zr80Pt20 liquids, respectively, which do not crystallize to an icosahedral quasicrystal. Taken together, these results significantly strengthen the validity of Frank's hypothesis.

Influence of minor additions on Icosahedral Short-Range Order (ISRO) and its consequences on nucleation and solidification growth kinetics in fcc alloys

The present contribution reviews the recent progress related to the influence of Icosahedral Short-Range Order (ISRO) and icosahedral Quasicrystals (i-QC) formation on the solidification of fcc alloys through minor solute element additions. From intensive crystallographic analysis of multi-twinned regions in as-cast Al-based and Au-based fcc alloys, Kurtuldu et al. have shown recently that a so-called “iQC-mediated” nucleation mechanism occurs when a few hundred ppm of Cr and Ir, respectively, are added to the melt [1] [2]. Similarly, it appears that the growth directions of dendrites in Al-Zn:Cr is also influenced by ISRO in the liquid, thus showing an attachment kinetics effect [3]. In a recent contribution, we have shown that iQC-mediated nucleation also occurs in pink gold alloys with Ir-additions, but two additional phenomena at high solidification speed [4]: (i) a spinodal-type decomposition of the liquid, leading to the formation of twinned Cu precipitates in addition to multi-twinned Au-rich grains; (ii) a change of the microstructure of the Au-rich grains, from 〈100〉 dendrites to 〈111〉 textured cells in the columnar zone.

The effect of atomic concentration on the structural evolution of Zr100-xCox alloys during rapid solidification process

The effects of the concentration on the glass formation, the crystallization process, and atomic structure of Zr100-xCox (x = 10, 30, 50, 70, and 90) alloys were investigated by classical molecular dynamics simulation based on the embedded atom method during rapid quenching. The structural evolution of these systems was evaluated by analyzing some structural functions and structural characterization methods. The calculated coordination number and structure factor were compared with the experimental data and it was observed that there was a good agreement between the molecular dynamics simulation and experimental results. Simulations performed for this study revealed that the local environment depends on the atomic composition in the ZrCo alloy. The liquid - glass transition was observed for the concentrations of Co in the range of 30–90%, which is consistent with the experimental glass forming range for Zr100-xCox alloys. The Voronoi and Honeycutt–Andersen pair analysis showed that the number of pairs and bonds reflecting the development of the icosahedral short range order increased with increased Co concentration and the Zr30Co70 alloy, having a large number of ideal and distorted icosahedrons with respect to other compositions considered in present study, has the highest fraction of Co-centered full icosahedral ⟨0,0,12,0⟩ clusters. Analysis results showed that in Zr90Co10 alloy, the number of ⟨0,6,0,8⟩ clusters denoting the transition to bcc structure remarkably increased, and ⟨0,3,6,4⟩, ⟨0,5,2,6⟩ and ⟨0,4,4,6⟩ clusters play a key role during the crystallization process.

Influence of Ir Additions and Icosahedral Short Range Order (ISRO) on Nucleation and Growth Kinetics in Au-20.5Wt Pct Cu-4.5Wt PctAg Alloy

Based on detailed EBSD analyses, Kurtuldu et al. (Acta Mater. 70:240–248, 2014) have explained the grain refinement of Au-12.5 wt pctCu-12.5 wt pctAg (yellow gold) by the addition of minute amounts of Ir in terms of “icosahedral quasicrystal (iQC)-mediated nucleation”, i.e., Ir induced the formation of Icosahedral short range order (ISRO) of atoms in the liquid, leading to the formation of iQC on which the fcc-phase forms. In the present contribution, we show that: (i) this mechanism is also responsible of the grain refinement in Au-20.5 wt pctCu-4.5 wt pctAg (pink gold) with Ir addition; (ii) ISRO also influences the morphology and growth kinetics of the fcc phase: at solidification rate of a few mm/s, $$\langle 100\rangle $$ dendrites are replaced by a cellular-type morphology growing along $$\langle 111\rangle $$ when 100 wt ppm of Ir is added to the melt; (iii) iQC-mediated nucleation is accompanied by a spinodal decomposition of the liquid, which is revealed at high cooling rate by the formation of Cu-rich particles or dendrites, some of them being also twinned, in parallel to iQC-mediated grain refinement and twin formation.

Effects of cooling rate on the atomic structure and glass formation process ofCo90Zr10metallic glass investigated by molecular dynamics simulations

In this study, the atomic structure and the glass formation process of Co90 Zr10 metallic glass alloy were studied by molecular dynamic simulation based on the embedded atom method using four different cooling rates. The average atomic volume and the potential energy of the system were observed to be strongly dependent on the cooling rate during rapid solidification, and the glass transition temperature decreased with decreasing cooling rate. The radial distribution functions and the structure factors derived from molecular dynamics simulations at 300 K agreed well with the experimental and other molecular dynamics results. The coordination numbers calculated at 300 K were consistent with the experimental results in the literature. The local structural atomic orders of the system have been characterized by Honeycutt-Andersen indices and the Voronoi tessellation method. We realized that the FCC and icosahedral shortrange order increases with decreasing cooling rate, the icosahedral clusters are predominant in the Co90 Zr10 metallic glass, and the fraction of the icosahedral polyhedra increases with decreasing cooling rate and temperature. We have seen that Co-centered Voronoi polyhedrals such as , , and play a dominant role in the development of the icosahedral order of the Co90 Zr10 alloy.

The icosahedral short-range order and its local structures in Cu50Zr40Al10 alloy

Local structures are the fundamental consideration in the formation and properties of metallic glasses. Icosahedral clusters are the essential basic clusters in amorphous alloys. The icosahedral short-range order structures and their local structures in Cu50Zr40Al10 alloy were investigated in this study based on the molecular dynamics simulation. The properties of the short-range order were analyzed by several structural characterization methods. The central atoms of 〈0 2 8 1〉, 〈0 2 8 2〉, 〈0 0 12 0〉 and 〈0 3 6 3〉 clusters were dominated by small Cu atoms, while that happened to prevail in 〈0 3 6 4〉, 〈0 2 8 3〉 and 〈0 2 8 4〉 clusters were the big Zr atoms. The Cu and Zr atoms have equal probability to be the central atoms of 〈0 1 10 2〉 clusters during the rapid solidification of liquid Cu50Zr40Al10 alloy. The icosahedral clusters were bonded with eight typical Voronoi polyhedrons by interpenetrating connection and formed the short-range order structures and their local structures. Moreover, in their local structures, icosahedral clusters are mainly interconnected with the defective icosahedral clusters.

The dissection of atomic clusters with local structure in Ga-Sn alloy melts

The statistic short-range order (SRO) structure parameters of the Ga-Sn alloy melt are obtained with the X-ray diffractometer. The local atomic and electronic structures around the absorption atoms of Ga and Sn elements are detected by X-ray absorption fine structure spectrum. Centered on the absorption atom, the atom cluster structure was dissected and the atom cluster model was established. Unlike the ordinary liquid metal structure, the local atomic structure demonstrates that the liquid Ga-Sn alloy is not characterized by a dominant icosahedral short-range order. Specially, Sn atoms aggregate in Ga91.6Sn8.4 alloy, giving rise to a phase separation that exhibits compositional disorder.

Probing structural changes during ductile fracture in metallic glasses via in situ straining inside a MeV transmission electron microscope

Structural changes in the vicinity of a crack during ductile fracture in various metallic glasses (MGs) with the propensity of forming shear bands were investigated by in situ straining inside a MeV transmission electron microscope (TEM). During tensile loading, a plastic deformation zone with a width of ∼100 nm and shear bands with a width of ∼15 ± 5 nm developed from crack tips of ductile fracture in MGs. In particular, crack propagation by nanovoid formation and coalescence was observed in Ti40Zr29Cu9Ni8Be14 MG with the highest compressive plasticity (∼6.7 ± 0.5%) among the tested MGs, whereas this process was hardly observable in other MGs. This difference may be attributed to atomic scale heterogeneities in Ti40Zr29Cu9Ni8Be14 MG induced by icosahedral short range orders (ISRO), which cause pronounced shear band branching and thus delay fracture process. Meanwhile, no crystallization of MGs even with ISRO or with lower Tx (crystallization onset temperature) was observed during in situ straining inside the MeV TEM and uniaxial tensile test with ribbon samples; a possible effect of normal stress and nanovoid formation during in situ straining is discussed to compare with crystallization during uniaxial compression test of the same MGs. Our study provides an insight into intrinsic toughening of MGs and thus can give useful guidelines on how to delay ductile fracture processes of MGs by tuning nanovoid formation, taking advantage of structural features through customized design of MGs.

Influence of Minor Additions on Icosahedral Short-Range Order (ISRO) and Its Consequences on Nucleation and Solidification Growth Kinetics in fcc Alloys

The present contribution reviews the recent progress related to the influence of icosahedral short-range order (ISRO) and icosahedral quasicrystal (iQC) formation on the solidification of fcc alloys through minor solute element additions. From intensive crystallographic analysis of multi-twinned region in as-cast Al-based and Au-based fcc alloys, it has been shown recently that a so-called iQC-mediated nucleation mechanism occurs when a few hundred ppm of Cr and Ir, respectively, are added to the melt (Kurtuldu et al. in Acta Mater 61(19):7098–7108, 2013; Acta Mater 70:240–248, 2014). Similarly, it appears that the growth directions of dendrites in Al–Zn/Cr are also influenced by ISRO in the liquid, thus showing an attachment kinetics effect (Kurtuldu in Influence of trace elements on the nucleation and solidification morphologies of fcc alloys and relationship with icosahedral quasicrystal formation, 2014). Similar observations have been made recently in pink gold alloys with Ir additions, when the solidification speed is increased (Zollinger et al. in Acta Mater, 2018).

Correlation between the atomic configurations and the amorphous-to-icosahedral phase transition in metallic glasses

Abstract

The characterisation of atomic structure and glass-forming ability of the Zr–Cu–Co metallic glasses studied by molecular dynamics simulations

In the present work, the glass formation process and structural properties of Zr50Cu50-xCox (0 ≤ x ≤ 50) bulk metallic glasses were investigated by a molecular dynamics simulation with the many body tight-binding potentials. The evolution of structure and glass formation process with temperature were discussed using the coordination number, the radial distribution functions, the volume–temperature curve, icosahedral short-range order, glass transition temperature, Voronoi analysis, Honeycutt–Andersen pair analysis technique and the distribution of bond–angles. Results indicate that adding Co causes similar responses on the nature of the Zr50Cu50-xCox (0 ≤ x ≤ 50) alloys except for higher glass transition temperature and ideal icosahedral type ordered local atomic environment. Also, the differences of the atomic radii play the key role in influencing the atomic structure of these alloys. Both Cu and Co atoms play a significant role in deciding the chemical and topological short-range orders of the Zr50Cu50-xCox ternary liquids and amorphous alloys. The glass-forming ability of these alloys is supported by the experimental observations reported in the literature up to now.

Structural essence of abnormal crystallization behaviors during the annealing of Cu33Zr67 metallic glass

During the non-isothermal annealing of Cu33Zr67 metallic glass (MG), two abnormal crystallization behaviors were detected. Nano-crystal growth ceasing leads to amorphous/nano-crystalline dual-phase structure, and residual amorphous crystallization is accompanied by grain coarsening. The structural evolutions of Cu33Zr67 MG at atomic-level were observed by high-resolution transmission electron microscopy. By analyzing structural factors, the nano-crystal growth ceasing is attributed to the spatially variation of icosahedral short-range order density arising from the structural heterogeneity of Cu33Zr67 MG, and the concurrence of crystallization and grain coarsening is due to higher thermal stability of residual amorphous as compared to bct-CuZr2 nano-structure.

Atomic-scale simulations of the local structures of molten Ni1-xCox and Ni1-xFex

In this work, ab initio molecular dynamics simulations have been performed to determine the local structures of molten Ni1-xCox and Ni1-xFex binary alloys. The results reveal that the spin-polarized setting is crucial to simulating Ni1-xFex alloys, while for Ni1-xCox the magnetic and nonmagnetic simulations yield similar results. The bonding tendency of cobalt is not very obvious while Fe atoms prefer hetero-coordination in nickel-based binary alloys. Pentagonal bipyramids are prevalent in both Ni1-xCox and Ni1-xFex, but icosahedral short-range order (ISRO) is not abundant as expected. Besides, the coordination short-range orders of Ni1-xCox and Ni1-xFex are rather similar, distinguishing from those in Ni1-xAlx, which means the local melt structure depends on the specific solute-solvent interaction in nickel-based binary alloys.

The effect of Ag addition on the non-isothermal crystallization kinetics and fragility of Zr56Co28Al16 bulk metallic glass

The non-isothermal crystallization kinetics and fragility of Zr56Co28Al16 and Zr56Co24Ag4Al16bulk metallic glasses (BMGs) were studied by differential scanning calorimetry (DSC). The X-ray diffraction (XRD), transmission electron microscopy (TEM) and microhardness test were used to investigate the glassy alloys structure. DSC traces exhibited two crystallization processes for both alloys. The activation energies of characteristic temperatures were obtained by Kissinger and Ozawa methods. Also, the heating rate sensitivity of characteristic temperatures was determined. The Ag-bearing Zr-based BMG presented higher activation energies and lower heating rate sensitivity in regard to characteristic temperatures, indicating a higher stabilization of the supercooled liquid, which can be correlated with the existence of strong icosahedral short range order (ISRO) clusters in the structure. In addition, the local Avrami exponents at various heating rates were calculated by the Johnson–Mehl–Avrami equation. The n value variations for each crystallization process pursued almost similar trends for both alloys with n>2.5, revealing that the transformation kinetics is dominated by a diffusion-controlled three-dimensional growth, as well as an increase in the nucleation rate. Moreover, both BMGs were classified into “strong glasses”, depending on the fragility index calculated values.

Structural signature and size-dependent mechanical response of frozen-in icosahedral phase in bulk metallic glasses

In the present study, we report structural signature and size-dependent mechanical response of frozen-in icosahedral phase (I-phase) embedded in Zr-(Ti, Nb, Al)-(Cu, Ni) bulk metallic glass (BMG)-forming alloys, which exhibit a narrow composition range overlapping between glass and I-phase forming region. Thus, the different sizes of the I-phase from tens of nanoscale precipitates to enhanced icosahedral short-range order in MG matrix can be obtained by controlling cooling history. I-phase particles with tens of nanoscale in BMGs do not contribute to extrinsic ductility due to limited blocking of the propagation of shear band. The shear band passes through the particle, splitting it into two pieces. On the other hand, BMGs with enhanced icosahedral short-range order, which can be evaluated by EXAFS analysis as well as calorimetric signal during isothermal annealing, exhibit enhanced ductility without strength reduction. Indeed, we examine the cut-off size of strain bursts and deformation dynamics of shear-avalanches through the statistical analysis of serration behavior. Icosahedral short-range order (ISRO)-embedded MGs exhibit weakened jammed state of the self-organized deformed zones, which results in the formation of numerous catastrophic deformed zones under relatively chaotic dynamics, and consequently multiple shear bands. These results would help deepen our understanding of the underlying mechanism that determines intrinsic ductility in BMGs via frozen-in ISRO, and ultimately give us a guideline for the design of promising BMGs with improved intrinsic ductility by manipulating local structural instability.

Interplay between structural and atomic transport properties of undercooled Al-Cu binary alloys

We perform ab initio molecular dynamics simulations in stable and undercooled Al1-xCux liquids, with copper composition x = 0.3 and 0.4, to examine the relationship between structure and dynamics as a function of temperature. The temperature evolution of the diffusivity, relaxation times and viscosity display a change from an Arrhenius behavior at high temperature to a non-Arrhenius one at low temperature, which occurs at a crossover temperature TX yet in the vicinity of the liquidus temperature. We find that TX corresponds also to an onset of dynamic heterogeneities (DHs) that develop in the undercooled states. The structure factors and pair-correlation functions display characteristic features compatible with the occurrence of the icosahedral short-range order (ISRO) as well as the development of a medium range order (MRO) upon cooling. A common neighbor analysis further identifies that ISRO undergoes a strong increase below TX. Finally, we demonstrate that the medium-range order is formed by interconnected fivefold bipyramids and is correlated with the emergence of DHs.

First principles determination of static, dynamic and electronic properties of liquid Ti near melting

Several static and dynamic properties of bulk liquid titanium at a thermodynamic state near its melting point have been evaluated by ab initio molecular dynamics simulations. The calculated static structure shows very good agreement with the available experimental data, including a marked shoulder on the second peak in the structure factor, which underlines a notable local icosahedral short-range order in the liquid. The dynamical structure reveals propagating density fluctuations whose dispersion relation and damping closely follow the inelastic X-ray scattering measurements. The dynamical processes behind the S(q,ω) have been analyzed in terms of a model with two decay channels (a fast and a slow) associated to the relaxations of the collective excitations. Finally, results are also reported for several transport coefficients.