Icosahedral Short-range Order Research Articles

Structure and dynamical properties during the solidification of the liquid aluminum induced by cooling and compression

Abstract The structural transformation from a liquid to a crystalline solid is an important subject in condensed matter physics and materials science. In the present study, firstprinciples molecular dynamics calculations were performed to investigate the structure and properties of aluminum during the solidification which was induced by cooling and compression. During both the cooling and compression processes, it was found that the icosahedral short-range order was initially enhanced and then began to decay, the face-centered cubic short-range order eventually became dominant before it transformed to a crystalline solid.

Development and optimization of novel sulfur-containing Ti-based bulk metallic glasses and the correlation between primarily crystallizing phases, thermal stability and mechanical properties

The effect of sulfur on the glass forming ability, thermal stability and mechanical properties of the eutectic alloy Ti33.4Zr33.3Cu33.3 was investigated by conventional X-ray diffraction, differential scanning calorimetry and 3-point flexural experiments. A novel region of bulk glass formation with a critical casting diameter of up to 4 mm was found in the quaternary Ti-Zr-Cu-S system, however, brittle fracture behavior was predominant. Various alloying strategies were employed to improve mechanical properties and a compositional transition from brittle to ductile fracture has been identified (e.g. for Ti36Zr33.5Cu24.5S6). A change of the primary precipitating phases from a C14 Laves to an intermetallic (Ti,Zr)2Cu phase can be observed, as well as a stabilization of the supercooled liquid. The origin of the thermally unstable behavior in Ti-based bulk metallic glasses is traced back to the easy formation of the icosahedral phase upon heating, which is structurally close to the supposedly predominant icosahedral short-range order in the amorphous state. The systematic study carried out in this work indicates a strong correlation between primary crystallizing phase and thermal stability, both pointing to the frozen short-range order in the amorphous state which is predetermining the mechanical properties. The transition from the Laves to the intermetallic (Ti,Zr)2Cu phase as well as the enlarged supercooled liquid region appear to be directly related to a destabilization of the icosahedral short-range order and ultimately to the improved mechanical properties.

Effects of cooling rate on the glass formation process and the microstructural evolution of Silver mono-component metallic glass

In this study, using molecular dynamics simulations in combination with the embedded-atom approach, we investigate the effect of cooling rate on the microstructural evolution and the glass formation process of Silver monatomic metallic glass. In order to accomplish our investigation, we have utilised a variety of analytical techniques, including the pair distribution function, the bond angle distribution, the Voronoi tessellation analysis, the coordination number, and the five-fold symmetry. The splitting of the second peak in the pair distribution function during the cooling process confirms that glass formation occurs. Via the Went-Abraham parameter, we have found that a faster cooling rate leads to a higher glass transition temperature Tg, and a less relaxed glass has a lower density due to its greater free volume and disordered structure. In the same context, the bond angle distribution revealed that the cooling rate clearly influences the icosahedral short-range order in the quenched system, and the Voronoi tessellation analysis indicated that the percentage of mixed-like and icosahedral-like clusters grows as the cooling rate increases. Furthermore, the coordination number indicated that when the temperature drops throughout the cooling process, the local environment and topological structure of the amorphous Silver change. Lastly, we have also revealed that the five-fold symmetry controls the formation of the amorphous structure and that the highest cooling rate yields the greatest amount of icosahedral-like clusters in the vitreous phase, implying that fast cooling rates subserve the formation of glassy states with icosahedral-like features.

Structure formation processes in fullerene mixtures

Glass formation processes in condensed matter are characterized by some specific short-range order changes in the arrangement of particles (atoms/molecules/ions). So, the short-range structural order in supercooled liquids and glasses is characterized by fivefold symmetry in the arrangement of particles, often referred to as icosahedral (ideal or distorted) short-range order. This article is devoted to the study of local structural features of the supercooled melt of the A20B80 fullerene mixture (where A = C60 and B = C70) obtained under various cooling protocols in order to elucidate the mechanism of formation of the icosahedral short-range order in binary molecular liquids. Comprehensive studies of the properties of a fullerene mixture melt were carried out using large-scale molecular dynamics simulations followed by structural and cluster analysis. The crystallization temperature and the critical glass transition temperature of the system were calculated to be T_m~1439 K and T_c~1238 K, respectively. It has been established that the crystallization of a binary fullerene mixture proceeds according to the polycrystalline scenario with the formation of clusters with fcc and hcp symmetries. It is shown that in a supercooled fullerene mixture, the short-range icosahedral order is formed by an insignificant number of ideal icosahedral clusters and a certain set of distorted icosahedral clusters, the fraction of which remains practically unchanged at temperatures below the critical glass transition temperature. Keywords: molecular dynamics, fullerenes, short-range order, structural ordering.

Процессы структурообразования в фуллереновых смесях

Glass formation processes in condensed matter are characterized by some specific short-range order changes in the arrangement of particles (atoms/molecules/ions). So, the short-range structural order in supercooled liquids and glasses is characterized by fivefold symmetry in the arrangement of particles, often referred to as icosahedral (ideal or distorted) short-range order. This article is devoted to the study of local structural features of the supercooled melt of the A20B80 fullerene mixture (where A = C60 and B = C70) obtained under various cooling protocols in order to elucidate the mechanism of formation of the icosahedral short-range order in binary molecular liquids. Comprehensive studies of the properties of a fullerene mixture melt were carried out using large-scale molecular dynamics simulations followed by structural and cluster analysis. The crystallization temperature and the critical glass transition temperature of the system were calculated to be Tm ≈ 1439 K and Tc ≈ 1238 K, respectively. It has been established that the crystallization of a binary fullerene mixture proceeds according to the polycrystalline scenario with the formation of clusters with fcc and hcp symmetries. It is shown that in a supercooled fullerene mixture, the short-range icosahedral order is formed by an insignificant number of ideal icosahedral clusters and a certain set of distorted icosahedral clusters, the fraction of which remains practically unchanged at temperatures below the critical glass transition temperature.

A 3D characterization of equiaxed grains induced by mediated nucleation in additively manufactured Inconel 718

In additive manufacturing of Ni-based alloy IN718, what causes Icosahedral Short-Range Order (ISRO) or iQC mediated nucleation of equiaxed grains is still not understood. In order to have a better comprehension of the microstructure formation, a more comprehensive characterization both on grain structure and chemical heterogeneities must be achieved. In this work, a scanning electron microscope equipped with a focused ion beam (FIB) allowed to perform a 3D EDS chemical analysis and EBSD orientation mapping of the volume in an equiaxed grain zone. After the 3D reconstruction of the equiaxed grains has been performed, grain-to-grain orientation analysis allowed to analyze multiple twin boundaries orientation relationships in a volume containing hundreds of grains. Chemical mapping using EDS allowed to identify TiC inclusions from which a five-fold symmetry axis seem to emerge, indicating a potential nucleation site. Such areas were analyzed in transmission electron microscopy (TEM) to investigate the different present phases. The nucleation sequence and potential origin of mediated nucleation are further discussed, opening new prospects for grain refinement in additively manufactured nickel-based alloys.

Medium Range Ordering in liquid Al-based alloys: towards a machine learning approach of solidification

Ab initio molecular dynamic simulations of liquid Al93Cr7 and Al83Zn10Cr7 alloy have evidenced the presence of an icosahedral short range order (iSRO) which develops into icosahedral medium range order (iMRO) as the melt is undercooled. This atomic arrangement accounts for the presence of Dynamic Heterogeneities characterized by Al fast-dynamics regions and Cr-rich slow-dynamics regions. Characterisation of the medium range order was carried out by a direct connectivity approach. However, given the small size of the simulation (256 atoms), such characterisation remains partial. In order to better describe both iMRO formation and more dilute alloys closer to industrial compositions, a new modelling strategy has been initiated to allow in the long term for large-scale atomic-level simulations. Molecular Dynamics (MD) of million to billion atoms may indeed lead to meaningful results. Exploitation of such large amounts of MD-generated big data can be carried out by means of Machine Learning (ML) tools which provide relevant and powerful analysis methods. An unsupervised ML approach based on topological descriptors using persistent homology concepts is proposed to reveal the structural features of atomic arrangements without a priori knowledge on the studied system. This approach has been applied so far to pure Al melts. Both translational and orientational orderings are thus evidenced together with nucleation pathways, whose revealed features are beyond the hypotheses of the Classical Nucleation Theory.

On the origin of grain refinement and twin boundaries in as-fabricated austenitic stainless steels produced by laser powder bed fusion

316 L austenitic stainless steel samples have been fabricated by laser powder bed fusion using two powder batches of slightly different compositions but keeping the same processing parameters. Compared to the first powder batch, the second powder batch led to a finer grain structure, a more random texture, and an extremely high density of Σ3 twin boundaries. A review of the literature shows that these two kinds of microstructures had been observed in previous works, but separately and not connected to changes in composition. Several hypotheses are reviewed to explain these microstructure differences. We conclude that local liquid ordering, also described as icosahedral short range order (ISRO) can have an effect on the microstructure formation during solidification. ISRO in the liquid is thought to be stronger in the second powder batch. Finding ways to enhance the influence of the ISRO in the liquid on the microstructure formation can be seen as a promising strategy to design alloys for additive manufacturing.

Evidence that the liquid structure affects the nucleation of the primary metastable L12-Al3Zr in additive manufacturing

The strategy aiming at promoting the precipitation of the primary metastable L12-Al3X phase is the most widely used to refine grain size in Al alloys designed for additive manufacturing. It is generally admitted that Al grains nucleate onto the primary L12-Al3X precipitates by epitaxy resulting in a cube-cube orientation relationship. Here we report TEM observations showing clusters of precipitates of this primary L12-Al3X phase. Using Automated Crystallographic Orientation Mapping in the TEM, we show that such clusters consist of various orientations. Analysis of those clusters reveals that some orientations share common 5-fold rotational symmetry axes suggesting that they have nucleated from an icosahedral template. We conclude that Icosahedral Short Range Order (ISRO) in the liquid can play a role in the nucleation of the primary metastable L12-Al3Zr phase through the ISRO-mediated nucleation mechanism.

Towards an alloy design strategy by tuning liquid local ordering: What solidification of an Al-alloy designed for laser powder bed fusion teaches us

High-strength aluminium alloys from the 2XXX, 6XXX, and 7XXX series suffer from severe hot cracking issues during 3D printing. Thus, there is a need to design new alloys with an improved hot cracking resistance while taking advantage of the non-equilibrium processing conditions of laser powder bed fusion to achieve optimized properties. Here, we report the successful fabrication of a new Al-Mn-Ni-Cu-Zr alloy designed for L-PBF. The as-built microstructure was characterized at different scales using synchrotron X-ray nano-tomography and microscopy with a special focus on automated crystallographic orientation mapping (ACOM) in transmission electron microscopy. Based on this multiscale microstructural investigation, we identify various mechanisms involving the presence of a locally ordered liquid during solidification under conditions typical of additive manufacturing. This local liquid ordering is often defined as Icosahedral Short Range Order (ISRO). The multiple consequences of ISRO in the liquid on the hierarchical microstructure inherited from additive manufacturing are discussed. ISRO enables refinement of the grain size thus improving the processability via an increase of the hot cracking resistance, producing equiaxed grains and twinned dendrites contributing to randomizing the crystallographic texture, and possibly extending supersaturated solid solutions through a cage effect. It is concluded that promoting ISRO should be considered a promising pathway to design alloys for additive manufacturing with good processability and optimized properties.

Atomic-scale icosahedral short-range ordering in a rejuvenated Zr-based bulk metallic glass upon deep cryogenic treatment

Tailoring the formation and distribution of loose-packing regions and dense-packing regions in the bulk metallic glasses (BMGs) has long been considered to facilitate macroscopic mechanical properties and break strength-ductility trade-off. Herein, studies on structural heterogeneity caused by the deep cryogenic treatment (DCT) were systematically carried out to not only probe the atomic-scale structure-property relationships but also further understand the deformation mechanisms involving improved plasticity without sacrificing strength on the basis of these experimental results in rejuvenated Zr-based BMGs. The internal stress due to the cooling contraction will activate the atomic motion in weakly bonded regions and meanwhile form perfect icosahedral short-range ordering (ISRO) via containing mobile atoms to become denser geometric configurations. The multiplication and propagation of shear bands are closely associated with their structural features originating from the optimization of perfect icosahedral ordered structures in the rejuvenated glassy matrix. It is believed that these findings are helpful to shed light on the correlations between atomic-scale structural heterogeneity and macroscopic mechanical properties and further provide new possibilities for the development of promising BMGs with strength-ductility synergy.

Is icosahedral short-range order presented in supercooled transition metals?

Supercooled transition metals are characterized by the absence of long-range order and the presence of a specific short-range order in the arrangement of atoms. So, the presence of shoulders and broadenings at the second maximum in the experimentally measured quantity—in the static structure factor S(k)—is usually interpreted as a manifestation of the icosahedral (ideal or distorted) short-range order (ISRO—Icosahedral Short-Range Order). Icosahedral short-range order is a structure with fivefold symmetry in the arrangement of atoms, which can lead to the possibility of achieving deep supercooling. In this work, we study the local structural features of equilibrium and supercooled nickel melts under various cooling protocols ( K s−1) in order to clarify the mechanism of formation of the icosahedral short-range order in pure transition metals. Comprehensive studies of the properties of nickel melts were carried out using experiments on x-ray diffraction, large-scale molecular dynamics (MD) simulations with subsequent structural and cluster analysis. A good agreement was found between the results of x-ray diffraction data and the MD simulations results for an equilibrium nickel melt. It was found that the nickel melt is characterized by a short-range order, formed by fragments of icosahedra and distorted icosahedral clusters. It was revealed that the ‘liquid-crystal’ phase transition in nickel is accompanied by the transformation of distorted icosahedral clusters into clusters with the fcc/hcp-symmetry. It is shown that, in contrast to the Voronoi tessellation method, the cluster analysis method based on the () rotational invariants does not allow sufficiently correct identification of the distorted icosahedral short-range order in metal melts.

Atomic-scale Nb heterogeneity induced icosahedral short-range ordering in metallic glasses

• Atomic-scale structural evolution has been depicted via thermal agitation during heating. • The Nb heterogeneity plays an important role in the formation of short-range icosahedral-like clusters. • The coupling development of atomic-scale structure and composition is simultaneously responsible for promoting the formation of perfect orders or even long-range quasicrystalline. The intrinsic origins and formation of atomic-scale structure in multicomponent alloys remain largely unknown owing to limited simulations and inaccessible experiments. Herein, we report the formation of three-dimensional periodicity from a disordered atomic-scale structure to an imperfect/perfect ordered cluster and finally to long-range translational and rotational symmetry coupled with Nb heterogeneity. Significant atomic-scale structural clustering and atomic arrangements involving solvent or solute atoms simultaneously occurred during isothermal annealing. A close relationship between atomic-scale structural evolution and composition variation has important implications in depicting the chemical and topological packing during the early crystallization stage in metallic glasses. This work can provide a comprehensive understanding of how short-range orders evolve into long-range periodicity and will further shed light on the origins and nature of metallic glasses.

Influence of La on the atomic structure of Al[sbnd]Cu alloy liquid

The influence of La element on the atomic structure of AlCu alloy liquid above the liquidus line was investigated through high-energy x-ray diffraction technology on synchrotron facility and constrained reverse Monte Carlo simulation. The addition of La induced a change in nearest neighbor environments and chemical short-range orders of the atoms, which makes the Cu and La atoms dispersed in the liquid and surrounded by Al atoms. The analysis results of atomic topological short-range order show La element does not promote the icosahedral short-range order in the Al86Cu9 liquid, but increases the number of cluster types and reduces the variance of the cluster fraction, leading to a more disordered structure. These results strongly support that the diversity and distribution of clusters are responsible for the glass-forming ability of alloys, not the specific cluster type. Moreover, La element affects the arrangement of clusters in space, enhancing structural heterogeneity of the liquid and the correlation between clusters with high five-fold symmetry, which can also result in an improvement in glass-forming ability. These findings reveal the role of La element in enhancing glass-forming ability, and have implications for understanding the liquid-glass transition mechanism.

Short-range order of liquid Ti72.3Fe27.7 investigated by a combination of neutron scattering and X-ray diffraction

Abstract In this work we report on investigations into the topological and chemical short-range order of binary Ti72.3Fe27.7 alloy melts. In order to analyse the structure, the melts were investigated by means of neutron scattering and energy dispersive diffraction of synchrotron radiation. The combination of both diffraction techniques allowed the determination of the partial Bhatia Thornton structure factors S NN(Q) and S CC(Q). The experimental results provide evidence of an icosahedral topological short-range order prevailing in the liquid that is accompanied by a pronounced chemical short-range order such that Ti –Fe nearest neighbours are preferentially formed.

Icosahedral quasicrystal enhanced nucleation in commercially pure Ni processed by selective laser melting

This work provides unambiguous evidence for the occurrence of icosahedral quasicrystal (iQC) enhanced nucleation during selective laser melting of gas atomized commercially-pure Ni powders. This solidification mechanism, which has only been recently reported in a few alloys and has to date never been observed in pure metals, consists on the solidification of grains of the primary phase on the facets of iQCs formed due to the presence of icosahedral short range order in the liquid. The occurrence of iQC enhanced nucleation has been inferred from the observation in the SLM processed pure Ni samples of an excess fraction of partially incoherent twin boundaries and of clusters of twinned grain pairs sharing common 〈110〉 five-fold symmetry axes. This work further evidences that additive manufacturing methods may constitute an invaluable tool for investigating the fundamentals of solidification and for the design of unprecedented grain boundary networks.

On the transferability of classical pairwise additive atomistic force field to the description of unary and multi-component systems: applications to the solidification of Al-based alloys.

Multi-component and multiphasic materials are continually being developed for electronics, aircraft, automotive, and general applications. Integrated Computational Materials Engineering (ICME) is a multiple-length scale approach that greatly benefits from atomistic scale simulations to explore new alloys. Molecular Dynamics (MD) allows to perform large-scale simulations by using classical interatomic potentials. The main challenge of using such a classical approach is the transferability of the interatomic potentials from one structure to another when one aims to study multi-component systems. In this work, the reliability of Zr, Al-Cu, Al-Cr and Al-Zr-Ti force field potentials is examined. It has been found that current interatomic potentials are not completely transferable due to the structure dependence from their parameterization. Besides that, they provide an appropriate description of unary and binary systems, notably for liquids, isotropic solids, and partially isotropic compounds. For solidification purposes, it has been found that coherent primary solidification of the FCC-phase in pure Al is highly dependent on the formalism to tune interatomic interactions. For Al-Cr alloys, the icosahedral short-range ordering (ISRO) increased by adding Cr to the melts. The different steps of solidification (formation of nuclei, effective germination of the α-Al phase and end of solidification) have been related to the evolution of the ISRO. The addition of Cr in melts prevented undercooling via icosahedral-enhanced nucleation of the α-Al phase. Precipitation of primary intermetallics in hyper-peritectic Al-Cr alloys was also tested. Contrary to classical thermodynamics predictions, α-Al phase was the primary precipitate for these alloys. This implies that Cr supersaturated the α-Al phase rather than forming intermetallic phases due to the high cooling rates.

Atomic-Scale Icosahedral Short-Range Ordering in a Rejuvenated Zr-Based Bulk Metallic Glass Upon Deep Cryogenic Treatment

Atomic-Scale Icosahedral Short-Range Ordering in a Rejuvenated Zr-Based Bulk Metallic Glass Upon Deep Cryogenic Treatment

Connecting short-range order for the liquid-solid transition of Al-Cu-Fe alloys

Liquid-solid transition processes have always been mysterious, especially those with the participation of quasicrystal (QC) structures, and are generally far less understood than the conventional solidification process. Here, the liquid structure variation of ternary QC-forming Al60Cu37Fe3 with crystal-forming Al50Cu40Fe10 alloy liquids was compared using synchrotron radiation based on high-energy X-ray diffraction (HE-XRD) and reverse Monte Carlo simulation (RMC). It was noted that the types of short-range order structures are almost the same in the Al60Cu37Fe3 and Al50Cu40Fe10 alloy liquids, while the former has more icosahedral short-range order (ISRO) and the latter has less. The medium-range order (MRO) was characterized by the cluster correlation method and visualization analysis. More obvious structural heterogeneity was found in the Al60Cu37Fe3 liquid, which is due to the strong correlation between clusters with high icosahedral order and the weak correlation between crystal clusters. Moreover, strongly correlated ISRO clusters can form icosahedral medium-range order (IMRO) structures by interpenetrating connections forming a stable structure upon cooling, and some other clusters can even enhance the IMRO stability. Such IMRO will be retained in the liquid upon cooling, leading to QC formation. All the findings provide a further understanding of the liquid-solid transition with ISRO participation.

Influence of stoichiometry on indentation-induced plasticity in CuZr glasses

Plasticity in metallic glasses depends on their stoichiometry. We explore this dependence by molecular dynamics simulations for the case of CuZr alloys using the compositions Cu_{64.5}Zr_{35.5}, Cu_{50}Zr_{50}, and Cu_{35.5}Zr_{64.5}. Plasticity is induced by nanoindentation and orthogonal cutting. Only the Cu_{64.5}Zr_{35.5} sample shows the formation of localized strain in the form of shear bands, while plasticity is more homogeneous for the other samples. This feature concurs with the high fraction of full icosahedral short-range order found for Cu_{64.5}Zr_{35.5}. In all samples, the atomic density is reduced in the plastic zone; this reduction is accompanied by a decrease of the average atom coordination, with the possible exception of Cu_{35.5}Zr_{64.5}, where coordination fluctuations are high. The strongest density reduction occurs in Cu_{64.5}Zr_{35.5}, where it is connected with the partial destruction of full icosahedral short-range order. The difference in plasticity mechanism influences the shape of the pileup and of the chip generated by nanoindentation and cutting, respectively.