Metallic Glass Formation Research Articles

Atomistic design favored compositions and atomic-level structure of Mg–Ca–Ag ternary metallic glasses

A realistic interatomic potential of Mg–Ca–Ag system is constructed under a proposed modified tight-binding scheme with the aid of ab initio calculations. Then, the favored and optimized compositions for forming Mg–Ca–Ag metallic glasses are investigated by means of the potential-based molecular dynamics simulations of atomistic modeling. The calculation shows that the Mg–Ca–Ag ternary system is liable to form amorphous alloys and the favored compositions for metallic glass formation locate at a sub-region, where the amorphization driving force is greater than that of other regions. Notably, we improve the conventional method of removing the facets in the amorphous structure analysis, and we introduce K-means clustering to eliminate the facets of the Voronoi polyhedra, which allows more reasonable description of various geometries of the different central atoms and their neighboring atoms. Then, both the Voronoi tessellation method and the Honeycutt–Andersen pair analysis are used to describe the disordered structure in Mg50Ca30Ag20 metallic glass based on the result of K-means clustering. Our work provides good guidance for composing the Mg–Ca–Ag metallic glasses, and further verifies the microstructure of the amorphous alloys.

Impact of hybridization on metallic-glass formation and design

The formation mechanism of glass at the atomic scale has been under debate over centuries. In this work, we demonstrate that hybridization, as manifested by Mott’s pseudogap, has a strong influence on the bond length as well as atomic packing, which can potentially tailor the formation of metallic glasses at microscopic time and length scales. A p–d orbital hybridization between the post-transition metal Al and the transition metal was shown by the 27 Al isotropic shifts and the spin–lattice relaxation time of Zr–Co–Al alloys using nuclear magnetic resonance. These bonds lead to a charge transfer between the specific atomic pairs and the shrinkage of interatomic distances. Such chemical bonding favors the formation of metallic glasses by introducing a string-like structure and further stabilizes metallic glasses via a reduction in the density of states at the Fermi level. Our work has implications for understanding the glass formation mechanism at the electronic level and may open up new possibilities on the design of glass from the perspective of atomic interactions.

Nucleation kinetics in a supercooled metallic glass former

We use molecular dynamics simulations to shed light on the mechanism underlying crystal nucleation in a supercooled metallic glass former characterized by a concurring crystal and amorphous local order based on icosahedral symmetry. At a crossover temperature, well below the melting point, we find that the supercooled phase exhibits glassy dynamics which includes a breakdown of the Stokes-Einstein relation and the emergence of spatially heterogeneous dynamics. In addition, we show that the origin of these phenomena can be related to a structural heterogeneity caused by the increase of icosahedral symmetry upon cooling. We also reveal that crystal nucleation occurs close to the glass transition and takes place in regions of high icosahedral symmetry, which can be interpreted by a strong reduction of the crystal-liquid interfacial energy. This scenario provides a framework for testing the estimation of the nucleation time according to the classical nucleation theory. More specifically, our findings allow to quantify how the heterogeneous character of the supercooled phase affects the predictions of this theory.

Ab initio simulations of the atomic and electronic environment around B in Fe–Nb–B metallic glasses

By using ab initio molecular dynamical simulation, the local atomic and electronic environment around B atoms in ternary Fe–Nb–B alloys was investigated in details. We find that, with a large amount of B addition, abundant direct B–B bonds promote the propagation of prisms-like B-centered <0 3 6 0> polyhedron instead of antiprisms-like <0 2 8 0> polyhedron in Fe–Nb–B system. <0 3 6 0> polyhedron has a similar atomic configuration as the only B-centered <0 5 4 0> polyhedron in metastable Fe23B6 phase. It causes that the Fe23B6 phase rather than simple bcc α-Fe phase forms as the primary crystallization phase in Fe-rich system via the direct B–B bonding. Such a strong B–B covalent bonding as indicated by an obvious pseudogap in the density of states of p-orbital of B in the vicinity of Fermi level and the high charge density distribution between B–B bonds, also benefits the formation of skeleton networks by the vertex, edge and face sharing of <0 5 4 0> polyhedra in Fe23B6 phase. These complex crystallization modes and directional bonding structures introduced by the covalent effect of metalloid B are the structural origin for the excellent glass-forming ability of B-rich systems. In addition, the B alloying also enhances the average magnetic moments of Fe. However, the negative spin polarization of both Nb and B eventually causes a worse magnetism. Thus, for maintaining a high magnetic performance and formability in Fe–Nb–B system, the proper metalloid addition should be considered for material design. Our studies help us better understand the structural origin for the formation of metallic glass and offer a potential strategy for designing ferromagnetic glassy alloys with the outstanding formability as well as magnetism.

Controllable thermoplastic forming of bulk metallic glasses in milliseconds by resistance welding forming

The work proposed a controllable and rapid thermoplastic forming approach–the resistance welding forming (RWF) method for bulk metallic glasses (BMGs). The rapid forming approach can finish the thermoplastic forming of BMGs in 150 milliseconds without crystallization and degradation in mechanical performance. Besides, the RWF is also proved to be competent in the fabrication of parts and structures with the length scale ranging from macro scale to micro scale. Our results may have promising applications in the rapid fabrication of macro to micro scale products and devices using metallic materials.

Research on the crystallization behavior occurred in the process of preparing bulk metallic glass with selective laser melting

The crystallization behavior occurred in the heat affected zone in the process of bulk metallic glasses preparation with selective laser melting has been studied with molecular dynamics simulation. Cyclical heating conditions are applied with three different temperature variation rates in the simulation cases, while the heating rate is ten times of the cooling rate in each case. The results reveal that the newly formed crystal cluster has more Ni atoms than the parent phase. When the cooling rate is about one third of the critical cooling rate for the formation of metallic glass, the crystallization behaviors happen in the first cooling process, which is controlled alternately by growth mechanism and nucleation mechanism. In the following heating process, the numbers of both crystal clusters and crystal atoms first increase and then decrease. When the cooling rate is about two fifths of the critical cooling rate, the crystallization behaviors happen in the second cooling process. When the cooling rate increases to about two thirds of the critical cooling rate, massive crystallization behavior is not observed, showing that the crystallization occurred in the HAZ in the process of preparing BMGs with SLM can be suppressed with a critical temperature variation rate.

Preparation of bulk metallic glasses by modifying local structure of icosahedral quasicrystals

An attempt is carried out to design metallic glasses by modifying the local structure of icosahedral quasicrystals. A series of metallic glasses are prepared by introducing beryllium to the quasicrystal former, Zr40Ti40Ni20, and the maximum diameter of the glasses reaches 20 mm. A phase transformation diagram is constructed for the alloys of (Zr40Ti40Ni20)100-xBex, revealing the transition from stable crystal to quasicrystal and then to metallic glasses with the increased Be content. This study shows a connection between the formation of metallic glasses and the specific thermodynamics of the initially precipitated quasicrystals. Interestingly, both improvements in the forming ability for glasses and quasicrystals are observed over a wide composition range. Ab initio molecular dynamic simulations show the evolution of the structural features of the metallic melts for the optimised glass forming region, which guarantee the primary precipitation of quasicrystal.

Thermal simulation and phase modeling of bulk metallic glass in the powder bed fusion process

One of the major challenges with the powder bed fusion process (PBF) and formation of bulk metallic glass (BMG) is the development of process parameters for a stable process and a defect-free component. The focus of this study is to predict formation of a crystalline phase in the glass forming alloy AMZ4 during PBF. The approach combines a thermal finite element model for prediction of the temperature field and a phase model for prediction of crystallization and devitrification. The challenge to simulate the complexity of the heat source has been addressed by utilizing temporal reduction in a layer-by-layer fashion by a simplified heat source model. The heat source model considers the laser power, penetration depth and hatch spacing and is represented by a volumetric heat density equation in one dimension. The phase model is developed and calibrated to DSC measurements at varying heating rates. It can predict the formation of crystalline phase during the non-isothermal process. Results indicate that a critical location for devitrification is located a few layers beneath the top surface. The peak is four layers down where the crystalline volume fraction reaches 4.8% when 50 layers are built.

Stability of the B2 CuZr phase in Cu-Zr-Al-Sc bulk metallic glass matrix composites

This study investigates the effect, that replacing Zr in a glass-forming Cu47.5Zr47.5Al5 alloy by Sc (0–2 at.%) has on the phase formation as well as on the thermal and mechanical properties. Even though it is not reflected in the thermal data, the glass-forming ability (GFA) is significantly reduced. This originates from the increased tendency to precipitate the shape-memory phase B2 CuZr(Sc), which, in turn, promotes the formation of bulk metallic glass (BMG) matrix composites. Sc appears to be very effective in stabilizing the B2 crystals because it forms the stable B2 CuSc phase with a similar lattice constant like B2 CuZr. By adjusting the casting parameters, the composite microstructure of Cu47.5Zr46.5Al5Sc1 can be controlled to a certain extent. The yield strength and the plasticity of the present composites depend on the crystalline volume fraction. In-situ high-energy X-ray diffraction reveals that deformation proceeds in three stages: (i) martensitic transformation of the B2 phase, (ii) yielding of the amorphous phase and continuing martensitic transformation, (iii) completion of the phase transformation and plastic deformation of all phases. Our work suggests that Sc is a promising candidate to adjust the microstructure and, thus, the mechanical properties of CuZr-based composites consisting of a glassy matrix and shape-memory crystals.

Dynamic slowing-down and crystal nucleation in a supercooled metallic glass former induced by local icosahedral order

A long-standing question in condensed matter and material science is understanding how local structural order determines the fate of a liquid on cooling, i.e., whether it crystallizes or experiences dynamic slowing down leading to glass formation. Here we address this issue by focusing on a metallic glass former with concurring crystal and amorphous local order based on icosahedral symmetry. Using atomistic simulations, we study its deep supercooled regime in which we clearly identify spontaneous crystal nucleation close to the glass transition. We find that regions of slow dynamics characterized by a high degree of icosahedral orientational order play a central role in the nucleation process. Our study brings a new perspective into the microscopic mechanism of crystallization from the supercooled phase, which could be of significance for the in silico design of materials with selected glass-forming ability.

Contrasting Bonding-Interaction-Induced Distinct Relaxation in La65Ni35 and La65Al35 Glass-Forming Alloys.

The α and β relaxations are two fundamental processes in glass-forming materials, and quite important for many of the properties. Although intensive studies have found that α and β relaxations can be tuned by changing the constituent elements, the underlying structural basis is still elusive. Here, we explored the effect of two key elements of Al and Ni on distinct β and α relaxations in La65Al35 and La65Ni35 glass-forming alloys via classical and ab initio molecular dynamics simulations combined with dynamical mechanical spectroscopy. Unexpected coupling of relaxation in both β and α relaxation time scales is observed for La and Al atoms in the La65Al35 system, which drastically suppresses the relaxation dynamics. It is revealed that the dynamic coupling of La and Al results from the covalent-like bonding interaction between Al atoms, which connect Al together, forming a network-like structure. The bonding network not only drastically slows down the dynamics of Al but also couples the motion of La and Al together. This finding elucidates the underlying basis of Al and Ni elements for distinct β and α relaxation and sheds light on tuning the formation and properties of metallic glasses by minor alloying.

Universal localization transition accompanying glass formation: insights from efficient molecular dynamics simulations of diverse supercooled liquids.

The origin of the precipitous dynamic arrest known as the glass transition is a grand open question of soft condensed matter physics. It has long been suspected that this transition is driven by an onset of particle localization and associated emergence of a glassy modulus. However, progress towards an accepted understanding of glass formation has been impeded by an inability to obtain data sufficient in chemical diversity, relaxation timescales, and spatial and temporal resolution to validate or falsify proposed theories for its physics. Here we first describe a strategy enabling facile high-throughput simulation of glass-forming liquids to nearly unprecedented relaxation times. We then perform simulations of 51 glass-forming liquids, spanning polymers, small organic molecules, inorganics, and metallic glass-formers, with longest relaxation times exceeding one microsecond. Results identify a universal particle-localization transition accompanying glass formation across all classes of glass-forming liquid. The onset temperature of non-Arrhenius dynamics is found to serve as a normalizing condition leading to a master collapse of localization data. This transition exhibits a non-universal relationship with dynamic arrest, suggesting that the nonuniversality of supercooled liquid dynamics enters via the dependence of relaxation times on local cage scale. These results suggest that a universal particle-localization transition may underpin the glass transition, and they emphasize the potential for recent theoretical developments connecting relaxation to localization and emergent elasticity to finally explain the origin of this phenomenon. More broadly, the capacity for high-throughput prediction of glass formation behavior may open the door to computational inverse design of glass-forming materials.

The relation between density, thermal expansion and glass-forming ability in polycrystalline Cu–Zr alloys

In contrast to the generally accepted practice for searching metallic glassformers by trial-and-error quenching, we propose using density and coefficient of thermal expansion of as-cast polycrystalline alloys as indicators of the glass-forming ability. We study experimentally these characteristics for Cu100−xZrx alloys over the whole concentration range and a wide temperature range from room temperature up to 1120 K. In the concentration range of 35 < x < 55 at% corresponding to high glass-forming ability, strongly non-monotonous behavior of both quantities is observed. We reveal that location of the extrema on concentration dependencies of both the coefficient of thermal expansion and the density deviation from the additive law correlates with those for critical thickness of Cu–Zr amorphous alloys. We argue that correlation between glass-forming ability and anomalous behavior of thermal expansion is because as-cast polycrystalline samples of glass forming alloys possess highly distorted structure, which cause phonon anharmonicity. Our findings propose new express method to search metallic alloys with high glass-forming ability.

Stretched and compressed exponentials in the relaxation dynamics of a metallic glass-forming melt

The dynamics of glass-forming systems shows a multitude of features that are absent in normal liquids, such as non-exponential relaxation and a strong temperature-dependence of the relaxation time. Connecting these dynamic properties to the microscopic structure of the system is challenging because of the presence of the structural disorder. Here we use computer simulations of a metallic glass-former to establish such a connection. By probing the temperature and wave-vector dependence of the intermediate scattering function we find that the relaxation dynamics of the glassy melt is directly related to the local arrangement of icosahedral structures: Isolated icosahedra give rise to a liquid-like stretched exponential relaxation whereas clusters of icosahedra lead to a compressed exponential relaxation that is reminiscent to the one found in a solid. Our results show that in metallic glass-formers these two types of relaxation processes can coexist and give rise to a dynamics that is surprisingly complex.

Evaluation of glass forming ability of Zr–Nb alloy systems through liquid fragility and Voronoi cluster analysis

In the present study, a molecular dynamics (MD) simulation-based investigation has been carried out for Zr-Nb binary system to analyze structural evolution during the glass formation along with its underlying physics for identifying suitable composition having better glass forming ability (GFA). The analysis is performed through the determination of liquid fragility, coordination numbers, and Voronoi cluster analysis. Results from Angell’s plot have shown that Zr50Nb50 specimen is a strong liquid and has the highest glass forming ability. It is revealed from Voronoi cluster analysis, that Zr50Nb50 metallic glass has a higher population fraction of icosahedral cluster, which also confirms its better glass forming ability. Apart from 〈0 0 12 0〉 icosahedral polyhedra, some Zr-centered clusters like 〈0 1 10 2〉, 〈0 2 8 5〉 and 〈0 2 8 4〉 are also found to be the key factors with respect to slowing down the dynamics of the system during liquid to glass transition. In addition, a pathway for Zr50Nb50 metallic glass formation is presented to show different stages of structural transformation from liquid to glass state.

Denary high entropy metallic glass with large magnetocaloric effect

We report the formation of a Gd10Tb10Dy10Ho10Er10Y10Ni10Co10Ag10Al10 denary high entropy metallic glass (HEMG). The magnetocaloric effect studies show that the denary HEMG possess large magnetic entropy changes over a wide temperature range, and thus a large refrigerant capacity. Combined with the simple alloy design of high entropy alloys and atomic level homogeneous single phase nature of metallic glasses, the magnetocaloric properties of HEMGs can be easily adjusted by changing the elements or configurational entropy. Our works show that the HEMG is a promising candidate material as the magnetic refrigerant.

Effects of Ag, Nd, and Yb on the Microstructures and Mechanical Properties of Mg‒Zn‒Ca Metallic Glasses

Mg‒Zn‒Ca metallic glasses are regarded as promising biodegradable materials. Previous studies on this alloy system have mostly focused on the composition regions with a large critical size (Dc) for the formation of metallic glasses, while this paper investigates the composition regions with a small Dc, which has been overlooked by researchers for a long time. The effects of the addition of Ag, Nd, and Yb elements on the microstructure and mechanical properties of Mg‒Zn‒Ca metallic glasses were studied. It was found that the Mg‒Zn‒Ca metallic glass exhibits a single and uniform amorphous structure with a compressive strength of 590 MPa. After the addition of a small amount of Ag into the alloy, the amorphous matrix is retained and new precipitate phases that lead to the decrease of the compressive strength are formed. The addition of the rare earth elements Nd and Yb changes the microstructure from a single amorphous matrix to a large number of quasicrystal phases, which results in an increase in compressive strength. The compressive strength of the Mg‒Zn‒Ca‒Yb alloy increases to 606.2 MPa due to the formation of multi-layered swirling solidified structure and a large number of small quasicrystals with high microhardness. Moreover, this study can be considered as a useful supplement to the existing studies on the Mg‒Zn‒Ca alloy system; it also provides new ideas for designing the microstructure and spatial structure of quasicrystal containing alloys with high performances.

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.

Achieving high uniformity of the elastic strain energy accumulation rate during the serrated plastic flows of bulk metallic glasses

The shear-band formation and propagation mechanisms of bulk metallic glasses (BMGs) have been studied extensively, however, the interpreting of the mechanisms of the serrated plastic flows is still challenging. In this work, the variation of the elastic strain energy accumulation rates of the serrations during the plastic flows of BMGs was examined using a statistical analysis. High uniformity of elastic strain energy accumulation rates was achieved in the serrated plastic flows under complex stress fields with three-parameter Weibull modulus reaching to 14.29, which is much larger than the value of 4.06 under conventional compression tests. The variations of the elastic strain energy accumulation rates are related to the formation and propagation of shear bands under varying loading conditions, and the high uniformity under complex stress fields is due to that the formation and propagation of shear bands can be confined to certain shearing path. The present findings are of significance for giving more insight into the mechanisms of the serrated plastic flows of BMGs.

Microscopic evidence of the connection between liquid-liquid transition and dynamical crossover in an ultraviscous metallic glass former

Liquid-liquid transitions are interesting to many researchers since they occur in systems as diverse as monoatomic liquids, multicomponent oxides, and metallic glass formers. In some cases, the crossover is accompanied by changes in the dynamical properties. By combining state-of-the-art synchrotron techniques, we followed the structure and atomic motion during quasistatic cooling of the $\mathrm{A}{\mathrm{u}}_{49}\mathrm{C}{\mathrm{u}}_{26.9}\mathrm{S}{\mathrm{i}}_{16.3}\mathrm{A}{\mathrm{g}}_{5.5}\mathrm{P}{\mathrm{d}}_{2.3}$ metallic glass former from the low-temperature supercooled liquid. With this thermal protocol, we were able to lower the glass transition temperature far enough to reveal a liquid-liquid crossover between two amorphous structures corresponding to two ultraviscous liquids with different kinetic behavior. This transition is in competition with vitrification, which occurs at conventional cooling rates, and is accompanied by structural changes not affecting the average density. Our results provide a direct connection between polyamorphism and dynamical crossover, and an alternative case to add to the highly debated topic on the low-temperature divergence of the dynamics in supercooled liquids.