Metallic Glass Formation Research Articles

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.

Static atomic-scale structural heterogeneity and its effects on glass formation and dynamics of metallic glasses

Molecular dynamics simulations were employed to investigate structural heterogeneity at the sub-nanometer scale for binary Cu-Zr liquids and glasses. The normalized local atomic number density fluctuates significantly in all the liquids and glasses, confirming existence of the static atomic-scale structural heterogeneity. The dynamic evolution of structural heterogeneity profile shows that the good glass former has better inheritance of structural heterogeneity from its liquid state than the poor one, suggesting a close relation between the structural inheritance and the glass-forming ability. Further analyses regarding evolution of dynamic properties, i.e., viscosity and the α relaxation time, as a function of undercooling demonstrate that the better inheritance of the structural heterogeneity in the better glass former stems from its higher liquid stability. Our atomic structural investigation indicates that the enhanced connectivity of short-range ordering clusters could be responsible for the better inheritance of structural heterogeneity and the larger glass-forming ability.

Power law relationship between diffusion coefficients in multi-component glass forming liquids.

The slow down of dynamics in glass forming liquids as the glass transition is approached has been characterised through the Adam-Gibbs relation, which relates relaxation time scales to the configurational entropy. The Adam-Gibbs relation cannot apply simultaneously to all relaxation times scales unless they are coupled, and exhibit closely related temperature dependences. The breakdown of the Stokes-Einstein relation presents an interesting situation to the contrary, and in analysing it, it has recently been shown that the Adam-Gibbs relation applies to diffusion coefficients rather than to viscosity or structural relaxation times related to the decay of density fluctuations. However, for multi-component liquids --the typical cases considered in computer simulations, metallic glass formers, etc.-- such a statement raises the question of which diffusion coefficient is described by the Adam-Gibbs relation. All diffusion coefficients can be consistently described by the Adam-Gibbs relation if they bear a power law relationship with each other. Remarkably, we find that for a wide range of glass formers, and for a wide range of temperatures spanning the normal and the slow relaxation regimes, such a relationship holds. We briefly discuss possible rationalisations of the observed behaviour.

Zr-Co(Cu)-Al bulk metallic glasses with optimal glass-forming ability and their compressive properties

Abstract The formation of bulk metallic glasses (BMGs) in the ternary Zr56Co28Al16 and quaternary Zr56Co28–xCuxAl16 (x=2, 4, 5, 6, 7, mole fraction, %) glassy alloys was investigated via the copper mold suction casting method. The main purpose of this work was to locate the optimal BMG-forming composition for the quaternary ZrCo(Cu)Al alloys and to improve the plasticity of the parent alloy. The X-ray diffractometry (XRD), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) were used to investigate the glassy alloys structure and their glass forming ability (GFA). In addition, the compression test, microhardness, nano-indentation and scanning electron microscopy (SEM) were utilized to discuss the possible mechanisms involved in the enhanced plasticity achievement. The highest GFA among Cu-containing alloys was found for the Zr56Co22Cu6Al16 alloy, which was similar to that of the base alloy. Furthermore, the plasticity of the base alloy increased significantly from 3.3% to 6% for the Zr56Co22Cu6Al16 BMG. The variations in the plasticity and GFA of the alloys were discussed by considering the positive heat of mixing within Cu and Co elements.

Comparing short–range and medium–range ordering in Cu[sbnd]Zr and Ni[sbnd]Zr metallic glasses – Correlation between structure and glass form ability

According to recent studies, CuZr and NiZr binary alloying systems are very similar based on known glass-forming ability criteria, however, they exhibit significant difference in glass-forming ability in practice. In this work, local atomic structures of CuZr and NiZr metallic glasses are studied by molecular dynamics simulation to explain the source of mentioned difference. The total and partial distribution functions, coordination number and voronoi analysis are utilized to characterize the local atomic structures around Zr, Ni and Cu atoms. It was found that the local environment around Zr atoms is almost similar in both systems. The difference in the atomic structure in these systems mainly arises from topologies of polyhedra around Cu and Ni atoms. In particular, full icosahedron is one of the most popular local structure in CuZr system, while its population is significantly smaller in NiZr system with the same composition. Additionally, networks formed by full icosahedra connections, medium range order, are more robust in CuZr system. Higher percentage of topologically ordered and compact structures and their connectivity could explain why bulk metallic glass formation is possible in CuZr system but not in NiZr system.

On the bulk glass formation in the ternary Pd-Ni-S system

We report on the formation of bulk metallic glasses in the ternary Pd-Ni-S system. In a large compositional range, glass formation is observed by copper mold casting with a glass forming ability of up to 2 mm in diameter for the composition Pd37Ni37S26. The best compromise of thermal stability upon heating from the as-cast state and glass forming ability was found for Pd31Ni42S27, having a critical diameter of 1.5 mm and an extension of the supercooled liquid region of 27.2 K (ΔTx = Tx – Tg). Differential scanning calorimetry and X-ray diffraction experiments were conducted in order to study the influence of the composition on the glass forming ability and thermal stability. The primary precipitating crystalline phases Ni3S2 and Pd4S are identified by in-situ high energy synchrotron X-ray scattering experiments upon heating from the glassy state as well as upon cooling from the equilibrium liquid. Finally, the origin of the bulk glass formation in this novel system is discussed regarding thermodynamics and kinetics and compared to current models for the prediction of the glass forming ability. Furthermore, the mechanical properties are investigated and discussed with respect to the rather fragile kinetic behavior. All in all, we gain new insights into the process of glass formation in this novel alloying system and give conclusions about the determining contributions for the glass forming ability and glass forming range.

Effects of pressure on structure and dynamics of metallic glass-forming liquid with miscibility gap

The metallic liquid with miscibility gap has been widely explored recently because of the increasing plastic deformation ability of phase-separated metallic glass. However, the poor glass-forming ability limits its application as the structural materials due to the positive mixing enthalpy of the two elements. Since high pressure is in favor of the formation of the glass, the effect of pressure on the structural and dynamical heterogeneity of phase-separated Cu50Ag50 liquid is investigated by molecular dynamics simulation in the pressure range of 0–16 GPa. The results clearly show that the pressure promotes the formation of metallic glass by increasing the number of fivefold symmetry cluster W and dynamical relaxation time; meanwhile, the liquid–liquid phase separation is also enhanced, and the homogenous atom pairs show stronger interaction than heterogeneous atom pairs with increasing pressure. The dynamical heterogeneity is related to the formation of fivefold symmetry clusters. The lower growing rate of W at higher pressure with decreasing temperature corresponds to the slow increase in dynamical heterogeneity. The pressured glass with miscibility gap may act as a candidate glass with improved plastic formation ability. The results explore the structural and dynamical heterogeneity of phase-separated liquid at atomic level.

Ultrastable metallic glasses formed on cold substrates

Vitrification from physical vapor deposition is known to be an efficient way for tuning the kinetic and thermodynamic stability of glasses and significantly improve their properties. There is a general consensus that preparing stable glasses requires the use of high substrate temperatures close to the glass transition one, Tg. Here, we challenge this empirical rule by showing the formation of Zr-based ultrastable metallic glasses (MGs) at room temperature, i.e., with a substrate temperature of only 0.43Tg. By carefully controlling the deposition rate, we can improve the stability of the obtained glasses to higher values. In contrast to conventional quenched glasses, the ultrastable MGs exhibit a large increase of Tg of ∼60 K, stronger resistance against crystallization, and more homogeneous structure with less order at longer distances. Our study circumvents the limitation of substrate temperature for developing ultrastable glasses, and provides deeper insight into glasses stability and their surface dynamics.

Atomic Scale Investigation of Structural Properties and Glass Forming Ability of Ti100−xAlx Metallic Glasses

In this work, we employed molecular dynamics (MD) simulations to study Ti-Al metallic glasses (MGs) using the embedded atom method (EAM) potential to model the atomic interaction with different compositions. The results showed evidence of the metallic glass formation induced by the split occurring in the second peak of the radial distribution function (RDF) curves implying both Ti and Al atoms. The common neighbor analysis (CNA) method confirmed the presence of the icosahedral clusters with a maximum amount observed for an alloy with 75 pct of Al. Analysis of coordination numbers (CNs) indicated that the total CNs are nearly unchanged in these systems. Finally, Voronoi tessellation analyses (VTA) showed a higher value of the number of icosahedral units at Ti25Al75 composition. This specific composition represents a nearby peritectic point localized at a low melting point in the Ti-Al binary phase diagram. The glass forming ability (GFA) becomes important when the fraction of Al increases by forming and connecting “icosahedral-like” clusters (12-coordinated 〈0, 0, 12, 0〉 and 13-coordinated 〈0, 1, 10, 2〉) and by playing a main role in the structure stability of the Ti-Al MGs.

Analysis of thermophysical properties of lead silicates in comparison to bulk metallic glasses

The equilibrium viscosity and the specific heat capacity of three lead silicate glass forming liquids (flint glasses) with low glass transition temperatures are experimentally measured and compared to the thermophysical properties of network glass-formers like SiO2 and GeO2 and of bulk metallic glasses. The data are analyzed using the Vogel–Fulcher–Tammann (VFT) equation, the Mauro–Yue–Ellison–Gupta–Allan (MYEGA) equation, and the Adam–Gibbs approach. The flint glasses depart from the Arrhenius equation in terms of liquid kinetics and their corresponding kinetic fragilities are intermediate between GeO2 and the bulk metallic glass-formers. Among the lead silicates the fragility increases with increasing content of network modifiers, as lead, sodium and potassium modify the silicate network by creating non-bridging oxygen that softens the network.

Glassy blast furnace pig iron and design of other glassy compositions using thermodynamic calculations

Formation of metallic glass largely depends on the composition of the alloy system and inherent glass forming ability (GFA). Among all the relevant GFA criteria, GFA parameter, PHSS, gives better quantitative description to explain higher GFA in multicomponent systems. Present work shows a set of glass forming alloys designed with the help of some GFA parameters including PHSS using a raw blast furnace (BF) grade metal, popularly known as pig iron, in the range of 75–100 wt% of the total alloy weight and addition combination of alloying elements (B, P, Nb, Cr, Mo). Good glass forming ability of the blast furnace hot metal (direct from blast furnace containing impurities) as determined by thermodynamic calculation has been experimentally validated by making its glassy ribbons by melt spinning in air as well 1 mm diameter partially glassy rod as confirmed by X-ray diffraction (XRD), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) studies.

Dynamic characterization of Cu–Zr binary bulk metallic glasses: A molecular dynamics study

In the present study, a molecular dynamics simulation employing embedded atom method potential is performed to investigate the formation and characterization of CuZr bulk metallic glasses (BMGs). To elucidate the effect of component concentration of three samples of BMGs including Cu25Zr75, Cu50Zr50, and Cu75Zr25 that are formed by melt quenching. The local structure of BMGs is analyzed by means of radial distribution function and local atomic number density, ρ. The mechanical behavior of three compositions is investigated using uniaxial compressive loading at a constant strain rate. It is revealed from the results that yield strength increases with increasing Cu concentration. Thermal expansion of CuZr BMGs is examined and variation in length and volume is measured. The analysis revealed that Cu25Zr75, and Cu50Zr50 exhibited the typical expansion behavior while Cu75Zr25 showed an anomalous behavior.

Metallic Glass Formation Upon Rapid Solidification of Fe60Cr8Nb8B24 (at%) Alloy through LASER Cladding and Remelting

Fe60Cr8Nb8B24 (at%) amorphous powder and ingot were submitted to LASER cladding and remelting process, respectively. Fe-based coatings were produced using different LASER processing parameters and characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD) and Vickers microhardness. All coatings obtained by LASER cladding (LC) and layers obtained by LASER surface remelting (LSR) with shorter and median interaction times (scanning speeds of 16.7 and 66.7mm/s), irrespective of overlap value, presented peaks of the crystalline phases α-Fe, Fe2B and the FeNbB intermetallic. In addition, layers obtained by LSR with shorter interaction time (150mm/s), in general, presented only peaks of the α-Fe and Fe2B phases, except that produced with overlap value of 20%, more amorphous. SEM results showed that, regardless of LASER process, the coatings presented cracks, some pores and FeNbB intermetallic phase. All coatings obtained by LC and that obtained with shorter interaction time, by LSR, showed a Vickers microhardness, 1086 to 1329HV and 2161 to 2294HV, respectively, significantly higher than the substrates, AISI steel: 170HV and Fe-Cr-Nb-B: 1921HV.

Atomic packing and fractal behavior of Al-Co metallic glasses

The rapidly solidified Al-Co alloy is studied in a favored composition range of 25–75 at.% of aluminum (Al) by performing a set of classical molecular dynamics simulations (MD) based on the Embedded Atom Method (EAM). We have examined particularly the structure using several analyzing techniques such as: the radial distribution function (RDF), the Voronoi tessellation analysis and the structure factor. We observed a splitting in the second peak of both the RDF and the structure factor, indicating metallic glass formation. The Voronoi analysis shows that it is difficult to define a dominant type of coordination polyhedra for all compositions, manifesting two different Voronoi polyhedral (VP): <0,3,6,4> and <0,2,8,4>, and this distribution varies with increasing Al content. The structure factor is further examined via illustrating two of their important parameters: the full width at half maximum (FWHM) and the positon of the first sharp peak (FSP) as a function of the composition and the atomic volume, respectively. The first parameter showed a maximum for the Al50Co50 alloy according to the dominance of unlike pairs, while the second one revealed a power law that allowed to compute the fractal dimension which is almost comparable to that found out from the analysis of the cumulative coordination number (CN).

Effect of temperature on shear bands and bending plasticity of metallic glasses

We report the effect of testing temperature on the bending plasticity of Pt57.5Cu14.7Ni5.3P22.5 and Zr35Ti30Cu8.25Be26.75 metallic glasses. Bend tests ranging from liquid nitrogen to the glass transition temperature were conducted using a customized setup. The bending strain, the shear band density, and the critical shear offset at fracture decreased with increasing temperature in both the metallic glass formers. The constant strain experiments (without fracture) revealed that low temperature generates more shear bands of smaller lengths to accommodate the same applied strain. The results can be rationalized in terms of temperature dependence predicted by the nucleation and propagation models.

Hierarchical aging pathways and reversible fragile-to-strong transition upon annealing of a metallic glass former

The change of physical properties during aging and the associated microscopic dynamics of the Au49Cu26.9Si16.3Ag5.5Pd2.3 bulk metallic glass are investigated using a broad collection of laboratory and synchrotron-based techniques, such as differential- and fast-scanning calorimetry, thermomechanical testing, and x-ray photon correlation spectroscopy. We observe multiple decays in the enthalpy change during aging. This is reflected by a microscopic ordering consisting of distinct stationary dynamical regimes interconnected by abrupt aging processes. The stationary regimes are representative of states of local and transient equilibrium with increasingly higher activation energies. Furthermore, the aging study is conducted with the kinetically fragile frozen-in structure and the underlying fragile-to-strong transition is accessed by the ultra-viscous liquid state during annealing on a long-time scale and corresponds to the last observed enthalpy equilibration decay. The experimental work verifies, for the first time, that in a metallic glass forming system, the fragile-to-strong transition can also occur below the conventional glass transition temperature. Upon reheating, the reverse transformation, i.e. the strong-to-fragile transition, is observed with an entropy change of 0.19 J/(g-atom K), which is 2.4% of the entropy of fusion.

Formation of Zr-based bulk metallic glass with large amount of yttrium addition

In this study, we systematically studied the effects of Y addition on the glass forming ability (GFA), thermal stability and mechanical property of Zr56Co28Al16 bulk metallic glass (BMG). The results show that two BMGs, i.e., Zr49Co28Al16Y7 and Zr46Co28Al16Y10 with diameters of at least 14 mm, were successfully prepared by the water quenching method. In addition, fully metallic glass ingot about 25 g can be obtained in the Zr46Co28Al16Y10 alloy by using the conventional arc-melting method. The origin of the high GFA of Zr46Co28Al16Y10 alloy was attributed to the smaller Gibbs free energy difference between the supercooled liquid state and crystalline state of the Zr46Co28Al16Y10 than that of Y-free alloy. On the other hand, the fracture strength of Zr-Co-Al-Y BMGs decreases with the increase of Y content. However, the Zr-Co-Al-Y BMGs containing 0–5 at.% Y still exhibit a high fracture strength of about 1690–1790 MPa. The fracture surface morphology of BMGs with 5–12 at.% Y shows a typical featureless mirror surface, implying the brittle deformation. This Zr-Co-Al-Y quaternary BMG could be regarded as a good candidate for developing a new series of BMGs.

Effect of high pressure on the formation and evolution of clusters during the rapid solidification of zirconium melts

The formation and evolution of clusters during the rapid solidification of liquid zirconium (Zr) under the pressure of 0–80GPa were examined through molecular dynamics simulations. Local atomic structures were analyzed using pair-distribution function, Honeycutt–Anderson bond-pair index method, and developed cluster-type index method. Results revealed that Zr melts can be completely vitrified into a glassy state under 30–60GPa. (13 1-1441 10-1551 2-1661) Kasper clusters, instead of (12 12-1551) icosahedra, dominate the formation of Zr metallic glasses. The numbers of Kasper clusters increase with increasing pressure. Under ≤20GPa, the coexistence structures of the stable hexagonal close-packed (HCP) with meta-stable body-centered cubic (BCC) configurations are obtained with phase separation in the final solids. The fraction of BCC structures increases with increasing pressure. The evolution of basic clusters in the crystallization of Zr melts always takes the path of Kasper clusters→(14 6-1441 8-1661) BCC→(12 6-1421 6-1422) HCP. Crystallization under ≥70GPa lacks an intermediate stage. Instead, the supercooled liquid (scl) Zr is directly transformed into A15 phase (scl→A15), which is composed of (12 12-1551) icosahedra and (14 12-1551 2-1661) Kasper clusters. The A15 structure is perfected with increasing pressure. Competition between densification and atomic diffusion contributes to the non-monotonic effect of high pressures on the rapid cooling of Zr melts.

On the Fragility of Bulk Metallic Glass Forming Liquids

In contrast to pure metals and most non-glass forming alloys, metallic glass-formers are moderately strong liquids in terms of fragility. The notion of fragility of an undercooling liquid reflects the sensitivity of the viscosity of the liquid to temperature changes and describes the degree of departure of the liquid kinetics from the Arrhenius equation. In general, the fragility of metallic glass-formers increases with the complexity of the alloy with differences between the alloy families, e.g., Pd-based alloys being more fragile than Zr-based alloys, which are more fragile than Mg-based alloys. Here, experimental data are assessed for 15 bulk metallic glasses-formers including the novel and technologically important systems based on Ni-Cr-Nb-P-B, Fe-Mo-Ni-Cr-P-C-B, and Au-Ag-Pd-Cu-Si. The data for the equilibrium viscosity are analyzed using the Vogel–Fulcher–Tammann (VFT) equation, the Mauro–Yue–Ellison–Gupta–Allan (MYEGA) equation, and the Adam–Gibbs approach based on specific heat capacity data. An overall larger trend of the excess specific heat for the more fragile supercooled liquids is experimentally observed than for the stronger liquids. Moreover, the stronger the glass, the higher the free enthalpy barrier to cooperative rearrangements is, suggesting the same microscopic origin and rigorously connecting the kinetic and thermodynamic aspects of fragility.

Effects of lutetium addition on formation, oxidation and tribological properties of a Zr-based bulk metallic glass

Enhancement of glass-forming ability (GFA) and surface properties are important for the application of Zr-based bulk metallic glasses (BMGs), and surface oxidation is an effective strategy for surface strengthening. In this paper, the effects of rare earth element Lu addition on GFA and oxidation properties of a Zr50Ti2Cu38Al10 bulk metallic glass were studied. The tribological properties of Lu-free and Lu-doping BMGs before and after oxidation were also investigated. It is found that 2 at.% Lu addition in this alloy significantly enhance the critical diameter (dc) from 5 mm to 20 mm. The oxidation rate of 2 at.% Lu-doping alloy is higher than Lu-free alloy, indicating that the addition of Lu facilitates the formation of oxidized scale on the surface of Zr50Ti2Cu38Al10 alloy. Moreover, surface oxidation treatment remarkably improves the wear resistance of Zr50Ti2Cu38Al10 and (Zr0.5Ti0.02Cu0.38Al0.1)98Lu2. This study is beneficial for the improvement of surface properties and further application of Zr-based BMGs.