Marginal Glass Research Articles

The mismatch entropy for bulk metallic glasses: A thermodynamic approach

Based on the thermodynamic fragility model, a new parameter (Sσ/kB)/M is proposed to evaluate the glass-forming ability (GFA) of metallic glasses, where M and Sσ/kB are the fragility of the superheated melts and the mismatch entropy normalized by the Boltzmann constant, respectively. A positive relationship between (Sσ/kB)/M and glass forming ability is observed for both bulk metallic glasses and marginal metallic glasses, and an eutectic alloy with (Sσ/kB)/M larger than 0.17 can be selected as a bulk glassy forming candidate. As most alloys can obtain the temperature dependence of viscosity above liquidus temperature, (Sσ/kB)/M can be used as a prediction parameter for high GFA. Although both high and low M values can work in determining GFA, the high GFA of bulk metallic glasses is primarily due to the large mismatch entropy (Sσ/kB>0.17).

Estimation of critical cooling rates for formation of amorphous alloys from critical sizes

The critical cooling rate for marginal glass formers, like Al-based alloys, is difficult to measure experimentally. In this work, we acquired the critical cooling rate for formation of amorphous alloys by estimating the cooling rates of the specimens with critical dimensions. Analytical solutions were given to estimate the cooling rates for the gas-atomized powders and melt-spun ribbons, and as an example the critical cooling rate to form an amorphous Al82Ni10Y8 alloy was estimated to be ~1.0×106Ks−1. The effect of melt temperature on the cooling rate was quantitatively evaluated and its effect on the glass forming ability was also discussed.

Ni-Hf非晶合金及其团簇式成分解析

Ni-Hf binary alloys are merely known as a category of marginal glass formers. In this system, the assessment of glass-forming abilities has not been made and the optimal glass-forming composition remains unclear. In the present work, Ni-Hf glass-forming compositions are explored using the atomic cluster plus glue atom model. Ni-Hf phase diagram shows a deep eutectic point at Ni65Hf35 which is associated two intermetallic phases, namely, Ni7Hf3 and Ni10Hf7. A predominant atomic cluster was first derived from the crystalline structures of the deep eutectic related phases, and several glass-forming compositions were determined considering the one or three glue atoms constraint condition. Rapid quenching results show that Ni71.43Hf28.57 and Ni68.75Hf31.25 metallic glasses can be made fully glassy. The latter can be well described with the model composition formula as [Ni-Ni7Hf5]Ni3. Among the melt-spun alloys, the Ni68.75Hf31.25 glass which locates in between the deep eutectic point and the Ni7Hf3 phase in the phase diagram, exhibited a highest crystallization temperature of 877 K and a lowest liquidus of 1482 K, and hence is suggested to be the optimal Ni-Hf glass former by the high reduced glass temperature criterion.

Dynamical study of a polydisperse hard-sphere system

We study the interplay between the fluid-crystal transition and the glass transition of elastic sphere system with polydispersity using nonequilibrium molecular dynamics simulations. It is found that the end point of the crystal-fluid transition line, which corresponds to the critical polydispersity above which the crystal state is unstable, is on the glass transition line. This means that crystal and fluid states at the melting point becomes less distinguishable as polydispersity increases and finally they become identical state, i.e., marginal glass state, at critical polydispersity.

Local structure in marginal glass forming Al–Sm alloy

The local structure in rapidly quenched Al (100− x) Sm x ( x = 8, 10, 11, and 12) and liquid Al 89Sm 11 has been investigated using a combination of transmission electron microscopy (TEM) and high-energy synchrotron X-ray diffraction (HEXRD). TEM analysis showed a featureless microstructure with diffuse scattering in rapidly quenched Al (100− x) Sm x ( x = 8, 10, 11, and 12) within the glass formation composition range under the bright field (BF) conditions. Total structure factor analysis of the liquid and as-quenched alloys revealed a pre-peak located well below the main amorphous peak and a distinct side peak. The presence of the pre-peak and the side peak is related to the formation of Sm rich medium range order (MRO) clusters in the liquid that is retained in the as-quenched alloys. Atomic structure models constructed using Reverse Monte Carlo (RMC) simulations from experimentally determined total structure factors and coupled with Voronoi Tessellation analysis indicated icosahedral and deformed bcc-like Voronoi polyhedron (VP) surrounding Al and Sm atoms, respectively. Sm atoms were found to be highly coordinated with Al atoms in the first shell neighborhood. The structural unit sizes corresponding to the extra broad peaks and the first shell neighborhood around Sm atoms have remarkable similarities with the high temperature metastable Al 11Sm 3 tetragonal phase. The existence of the Sm rich MRO clusters in the as-quenched state is believed to promote the high nucleation density of fcc-Al nanocrystals that form when the material is devitrified by acting as catalyst sites.

Primary crystallization in Al-rich metallic glasses at unusually low temperatures

The initial stage of the primary crystallization reaction and the glass transition of the marginal metallic glass Al 89Y 6Fe 5 were investigated by conventional differential scanning calorimetry (DSC) and modulated differential scanning calorimetry (MDSC), microcalorimetry, X-ray diffraction (XRD) and transmission electron microscopy. A sharp onset of the primary crystallization was found by microcalorimetry and XRD studies at temperatures which were 120 °C below the primary crystallization peak observed in conventional DSC. A systematic MDSC study of annealed samples revealed a wide spectrum of glass transition onsets, which show a strong dependence on the annealing conditions. In addition, the glass transition onsets can be linked to the initial stage of the primary crystallization. The spectrum of glass transition onsets observed is discussed with respect to the occurrence of phase separation preceding the nucleation and growth of dendritic aluminium nanocrystals.

Nanostructure development during devitrification and deformation

The devitrification of amorphous phases can be controlled to yield a nanoscale microstructure based upon either a high density of nanocrystals dispersed in an amorphous matrix or a completely nanocrystalline solid. For both rapidly solidified marginal glass forming alloys and slowly cooled bulk glass forming alloys, the high nanocrystal densities are related to nucleation reactions that are sensitive to the initial as-prepared state of the amorphous phase. For example, nucleation densities of 10 21 m −3 can be enhanced to 10 23 m −3 by selective doping of Al-based glasses and the glass forming ability of bulk amorphous alloys is known to be sensitive to minor impurities. The addition of only 1 at.% of Cu to amorphous Al–Sm–Ni or Al–Y–Fe alloys is an effective microstructure control by narrowing the size distribution of Al nanocrystals and reducing the average size of the nanocrystals from 10 nm to about 7.5 nm while increasing the particle number density. Calorimetry and microstructural analyses of the primary Al crystallization reaction suggest that the addition of Cu modifies the atomic arrangement and induces structural heterogeneities based upon medium range order (MRO) that can act as nucleation sites. The strong composition dependence of the crystallization reactions and resulting microstructures reflect not only underlying thermodynamic constraints, but also indicates a strong composition dependence of the amorphous phase atomic arrangement. These features point to a central role for heterogeneities in the evolution of nanoscale microstructures. During nanostructure synthesis by deformation of multilayers the thermodynamic constraints are also important, but the co-deformation of layers governs the generation of interfacial area that allows for the nanoscale alloying reaction. Moreover, kinetics studies have also revealed that the governing reactions often proceed under transient conditions and can lead to relatively stable and robust nanoscale microstructures.

The correlations among the fragility of supercooled liquids, the fragility of superheated melts, and the glass-forming ability for marginal metallic glasses

In an individual alloy system for marginal metallic glasses (MGs), the fragility of supercooled liquids (m) is inversely proportional to the fragility of superheated melts (M), which depends on the differently inherent characteristic temperatures. The more stable the superheated melt, the more fragile the supercooled liquid becomes and the better the glass-forming ability (GFA). The strong supercooled liquids correspond to the bad GFA resulting from heterogeneous nucleation in liquids. For the whole Al-based MGs, m has general linear relations with the GFA and M, respectively.

Characterization of a marginal glass former alloy solidified in gas atomized powders

Al 90Sm 10, a marginal glass former, was rapidly solidified using high pressure gas atomization (HPGA). Rapid solidification is a non-equilibrium process, with different degrees of departure from full equilibrium constituting a microstructural hierarchy that correlates with increasing solidification rate. In accordance with this the resultant HPGA powders show a variety of microstructures according to their particle diameters, corresponding to degree of undercooling, with an amorphous structure appearing at high cooling rates. Five distinct phases and microstructures have been identified at different undercoolings; Al solid solution, tetragonal Al 11Sm 3, two different orthorhombic phases, and an amorphous phase which exists in company with a high number density of Al nanocrystals. The product phases of the rapid solidification were identified and analyzed using high energy transmission X-ray diffraction (HEXRD), high resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS) and thermal analysis (DSC). The results of the study will be helpful in identifying metastable phase hierarchy and glass formation during vitrification of marginal glass formers.

Crystallization behavior in a highly driven marginal glass forming alloy

Al 90Sm 10, a marginal glass former, was rapidly solidified using Cu-block single roller melt spinning at wheel speeds of 30 and 40 m/s. The product phases of rapid solidification were identified and analyzed using high energy synchrotron X-ray diffraction (HEXRD), high resolution transmission electron microscopy, and atom probe tomography. The as-quenched structure consists of a saturated amorphous phase and nanocrystalline Al with typical length scale of about 5 nm. The appearance of a pre-peak on HEXRD diffraction patterns and a low activation energy for first crystallization as determined using the Kissinger and Ozawa methods indicate some local ordering in the amorphous phase. The devitrification phase transformation path was determined using in situ high energy synchrotron radiation. Three phases, MS1, H1, and Al 4Sm, were identified during decomposition of the amorphous phase. MS1, H1 and Al 4Sm are cubic, hexagonal and orthorhombic metastable phases, respectively.

Excellent Thermal Stability and Bulk Glass Forming Ability of Fe-B-Nb-Y Soft Magnetic Metallic Glass

Effects of Y addition on the glass forming ability (GFA) of a Fe-B-Nb marginal glass former, and annealing effects on glassy/supercooled liquid phases as well as soft magnetic properties in the multicomponent Fe-B-Nb-Y alloy system were investigated. The origin of highly improved GFA in the multicomponent system is discussed with related to a characteristic exothermic phase transformation, chemical short range ordering, in the supercooled liquid region due to the positive heat of mixing between Nb-Y elements. The separating tendency between Nb and Y elements is considered to suppress precipitation of metastable Fe23B6 and � -Fe crystalline phases, thus to result in highly improving GFA and distinct high thermal stability against heat treatment of the alloy system. In addition, a glassy ring was fabricated by copper mold casting and magnetic properties were investigated before/after heat treatment. [doi:10.2320/matertrans.MBW200732]

Kinetic Aspects of the Nanocrystallization and Evolution of Microhardness in Al92−X Ni8La X Amorphous Alloys

Marginal glass forming Al-based alloys have attracted considerable attention due to the occurrence of a primary crystallization reaction that yields a microstructure consisting of uniformly dispersed Al-nanocrystals embedded in an amorphous matrix. Rapidly solidified Al-alloys ribbons containing 8 at.% Ni and 4–6 at.% La were prepared by single roller melt-spinning technique. As-melt-spun and annealed ribbons were investigated by means of differential scanning calorimetry (DSC), X-ray diffractometry (XRD) and hardness measurements. XRD studies revealed that ribbons are fully amorphous in the as-melt-spun state. DSC studies showed that crystallization proceeds in two or three stages that are sensitively influenced by La content. Crystallizations kinetics were analysed by Kissinger and Johnson-Mehl-Avrami approaches. Microhardness of all the ribbons was examined at different temperatures and correlated with the corresponding structural evolution.

Glass Transition Dynamics and Boiling Temperatures of Molecular Liquids and Their Isomers

The relation between a dynamic and a thermodynamic temperature, glass transition Tg and boiling point Tb, is investigated for various glass-forming liquids, with emphasis on monohydroxy alcohols. As is well known, Tb and Tg are positively correlated across a large variety of liquids. However, we found that the same quantities show a negative correlation within an isomeric series, i.e., Tb decreases with increasing Tg for different isomers of the same chemical formula. For the alcohol series, CnH2n+1OH with 3 < or = n < or = 10, a master curve of the negative Tg - Tb correlation is obtained if the temperatures are normalized to the respective values of the n-alkanols. This Tg - Tb dependence of isomeric liquids is linked to entropic effects and responsible for much of the scatter of the correlation observed for a large number of molecular organic glass-formers with 45 < Tg < 250 K. Dielectric relaxation is measured for three groups of isomers: (a) 3-methoxyl-1-butanol and 2-iso-propoxyethanol, (b) 1,4-, 1,2-, and 2,4-pentanediol, and (c) di-n- and di-iso-butyl phthalate. Two key parameters of the dynamics, fragility m and stretching exponent beta, are found to be indistinguishable within isomers of moderately different Tgs. Larger fragility differences are readily expected with pronounced structural change, but no systematic trend is observed within an isomer series. The results provide a useful tool for assessing Tg, m, and beta for marginal glass formers on the basis of their isomers.

Hierarchical Nature of the Vortex Matter in Type II Superconductors due to Competition Between Interactions, Thermal Fluctuations and Disorder

We describe quantitatively the combined effects of both the thermal fluctuations and of the quenched disorder via the replica trick applied to the Ginzburg–Landau (GL) theory. We show that the vortex state can appear in either of the three disordered phases: (i) unpinned vortex liquid, (ii) amorphous vortex glass (pinned), and (iii) the crystalline (pinned but not containing topological defects) Bragg glass. The formation of the vortex glass is associated with the continuous replica symmetry breaking (RSB) reflecting the hierarchial structure of the potential barriers in a vortex glass state. An earlier analysis in the framework of London approximation have established that activation barriers controlling vortex dynamics obey the extreme value statistics within roughly the same domain of the phase diagram. We show that the disordered GL model in which only the coefficient at the quadratic term |ψ |2 is random, first considered by Dorsey et al., exhibits, in the gaussian approximation, an additional non-hierarchical state possessing certain glassy properties like nonzero Edwards–Anderson order parameter. We associate this state with the “marginal glass phase” predicted in the earlier work of one of the authors; the marginal glass state being characterized by the marginally glassy dynamics. We show further that when the random component of the coefficient of the quartic term |ψ |4 in GL free energy is taken into account, RSB effects appear. Application of the obtained results to description of various disorder-generated phenomena in vortex matter are briefly considered. The location of the glass transition line is determined and compared to experiments. This line is clearly different from both the melting line and the second peak line describing the translational and rotational symmetry breaking at high and low temperatures respectively. The phase diagram is separated by these two lines into the four phases described above.

Thermodynamic Basis for Cluster Kinetics: Prediction of the Fragility of Marginal Metallic Glass-Forming Liquids

Due to the inaccessibility of the supercooled region of marginal metallic glasses (MMGs) within the experimental time window, we study the cluster kinetics above the liquidus temperature, Tl, to acquire information on the fragility of the MMG systems. The thermodynamic basis for the stability of locally ordered structure in the MMG liquids is discussed in terms of the two-order-parameter model. It is found that the Arrhenius activation energy of clusters, Deltah, is proportional to the chemical mixing enthalpy of alloys, DeltaH(chem). Fragility of the MMG forming liquids can be described by the ratio of the absolute DeltaH(chem) value to the glass transition temperature, Tg. The manner of vitrification during rapid solidification is an important factor for the discrepancy between the data presented in this paper and the prediction of the two-order-parameter model concerning the relation between Delta h and the liquid fragility.

Amorphization and devitrification reactions in metallic glass alloys

The devitrification of amorphous phases can be controlled to yield a high nucleation density and a nanoscale microstructure based upon either nanocrystals dispersed in an amorphous matrix or a completely nanocrystalline solid. For both rapidly solidified marginal glass forming alloys and slowly cooled bulk glass forming alloys, the high nucleation densities are related to heterogeneous nucleation reactions that are sensitive to the initial as-prepared state of the amorphous phase. Selective doping of Al based glasses, for example, can enhance the nucleation densities from 10 21 to 10 23 m −3. The heterogeneous nature of the local structure is reflected also in the response of amorphous alloys to deformation-induced synthesis of nanocrystals. Moreover, kinetics studies have revealed that the governing devitrification reactions often proceed under transient conditions that can lead to relatively stable and robust nanoscale microstructures.

Hydrogen-induced structural relaxation in bulk metallic glasses

In order to clarify the hydrogen-induced structural relaxation (HISR) of hydrogenated bulk and marginal metallic glasses (BMG and MMG), the hydrogen concentration ( C H) dependence of the peak temperature ( T p) and the peak height ( Q p − 1 ) of the hydrogen internal friction peak (HIFP) in a-Zr 55Cu 30Al 10Ni 5 (numbers indicate at.%) (BMG), a-Zr 54Cu 30Al 10Ni 5Si 1 (MMG) and a-Zr 40Cu 49Al 10Si 1 (MMG) were studied. It is found that the T p versus C H data and the Q p − 1 versus C H data are well explained by the relationships of T p = Δ T p exp(− C H/ τ H) + T p,0 and Q p − 1 ∝ ln ( C H / τ H ) , respectively, not only for all the present metallic glasses but also for various Zr-base BMG and MMG reported. The characteristic C H dependence of T p and Q p − 1 was attributed to effects of the HISR. The detailed features of the C H dependence of T p and Q p − 1 were discussed in the light of material parameters and HISR.

Fragility, kinetic stability and phase separations in the undercooled state of bulk glass formers – a case study on metallic model systems

Abstract Several Pd-base alloys display a large tendency for glass formation that renders them especially suitable for investigations concerning (metastable) equilibrium properties of the deeply undercooled liquid including the glass transition, since the detrimental interference of premature crystallization can be avoided rather easily compared to other alloy systems. Here, thermodynamic, dynamic, and transport properties were analysed and compared to an Al-rich marginal glass former in the context of a possible relation between fragility, thermodynamic excess and kinetic stability against crystallization. Additionally, the possibility for liquid-phase separation occurring in the undercooled state of bulk glass-forming alloys is analysed as a function of thermal history, and critically discussed with respect of alternative mechanisms that are often summarized as ‘short-range ordering’.

Microstructure Selection in Severely Deformed Al-Base Systems

Many Aluminum base glass forming alloys require extremely high cooling rates for vitrification and often do not display a clear glass transition signal upon reheating. This marginal glass formation behavior is related mainly to high nucleation rates coupled with growth limitations that prevent a complete crystallization during quenching. The same kinetic control also provides the basis for the development of a high number density (10 2 2 m - 3 ) of nanocrystals (diameter about 10-20 nm) during a primary crystallization reaction - the so-called nanocrystallization - upon reheating. With alternate synthesis routes based upon solid state alloying resulting from severe plastic deformation, e.g. by repeated cold rolling and folding, the kinetic pathways to glass formation can be altered to avoid nanocrystallization reactions in the marginal glass forming alloys. In other systems, a deformation-induced nanocrystal synthesis can be observed during repeated cold rolling of amorphous ribbons. The microstructure selection in dependence of the processing pathway was monitored by X-ray diffraction and electron microscopy techniques. Additionally, modulated-temperature calorimetry has been used to detect the calorimetric signature of the glass transition on melt-quenched samples directly. The results are discussed with respect to the origin of the nanocrystallization, especially the formation of nanocrystal precursors during rapid melt quenching. Moreover, the development of the microstructure during cold-rolling and the comparison of the compositional ranges that allow the synthesis of vitreous product structures by severe plastic deformation or rapid quenching are analyzed in terms of the major thermodynamic - and mechanical properties that govern intermixing during the deformation process.

Aging in the Glass Phase of a Two-Dimensional Random Periodic Elastic System

Using the renormalization group method we investigate the nonequilibrium relaxation of the (Cardy-Ostlund) 2D random sine-Gordon model, which describes pinned arrays of lines. Its statics exhibit a marginal (theta = 0) glass phase for T < Tg described by a line of fixed points. We obtain the universal scaling functions for two-time dynamical response and correlations near Tg for various initial conditions, as well as the autocorrelation exponent. The fluctuation dissipation ratio is found to be nontrivial and continuously dependent on T.