Glass-forming Composition Range Research Articles

Structures, interfaces and thermodynamic stability of nanocrystalline phases in rapidly solidified Fe-based amorphous nanocomposite ribbon, powder and coating

Fe-based amorphous nanocomposite with nominal composition Fe 58.82 Cr 11.12 Mo 1.52 Si 4.16 B 15.12 P 8.88 C 0.39 (at%) has been processed through melt-spinning, melt-spinning followed by ball milling and thermal spray deposition of coating onto a mild steel substrate. X-ray diffraction and electron microscopy characterization of the processed alloy reveals that the melt-spun ribbon is mostly amorphous with uniform distribution of very fine crystals of α-Fe and oP30 (Fe 2 B 7 ) phase. In the melt-spun followed by ball milled powder mainly cI58 (Fe 18 Cr 6 Mo 5 ), tP30 (Fe 62 Cr 34 Mo 4 ) and mI32 (Fe 5 C 2 ) phases are obtained. However, the tP30 (Fe 62 Cr 34 Mo 4 ) phase while ball milling undergoes distortive changes locally to produce oC68, tP60 and tP58 phases with similar composition. The interface between the two structures is considerably strained. In the coating, cF118 (Fe 18 Cr 6 Mo 5 ) and oI92 (Fe 62 Cr 34 Mo 4 ) are observed in the amorphous matrix. Essentially the cF118 and oI92 can be seen as a structural derivative of cI58 and tP30 phases respectively. Local composition fluctuation and availability of a suitable nucleation condition leads to the nucleation of crystalline phases in the amorphous matrix in the melt-spun ribbon. In the melt-spun followed by ball milled powder further distortive changes are observed locally, which leads to the formation of polymorphically related phases to tP30 phase. Even though all the phases are crystallographically quite complicated, they are all derived from either the tP30 (Fe 62 Cr 34 Mo 4 ) phase or cI58 (Fe 18 Cr 6 Mo 5 ) phases. Thermodynamic calculations through Miedema’s model indicates that the nominal composition of the alloy lies at the boundary of glass forming composition. The crystal compositions, although different from the nominal composition of the alloy, lies in the crystal forming composition range. Local composition fluctuation, favorable thermodynamic conditions for nucleation and growth leads to the crystal nucleation. Although all the phases are crystallographically different, they have similar polyhedral order in them. The polyhedral structures cannot be correlated with Frank-Kasper or Bernal deltahedral phases. This is a significant difference between metal-metalloid glasses with its metal-metal counterpart. • Hagg phases and Fe-Cr-Mo intermetalic phases evolve in a Fe-based BMG forming alloy after processing through different solidification routes. • Different polymorphs of Fe-Cr-Mo intermetallics evolve after processing through different solidification routes. • Contrary to the metal-metal glasses, polyhedral structures of the intermetallics do not directly corroborate with Frank-Kasper polyhedra or Bernal deltahedra. • Formation of structurally different polymorphs in the same alloy indicates that the energy difference between two polymorphs is very little. • Miedema calculations show that the alloy composition lies in the boundary region of the glass forming composition range and composition fluctuation is responsible for nucleation of nanocrystalline phases.

Ionic glasses: Structure, properties and classification

Due to the absence of microstructure and virtually infinite compositional versatility, glassy materials are perfect candidates for rational methods to predict structure-property relationships. In the most common models of glass structure, a three-dimensional network backbone and less-localized interstitial species are taken as the fundamental constituents of a glass. Such theoretical frameworks break down for the wide range of glass-forming compositions in which covalent bonding does not percolate and, therefore, there is no network in the classical sense. Most prominently, this applies to the class of ionic glasses, presently emerging as a separate class of amorphous materials, which includes classical invert glasses, geometrically frustrated compounds, mixtures of simple salts and ionic liquids, and organic–inorganic hybrids. Here, we will critically review ionic glasses as a distinct group of materials in which structural predictions are complicated by the dominance of long-range bonding interactions which lack directionality. First, we will reassess Zachariasen’s and Hägg’s rules for conventional glass formation in an attempt to broaden the current understanding of glass formation to include ionic glasses. By reviewing the compositions and properties of many simple and complex glass-forming salts, we discern the commonalities shared by ionic glasses and their unique characteristics as compared to other classes of glass. Specifically, we discuss the importance of non-percolating topology to dynamic properties such as mechanical behavior, ionic conductivity, relaxation processes and crystallization. We also demonstrate the relevance of the adjacent fields of ionic liquids and amorphous metal organic frameworks, and highlight possibilities for fruitful interdisciplinary exchange. The structure of ionic glasses, being dominated by Coulombic interactions, lends itself to high compositional flexibility and therefore, creates significant opportunities in the search for optical materials with enhanced transmission windows, fast ion conductors, phase-change materials, or, e.g ., adapted solders, hermetic seals and dielectric insulators. • The field of ionic glasses is reviewed with a focus on structure and properties. • Classification criteria are proposed. • Invert, halide, oxoanionic, halide-oxide and superionic glasses are included. • Links are drawn to the adjacent fields of ionic liquids and metal organic framework glasses.

Microstructural characterization and mechanical behavior of nanocomposite Ti-Ni-Nb surface alloys synthesized on TiNi SMA substrate by additive thin-film electron-beam mixing

An important factor limiting the wide application of magnetron-sputtered amorphous or nanocomposite thin-film coatings for improving surface-sensitive properties of structural alloys is the poor adhesion of coating to substrate. This problem can be overcome through the synthesis of surface alloys (SAs) by additive pulsed electron-beam melting of film/substrate systems of high glass-forming ability (GFA). In this work, this approach is applied to the “film (Ti85/70Nb15/30, at.%), 100 nm/substrate (TiNi SMA)” system using low-energy, high-current electron beam (~2.5 μs, ~15 keV, 1.7 J/cm2) at 10 synthesis cycles and 10 pulses per cycle. The melting/solidification conditions were evaluated using numerical modeling of pulsed heating, taking into account the formation of eutectic in the Ti-Ni-Nb system and initial temperature risen up to 473 K to the end of the synthesis. Using surface SEM/EDS, AES, XRD and cross-sectional HRTEM/EDS/SAED analyses it has been found that both SAs of thickness ~1.5–2 μm have depth-graded structure, in which outer Ni-depleted layer is the multiphase nanocomposite with quasicrystals followed by predominantly Ti-Ni-based amorphous sublayer. Beneath, the ~1 μm thick intermediate sublayer with eutectic columnar structures (B2 + Ti2Ni and B2 + Ti3Ni4 for first and second SA, correspondently), monotonic Nb substitution for Ni and diffusion transition to TiNi substrate is formed. Depth-graded structure of modified surface layers provides monotonic changes with depth in nanohardness, elastic modulus, depth recovery ratio and plasticity to the values of the TiNi substrate. For both SAs the evolution of structure in depth is in a reasonable agreement with glass forming composition range evaluated for Ti-Ni-Nb system using Miedema's model.

Ultra magnesium-rich, low-density Mg–Ni–Ca bulk metallic glasses

A range of low-density Mg-rich glass-forming compositions from the Mg–Ni–Ca alloy system have been discovered. Distinct maxima in the glass-forming ability were located and shown to produce samples of bulk metallic glass dimensions. These bulk glass-forming alloys exhibit the lowest density (<2 g cm−3) and highest Mg content (86 at.%) of all Mg-based bulk metallic glasses reported to date. Here, the specific alloy design method and a range properties of these amorphous alloys are revealed.

Structure and triclustering in Ba-Al-O glass

Glass-forming ability in the (BaO)${}_{x}$(Al${}_{2}$O${}_{3}$)${}_{1\ensuremath{-}x}$ system ($0\ensuremath{\le}x\ensuremath{\le}1$) was investigated by using the containerless aerodynamic levitation and laser-heating method. The main glass-forming region was found to occur for 0.40(2) $\ensuremath{\le}x\ensuremath{\le}$ 0.48(2), where there is insufficient oxygen to form an ideal network of corner-sharing AlO${}_{4}$ tetrahedra in which the oxygen atoms are twofold coordinated, with another narrow glass-forming region at $x$ $=$ 0.62(2) around the eutectic composition. The glass corresponding to $x$ $=$ 0.4 was chosen for further investigation by using both neutron and x-ray diffraction, and a detailed atomistic model was built by applying a combination of molecular dynamics and reverse Monte Carlo methods. The results show a network structure based predominantly on corner-sharing tetrahedral AlO${}_{4}$ motifs in which triclusters (OAl${}_{3}$ units formed by three tetrahedral Al atoms sharing a common vertex) play an integral part, with as many as 21$%$ of the oxygen atoms involved in these configurations. The barium ions bind to an average of 7.4 O atoms, most of which are twofold-coordinated bridging oxygen atoms. The larger size of barium compared to calcium narrows the range of glass-forming compositions in alkaline-earth aluminates such that the main glass-forming range corresponds to a regime in which an oxygen-deficient Al-O network is stabilized by the formation of triclusters.

Metallic glass-forming composition range of the Cu–Zr–Ti ternary system determined by molecular dynamics simulations with many-body potentials

Abstract

Microscopic origin of slow dynamics at the good glass forming composition range in Zr1−xCux metallic liquids

We have studied the structure and dynamics of a series of Zr1−xCux metallic liquids at temperatures of 1500 and 1300 K. We found that as the Cu composition increases, the Zr–Zr order undergoes considerable transition, which can be attributed to the different size of Zr and Cu atoms. The diffusivities of both Cu and Zr atoms become lower at the composition range corresponding to good glass forming region, i.e., 0.50≤x≤0.70. We found that the lower diffusivities for the intermediate composition range are correlated with the concentration of some specific clusters, including icosahedra, which can be screened out automatically and unbiasedly by using direct atomic dynamics analysis. Our analysis for the dynamic properties of high temperature liquids of ZrCu metallic alloys shows that the icosahedral clusters together with some other pentagon-rich Voronoi clusters are responsible for slowing dynamics. Cluster lifetime analysis indicates that the slow clusters mobility could be originated in their energy.

Amorphous phase formation, spinodal decomposition, and fractal growth of nanocrystals in an immiscible Hf–Nb system studied by ion beam mixing and atomistic modeling

In the equilibrium immiscible Hf–Nb system characterized by a positive heat of formation, five Hf–Nb metallic glasses with overall compositions of Hf84Nb16, Hf65Nb35, Hf45Nb55, Hf38Nb62, and Hf20Nb80 are obtained by ion beam mixing with properly designed Hf–Nb multilayered films, suggesting a glass-forming composition range of 16–80 at.% of Nb. For the special case of Hf45Nb55 located at the ridge point on the convex free energy curve, dual-glass phases are formed at a dose of 2×1015 Xe+/cm2, which results from a spinodal decomposition of the expected Hf45Nb55 amorphous phase. With increasing irradiation dose, fractal growth of nanocrystals (around 20 nm) appears in the major glass phase and the dimension is determined to be from 1.70 to 1.84 within a dose range of (4–7)×1015 Xe+/cm2. In atomistic modeling, a n-body Hf–Nb potential is first constructed with the aid of ab initio calculations. Applying the constructed potential, molecular dynamics simulations using the hcp and bcc solid solution models, reveals an intrinsic glass-forming range to be within 15–83 at.% of Nb, which is compatible with the ion beam mixing experiments. Moreover, the formation of the metallic glasses and the fractal growth in association with the amorphous spinodal decomposition are also discussed in terms of the atomic collision theory and cluster-diffusion-limited-aggregation model.

Ionic conductivity of glasses in the MI + M 2S + (0.1Ga 2S 3 + 0.9GeS 2) system (M = Li, Na, K and Cs)

In this study, the base 0.1Ga 2S 3 + 0.9GeS 2 was chosen because it was found that it could be combined with all of the alkali sulfides to yield glasses over the same relatively wide glass-forming composition range. Both M 2S and MI, where M = Li, Na, K and Cs, were added to form broad glass-forming composition ranges to systematically study the ionic conductivity in these ionic conducting chalcogenide glasses as a function of the alkali ion radius. As expected, additions of both alkali sulfide (M 2S) and alkali iodide (MI) increase the conductivity of the glass. For the 0.6M 2S + 0.4(0.1Ga 2S 3 + 0.9GeS 2) glasses, however, the conductivity only increases slightly with the addition of NaI. The ionic conductivities show significant differences between the Li 2S, Na 2S and K 2S, Cs 2S glasses due to the large differences in their activation energies. The alkali radius effect on the activation energy was analyzed and discussed based on the modified Anderson and Stuart model.

Glass forming ability: Miedema approach to (Zr, Ti, Hf)–(Cu, Ni) binary and ternary alloys

Miedema’s approach has been useful in determining the glass forming composition range for a particular alloy system. The concept of mixing enthalpy and mismatch entropy can be used in order to quantify Inoue’s criteria of bulk metallic glass formation. In the present study, glass forming composition range has been determined for different binary and ternary (Zr, Ti, Hf)–(Cu, Ni) alloys based on the mixing enthalpy and mismatch entropy calculations. Though copper and nickel appear next to each other in the periodic table, the glass forming ability of the copper and nickel bearing alloys is different. Thermodynamic analysis reveals that the glass forming behaviour of Zr and Hf is similar, whereas it is different from that of Ti. The smaller atomic size of Ti and the difference in the heat of mixing of Ti, Zr, Hf with Cu and Ni leads to the observed changes in the glass forming behaviour. Enthalpy contour plots can be used to distinguish the glass forming compositions on the basis of the increasing negative enthalpy of the composition. This method reveals the high glass forming ability of binary Zr–Cu, Hf–Cu, Hf–Ni systems over a narrow composition range.

Homogeneous–inhomogeneous models of Ag x(Ge 0.25Se 0.75) 100−x bulk glasses

Ge–Se system presents an extensive glass forming composition range even when different metals (Ag, Sb, Bi) are added. In spite that the addition of Ag (up to 30 at%) to Ge–Se does not affect substantially the glass forming tendency, it impacts significantly on the transport properties. (Ge 0.25Se 0.75) 100− x Ag x is a fast ionic conductor with x⩾8 at% whereas it is a semiconductor for x<8 at%. This behavior is also reported in other chalcogenide systems containing Ag. One of the structural models proposed to explain the relation between transport and structure is based on an intrinsically inhomogeneous structure where zones rich in metals coexist with zones of the host material. Field effect scanning electron microscopy (FE-SEM) is performed on amorphous (Ge 0.25Se 0.75) 100− x Ag x bulk samples. These results appear to sustain this model. However previous structural and thermal studies oppose it. Mössbauer spectrometry on samples (0⩽ x⩽25) containing 0.5 at% of 57Fe is performed at T⩽300 K. The main contribution to the glasses spectra correspond to low spin Fe 2+ in octahedral coordination and high spin Fe 2+ in distorted octahedral environments. The relative population of both sites changes continuously as Ag concentration varies denoting that the change in the transport behavior obeys to a percolation phenomenon. The low temperature results are discussed with the aim to throw light on the controversy about the homogeneity–inhomogeneity of the studied bulk glasses.

Competitive formation of glasses and glass–matrix composites

By systematically investigating the effect of chemical composition on the competitive formation of glasses in various systems, we attempt to address two long-standing scientific puzzles upon metallic glasses, i.e., (i) which composition is the best for forming glasses and glass–matrix composites and (ii) what determines the easy glass-forming composition range in a given alloy system. Our findings have led to the construction of a qualitative microstructure selection map, which is useful for guiding the design of bulkier metallic glasses and glass–matrix composites. In addition, our analysis demonstrates that the classical kinetic treatment of glass formation is insufficient; to analyze glass formation properly, it is necessary to go beyond simple assumptions of single polymorphic solidification during crystallization.

Glass formation and phase separation in the Ag–Cu–Zr system

Glass formation and phase separation in Ag–Cu–Zr alloys have been investigated by energy-filtered transmission electron microscopy to discuss the possibility of phase separation in bulk metallic glasses. The experimentally determined glass forming composition range overlaps marginally with the calculated miscibility gap. As-quenched amorphous Ag20Cu48Zr32 alloy shows a composition contrast in a fully amorphous structure. Subsequently crystallized microstructure shows a good size correlation with the prior compositional fluctuations, indicating preferred nucleation in one of the phase-separated regions.

Formation and properties of Pr-based bulk metallic glasses

The data on the compositional dependence of glass-forming ability, glass transition, and properties of bulk metallic glasses (BMGs) are important for understanding the nature and glass-forming ability of metallic glasses and for their application. In this paper, we report the formation of rare-earth-based Pr–(Cu,Ni)–Al pseudo-ternary BMGs with a large bulk glass-forming composition range and distinct glass transition. The compositional dependence of glass-forming ability, glass transition, and properties were systematically studied. The contrasting effects of Al and Pr on glass formation and glass transition, unique elastic properties, and phonon softening of the BMGs are discussed from the structural point of view.

Leaching of Lead and Connectivity of Plumbate Networks in Lead Silicate Glasses

The formation of a plumbate network in binary lead silicate glasses was examined based on the leaching behavior of Pb2+ in lead silicate glasses over a wide composition region. The effective diffusion coefficient of Pb2+ at 40°C was on the order of 10−17 m2/s for PbO&lt;35 mol% glasses, and increased three orders of magnitude for 35–50 mol% PbO contents. Such a steep composition dependence is considered to be because of changes in the medium or longer range structure. That is, it is proposed that the plumbate network forms a percolative 3D network in the composition region to form diffusion paths for the lead ions. In addition, the present results indicate that the lead ion exists as a network former over the entire glass forming composition range of the binary silicate glasses.

Structural Study on PbO–SiO2 Glasses by X‐Ray and Neutron Diffraction and 29Si MAS NMR Measurements

Structure of xPbO–(100−x)SiO2 (x=25–89) glasses has been investigated by means of the X‐ray and neutron diffraction and 29Si MAS NMR measurements. In the radial distribution functions of all the glasses, the Pb–O correlation was observed at 0.23 nm, indicating that the PbO3 trigonal pyramids units do exist in the whole glass forming composition range. Furthermore the existence of the first Pb–Pb correlation at ∼0.385 nm in the whole composition range suggests that the basic structural unit is considered to be a Pb2O4 unit, which consists of the edge‐shared PbO3 trigonal pyramids. These results strongly imply that the Pb2O4 units participate in the glass network constructed by SiO4 tetrahedra even at low PbO content. Differing from other lead‐containing glass systems, these structural characteristics of Pb ions in the PbO–SiO2 glass system are responsible for the extremely wide glass‐forming region.

Excellent glass-forming ability in simple Cu 50Zr 50-based alloys

On the basis of the novel binary CuZr bulk metallic glass (BMG), we report the excellent glass-forming ability (GFA) in the simple Cu 50Zr 50-based BMG-forming alloys with minor Al addition. When Al addition changes from 4 to 8 at.%, Cu 50Zr 50-based BMGs can be produced to cylindrical rods with a diameter of at least 5 mm. The alloys have a broad glass-forming composition range with the excellent GFA. The elastic moduli were derived from the ultrasonic and density measurements. The physical origin of this unique GFA in the alloys is investigated. Our results might provide an effective route for exploring new BMGs.

Binary Cu–Zr Bulk Metallic Glasses

We report that bulk metallic glasses (BMGs) can be produced up to 2 mmby a copper mould casting in CuxZr1−x binary alloy with awide glass forming composition range (45<x<60 at.%). We find thatthe formation mechanism for the binary Cu–Zr binary BMG-forming alloy isobviously different from that of the intensively studied multicomponentBMGs. Our results demonstrate that the criteria for the multicomponentalloys with composition near deep eutectic and strong liquid behaviourare no longer the major concern for designing BMGs.

Experimental and Theoretical Studies on Composition Limits of Metallic Glass Formation in the Ni-Mo System

The Ni-Mo system was selected as a model system to study the composition limits of metallic glass formation by ion mixing experiments as well as by molecular dynamics simulation. The 200 keV xenon ion mixing was conducted for ten Ni-Mo mutilayered samples with various concentrations and the results showed that the glass-forming composition range was 39.8-80.4 at.% Mo, which is in good agreement with the assessment from an empirical model. Through comparing the stability of the Ni-rich and Mo-rich solid solutions with various concentrations versus their amorphous counterparts, two critical supersaturated solid solubilities were found by molecular dynamics simulation to be 21 at.% Mo in Ni and 25 at.% Ni in Mo, from which the glass forming range was deduced to be within 21-75 at.% Mo. The correlation of the simulated glass-forming range with those obtained by ion irradiation and solid-state reaction is also discussed.

Amorphous forming ability in the ternary Cu–Sn–P system by mechanical alloying

Highly pure (99.9%) copper, tin and phosphorus powders of 250 mesh were thoroughly mixed to obtain the required ternary Cu86−xSnxP14 (x=2–15) alloy compositions. These mixtures were milled in a planetary ball mill. The powder samples at different intervals of milling were characterized by X-ray diffraction (XRD) and transmission electron microscope (TEM). It has been found that a powder mixture of Cu84Sn2P14 amorphized after 28 h of mechanical alloying, but still some crystalline phase could be seen. When the period of milling was greater than 28 h, the intensity of the crystalline Bragg reflexes increased with further milling. The period of complete amorphization for Cu86−xSnxP14, x=4, 5, 8 and 10 are 28, 20, 12 and 32 h, respectively. However, no typical amorphous phase was detected for the Cu71Sn15P14 mixture powders. The results show that the amorphization tendency increases first then decreases with increase of tin content, but no amorphous phase forms after the content of tin is greater than 15 wt%. The glass forming composition range in this alloy system is 4–10 wt% Sn. The amorphous forming ability for the ternary system is explained by the formation theory of amorphous alloys by one of the present authors. The formation enthalpies of the system are also calculated using Miedema's model, in order to explain the glass forming composition range.