Main α relaxation and slow β relaxation processes in a La30Ce30Al15Co25 metallic glass

Dynamic relaxation processes are fundamental to understand the mechanical and physical properties of metallic glasses. In the current work, mechanical relaxations in a La₃₀Ce₃₀Al₁₅Co₂₅ bulk metallic glass were probed by dynamic mechanical analysis. In contrast to many metallic glasses, La₃₀Ce₃₀Al₁₅Co₂₅ metallic glass shows a pronounced slow β relaxation peak. Physical aging below the glass transition temperature T_g leads to an increase of the apparent activation energy and a decrease of the slow β relaxation magnitude. The correlation between the slow β relaxation and the main α relaxation is discussed.     

Metal induced effects on some physical properties of Se0.8Te0.2 amorphous system

Thermally evaporated selenium and tellurium-based thim film glasses were carefully characterized to establish the interdependence between their chemical composition and some physical parameters, such as density and optical energy gap.The effects of different metallic additions and annealing at a temperature below the glass transition temperature T_g on the optical characteristics was explored. The optical energy gap E_opt was found to decrease with the addition of metal, the extent of the diminution dependings on the chemical character of the added metal.     

Thermal rejuvenation in metallic glasses

Abstract

Structural rejuvenation in metallic glasses by a thermal process (i.e. through recovery annealing) was investigated experimentally and theoretically for various alloy compositions. An increase in the potential energy, a decrease in the density, and a change in the local structure as well as mechanical softening were observed after thermal rejuvenation. Two parameters, one related to the annealing temperature, T_a/T_g, and the other related to the cooling rate during the recovery annealing process, V_c/V_i, were proposed to evaluate the rejuvenation phenomena. A rejuvenation map was constructed using these two parameters. Since the thermal history of metallic glasses is reset above 1.2T_g, accompanied by a change in the local structure, it is essential that the condition of T_a/T_g ≥ 1.2 is satisfied during annealing. The glassy structure transforms into a more disordered state with the decomposition of icosahedral short-range order within this temperature range. Therefore, a new glassy structure (rejuvenation) depending on the subsequent quenching rate is generated. Partial rejuvenation also occurs in a Zr₅₅Al₁₀Ni₅Cu₃₀ bulk metallic glass when annealing is performed at a low temperature (T_a/T_g ~ 1.07) followed by rapid cooling. This behavior probably originates from disordering in the weakly bonded (loosely packed) region. This study provides a novel approach to improving the mechanical properties of metallic glasses by controlling their glassy structure.     

Electrical permittivities and subglassy relaxations in sulphate glasses

The dielectric behaviour of K₂SO₄-ZnSO₄ glasses have been investigated. These glasses exhibit subglassy relaxations close to glass transition temperature. The relative dielectric permittivities exhibit sharp rise nearT _g which may be due to loosening of glass constituents leading to ionic dipolar or electrode polarisation contributions.     

The effect of nitrogen on viscosity of La-Si-Mg-O-N glasses by compressive creep and dilatometry

Viscosity as a function of temperature and nitrogen content and the glass transition temperature (T_g) were investigated in 20La-60Si-20Mg-O-N glasses with nitrogen contents varying from 0 to 28 eq.% (e/o) using compressive creep and dilatometric experiments. Although T_g obtained from dilatometry were 6–12°C lower than the lower limit of the transition temperature range from creep, both methods revealed identical and linear dependencies of T_g on N content. The average activation energy was 1184 ± 36 kJ/mol and viscous flow remains the deformation mechanism in all glasses regardless of nitrogen content. Addition of 28 e/o N in oxide glass resulted in an increase of T_g by 94–105°C and an increase in viscosity of around 5 orders of magnitude. These changes are greater than those reported in similar Al-containing glasses. Linear increase of T_g and compactness of the glass with nitrogen content result from enhanced cross-linking of the glass network.     

Synthesis, IR, crystallization and dielectric study of (Pb, Sr)TiO 3 borosilicate glass–ceramics

Eleven glass compositions were prepared by melt and quench method with progressive substitution of SrO for PbO (0?≤?x?≤ 1·0) with a step-wise increment of 0·10 in the glass [(Pb _x Sr_1???x )OTiO₂]–[(2SiO₂B₂O₃)]–[BaO·K₂O]·Nb₂O₅ (mol percentage) system. The infrared spectra (IR) of various glass compositions in the above mentioned glass system was recorded over a continuous spectral range 400–4000 cm^???1 to study their different oxides structure systematically. Differential thermal analysis (DTA) was recorded from room temperature (~27 °C) to 1400 °C employing a heating rate of 10 °C /min to determine glass transition temperature, T _g and crystallization temperature, T _c. The melting temperature, T _m, of these glass compositions was found to be in the range 597–1060 °C depending on the composition under normal atmospheric conditions. T _g and T _m of glasses were found to increase with increasing SrO content. X-ray diffraction analysis of these glass–ceramic samples shows that major crystalline phase of the glass–ceramic sample with x ≤ 0·5 was found to have cubic structure similar to SrTiO₃ ceramic. Scanning electron microscopy has been carried out to see the surface morphology of the crystallites dispersed in the glassy matrix.     

Synthesis and properties of GeS2-Ga2S3-PbI2 chalcohalide glasses

The glass-forming region in the GeS₂-Ga₂S₃-PbI₂ system was determined and the basic parameters of thermal and optical properties (glass transition temperature, density, microhardness and transmission window) for these glasses have been measured. Better thermal stability originated from their larger difference between T_x and T_g in the range of 107-161 °C, higher glass transition temperatures between 252 and 398 °C and wide optical transmission window from 0.5 to 12.7 μm make these glasses the promising candidate materials for rare earth doped fiber amplifiers and nonlinear optical devices.     

Amorphous phase separation of ionomer glasses

Ionomer glasses of generic composition SiO₂-Al₂O₃-P₂O₅-CaO-CaF₂ were studied using differential scanning calorimetry (DSC) and high temperature dynamic-mechanical thermal analysis (DMTA). High temperature DMTA was used to measure the glass transition temperatures (T _g) of the original starting glass compositions, as well as being able to follow amorphous phase separation (APS) within the glass. High temperature DMTA traces of all the glasses studied exhibited two maxima in tan . These maxima correspond to two glass transition temperatures and demonstrate that amorphous phase separation of the parent glass into two glass phases had occurred. A DMTA study of a Sodium-Boro-Silicate glass, which is known to undergo amorphous phase separation yielded similar results. DSC studies showed that the ionomer glasses underwent a nucleation process at temperatures just above the glass transition temperature which is probably associated with APS. The glasses exhibited optimum nucleation temperatures which moved to lower temperatures with longer hold times indicating the time dependency of the APS process.     

Metallic glass formation in the binary Cu–Hf system

Glass formation, structure and thermal properties of alloys in the binary Cu_100?x Hf _x alloy system, where x = 25–50 at.%, are reported and discussed. This work also presents a comparison between copper casting techniques, from thick melt-spun ribbons to suction cast cylindrical rods, and the prediction of critical diameter, d _c, based on maximum ribbon thickness, x _c. Ribbons of Cu₆₀Hf₄₀ and Cu₆₅Hf₃₅ exhibited a fully glassy phase up to a thickness of 170 μm. Suction casting lead to an increase in the largest diameter over which both alloys could be cast, in comparison to melt-spun ribbons, and remain amorphous, with Cu₆₅Hf₃₅ showing a large critical diameter of 1 mm. This result is rationalised by a lower liquidus temperature, T _l, which maximises the reduced glass transition temperature, T _rg, and also correlates closely with the eutectic point. Finally, there were remarkable similarities between the Miedema model and the efficient packing model for predicting the range for metallic glass formation in this binary system.     

The dynamic shear response of the Zr base bulk metallic glass around the calorimetric glass transition temperature

The dynamic shear response of a bulk Zr₅₅Cu₂₅Ni₅Al₁₀Nb₅ metallic glass has been investigated in the vicinity of calorimetric glass transition temperature, T_g. Similar to other glass formers, the Zr base bulk metallic glass exhibits typical features of the dynamic glass transition. The intermediate fragility index, m, indicates that the Zr base alloy belongs to the category of relatively strong glass formers in the general classification scheme. On the other hand, the time temperature equivalence principle is found to be applicable for constructing the master curve of dynamic shear modulus as well as shear viscosity over the temperature range studied. Theses master functions can be fitted with phenomenological models, such as generalized Maxwell model, or a Kohlrausch-Williams-Watts (KWW) relation, and therefore reflect a broad distribution of local structural relaxation time for dynamic glass transition in the Zr base bulk metallic glass.     

Thermoanalytical study of some fluoroberyllate glasses

Some glasses in the BeF₂-KF-CaF₂-AlF₃ (BeF₂ = 50 mol%) system were examined by thermoanalytical methods (DSC, TG) and powder X-ray diffraction to obtain glass transition and crystallization temperatures. Values of T_g varied from 165 to 265°C as the KF content decreased from 40 to 12.5 mol%. Those glasses with KF contents ? 30 mol% exhibited crystallization exotherms in the region of 280–300°C. The ability to clearly establish T_c decreased markedly for the better glass forming composition. Various phases crystallizing in these glasses as a function of temperature and time of heat treatment were identified.     

Glass transition temperature of glasses in the SiO2-Na2O-CaO-P2O5-Al2O3-B2O3 system

A total of 24 glasses in or near the bioactive region in the system SiO₂-Na₂O-CaO-P₂O₅-Al₂O₃-B₂O₃ were studied. By differential thermal analysis their glass transition temperatures,T _g, were determined. On basis of an experimental plan for 16 glasses, two phenomenological equations describing the relationship betweenT _g and glass composition were developed. The equations describeT _g within the compositional ranges: SiO₂, 38.0–65.5 Na₂O, 15.0–30.0; CaO, 10.0–25.0; P₂O₅, 0–8.0; B₂O₃, 0–3.0; Al₂O₃, 0–3.0 wt%. The glass transition temperature shows a linear dependence of the Na₂O content. The higher the Na₂O content, the lower theT _g. Compositional alterations not including Na₂O influencesT _g little in comparison with changes in the Na₂O content.     

Effects of BaO on the glass formation of the PbO-B2O3-TiO2-BaO system in relation to transition temperatures

The variation of the BaO content on the quaternary PbO-B₂O₃-TiO₂-BaO system's glass formation tendency was investigated in relation to transition temperatures, such as melt temperature (T _f), liquidus temperature (T _l), crystallization temperature (T _c) and glass transition temperature (T _g). Compositions were melted between 800°C and 1300°C. In order to obtain bulk glass samples, glass formation was carried out using a preheated cylindrical brass mould without forcing the cooling rate to increase. Glass formation tendency increased with increasing the temperature ratios of T _g/T _l and T _c/T _l and with decreasing T _g/T _c. Eutectic compositions preferentially formed glasses from melts due to their low melting temperatures.     

An investigation into the amorphous phase separation characteristics of an ionomer glass series and a sodium-boro-silicate glass system

Amorphous phase separation of ionomer glasses, also known as fluoro-phospho-alumino-silicate glasses, of generic composition SiO₂-Al₂O₃-P₂O₅-CaO-CaF₂ were investigated. A sodium-boro-silicate glass system, which is known to undergo amorphous phase separation was also investigated. High Temperature Dynamic-Mechanical Thermal Analysis (DMTA), combined Differential Thermal Analysis/Thermo Gravimetric Analysis (DTA/TGA) and Scanning Electron Microscopy (SEM) were the principal analytical techniques used in this study. High temperature DMTA was used to measure the glass transition temperatures (T _g) of the original starting glass compositions, as well as being able to follow amorphous phase separation (APS) within the glass. High temperature DMTA traces of both the ionomer glasses and the sodium-boro-silicate glasses exhibited two maxima in tan , corresponding to two glass transition temperatures and demonstrating that amorphous phase separation of the parent glass into two glass phases had occurred. DTA/TGA of the ionomer glasses detected a glass transition and two crystallisation peaks for apatite and mullite, accompanied by a gradual weight loss of 1–3% on passing through the crystallisation region. The sodium-boro-silicate base glass showed no evidence of a glass transition, but a prominent glass transition was detected for a second sample which had undergone a heat-treatment of 240 min at 580°C. SEM analysis of the ionomer glass compositions revealed smooth spherical droplets of 2–15 nm while the background morphology appeared rough and speckled. A classic interconnected structure was observed for the sodium-boro-silicate glass.     

Cu-based metallic glass particle additions to significantly improve overall compressive properties of an Al alloy

We report the development of a novel light-weight Al (520) alloy-based composite reinforced with particles of a Cu-based (Cu₅₄Zr₃₆Ti₁₀) metallic glass by mechanical milling followed by induction heated sintering. The consolidation of the composite is performed at a temperature in the super-cooled liquid region of the metallic glass just above its glass-transition temperature (T_g). Metallic glasses are a promising alternative reinforcement material for metal-matrix composites capable of producing significant strengthening along with a «friendly» sintering behavior. The mechanical milling procedures were properly established to allow reduction of the size of the metallic glass particles and their uniform distribution in the matrix. Microstructural observation of the composite did not reveal any porosity. The interface between the glassy particles and the matrix remained free of such defects. The fully dense consolidated composite showed a drastic gain in specific yield strength under compression relative to the matrix alloy and appreciable plasticity at fracture.     

Mechanical property degradation of a CuZr-based bulk metallic glass composite induced by sub-Tg annealing

The annealing effect at temperatures below glass transition temperature (T_g) on the mechanical behavior of a ductile Cu₄₈Zr₄₈Al₄ bulk metallic glass composite (BMGC) containing a B2-CuZr phase was investigated. It was found that remarkable plasticity and fracture strength degradations of the CuZr-based BMGC occurred with increasing the annealing temperature in the condition without significantly changing the crystalline volume fraction in the BMGC, which were correlated with the annealing-induced microstructure variations. It is expected that the free volume in the glassy matrix of the BMGC still played an important role for its deformation behavior, in despite of the existence of the B2-CuZr phase. The sub-T_g annealing-induced free volume annihilation depressed the shear band generation in the glassy matrix, reduced the synchronous contribution of the “blocking effect” and “deformation-induced martensitic transformation effect” of the B2-CuZr phase to the multiplication of shear bands, resultantly caused the plasticity degradation. The annealing-induced martensitic transformation of the B2-CuZr phase at the temperature close to T_g would further expand the plasticity degradation due to the absence of the “deformation-induced martensitic transformation effect”. Furthermore, the plasticity degradation simultaneously resulted in the fracture strength reduction of the BMGC because its work-hardening-like behavior was conditioned by the plastic deformation ability. The present results indicate that the ductile CuZr-based BMGC reinforced by the B2-CuZr phase similarly suffers from sub-T_g annealing-induced embrittlement, as is the case for most monolithic BMGs.     

Calorimetric investigations on As-Te-Se and Ge-As-Se glasses

Results of calorimetric investigations performed on two groups of glasses, viz. As-Te-Se and Ge-As-Se, are described. The glass transition temperatureT _g and specific heat at constant pressureC _p of these two families of glasses have been determined using a differential scanning calorimeter. The composition dependence of glass transition temperature and variation of heat capacity during transition are discussed.     

A critical study of the emergence of glass and glassy metals as “green” materials

In 2008, The National Academy of Engineering (NAE – Washington, DC) identified the glass family (glasses, glass ceramics and glass composites) as central to many of the great engineering achievements of the twentieth century: the development of solid state lasers and optical glass fibers, biomaterials, glasses for imaging technologies, and glass films in microelectronic devices.The work reported in this paper discusses the importance of glass and metallic glasses as environmental friendly materials and also provide some points of view about the future influence of these materials for the related fields of industrial engineering and industrial ecology. The environmental capabilities of metallic glasses (MGs), which are considered to be among the important materials of the future, have not been sufficiently investigated. However, some aspects have yet to be done: the biocompatibility of most MGs, obtaining valuable MGs from waste materials, using MGs in green energy applications (solar cells and hydrogen production), using MGs in catalyst systems, as well as the possibility for using MGs in systems for retention and purification of dangerous pollutants and in the nuclear industry.     

Compositional effects on the optical and thermal properties of potassium aluminophosphate glasses

The K₂O–Al₂O₃–P₂O₅ glass system has been examined and various compositions have been melted. Their optical and thermal properties have been measured to assess their potential for athermalisation. The addition of alumina (Al₂O₃) increases the refractive index (n) and glass transition temperature (T_g) and decreases the thermal expansion coefficient (α), consequently leading to positive thermo-optic coefficient (dn/dT). In addition to thermal expansion, polarisability of the glass also affects dn/dT. Generally, glasses must exhibit negative dn/dT to counter thermal expansion, in order to have potential application in athermalisation.     

Synthesis and properties of new CdSe-AgI-As2Se3 chalcogenide glasses

The glass-forming region in the pseudo-ternary CdSe-AgI-As₂Se₃ system was determined. Measurements including differential scanning calorimetry (DSC), density, and X-ray diffraction were performed. The effect resulting from the addition of CdSe or AgI has been highlighted by examining three series of different base glasses. The characteristic temperatures of the glass samples, including glass transition (T_g), crystallisation (T_x), and melting (T_m) temperatures are reported and used to calculate their ΔT = T_x − T_g and their Hruby, H_r = (T_x − T_g)/(T_m − T_x), criteria. Evolution of the total electrical conductivity σ and the room temperature conductivity σ₂₉₈ was also studied. The terahertz transparency domain in the 50-600 cm⁻¹ region was pointed for different chalcogenide glasses (ChGs) and the potential of the THz spectroscopy was suggested to obtain structural information on ChGs.