The correlation between the pressure sensitivity and the fragility/glass transition temperature in bulk metallic glasses

The correlations between the pressure sensitivity and the fragility/glass transition temperature have been addressed in various bulk metallic glasses in the present work. The results demonstrate that the pressure sensitivity of bulk metallic glasses is closely related to both the fragility index (m) and the glass transition temperature (T_g). The physical origin of the correlations has been discussed from their disordered structure, which is determined by the glass transition behavior and the glass transition temperature.     

Indentation response of glass with temperature

The mechanical response of different glasses to a Vickers indentor has been investigated between room temperature and T_g+50 °C. The permanent deformation, from which hardness is estimated, as well as the brittle fracture characteristics, allowing for an evaluation of the fracture toughness, were measured and analysed. Comparison between a standard float glass and advanced glasses such as chalcogenide (with mainly covalent bonding) and metallic glasses was made to get a more general insight into high temperature indentation behaviour. As temperature increases, the glass response becomes more and more time-dependent, and in the vicinity of T_g the permanent deformation was observed to increase rapidly for all glasses. Further, while the standard float glass showed an enhanced apparent toughness at elevated temperatures due to a brittle to ductile transition, almost no change in apparent toughness was revealed in the GeAsSe glass emphasizing the time-dependent response of glass at elevated temperature.     

Characterization of the elasticity and anelasticity of bulk glasses by dynamical subresonant and resonant techniques

The determination of the elastic and anelastic characteristics by means of original non-destructive techniques has been applied to glasses and glass composites in order to link together the macroscopic data with structural aspects. The dynamical Young’s modulus determined by a free resonance technique allows a good accuracy measurement. Some examples concerning oxides, Ge(As)Se or metallic glasses are presented: the abrupt drop of the modulus in the range of the glass-transition temperature T_g is a general observation, which leads us to an attempt at normalization of the curves E versus T on master curves E/E(T_g) versus T/T_g. To study the viscoelastic properties, a low frequency torsional spectrometer is preferentially used to measure the damping due to viscous movements at a microscopic scale. A study of MgSiAlON glasses allows us to show that the intrinsic activation energy is much smaller than the one measured by creep or relaxation tests and that the glassy transition is characterized by a smooth change from vitreous solid (highly correlated) to quasi-liquid behavior; this has been confirmed on a metallic glass.     

Composition dependence of the glass transition temperatures of PdNiP and PtNiP glasses

Thermal properties of glassy PdNiP and PtNiP alloys have been measured as a function of the concentration of transition metals. The glass transiion temperature, T_g, of these alloys glasses exhibits a negative linear deviation with transition metal content - which is in contrast to the increasing T_g of binary glassy alloys with increasing metalloids.It is suggested that the suppression of the glass transition temperature of these glassy alloys may be attributed to the excess configurational entropy of disorder associated with a mixture of hard spheres differing in radius. In contrast, the increasing T_g of binary glassy alloys with the metalloid content may be associated with the short-range order resulting from strong interactions between metal and metalloid atoms.     

Alloying effect on the viscous flow in metallic glasses

The temperature dependence of viscosities near the glass transition is measured from the rates of thermal transformation for metallic glasses PtNiP, PdNiP, NiPBA1 and (Fe, Co)PBA1. Alloying among metallic elements which lowers the glass transition temperature T_g lowers the ideal glass transition temperature T₀, but raises the residual configurational entropy S_g and the activation energies for “diffusive” rearrangement, $\text{Δμ}{}^1$, of the alloying glasses, while compositional ordering associated with the addition of metalloids raises the T_g and T₀ and lowers the S_g and $\text{Δμ}{}^1$. Results are correlated to the atomic ordering and stability of the glasses. The extracted free volume and the critical diffusive volume are much smaller, by a factor of 4, for metallic glasses than for many other glasses.     

Correlation between dynamic fragility and glass transition temperature for different classes of glass forming liquids

Here we compile literature data for dynamic fragility m for six types of glass forming liquids: polymers, small molecule organics, hydrogen bonding organics, inorganics, ionic and metallic glass formers. Our analysis of the data shows that different categories of glass forming liquids exhibit different behaviors in terms of the correlation between m and T_g, a correlation not previously examined. For example, for hydrogen bonding organics, polymeric and metallic glass formers, there is an approximately linear increase in m with increasing T_g. While for inorganic glass formers, m appears almost independent of T_g, remaining nearly constant over a wide range in T_g. At the same time, another important parameter, the apparent activation energy E_g at T_g has been investigated. It was found that E_g increases with T_g to the 2nd power for hydrogen bonding organics, polymeric and metallic glass forming liquids, while E_g of inorganic glasses has a linear dependence on T_g.     

Structure and properties of Ni60(Nb100−xTax)34Sn6 bulk metallic glass alloys

Refractory bulk metallic glasses and bulk metallic glass composites are formed in quaternary Ni-Nb-Ta-Sn alloy system. Alloys of composition Ni₆₀(Nb_100−xTa_x)₃₄Sn₆ (x = 20, 40, 60, 80) alloys were prepared by injection-casting the molten alloys into copper molds. Glassy alloys are formed in the thickness of half mm strips. With thicker strips (e.g., 1 mm), Nb₂O₅ and Ni₃Sn phases and the amorphous phase form an in situ composite. Glass transition temperatures, crystallization temperatures, and ΔT_x, defined as T_x1 − T_g (T_x1: first crystallization temperature, T_g: glass transition temperature) of the alloys increase dramatically with increasing Ta contents. These refractory bulk amorphous alloys exhibit high Young’s modulus (155-170 GPa), shear modulus (56-63 GPa), and estimated yield strength (3-3.6 GPa).     

A new criterion for the glass-forming ability of liquids

Based on theoretical calculations using the fragility concept and the nucleation theory for a model glass-forming system, we propose a dimensionless criterion, ?, expressed by T_rg(ΔT_x/T_g)^a, with T_rg, the reduced glass-transition temperature, ΔT_x, the width of the supercooled liquid region when heating a glass, T_g, the glass transition temperature, and a, the exponent. The application of this simple criterion to various glasses, including network, metallic, and molecular glasses (except pure water), indicates an excellent correlation between the critical cooling rate R_c and ? in a Log R_c-? single master plot with a = 0.143.     

Thermophysical properties of Zr-Cu-Al metallic glasses during crystallization

The crystallization behavior of Zr-Cu-Al metallic glasses was studied using thermophysical property measurements. When the Zr content of Zr-Cu-Al metallic glass decreased from 65 at.% to 45 at.%, the thermal conductivity gradually increased and the maximum value obtained was the composition of Zr:Cu:Al = 50:39.3:10.7(at.%). These metallic glasses were not crystallized upon heat treatment below the glass transition temperature T_g, and the thermophysical properties of these metallic glasses were almost constant. In contrast, these metallic glasses started to crystallize upon heat treatment above T_g after a certain derived time, and their thermal conductivity increased with the crystallinity of the metallic glass.     

Characteristic temperatures of enthalpy relaxation in glass

The glass transition temperature, T_g, directly measured by differential scanning calorimetry at 10 K/min is compared with the T_g indirectly determined by fitting viscosity data to a viscosity model for oxide glasses. The results show good match between the two T_g values. A standard, unified approach for measuring T_g is proposed. Characteristic temperatures of enthalpy relaxation in glass are defined, and the relationships between these temperatures are illustrated by performing aging and calorimetric experiments on hyperquenched glasses. The features of the energy release peak, the endothermic pre-peak, and the real glass transition are discussed with respect to their physical origins.     

Thermal and some physical properties of glasses in the La2O3−CaO−B2O3 ternary system

Glasses in the La₂O₃−CaO−B₂O₃ ternary system were studied. The glass forming range as determined by the appearance of the annealed cast was found to match previously published findings. Clear glasses were formed in the composition range of 5.7−19.1 mol% La₂O₃ with constant B₂O₃ content of 71.4 mol%, and in glasses of constant La₂O₃:CaO ratio of 1:4 with B₂O₃ content in the range of 71.4-55.0 mol%. The non-linear optical crystalline phase La₂Ca₂B₁₀O₁₉ was crystallized from the clear glasses after heat treatments, as determined by powder XRD. Two types of the LaBO₃ crystalline phases were detected in the partially and the fully crystallized glass compositions outside the glass forming range. Data are reported for the glass transition temperature (T_g), dilatometric softening point (T_d), linear coefficient of expansion (α), onset crystallization temperature (T_x), exothermal peak temperature (T_P), density (ρ) and index of refraction (n_D) in the clear glasses.     

Mg based bulk metallic glasses: Glass transition temperature and elastic properties versus toughness

In this work, optimal compositions for bulk metallic glasses (BMGs) formation in the ternary Mg-Cu-Nd and Mg-Ni-Nd systems are located at the Mg₅₇Cu₃₄Nd₉ and Mg₆₄Ni₂₁Nd₁₅, respectively, with the critical diameter of 4 mm for the rods fabricated by copper mold casting. As indicated by notch toughness testing, Mg₆₄Ni₂₁Nd₁₅ BMG (K_Q = 5.1 MPa√m) manifests higher toughness with respect to the Mg₅₇Cu₃₄Nd₉ (K_Q = 3.6 MPa√m), even though both BMGs have similar compressive fracture strength of 870-880 MPa. Such an improvement in toughness for Mg BMGs correlates with the reduction of shear modulus and the enhancement of thermal stability to resist to the structural relaxation at room temperature, which is indicated by the elevated glass transition temperature T_g. Under the Mode I loading condition, morphology in fracture surface of the Mg₆₄Ni₂₁Nd₁₅ BMG varies along the crack propagation path. Fractographic evolution of the fracture surface follows the Taylor's meniscus instability criterion. For the Mg-based BMGs, shear modulus scales with the glass transition temperature, and can be expressed as μ = 4.7 + 625T_g/V_m[1-4/9(T/T_g)^2/3]. Meanwhile, correlation between the calorimetric T_g and elastic properties at T_g can be rationalized with Egami's model.     

Surface structure and properties of NiZr model metallic glasses: A molecular dynamics simulation

A systematic study of the surface structure and properties of NiZr model metallic glasses is reported using atomistic simulations. It is found that at low temperatures below the glass transition temperatures, the surface retains the amorphous structure and the surface energy γ is significantly lower (∼50%) than that of the corresponding crystalline alloy constituents. The variation of alloy concentration has little effect on γ; but increase in cooling rate and annealing temperature can lead to large decrease in γ. At elevated temperatures, γ increases with temperature and surface melting occurs at a temperature about 30% below T_g. At all temperatures up to T_g, the surface remains atomically smooth.     

Nanophase separation and effects on properties of Ge–As–Se chalcogenide glasses

Nanophase separation in the bulk Ge–As–Se chalcogenide glasses was observed by SEM and supported by XRD and IR measurements. Effects of nanophase separation on glass transition temperature (T_g), microhardness (H_v), optical band gap (E_opt) and thermal expansion coefficient (α) were investigated in terms of glass rigidity transitions. According to the correlations between the properties and average coordination number Z, it is established that nanophase separation becomes more intensive when Z is larger than 2.64.     

Structural behavior of amorphous and liquid metallic alloys at elevated temperatures

The thermal behavior of the short-range order of Pd₄₀Cu₃₀Ni₁₀P₂₀ bulk metallic glasses has been investigated in situ by means of high-temperature X-ray synchrotron diffraction. The dependence of the X-ray structure factor S(q) of the glassy state on temperature follows the Debye theory up to the glass transition. Above the glass transition temperature T_g, the temperature dependence of S(q) is altered toward a continuous development of structural changes in the liquid state with temperature. The behavior of the structure factor during heating and cooling through the glass transition gives experimental evidence for melting the glass, and for freezing the liquid, respectively at the caloric glass temperature.     

Properties and structure of (50 − x)BaO–xZnO–50P2O5 glasses

The thermal, mechanical, chemical properties and the structure of (50 − x)BaO–xZnO–50P₂O₅ (0 ⩽ x ⩽ 50 mol%) glasses were investigated. For these glasses, the density (ρ), glass transition temperature (T_g), dissolution rate (DR), ³¹P magic angle spinning nuclear magnetic resonance (MAS-NMR) spectra and Fourier-transformed infrared (FTIR) spectra were determined. As BaO was replaced by ZnO, all the properties were similarly decreased in density, Young’s modulus, T_g and water resistance. FTIR analyses revealed a shortening of phosphate chains by the shift of (P–O–P)_as band to a higher wave number owing to the substitution ZnO of BaO. The NMR spectra showed that the replacement of BaO by ZnO decreased the concentration of Q²-tetrahedral sites and increased that of Q¹-tetrahedral sites.     

Elastic constants,hardness and their implications to flow properties of metallic glasses

The longitudinal and transverse sound velocities and Vickers hardness of metallic glasses (Pd_{1 ? x}Ni_x)_0.80P_0.20, (Pd_{1 ? x}Fe_x)_0.80P_0.20 and (Pt_{1 ? x}Ni_x)_0.75P_0.25 have been measured. The elastic constants at room temperature exhibit a positive deviation with composition χ from linearity whereas the hardness shows a negative deviation. The increase in elastic constants has been attributed to a denser packing of the alloys on mixing. The reduced hardness H_rH/μ versus χ exhibits a remarkable similarity to a T_g versus χ relationship. This seems to indicate that flow mechanisms involved in metallic glasses above and below the glass-transition temperature are of similar origins. It is the excess entropy of disorder associated with alloying which lowers the hardness as well as the viscosity of metallic glasses. The metallic glasses possess in general a relatively high Poisson's ratio ν ≈ 0.40 and a shear modulus approaching that of the noble metals Cu, Ag and Au. Among the metallic glasses observed, the PtP glasses exhibit the highest ν = 0.42, whereas the glasses containing Fe tend to have lower values. The phenomenon that the conduction electrons in the glassy alloys behave as in the noble metals may be partly attributed to the filling of d shell orbitals of the transition metals in the PtP, PdP and NiP alloys. The high ν of metallic glasses is believed to be responsible for the ductile behavior of these glasses. Poisson's ratio ν of metallic glasses was observed to decrease with decreasing temperature. It is suggested that the decreasing ν with falling temperature causes the rapid increase in the fracture strength of Fe-based glasses.     

Modulus and internal friction in phosphate-silicate bioactive glass

The real and imaginary parts of the dynamic shear modulus, G′ and G″, respectively, of a “bioactive glass” Na₂O, CaO, SiO₂, P₂O₅ have been measured from 10^?4 Hz to 1 Hz at temperatures from 160 K to 610 K. The mechanical loss tangent of the glass (T_g = 816 K) at 1 Hz shows two relaxation regions: one centred at 312 K and the second at 585 K. The low-temperature peak appears in the same region as the peak attributed to Na ion motion in silicate glasses. The modulus of the glass is 32 GPa at 295 K. The spectrum of the high temperature peak has a broad distribution of relaxation times with a half width of five decades of frequency, and Arrhenius “activation energy” of 164 kJ mol.^?1, and the amplitude of this relaxation decreases on cooling. The similarity of these features with those observed in rigid molecular and other types of glasses suggests that the relaxations observed in the bioglass may be more appropriately considered as an intrinsic property of the non-periodic arrangement of atoms or ions in a solid than as being due to the diffusion of specific types of ions in it. It is pointed out that isothermal and isochronal measurements of internal friction in glasses do not give the same relaxation rate at the same temperature, due mainly to a rapid decrease in the amplitude of relaxation with temperature. The shear, Young's and bulk moduli of the glass are, 31.9, 76.6 and 43.0 GPa, respectively, and the Poisson's ratio is 0.20.A sample of the same bioglass containing 0.05% more water showed a shoulder in its tan φ at ～ 400 K and a peak at ～ 560 K, with little change in its modulus or T_g. Thus the presence of water above a certain amount had less significant effect on the internal friction of the bioactive glass.     

Influence of TM and RE elements on glass formation of the ternary Al-TM-RE systems

The effects of TM and RE elements on glass formation ability (GFA) of the Al-TM-RE systems are studied systematically. The TM elements show distinct differences: critical sizes of the Al-TM-Ce systems are in the order of Ni > Co > Fe > Cu. However, the RE elements show similar effect on glass formation. These results are discussed in terms of the GFA-related factors, i.e., reduced glass transition temperature (T_rg = T_g (T_x)/T_L, where T_g, T_x and T_L are the glass transition temperature, onset crystallization temperature and liquidus temperature, respectively) and the mixing enthalpy ΔH_Al-TM. A new parameter of T_L − T_g (T_x) is potentially proposed and used to evaluate the GFA of the Al-based alloys, as well as bulk glass-formers.     

The slow β-relaxation observed in Ce-based bulk metallic glass-forming supercooled liquid

Dynamic mechanical relaxation measurements are performed on a Ce-based metallic supercooled liquid close to its glass transition temperature T_g. An obvious excess wing is observed both in the temperature and frequency dependent loss modulus curves by the calculation the relaxation time of the α-relaxation in supercooled liquid with the fit by the combination of the Kohlrausch–Williams–Watts and Vogel–Fulcher–Tamman equation. The results indicate that the slow β-relaxation process exists in the metallic liquid and arises from the small-scale translational motions of atoms that are linked in its metastable islands.