Thermal expansion and density of glassy PdNiP and PtNiP alloys

Thermal expansion and density of (Pd_1?xNi_x)_0.80P_0.20 and (Pt_1?yNi_y)_0.75P_0.25 alloys in their various states have been measured from room temperature to the glass transition temperature T_g. The thermal expansion of the glassy alloys at room temperature varies linearly with x and y and is 10 to 20% higher than that of corresponding pure metals. The thermal expansion of the undercooled alloy liquids near T_g as well as the molar volume $\text{v}\text{?}$ deduced from the density of glasses in contrast exhibits a negative deviation with composition x and y.This behavior is in line with the previously reported negative deviation of the glass transition temperature of these glassy alloys with metal content and may be explained in terms of excess volume associated with a mixture of hard spheres.     

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.     

X-ray diffraction studies of the structure of LaA1Ga metallic glasses

The LaLa and LaA1 partial atomic distribution functions have been determined for glassy La_1?xA1_x from X-ray diffraction studies of isomorphous alloys of La_1?xA1_x, La_1?x(A1Ga), and La_{1 ?x}Ga_x for x = 0.20, 0.24 and 0.28. The atomic short range order of these La based metallic glasses is quite different from that of typical amorphous transition metal-metalloid alloys and from dense random packing models. A relatively short and well defined LaA1 nearest neighbor distance suggests some covalency in the bonds between unlike atoms and possible chemical ordering in the alloys.     

A method for evaluating viscosities of metallic glasses from the rates of thermal transformations

The apparent activation energies E(T) for the glass transition and crystallization in Pd_77.6Cu₆Si_16.5 and Pd₄₈Ni₃₂P₂₀ glass are seen to coincide with those for the viscous flow. This implies that both the rates of glass transition and crystallization in metallic glasses scale as the viscosity. Based on this proposition, the viscosity of a Pt₄₅Ni₃₀P₂₅ glass, for example, has been evaluated, by means of thermal measurements, from the glass transition far into the crystallization temperatures. The viscosity η decreases rapidly from 10¹³ P at 480 K to 10⁷ P at 580 K and is described by a Fulcher's expression as η = 10^?3 exp[5950/(T ? 320)].     

Crystallization behaviour of Al-based metallic glasses below and above the glass-transition temperature

The present paper aims to report an effect of a supercooled liquid region on crystallization behaviour of the Al₈₅Y_8−xNd_xNi₅Co₂ metallic glasses produced by rapid solidification of the melt. The paper describes the crystallization process at different regimes of heat treatment. It is found that crystallization behaviour of the above-mentioned Al-based metallic glasses above the glass-transition temperature and below it follows different transformation mechanisms. Formation of the primary nanoscale α-Al particles was observed during continuous heating or after isothermal annealing above the glass-transition temperature. During isothermal annealing below the glass-transition temperature an unknown metastable phase is formed conjointly with α-Al. The metastable phase formed in the Nd-free alloy varies from that in the Nd-bearing alloys. Al₈₅Nd₈Ni₅Co₂ amorphous alloy exhibiting no glass transition crystallizes equally during isothermal calorimetry at different temperatures and during continuous heating.     

Quenched-in excess volume and structural relaxation in Ni- and FeNi based metallic glasses

The amount of quenched-in excess volume (ΔV_R) in various Ni- and FeNi based metallic glasses, which is defined by the volume change due to the structural relaxation, was estimated from measurements of length changes caused by isochronal annealing. It was found that the values of ΔV_R depend greatly on the kind of transition metals (Fe, Ni) and metalloids (Si, B, P) and their ratios (Fe/Ni, Si/B). In non-magnetic Ni-based metallic glasses, almost a linear relationship was observed between the changes in volume and electrical resistivity during the low temperature annealing below around 275°C. Furthermore, a large ΔV_R was observed in the metallic glasses which have a large thermal expansion coefficient such as Ni₇₅Si_12.5B_12.5 or which exhibit a well-defined glass transition such as Fe₂₇Ni₅₃P₁₄B₆, as a general trend. These results indicate that the amount of quenched-in excess volume is very sensitive to the local atomic arrangement and is attributed mainly to the distribution in bond lengths, angles and coordination numbers in the short range structure and the difference in packing of basic structural units between as-quenched and relaxed glassy states.     

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).     

Glass-forming ability and crystallization of bulk metallic glass (HfxZr1−x)52.5Cu17.9Ni14.6Al10Ti5

We have produced a series of bulk metallic glasses of composition (Hf_xZr_1−x)_52.5Cu_17.9Ni_14.6Al₁₀Ti₅ (with x=0-1) by an arc melting/suction casting method. The density of these alloys increases by nearly 67% with increasing Hf content from 6.65 g/cm³ (x=0) to 11.09 g/cm³ (x=1). Over the same composition range the glass-forming ability decreases, as demonstrated by the size of the largest amorphous ingots that can be cast without crystallization. Although both the glass transition temperature and the melting temperature increase linearly with increasing Hf content, the reduced glass transition temperature (T_g/T_m) decreases, from 0.64 (x=0) to 0.62 (x=1), which suggests that the `confusion principle' correlating increased glass-forming ability with increased number of components, does not apply in this case due to the chemical similarity between Zr and Hf. A different crystallization behavior is observed for Zr-based and Hf-based glasses. The final crystalline phases are CuZr₂ and Zr₂Ni for Zr-based alloys, and Al₁₆Hf₆Ni₇ and CuHf₂ for Hf-based alloys.     

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.     

Formation of bulk Pt–Pd–Ni–P glassy alloys

The thermal behavior of (Pt_0.4Pd_0.3Ni_0.3)_100−xP_x (x = 16–25 at.%) glassy alloys has been investigated. It is found that the crystallization behavior of the (Pt_0.4Pd_0.3Ni_0.3)_100−xP_x glassy alloys changes from a single-stage exothermic reaction to a two-stage exothermic reaction depending on phosphorous content. When the phosphorous content is 23 at.%, the glassy alloy exhibits the largest supercooled liquid region (ΔT_x) and a sharp single exothermic peak. Fixing the phosphorous content at 23 at.%, the Pt_77−x−yPd_xNi_yP₂₃ (x = 7.7–61.6 at.%, y = 7.7–61.6 at.%) glassy alloys have a wide composition range in which the glassy alloys exhibit a large supercooled liquid region (ΔT_x beyond 60 K). In this range, the Pt_30.8Pd_23.1Ni_23.1P₂₃ glass has the largest ΔT_x (77 K) and a high reduced glass transition temperature (T_rg) of 0.60. This alloy can be cast into fully glassy rods with a diameter of 3 mm. Under uni-axial compression, bulk Pt_30.8Pd_23.1Ni_23.1P₂₃ glassy alloy has an elastic strain of ∼2%, an ultimate strain (to fracture) of ∼6.4%, a Young’s modulus of ∼106 GPa and a failure strength of ∼1390 MPa.     

Electrical transport in n-type semiconducting Ge120BixSe70−xTe10 glasses

Resistivity and thermoelectric power were measured as a function of temperature and composition for Ge₂₀Bi_xSe_70?xTe₁₀ glasses (x = 0–11). The results were compared with the case of of Ge₂₀Bi_xSe_80?x glasses to see on the electrical properties the influence of the substitution of Te for a part of Se. The glasses show n-type conduction for x ? 9, which was not affected by the substitution of Te. The resistivity was about three orders of magnitude lower for the glasses with x < 10, and remained almost the same for x ? 10, compared with the glasses not containing Te. From the composition dependence of the calculated concentration of covalent bonds in the glasses, it was proposed that the appearance of n-type conduction was closely related to the formation of a sufficient number of BiSe bonds and the disappearance of the bonds between two chalcogen atoms such as TeSe or SeSe bonds, and that the remarkably low resistivity in the present glasses with x < 10 was likely to be attributed to the formation of TeSe bonds.     

Polynary silicon arsenic chalcogenide glasses with high softening temperatures

The introduction of Ag in SiAsTe glasses permits the incorporation of Se, otherwise volatile and/or degradable as a constituent in Si-containing chalcogenide glasses. SiAsAgTeSe glasses exhibit much higher softening ranges and glass transition temperatures than encountered in known chalgogenide systems. A glass Si₃₅As₁₅Ag₁₀Te₂₀Se₂₀ had the viscosity log ν = 13 at about 500°C, as compared to 370°C for the base glass Si₃₅As₂₅Te₄₀, the viscosity of log ν = 9.8 at about 560°C, as compared to 442°C for the base glass. Phase separation occurs in the system SiAsAgTeSe and becomes manifest in two glass transitions indicated by changes in the slopes of the expansion curves and breaks in the softening point-composition relations. The existence and behavior SiAsAgTeSe glasses suggests the possible development of higher T_g i.r. transparencies and higher T_g semiconductor glasses than described so far.     

Atomic dynamics in different regions of the potential energy surface

The atomic dynamics in two (bulk) metallic glasses, Ni₄₀Pd₄₀P₂₀ and Zr₅₅Cu₃₀Al₁₀Ni₅, were investigated by neutron inelastic scattering in different regions of the potential energy landscape, which are reached by slow cooling the bulk glasses and by hyper-quenching the same alloys. The results prove that the atomic dynamics depends also on the fictive temperature, i.e. the region of the potential energy surface, in which the glass is frozen in. Obviously the shapes of the basins or inherent structures are not the same everywhere on the potential energy surface, and the glass with a higher fictive temperature has more low energy modes than has the same glass with a lower fictive temperature. As results from computer simulation have suggested already, on moving to regions of lower mean potential energy (aging), part of theses low energy modes are transferred to the energy region of the calculated Debye cut-off energy. The difference between the vibrational entropies, calculated from the generalized vibrational density-of-states, which have been determined for both fictive temperatures, shows that the contribution from the vibrational entropy to the total entropy change, when moving through the potential energy landscape, is small for the two metallic glasses investigated. Structural relaxation of the hyper-quenched glass removes part of the additional low energy modes, but quantitatively possibly only at the low and perhaps also at the high-energy limit of the density-of-states. The wavelength dependence of the dynamics suggests that the additional low energy modes in the glass with the higher fictive temperature are not dominated by extended but more likely by localized modes.     

Raman spectra and structure of sodium aluminogermanate glasses

Polarized Raman spectra of x NaAlO₂·(100 ? x) GeO₂ glasses (x = 0, 5, 10, 15, 20, 25, 33, 42, and 50) are presented. Analyses of the Raman data indicate that the aluminogermanate glasses have three-dimensional network structures consisting of interconnected AlO₄ and GeO₄ tetrahedra; Na⁺ ions are present in cavities and charge balance the Al³⁺ ions. Systematic changes are observed in the frequencies, intensities and polarization characteristics of spectral bands with variations in the NaAlO₂ content of these glasses. The antisymmetric stretching mode [ν_as (TOT), where T = Al, Ge] in the high-frequency region of the spectra (800–1000 cm^?1) appears as a doublet consisting of well-defined bands in the spectra of glasses along the entire join. Both components of the high-frequency doublet shift to a lower frequency with increasing NaAlO₂ content, indicating that the ν_as (GeO₄) and ν_as(AlO₄) stretching modes are coupled. The variations in the TO force constants and TOT bond angles with change in composition most likely cause the bands to shift. The frequencies of the Raman bands of sodium aluminogermanate glasses are compared with those of the corresponding bands in isostructural sodium gallogermanate glasses. On the basis of this comparison, the origin and delocalization of the vibrational modes producing characteristic Raman bands in the spectra of these glasses are discussed. The changes observed in the Raman spectra of aluminogermanate glasses with variation in NaAlO₂ content are analogous to those observed in the spectra of glasses along the NaAlO₂SiO₂ join.     

Electrical properties of phosphate glasses

The electrical properties of glasses in the Na₂OP₂O₅, Ag₂OP₂O₅ and (1?x)Na₂OxAg₂OP₂O₅ systems have been measured over a range of temperature and composition.The properties of the Na₂OP₂O₅ and Ag₂OP₂O₅ glasses have been compared within the phosphate system as well as with silicate glasses. The silver-containing glasses show higher conductivity and lower temperature coefficients when compared with the sodium-containing glasses. A maximum in the room temperature resistivity of the (1?x)Na₂O?xAg₂O?P₂O₅ system was found around the mole ratio of 0.16:0.84 Ag₂O:Na₂O, indicating a mixed-alkali effect. A similar effect was seen in the tan δ, but not in the T_g-against-composition plots. A linear relationship was noted for the tan δ-versus-log₁₀ (resistivity) plot, as has been seen in other glass-forming systems.     

Theoretical calculation of dense random packings of equal and non-equal sized hard spheres applications to amorphous metallic alloys

A model describing the structure of amorphous metallic alloys is proposed using a packing of non-equal-sized hard spheres. Models containing up to 1000 spheres were generated by computation. Hypotheses guiding the calculation are chosen in order to obtain a model as compact as possible and to take into account affinity between metal and metalloids: therefore two small spheres are not allowed to be in hard contact. This method of calculation is first tested by building up heaps of equal-sized spheres. Both the radial distribution function and the interference function are calculated for each model and the variations of these curves with the number of spheres are studied. In the case of two sizes of spheres heaps calculations are devoted to the study of the so-called ‘shoulder interference function’ which characterizes many amorphous alloys such as NiP, PdSi, etc. It is shown that the existence of such a shoulder depends on two parameters: the ratio of sphere diameters and the relative concentration of small and large spheres. These results are in good agreement with some recent experimental observations. This shoulder is well-marked when the diameter ratio reaches 10% and when the concentration in small spheres lies in the range 10–15%. In such a structure (e.g. amorphous NiP and PdSi alloys) it is observed that small spheres are always surrounded by nine large spheres such as P or Si atoms in crystalline Ni₃P or Pd₃Si. Large spheres, i.e. metallic atoms, form distorted icosahedra, the distortion of which varies from one to the other. The short-range order has a five-fold symmetry which is not compatible with any long-range order.     

A metastable liquid state miscibility gap in undercooled Pd–Ni–P melts

Recently, phase separation was found to occur in Pd_41.25Ni_41.25P_17.5 bulk metallic glasses (BMG), which have a negative heat of mixing among the constituent elements. In this work, Pd_40 + 0.5xNi_40 + 0.5xP_20 ? x BMG with x = 0 to 3.5, were studied for amorphous phase separation. It occurs for x ? 1, but absent for x ? 1. In addition, in phase-separated specimens, the characteristic sizes or wavelengths of the decomposed phases were measured. It was found that they obey the lever rule. The experimental results suggest the existence of a metastable liquid/amorphous miscibility gap. Its origin is attributed to unique short range orders in the undercooled Pd–Ni–P melts.     

Photo-acoustic spectroscopy of SixSe1−x glasses

The optical properties near the fundamental absorption edge has been studied for a series of Si_xSe_1−x glasses using photoacoustic spectroscopy. The compositional dependence of the bandgap E_O, derived from these measurements, is presented and contrasted with the GeSe and the SiS systems. This data is qualitatively explained with a model which accounts for differing numbers of homopolar and heteropolar and heteropolar bonds as the composition is varied. Additional support for this interpretation is found in the compositional behavior of the glass transitions of these alloys.     

Formation and mechanical behavior of (Fe0.5Ni0.5)80−xMoxB20 (x = 0–8) amorphous alloys

Glass forming ability, thermal stability and mechanical behavior of (Fe_0.5Ni_0.5)_80?xMo_xB₂₀ (x = 0, 2, 4, 6, 8) amorphous alloys were studied by XRD, TEM, SEM, DSC, tensile test, microhardness test and tearing test. The effects of Mo addition on glass formation, strength and ductility of (Fe_0.5Ni_0.5)_80?xMo_xB₂₀ amorphous alloys were discussed. The substitution of Mo for Fe and Ni simultaneously causes improvement in glass forming ability and thermal stability, and changes the crystallization process. The tensile fracture strength of amorphous alloy depends on both hardness and ductility; the alloy with high hardness and good ductility simultaneously also has a high tensile fracture strength. The (Fe_0.5Ni_0.5)₇₈Mo₂B₂₀ amorphous alloy exhibits good glass forming ability and the highest tensile fracture strength among (Fe_0.5Ni_0.5)_80?xMo_xB₂₀ alloys. Micro-plastic deformation occurred in ductile and brittle amorphous alloys that both show viscous flow characteristics. The mechanical behavior of (Fe_0.5Ni_0.5)_80?xMo_xB₂₀ amorphous alloys is related to the average outer shell electron concentration of metal atoms.     

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.