Mechanically alloyed multi-component Mo-based amorphous alloys with wide supercooled liquid region

A Mo₄₄Si₂₆Ta₅Zr₅Fe₃Co₁₂Y₅ multi-component amorphous alloy was developed via mechanically alloying (MA). It exhibits a record high glass transition temperature of 1202 K and crystallization temperature of 1324 K, an ultrahigh hardness of 18 GPa, as well as a wide supercooled liquid region (122 K) promising for processing through powder metallurgy routes. Here we present the details of the phase evolution during MA and discuss the effects of alloying elements, starting from the Mo–Co and Mo–Si binary systems, through two series of ternary alloys, eventually reaching the desired properties by selecting additional components. The propensity for glass formation and the high thermal stability were interpreted in terms of the negative heat of mixing of the elements introduced, as well as a uniform coverage spanning a wide range of atomic sizes.     

Spinodal decomposition in amorphous systems

The theory of spinodal decomposition in amorphous systems is extended to include the effect of internal stress due to coherency strain. A quantitative treatment is developed for the kinetics of internal stress relaxation via plastic deformation. The general solution for the evolution of the composition and internal stress distributions during the early stage of spinodal decomposition is derived for a system obeying the visco-elastic constitutive relations of a Maxwell solid. A kinetic equation for the structure factor is obtained for the limiting case in which coherency strain is relatively large. In this case, it is found that plastic deformation to relieve internal stress can become the rate-limiting step in the evolution of Fourier components having large wavenumbers. The predicted shape of the structure factor as a function of wavenumber, time, and temperature can be very different from that obtained when the kinetics of internal stress relaxation are neglected. The theory is expected to be particularly applicable to amorphous systems in which the species of interest have very different mobilities, and can explain the discrepancies found in previous analyses of scattering data for the lead-aluminum-borate system and alkali-silicate systems.     

Electrical current and laser radiation effects on amorphous solids

In the present paper a phenomenological theory of external factor effects upon amorphous solid properties is presented and applied to some examples. Electric current and laser light effects are considered, but the theory can be generalized to include other effects, e.g., ionizing radiation, chemical reactants etc. Changes of amorphous solid properties are considered as initiated locally and propagated along certain lines, narrow stripes or tubes, which will be called current- or laser traces. The proposed theory consists of the kinetics of the traces’ appearance, development and disappearance, and of the kinetics of processes (physical, chemical a. o.) which occur inside traces. This phenomenological theory is applied to determine the molecular mechanisms of the considered processes by comparing the kinetics obtained theoretically and experimentally for relevant cases. The comparison also allows determination of constants and functions that cannot be calculated theoretically. A possibility was considered that properties change by crystallization, e.g., initiated by resonance electron collisions with atoms, molecules and ions within the amorphous solid. It is shown how the obtained kinetic dependencies can be compared with experimental data to check the hypothesis itself on the resonance mechanism and to determine what are namely resonances contributing to these processes.     

The structural relaxation and glass transition of La---Al---Ni and Zr---Al---Cu amorphous alloys with a significant supercooled liquid region

An amorphous phase with a wide supercooled liquid region, > 50 K, was found to form over wide composition ranges in the La---Al---Ni and Zr---Al---Cu systems. The largest values for the temperature span between the crystallization temperature, T_x, and the glass transition temperature, T_g, ΔT_x(-T_x−T_g), are 69 K for La₅₅ Al₂₅Ni₂₀ and 88 K for Zr₆₅Al_7.5Cu_27.5. The structural relaxation behavior on annealing was examined for the two amorphous alloys with the largest ΔT_x values. The magnitude of the structural relaxation increases gradually with increasing annealing temperature, T_a, and then rapidly in the T_a range slightly below T_g and decreases significantly on annealing T_g. The rapid increase in the magnitude of the structural relaxation on annealing near T_g is due to the glass transition. The single-stage structural relaxation indicates that there is no distinct difference in relaxation times (atomic bonding forces) between the constituent atoms in the two metal-metal-type amorphous alloys. The existence of an optimum bonding state is thought to cause the wide supercooled liquid region for the two amorphous alloys.     

Optical properties and X-ray diffraction of amorphous systems

By appealing to the persistence of short range order in amorphous materials a simple estimate of the form of the static structure factor can be obtained. It is also possible to derive an expression for the optical absorption of amorphous materials involving a convolution of the corresponding crystalline expression and a simple function.     

Structure simulation of amorphous monatomic and binary systems

A simplified approach to computer simulation of amorphous structures by the dense random packing of hard spheres (DRPHS) has been developed and applied to amorphous Tb and TbFe₂. The computer algorithms devised are rapid and result in uniform, isotropic packing for both the monatomic as well as binary systems; in the latter case, small-atom drift toward the center of the cluster is reduced to the uncertainty in the random number generator. Energy relaxation has been performed using the first term in Keating's expression for the elastic energy; relaxation causes the peaks in the pair correlation functions to broaden, but their relative intensities (peak areas) remain essentially the same. In all cases investigated, the packing fractions are 2% (or less) smaller than those of Finney's ball bearing measurements, and the calculated densities are 13–23% lower than their crystalline counterparts. The pair correlation functions of the monatomic and binary models have been compared with X-ray results on CoP and neutron results on TbFe₂ and found to be in good agreement.     

High field effects in amorphous germanium

Measurements of high field current have been made on amorphous Ge (a-Ge) films over the temperature range from 100 to 300 K. Non-ohmic conduction in a-Ge occurs at electric fields greater than 1–2 × 10⁴ V/cm. Field dependence of the conductivity has been explained in terms of the enhanced emission probability of carriers from the screened coulombic trap centers. Assuming the optical dielectric constant for a-Ge films, the screening length of the trap centers and the density of states at the Fermi level are estimated to be 12 Å and ～6.1 × 10²⁰ cm^?3 eV^?1, respectively.     

Formation and stability of amorphous molecular systems: As the prototype of functional amorphous organic materials

Structural changes in vapor-deposited amorphous states of simple organic compounds were studied by Raman scattering, X-ray diffraction, and light interference in the sample. Two types of processes, namely direct crystallization and structural relaxation leading to glass transition, were observed depending on the flexibility of the molecule. Gradual relaxations were also found to occur in amorphous states of butyronitrile and 1,2-dichloroethane at temperatures much lower than their T_g or crystallization temperature. The observed structural changes are discussed by referring to the high enthalpy of the vapor-deposited amorphous systems.     

One- and two-phonon vibrational spectra of amorphous As- and Sb-chalcogenide systems

One- and two-phonon infrared and Raman spectra of As- and Sb-chalcogenide systems are reported. Both one- and two-phonon Raman and infrared vibrational spectra exhibit two modes type behaviours characteristics to the vibration modes of AsSe₃, AsS₃, and SbSe₃ pyramids. In the infrared spectra, the larger peak shifts are observed for one phonon frequencies than for two phonon frequencies with the change of the composition x, and its suggests rather big local field effects. The two-phonon coupling parameters of the combination modes are estimated.     

On the characteristic structural clustering in metal-metalloid amorphous systems

A Voronoi analysis of selected Fe---B, Fe---P, Ni---B and Ni---P amorphous alloys is made. The alloys were modeled by molecular dynamics, using pseudo- and Morse-pair potentials. Characteristic properties of the cluster formation are discussed.     

One- and two-phonon Raman spectra of amorphous AsSe systems

The polarization dependence of the one- and two-phonon Raman scatterings of the amorphous semiconductor As_xSe_1?x systems for 0 ? x ? 0.5 was measured. The analysis of the Raman data by the superposition of the vibrations of AsSe₃ pyramids and SeSe bonds suggests the existence of the intermediate range interaction in the As_xSe_1?x systems.Two phonon Raman spectra for 0.3 ? x are explained by the combination plus overtone of the first order Raman scatterings. For x ? 0.2, however, the overtone processes become predominant.The dependence of the Raman spectra on the As concentration suggests that the chemical orderings are predominant.     

Medium range order in amorphous AsSe systems

The effects of photon and As on the crystallization of a-Se were investigated and the structure factors of a-As_xSe_1-x (0≤×≤0.36) systems were measured by neutron diffraction. We obtained the information about the medium range order (MRO) in these systems. The length of the MRO in these systems is 10～15A.     

On-state behaviour and degradation effects in thin-film amorphous switching devices

The on-state behaviour of thin-film chalcogenide threshold switches was studied with special reference to the influence of electrode materials and the progressive effects of device degradation. For operations during the forming period, the on-state current-voltage characteristics remain identical for both rising and falling current. The on-state minimum voltage is found to be closely proportional to the square root of the electrode thermal conductivity and, in addition, shows some polarity dependence with dissimilar electrode combinations. This behaviour is interpreted in terms of a model in which the voltage drop across the on-state current channel is dominated by constrictions and associated hot spots near the channel/electrode boundaries. After forming and after a further number of operations depending both on electrode material and ambient temperature, one or more stable intermediate steps or “branches” develop in the I–V characteristic of the transition from off-state to on-state. The exact pattern shows some polarity dependence. Branching is identified with the progressive precipitation of tellurium-rich “islands” in the glass as the number of device operations increases and as a result of which the growth of the on-state channel proceeds via a number of “island-hopping” stages.     

Composition and polyamorphism in supercooled yttria-alumina melts

By extending recent work on liquid-liquid transitions in supercooled yttria-alumina AYx liquids we draw attention to the compositional dependence of the structure factor of the high density liquid, arguing that this is sufficiently sensitive to discriminate between liquids at the level of a few %. Comparing structure factor differences between liquids of different compositions and in the same liquid AY20 between high and low temperatures straddling the transition at 1788 K between a high density liquid (HDL) and a low density liquid (LDL) enables compositional phase separation to be ruled out. It points instead to kinetic changes in polyhedral configurational order being the drivers for this polyamorphic transformation. Rotor behaviour observed in levitated liquid drops used in the high temperature experiments enables the reversibility of the LLT transition (LLT) and the associated changes in entropy and density to be identified. Evidence for critical-like behaviour in the structural relaxation time and in the fluctuation correlation length is presented. By re-examining recent work which failed to find the structural and thermal signatures for the LLT in liquid AY20 at 1788 K we present evidence for the LLT occurring instead in liquid AY15 at 1940 K, suggesting that the liquid-liquid transition temperature in AYx liquids decreases with increasing yttria content.     

Relaxational features of supercooled and glassy m-toluidine

Recent investigations performed by dielectric and mechanical spectroscopy have provided detailed information about the dynamical response of m-toluidine in the supercooled and glassy states. In the present work we analyze the properties of the α-relaxation process on the basis of dielectric and shear mechanical data, discussing the unusual behavior of this fragile glass-former. The nature of the secondary relaxation is elucidated by high resolution dielectric measurements and discussed in terms of the Coupling Model and literature results on other small molecule glass-forming liquids.     

Crystal field effects in amorphous rare earth magnetic materials

A model proposed by Harris et al. in which crystal field affects are important, is extended to consider the magnetic properties of amorphous materials containing localized ionic moments of a single, heavy rare earth, magnetic element. The paramagnetic and ferromagnetic properties are investigated for all the heavy rare earths. It is found that the average spontaneous moment per ion in the amorphous state can be lowered below the free ion moment by more than 50%, but the relative reduction in Curie temperature is much less marked than found by Harris et al.     

The effects of incorporating fluorine in amorphous germanium-silicon alloy films

The structural, electrical and optical properties of rf sputtered fluorinated amorphous GeSi alloy films have been studied as a function of the amount of fluorine incorporated in the films. The results indicate an increased coordination in the binary semiconductor alloys with the increasing fluorine content. Fluorine attaches with extreme preference to silicon over germanium in the alloys. Due to this high preferential ratio, the increase in coordination on fluorination is much less than that observed in fluorinated amorphous silicon. The thermal stability of fluorinated alloys is found to be higher than similarly prepared hydrogenated samples.     

Mössbauer studies of glasses and supercooled liquids

Several examples of Mössbauer spectra taken from amorphous materials are presented. The purpose is to help assess whether this technique is likely to be useful in the determination of structure for non-crystalline solids. One conclusion is that, except for iron, the resolution is poor and unique interpretations are unlikely. A second conclusion is that in a glass formed from FeCl₂·9H₂O the microenvironment is very uniform, as might be expected from a continuous-random-network model.     

Chemical shifts of the K-absorption discontinuities of germanium and selenium in some amorphous systems

In the present investigation the chemical shifts of the K-absorption discontinuities of germanium and selenium have been studied in their amorphous chemical compounds Ge₁₀Se₉₀, Ge₃₀Se₇₀, Ge₁₀Se₈₀Te₁₀ and Ge_33.3Se₄₀Te_26.7 using a 1 m Cauchois type bent crystal (mica) X-ray spectrograph. A graph of the chemical shift ΔE in binary compounds against the effective charge q on the absorbing atoms has been plotted. This plot is helpful in determining the effective charges in ternary compounds in which they cannot be calculated theoretically.     

2H NMR studies of supercooled and glassy aspirin

Acetyl salicylic acid, deuterated at the methyl group, was investigated using ²H NMR in its supercooled and glassy states. Just above the glass transition temperature the molecular reorientations, studied using stimulated-echo spectroscopy, demonstrated a large degree of similarity with other glass formers. Deep in the glassy phase the NMR spectra look similar to those reported for the crystal and below 20 K they are indicating that rotational tunneling plays a role. Measurements of the spin–lattice relaxation times for temperatures below 150 K reveal a broad distribution of correlation times in the glass. The dominant energy barrier characterizing the slow-down of the methyl group motion is significantly lower than the well-defined barrier in the crystal.