Diffusion in amorphous NiZrAl alloys

Tracer diffusion coefficients for large and small sized impurities were measured in the bulk metallic glass Ni_xZr_yAl_z for several compositions. Molecular beam deposition was employed to grow thin films and secondary ion mass spectrometry was used to determine the concentration-depth profiles of several tracers. A dependence of the atomic mobility of the tracer on its size was found, as observed in the binary Ni_xZr_y amorphous alloy system. The presence of the Al reduces the diffusion coefficients in general, with a stronger decrease for the small sized tracer. The results support the existence of two different diffusion mechanisms proposed for amorphous metallic alloys.     

Crystallization of NANOPERM-type amorphous alloys

In the present work, we have investigated progress of crystallization in the Fe_91?xMo₈Cu₁B_x (x = 12, 15, 17, 20) alloy as a function of annealing temperature. This material belongs to the family of the Fe–M–B–(Cu) alloys (transition metal M = Zr, Nb, Hf, Mo, …) called also NANOPERM. The alloy was found to contain small amounts (<5%) of bcc Fe(Mo) and Mo₂FeB₂ nanocrystallites (<2 nm) located on the surface of the ribbon-shaped samples already in the as-quenched state. Depending upon composition, the nanocrystallites are formed on the air and/or wheel side of the ribbon. They are characterized by atomic force microscopy, conversion electron Mössbauer spectroscopy, and by X-ray diffraction of synchrotron radiation. Fast detection (every 10 s) of the latter during continuous heating of as-quenched specimens enabled an in situ observation of the evolution of the crystallization. For x = 12, the crystallization starts earlier at the wheel side of the ribbon but its progress is more rapid at the opposite, i.e. air side.     

Description of chemical ordering in amorphous alloys

Partial coordination numbers $\text{Z}{}_{\mathrm{ij}}{}^1$ for a chemically disordered binary amorphous alloy A_XAB_XB are obtained in terms of total coordination numbers Z_i and average coordination numbers 〈Z〉 $\text{Z}{}_{\mathrm{ij}}{}^1\text{=x}{}_{\mathrm{j}}\text{Z}{}_{\mathrm{i}}\text{Z}{}_{\mathrm{j}}\text{/〈Z〉}$. Departures from complete chemical disorder are characterized by a short-range order coefficient with η_AB^max=x_BZ_B/x_AZ_A for X_BZ_B<x_AZ_A and η_AB^max=x_AZ_A/x_BZ_B for x_BZ_B>x_AZ_A.For complete chemical disorder ν_AB⁰ = 0; for complete chemical order, i.e. with chemical preference for AB nearest neighbor pairs, ν_AB⁰ = 1. For departures from chemical disorder associated with clustering, i.e. chemical preference against AB nearest neighbor pairs, ν_AB⁰ < 0. Experimental results for several amorphous alloys are used to demonstrate the usefulness of these results. A more precise calculation of η_AB^max is necessary to compare the degree of ordering in the metal-metalloid and metal-metal alloys.     

Infrared spectra of amorphous thallium-selenium alloys

Amorphous thallium-selenium alloys (a-Tl_xSe_1?x) have been studied by Fourier transform infrared spectroscopy within the composition range 0 ? x ? 0.53. Differential scanning calorimetry measurements were used to check the selenium content and the homogeneity of the samples. For the stoichiometric composition a-TlSe there is strong evidence of short-range order similar to that found in the crystalline phase, c-TlSe. The spectra of both amorphous and crystalline TlSe display features associated with bond-bending and bond-stretching modes of distorted tetrahedra at ～97 cm^?1 and ～163 cm^?1, reflecting the similarity of tetrahedral coordination within the [(TlSe₂)^?1]n chains. At the Se-rich side with x ? 0.24, a superposition of typical infrared bands of a-TlSe and amorphous selenium (a-Se) is observed. The results are discussed in terms of a structural model for a-Tl_xSe_1?x.     

Random network model for amorphous alloys

A random network model has been developed for amorphous alloys which characterizes their band structure in terms of tight binding matrix elements. This model is applied to the arsenic-selenium system as well as the germanium-sulfur system. In the latter case it predicts a local maximum in the band gap in the vicinity of the compound composition GeS₂.     

Electric field gradient in transition metal amorphous alloys

The mean atomic configurations in Hf- and Zr-based amorphous alloys prepared by different routes were studied using the perturbed angular correlations (PAC) technique. For all the as-prepared amorphous samples, the PAC spectra are well described by a wide distribution of frequencies revealing atomic configurations around the probes compatible with those in a dense random packing of ions (DRP). The measured electric field gradient strength V_zz seems to be independent of the preparation method and of the addition of second phase nanoparticles (CaO and ZrC) for the same early transition metal content. The V_zz values observed in Hf based alloys are systematically higher than that in Zr-based alloys of similar composition and show a greater variation with composition. These behaviors are discussed in terms of the different contributions to the EFG, its dependence with the density of states at the Fermi level and in comparison with similar trends observed in binary amorphous alloys. Also, the Ta probe role on the measured V_zz in Hf and Zr systems was analyzed.     

Structure formation in liquid and amorphous metallic alloys

Bulk metallic glasses developed in last 15 years represent a new class of amorphous metallic alloys. These multi-component metallic alloys can be obtained at relatively low cooling rates, which allow the production of large-scale materials by conventional casting processes. Furthermore, bulk metallic glasses show a glass transition well below the crystallization temperature enabling hot deformation, but also to investigate the glass transition phenomenon in a metallic system. The thermal behavior of Zr- and Pd-based bulk metallic glasses was studied by in situ X-ray diffraction at elevated temperatures. The temperature dependence of the X-ray structure factor of the glassy state can be well described by the Debye theory. At the caloric glass transition the temperature dependence of the structure alters, pointing to a continuous development of structural changes in the liquid state. The short-range order of the glass, of the super-cooled liquid, and of the equilibrium melt is found to be very similar. The existence of complex chemically ordered clusters in the melt is supposed to be related to the high glass-forming ability of the alloys. The microstructure of metallic glasses consisting of elements with negative enthalpy of mixing is homogeneous at dimensions above 1 nm. Phase separation in the liquid state appears in metallic systems with large positive enthalpy of mixing of the elements like Nb–Y. Thermodynamic calculations of the Ni–Nb–Y phase diagram show that the miscibility gap of the monotectic binary Nb–Y system extends into the ternary up to large Ni content. Experimental evidence of the phase separation in ternary Ni–Nb–Y melts is obtained by in situ X-ray diffraction at elevated temperatures and differential scanning calorimetry. The phase separated melt can be frozen into a two-phase amorphous metallic alloy by rapid quenching from the liquid. The microstructure depends on the chemical composition and consists of two amorphous regions, one Nb-enriched and the other Y-enriched, with a size distribution from several nanometers up to micrometer dimension. The experimental results confirm the close relationship between the structure of metallic glasses and the corresponding under-cooled liquids.     

Physical aging and molecular mobility of amorphous polymers

The classical approaches of the slow dynamics of liquid glass formers and glasses are recalled. Actually, it is well acknowledged that several features are common to glasses and glass forming liquids. For example, the quasi universal occurrence of secondary β slow (or so-called Johari–Goldstein) relaxation and its correlation with the α relaxation process has to be included in any physical model. This ingredient is now introduced in the coupling model proposed by Ngai. However, recent experimental data, mainly dealing with physical aging, show that some aspects remain unsolved and many questions are still to be answered.     

Comparison of amorphous and liquid palladium-silicon alloys

The Pd_82,5Si_17,5 alloy has been investigated by X-ray diffraction in an amorphous state obtained by rapid quenching from the liquid state and also in the liquid state at a temperature just above T_f. The differences seen in the interference functions obtained relative to both states are not only due to a temperature effect. From the two atomic radial distribution functions it was possible to evaluate the Pd–Pd distances and the Pd–Si distances as well as the number of nearest neighbours. These distances have the same values, but the displacements of the atoms are more important in the liquid. The contribution of Pd–Pd bonds, for the metal-metal distances equal to 1.65 times the particle diameter, decreases in the liquid.     

The amorphous structure of immiscible Fe---Cu---Ag alloys

By means of facing target dc sputtering, an amorphous phase has been produced for immiscible Fe---Cu---Ag alloys at around the central concentration of the ternary phase diagram. The radial distribution function estimated from X-ray diffraction measurements indicates that a tetrahedron is the dominant structure unit and that the average coordination number is about 11. In the extended X-ray absorption fine structure radial structure function, a clear peak is detected at the nearest neighbor distance, but the peak intensity is weaker than those for binary crystalline Fe---Cu and Fe---Ag alloys and no clear peak is detectable at larger distances. A hard or soft sphere dense random packing model is appropriate for the amorphous structure in these immiscible alloys.     

Decomposition of La- and Dy-rich amorphous alloys

An Al₂₅La₅₀Ni₂₅ and newly discovered Al₃₈Dy₅₀Ni₁₂, Al₃₈Dy₅₀Co₁₂ and Al₄₁Dy₄₇Ni₆Co₆ glasses prepared by melt-spinning were studied by means of electrical resistance (ER) and calorimetric measurements at constant and at linearly increasing temperatures as well as by X-ray diffraction (XRD) and microhardness studies after different heat treatments. Changes in short-range order during relaxation lead to prepeaks in XRD spectra, ER and hardness increases. The glass transitions are accompanied by endothermal heat effects and ER drops. In Al₂₅La₅₀Ni₂₅, crystallisation sets in above 240 °C via the formation of metastable LaNi and AlLa₃ phases which may transform into La(Ni, Al). The Al₃₈Dy₅₀Ni₁₂ glass crystallises only above 400 °C; fine precipitates of Dy₃Ni and probably DyAl appear before a Dy(Ni, Al) phase forms. The Co-containing materials possess still increased stability but are very brittle.     

Stability and crystallization of Fe-Co-Nb-B amorphous alloys

New soft magnetic amorphous alloys with a wide supercooled liquid region were obtained by increasing the boron content (up to 30 at.%) and by addition of cobalt (composition range between 5 and 42 at.%) in FeNbB based materials. Upon thermal treatments, the stability and the crystallization behavior of Fe₅₂Co₁₀Nb₈B₃₀ and Fe₂₂Co₄₀Nb₈B₃₀ have been studied by using X-ray diffraction (XRD) and transmission electron microscopy (TEM). On the other hand, the Curie temperature change, T_c, of the remaining amorphous phases has been determined by thermomagnetic analysis (TMG). Sub-micrometric fcc-(Co,Fe) and fcc-(Fe,Co)₂₁Nb₂B₆ grains were observed to crystallize at a relatively high temperature (950-970 K) providing an excellent stability of the amorphous states. Due to the partitioning behavior, occurring during crystallization, the Curie temperature of the remaining amorphous structure is observed to increase in the case of the Fe₅₂Co₁₀Nb₈B₃₀ composition while a decrease of T_c to the lower temperatures is observed with the Fe₂₂Co₄₀Nb₈B₃₀ composition.     

Thermodynamics of formation of aluminum-iron-germanium amorphous alloys

Thermodynamic properties of ternary liquid Al-Fe-Ge alloys were studied by electromotive force method at 1050-1250 K and by high-temperature isobasic calorimetry at 1740 ± 5 K. The heat capacity change at ternary alloy formation (Δ_mixC_p) was estimated using the temperature dependence of integral enthalpy of mixing. Thermodynamics of the formation of Al-Fe-Ge amorphous alloys was evaluated by extrapolation of thermodynamic functions of mixing to the temperature of amorphization. The process of glass forming is preferable by both enthalpy and entropy for compositions of (0.1<x_Fe<0.6, x_Ge<0.5). The area with most negative integral Gibbs free energy and enthalpy of amorphous alloy formation corresponds well to the area of amorphization estimated by a glass-forming tendency (GFT) criterion.     

Ti-Zr-Fe-Si system amorphous alloys with excellent biocompatibility

Ti-based amorphous alloys Ti70Zr6Fe7Si17 (at.%) and Ti64Zr5Fe6Si17Mo6Nb2 (at.%) are fabricated by a single roller spun-melt technique. The feature of the alloy composition satisfies Inoue’s three empirical rules. Amorphous structures of the both alloys were confirmed by the X-ray diffraction patterns. The both alloys were cultivated in the simulate body fluid (SBF) for 15 days and the experiment result shows that Ca phosphates depositions on alloys surfaces were gained. Moreover, n(Ca)/n(P) atom ratio of the deposition is about 1.6/1, which approach to that of human bone - 1.66/1, suggesting that the both alloys were with an excellent biocompatibility.     

Temperature variation of the mobility gap in non-polar amorphous semiconductors

For a non-polar amorphous semiconductor such as a-Si, we derive an explixit formula for (?E_g/?T)_V, the derivative with temperature of the mobility gap E_g at constant volume V. Within the framework of second-order perturbation theory for the electron-phonon (eφ) interaction, many of our physical assumptions are fundamentally different from those that apply to the crystal phase. The principal ingredients of our model are: (1) the random-phase-model (RPM); (2) the principle of non-conservation of particle momentum in the eφ interaction; and (3) the deformation potential approximation. Narrowing of E_g is found with increasing values of the temperature T. At very low T, we have $\text{(?E}{}_{\mathrm{<mtext>g</mtext>}}\text{/?T)}{}_{\mathrm{<mtext>V</mtext>}}\text{≌ ? ¢A · c}{}_{\mathrm{V}}\text{(T)}$, where c_V(T) is the average lattice specific heat per mode at constant volume and ¢A is a positive dimensionless quantity in the model. By contrast with low-temperature behavior of the crystal, this result implies that the mobility gap at constant volume dynamically responds to the phonomic “gas” of the disordered lattice. The high-T limit yields behavior quite similar to that of the crystal phase. We find $\text{(?E}{}_{\mathrm{<mtext>g</mtext>}}\text{/?T)}{}_{\mathrm{V}}\text{≌ ? x ¢A · k}{}_{\mathrm{<mtext>B</mtext>}}$, where k_B is Boltzmann's constant and the parameter x, expected to be confined to the interval $\text{1}\text{2}\text{? x ? 1}$, measures the admixture of the optical-phonon and acoustical-phonon coupling strengths.     

Principles of electrical behavior of amorphous semiconductor alloys

A new approach to understanding amorphous semiconductors is provided by a model whose novel concepts are necessary consequences of transport and recombination fundamentals. The amorphous semiconductors are examples of the ‘relaxation semiconductor’: When dielectric relaxation is slow compared with recombination, familiar transport behavior is drastically altered and the recombination characterized by cross sections for carrier capture which are extremely large. At thermal equilibrium, ‘equality recombination’ obtains, with which local space-charge neutrality is maintained through equal total capture rates of electrons and holes per unit volume. In the alloys, such recombination after single free paths gives equal electron and hole conductivities, and thus pins the Fermi level at the position of minimum conductivity for a given band gap. Both mean free paths are equal and of the order of the electron wavelength. They determine the concentrations of centers positively and negatively ionized, which are equal and large compared with mobile carrier concentrations. Nearly all the centers are neutral, are donors near the valence band and acceptors near the conduction band, and may be in fairly short band tails. The recombination is mainly at the deeper levels of the acceptor centers, which act also as shallow electron traps. On a simple and unified electronic basis, this model accounts in detail for the various aspects of observed behavior. The threshold switching is due to characteristics ‘recombinative injection’ of minority electrons. This produces a region of injected space charge in which the net recombination rate is zero, and puts at the anode an adjoining ‘recombination front’ of a type in which carriers are activated by field equal to the threshold field. Other aspects treated include, for example: the small negative Hall coefficient; the positive thermoelectric power; preswitching; delay time; integrated current or total charge during preswitching in its quantitative dependence on delay time; the ‘blocked ON state’; and the field effect, analysis of which gives the ionized concentration. The model has further implications for theory of the band structure.