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.     

Ductility improvement of Zr-Cu-Ni-Al glassy alloy

We studied the relationship between the phase diagrams and formation of crystalline inclusions in cast samples in order to improve the ductility of Zr-Cu-Ni-Al bulk amorphous alloy. A Zr₅₀Cu₄₀Al₁₀ bulk amorphous alloy with a composition close to the ternary eutectic point has no crystalline inclusions and possesses superior mechanical properties of tensile strength σ_B=2000 MPa, Young’s modulus E=107 GPa and Vickers hardness HV=580. Oxygen embrittlement of Zr-Cu-Ni-Al bulk amorphous alloys can be avoided in the Zr₅₀Cu₃₀Ni₁₀Al₁₀ bulk amorphous alloy, which exhibits superior ductility and resistance to oxidation. Furthermore, we tried to improve ductility by cold rolling by making use of the apparent work-softening phenomenon of Zr-Cu-Ni-Al bulk amorphous alloys. Preliminary studies indicated that cold rolling to produce a fine slip-band structure, which enabled uniform deformation and superior deformability, is an important procedure for improving the ductility of Zr-Cu-Ni-Al bulk amorphous alloys.     

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.     

Experimental studies of phase equilibria in high-temperature ternary immiscible metallic melts

Shear viscosity measurements were carried out for a liquid immiscible ternary system In–Se–Te in a temperature range from the monotectic to about 1200 K using an oscillating-cup viscometer. It was revealed that the ternary In₈₀Se_xTe_20−x (0 ⩽ x ⩽ 20 at.%) system may be considered as a set of quasibinary In–(Se/Te) alloys of almost critical concentrations. A variation of the Se to Te ratio at constant content of In changes the properties of coexisting liquids and affects the binodal temperature. The critical parameters describing the peculiarities of viscosity behavior in the vicinity of the critical point are evaluated. The analysis based on the dynamic theory of phase transitions for viscosity in the phase separation region is proposed. The results are compared with available data for immiscible dielectric solutions and metallic melts.     

Transformations in liquid state and microstructure development in immiscible Fe60Cu20P10Si5B5 alloy

A five-component Fe₆₀Cu₂₀P₁₀Si₅B₅ immiscible alloy was arc-melt and melt-spun from various temperatures. The microstructure and chemical composition of the ingot and melt-spun ribbons were analyzed using scanning electron microscope SEM/EDS. The melt-spun ribbon was investigated by transmission electron microscope (TEM). The melting range of the alloy was investigated by means of differential thermal analysis (DTA) and for reference, the temperature change during free cooling of the alloy was controlled by pyrometer in the melt spinning device. Processing with the slow cooling rate produced the fractal surface structures formed by the Fe-rich regions and Cu-rich regions. The arrangement of the regions was characteristic for the liquid immiscible alloys. It was found that the microstructures of the melt-spun ribbons depended on the ejection temperature. The lower ejection temperatures resulted in the formation of the lamellar structures separated into Fe-rich and Cu-rich regions. This was due to rapid cooling within the miscibility gap. Ejection at higher temperatures, above the miscibility gap led to the formation of a uniform amorphous/crystalline composite.     

Hydrogen in Cu?Ti and Ni?Ti Metallic Glasses

The hydrogen solubility and its effect on the crystallization of Cu Ti and Ni Ti glasses were studied by differential scanning calorimetry, thermogravimetry, and X-ray diffraction. Dependence of the crystallization products of the hydrogenated Ti-based alloys on the hydrogen content was found. Whereas in Cu-Ti alloys hydrogenation leads to drastic decreasing in the thermal stability due to phase separation in the amorphous state and to formation of microcrystalline structure during crystallization, in Ni Ti system hydrogen produces hydrides with Ni as well with Ti, which after heat treatment decompose, and finally the same crystalline phases as in unhydrogenated alloy are formed. The isothermal crystallization kinetics of the maximum hydrogenated Cu₅₀Ti₅₀ amorphous alloy was also investigated to obtain additional information about this transformation leading to nano-crystalline material.     

The microstructure development in arc-melt and melt-spun Fe50Ni10Cu20P10Si5B5 immiscible alloy

A six-component Fe₅₀Ni₁₀Cu₂₀P₁₀Si₅B₅ immiscible alloy was arc-melt in argon and it was melt-spun from various temperatures. The morphology and chemical composition of the cross-section of the ingot and melt-spun ribbons were analysed with a scanning electron microscope SEM/EDS. The melt-spun ribbon was investigated by a transmission electron microscope (TEM). The melting range of the alloy was investigated by means of differential thermal analysis (DTA) and for reference, the temperature change during free cooling of the alloy was controlled by pyrometer in the melt spinning device. The slow cooling rate resulted in the fractal surface structures formed by the Fe-rich regions and Cu-rich regions typical for the alloying system with a miscibility gap. The structures of the melt-spun ribbons were dependent on ejection temperatures before the melt spinning. The lower ejection temperatures resulted in the formation of the structures separated into Fe-rich and Cu-rich regions. This was due to rapid cooling within the miscibility gap. Ejection at higher temperatures led to the formation of a uniform amorphous/crystalline composite.     

Crystallization processes of Ag–Ge–Se superionic glasses

The interest in superionic systems has increased in recent years because of the potential application of these materials as solid electrolytes. In this field, amorphous materials present important advantages when compared to the crystalline solids: larger conductivity, isotropy and absence of grain boundaries. In this work, amorphous alloys of compositions (Ge₂₅Se₇₅)_100−yAg_y with y=10, 15, 20 and 25 at.% have been studied. Amorphous samples in bulk were obtained from the liquid by water quenching (melt-quenching technique). The crystallization kinetics of the amorphous alloys have been studied under continuous heating and isothermal conditions by means of differential scanning calorimetry. A glass transition and two exothermic transformations were observed in all the samples. The quenched samples and the crystallization products have been characterized by X-ray diffraction. The primary crystallization of the ternary phase Ag₈GeSe₆ and the secondary phase GeSe₂ was observed. The glass and crystallization temperatures, the activation energy and the crystallization enthalpy are reported. The first step of the crystallization of the Ag₈GeSe₆ phase in all the (Ge₂₅Se₇₅)_100−yAg_y samples is modelled taking into account the Johnson–Mehl–Avrami–Kolmogorov theory and considering that the changes in the composition only modify the viscosity of the undercooled liquid. The transformation diagrams (TTT and THRT) are calculated and the glass forming ability is analyzed. The experimental results are discussed and correlated with the structures proposed for the glass. The present results and conclusions are also compared with those reported by other authors.     

The effect of solute supersaturation on the ageing process of an Al 59,5 wt% zn alloy

The effect of the solution treatment temperature on the mechanism of the decomposition at room temperature of an quenched Al 59,5 wt.-% Zn alloy has been examined. The investigations were carried out using X-ray techniques, optical and transmission electron microscopy and measuring the changes in the dimensions, electrical resistivity and the hardness. The sequence of the phase transformations in the alloy quenched from 370°C were as follows. The transformation begins with spinodal decomposition. After some development of the spinodal structure, which is associated with increase in wavelength, the heterogenuous nucleation and growth of the zinc-rich particles occur within the spinodal structure. Simultaneously the discontinuous transformation was observed to grow into the spinodal structure. The decomposition of the same alloy quenched from 300°C (miscibility gap region) took place much more rapidly than when quenched from 370°C. Directly after quenching the spinodal structure was also observed using JEM-1000, coincidental with a remarkable increase of electrical resitivity. At the same time the heterogenuous nucleation and growth of the zinc particles, on the interfaces of the α-α′ phases was observed. The discontinuous transformation was also found within the α′ supercooled phase. The decomposition of the Al 38 at.-% Zn alloy quenched from 300°C appeared to be divided into two stages. In the first the decomposition of the zinc-rich α′ phase takes place and in the second one the transformation of the zinc-poor a phase occurs. These two transformations are distinctly marked on the curves of the changes in the dimensions, electrical resistivity as well as in the intensity of the (101) reflexion of the zinc phase.     

Amorphization from the quenched high-pressure phase of silicon and germanium

The phase transitions from the quenched high-pressure phase, including amorphous state, have been investigated for crystalline silicon and germanium at various pressures. X-ray diffraction patterns have been measured at pressures up to 15 GPa and temperatures down to 90 K by an energy-dispersive method using synchrotron radiation and a diamond anvil cell. The quenched β-Sn phase undergoes an amorphous phase transition when heated at 1.5 GPa for c-Si and 2.0 GPa for c-Ge. On the other hand, the quenched β-Sn phase transforms into a metastable crystalline phase when heated at higher pressures. The phase behavior is discussed in relation to the pressure dependence of the height of potential barrier between the β-Sb and amorphous phases and that between the β-Sn and metastable phases. The coordination number for the pressure-induced amorphous germanium, obtained through amorphization from the quenched high-pressure phase, is estimated to be about 4.     

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.     

The structure and properties of nanocrystalline Fe78B13Si9 alloy

A new method has been employed to prepare a nanocrystalline alloy from amorphous ribbons and some of the structure-dependent properties of this alloy have been studied and compared with those of coarse-grained crystalline and amorphous FeBSi alloys. The new method of preparing nanocrystalline alloys involves annealing isothermally an amorphous ribbon of the same composition. By comparing the structure-dependent properties, it can be concluded that the novel properties of the nanocrystalline alloy reflect the unusual nature of its grain boundaries.     

First-principle molecular dynamics study of the structural and electronic properties of liquid and amorphous Ni-Al alloys

Atomic and electronic structures of liquid and rapidly quenched Ni-Al (NiAl₃, NiAl, and Ni₃Al) alloys were studied by ab initio molecular dynamics simulation. Pair correlation function, structure factor, bond pair, electron population, and density of states were calculated. It is found that amorphous Ni-Al alloys can be prepared by rapid quenching of liquid alloys, with the former bearing similar structures to the latter, although amorphous alloys have a more ordered structure. Bond pair analysis indicates that both the liquid and amorphous Ni-Al alloys consist mainly of 1441, 1431, 1421, and 1422 pairs of tetrahedral local order. The positions of the first peaks of the Al-Ni pair correlation functions are lower than the sum of the metallic radii of Ni and Al, suggesting the occurrence of chemical bonding between Ni and Al in Ni-Al alloys. Electronic structure analysis further revealed that the interaction between the d-electrons of Ni and the p-electrons of Al is responsible for the bonding. The main peak positions of the total DOS of amorphous Ni-Al alloys become more and more positive when the content of Ni in the alloys increases.     

Quasicrystalline and related phases in titanium based alloy systems

Amongst the non-aluminium based quasicrystalline alloys, investigations of titanium containing alloys in regard to the occurrence and stability of quasicrystalline phases have aroused considerable interest in recent years. Employing X-ray and TEM techniques a systematic investigation of the influence of substitution for Fe by Si and Ni on the stability of icosahedral phase in rapidly quenched Fe-Ti alloy has been carried out. The occurrence of metastable phases including commensurately modulated phase and the decagonal phase in Fe-Ti-Si system have been found. In addition, the occurrence ofstructural disorder manifested by arcs of diffuse scattering in diffraction patterns and anisotropy in the shape of idffraction spots has also been pointed out. It has been observed that 6 at.% of Si in Fe-Ti-Si system results in the formation of single-phase icosahedral quasicrystals. We have shown that contrary to some earlier reported results on Ti₂Ni, Ti₂Ni(Si) and Ti₅₆Ni₂₃Fe₅Si₁₆ alloys do not possess the icosahedral phase. In Ti₆₈Fe_26−xNi_xSi₆ alloy system, icosahedral phase formation ability is limited to the value of x < 9. The occurrence of icosahedral phase in these alloy systems has been analysed in terms of e/a ratio.     

EXAFS studies of the Ni66Y33 and Cu60Zr40 amorphous alloys: A modelling of the pair distribution function with two subshells

Disorder effects in amorphous Ni₆₆Y₃₃ and Cu₆₀Zr₄₀ alloys have been investigated by the EXAFS technique. EXAFS spectra have been fitted using a double pair distribution function for the TM-TM (transition metal) pairs and for the unlike atom pairs. Both alloys have been found to be chemically ordered. For Cu₆₀Zr₄₀ the amorphous structure is very similar to the crystalline one.     

Formation and properties of Fe-based amorphous/nanocrystalline alloy coating prepared by wire arc spraying process

Wire arc spraying process was used to deposit FeBSiCrNbMnY amorphous/nanocrystalline alloy coating onto stainless steel substrate. The microstructure of the coating was characterized by using X-ray diffraction (XRD), scanning election microscopy (SEM) equipped with energy dispersive X-ray analysis (EDXA) and transmission electron microscopy (TEM). The coating is about 500 μm in thickness with fully dense and low porosity. The microstructure of the coating is classified into two regions, namely, a full amorphous phase region and homogeneous dispersion of α Fe, Cr nanoscale particles with a scale of 30–60 nm in a residual amorphous matrix region. The formation mechanism of the amorphous and nanocrystalline alloy was discussed. Mechanical properties, such as microhardness and wear resistance of the coating were also analyzed. The Vickers hardness of the coating is around Hv = 900–1050. The relatively wear resistance of the amorphous/nanocrystalline alloy coating is about 3× than that of crystalline structure 3Cr13 martensite stainless steel coating under the same wear testing condition. The FeBSiCrNbMnY amorphous/nanocrystalline alloy coating has high microhardness and excellent wear resistance.     

Phase separation and crystallization in amorphous Pd-Si-Sb

It is found that replacing some of the Si in a Pd-Si-Sb glass with Sb can make it more resistant to crystallization than the pure binary material. This is shown by the calorimetric results, in which the crystallization temperature increases and the glass temperature decreases with increasing Sb substitution, and also by the finding that certain alloys, such as Pd_80.5 Si_16.5Sb₃ may be quenched into amorphous rods (by the gradient-quench method) up to 0.5 mm thick.

Calorimetry and small-angle X-ray scattering data show that certain alloys in this system phase separate in the amorphous state. The two phases thus produced independently crystallize. Isothermal measurements indicate that the rate of crystallization of one of the phases is interface-limited, with most nuclei initially present. The other crystallization process does not fit an Avrami law, possibly due to a transformation of the crystalline phase.

A method called “gradient-quenching” (GQ) was used to prepare samples from the melt in such a way that the quench-rate was non-uniform, so all stages of crystallization in slowly-quenched alloys are displayed. Nucleation was frequently found to take place on voids and inclusions.

The use of the calorimetric and GQ methods in conjunction to gather information about crystallization processes in metallic glasses is discussed.     

Crystallization of amorphous Ni35Zr65 and Fe40Ni40P14B6 under proton irradiation

Two amorphous alloys, Ni₃₅Zr₆₅ and Fe₄₀Ni₄₀P₁₄B₆, were irradiated using 400 keV protons at several temperatures below the crystallization temperature, T_x, to peak doses in the neighborhood of 3.5 to 4.5 dpa. Irradiation at 250°C resulted in the crystallization of both alloys, which were examined by transmission electron microscopy of samples electrolytically polished to various distances from the irradiated surface to study the effect of dose. Samples masked from the proton beam remained amorphous during irradiation. In the Ni₃₅Zr₆₅ alloy crystallization of the equilibrium phases propagated throughout the entire sample, while the in the Fe₄₀Ni₄₀P₁₄B₆ alloy crystallization was observed only in those parts of the samples lying within the proton range. Neither alloy crystallized during irradiation at 100°C. In both these alloys the amorphous phase is therefore evidently stable at irradiation temperatures below approximately 0.6 T_x. An examination of the literature on irradiation damage of binary alloys and intermetallic compounds suggests that there is a tendency for initially amorphous alloys to remain amorphous at irradiation temperatures, T_irr < 0.3 T_L, where T_L (≈T_x) is the “melting” temperature (either a eutectic, peritectic or congruent melting temperature). Also, these same alloys, even when they are initially crystalline, transform to the amorphous state during irradiation at T < 0.3 T_L. Some other crystalline alloys have also been shown to transform to the amorphous state at T_irr < 0.3 T_L even though they have never been prepared in this condition by rapid quenching techniques. The temperature 0.3 T_L appears to be a lower limit, however, since the crystalline to amorphous transformation occurs in many of these alloys at temperatures greater than 0.3 T_L. It is suggested, by analogy with results on void formation in irradiated metals, that this low temperature limit is related to the low mobility of vacancies in these materials, although the mechanism of crystallization, or conversely amorphization, is not fully understood.     

Degree of local structural ordering and its correlation with the glass forming ability of alloys

The degree of local structural order in amorphous solids and liquids has been investigated in CuHf alloys for the first time. X-ray diffraction experiments and thermal analysis show that the local structural order is strongly composition-dependent. For the CuHf alloys, it has been found that a close relationship exists between the degree of local ordering and the glass forming ability. Comparisons between the present alloys and those studied previously show that local structural order in glass formation is determined by the dominant vitrification conditions (diffusion-controlled or nucleation-controlled). The local ordering structures act as random fields against crystallization when the primary crystallization mechanism for the undercooled melts is the formation of intermetallic crystals.     

Metastable microheterogeneity of melts in eutectic and monotectic systems and its influence on the properties of the solidified alloy

Experimental results that are the basis for the existence of metastable microheterogeneities in melts of eutectic and monotectic systems are summarized. Among these, recent results of small angle neutron scattering experiments are discussed in some detail. It is shown that the influence of a homogenizing heat treatment of a melt on the structure of crystalline castings and amorphous ribbons obtained after its solidification is very important.