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.     

Magnetic resonances of Fe and Ni nanoparticles in films of silicon suboxide produced by ion irradiation of triethoxysilane gels containing Fe or Ni solute atoms

The structure and magnetic properties of composite films obtained by ion irradiation or thermal treatment of triethoxysilane gels containing iron or nickel in solution are investigated by means of X-ray diffraction, electron microscopy and electron spin resonance. Ion irradiated gels are converted into silica glasses containing metallic nanoparticles with a narrow range of sizes. These particles exhibit a magnetic resonance with a preferential alignment of magnetic moments perpendicular to the surface. This unusual out of plane anisotropy seem to be ascribed to an interaction of the moments with radiation defects. The magnetization in irradiated film is comparable to that in films of same materials submitted to a thermal conversion at 1000 °C in vacuum. But the latter films are porous and contain particles of metal and silicide or silicate with a wider range of sizes.     

Characterization of the structure of calcium alumino-silicate and calcium fluoro-alumino-silicate glasses by magic angle spinning nuclear magnetic resonance (MAS-NMR)

Calcium aluminosilicate and calcium fluoro-aluminosilicate glasses have been characterized by ²⁹Si, ²⁷Al and ¹⁹F MAS-NMR. The two calcium aluminosilicate glasses examined were based on the composition 2SiO₂ · Al₂O₃ · 2CaO (ART1) and the mineral anorthite 2SiO₂ · Al₂O₃ · CaO (ART2). The observed chemical shifts for ²⁹Si and ²⁷Al agreed with previous studies. The fluorine containing glasses were based on 2SiO₂ · Al₂O₃ · (2−X)CaO · XCaF₂. The ²⁹Si chemical shift moved in a negative direction with increase fluorine content indicating a progressive reduction in the average number of non-bridging oxygens, NBO, attached to a silicon. The ²⁷Al spectra indicated the presence of four coordinate aluminium in the glasses with X=0.0-0.75, but aluminium was present in Al(IV), Al(V) and Al(VI) coordination states in the highest fluorine content glass with X=1.0. The ¹⁹F spectra indicated the presence of F-Ca(n) in low fluorine content glasses and both F-Ca(n) and Al-F-Ca(n) in high fluorine content glasses. We speculate here that the Al-F-Ca(n) species are oxyfluorides [AlO_xF_y]ⁿ⁻, where x=1-6, y=1-6 and n is the charge on the total complex when aluminium is in Al(IV), Al(V) and Al(VI) coordinate states. The reduction in the average number of NBO per silicon with increasing fluorine content is explained by fluorine converting Ca²⁺ to F-Ca(n).     

The influence of solute distribution on the high nucleation density of Al crystals in amorphous aluminum alloys

An extension of a previous model that describes the role of solute atoms on glass formability leads to the conclusion that solute distribution plays an important role in the formation and stability of amorphous metals. A random distribution of solutes is shown to produce local solute-depleted regions (on the size scale of the mean inter-solute spacing) that provide preferred sites for the formation of crystalline nuclei. The possibility that these solute-depleted regions are responsible for the exceptionally high number density of critical Al nuclei is explored for three Al-Y binary alloys using a computer simulation. Up to 10⁷ Y atoms were placed at random locations in the system, and the number of solute-free regions were counted as a function of the size of these regions. The experimentally observed number density of critical nuclei (∼3 × 10²¹ m⁻³) is reproduced for a critical nucleus about 5 Al atoms in diameter, containing ∼60 Al atoms in an fcc array. Good agreement with previous suggestions of the size of a critical nucleus (about 6 atoms in diameter, containing about 100 atoms) support the conclusion that the current model provides a reasonable physical explanation for the quenched-in features responsible for the exceptionally high nucleation density in some amorphous Al alloys.     

Effect of Co substitution for Ni on the Ti–Zr–(Ni, Co) pseudo-ternary quasicrystal formation

A series of alloys designed along a Ti₉(Ni,Co)₄–Zr e/a-variant line in the Ti–Zr–(Ni, Co) pseudo-ternary alloy system, are investigated by XRD and TEM. Experimental results show that bulk icosahedral quasicrystals (IQC) prepared by copper mold suction casting, can be obtained in a large region from Ti₅₅Zr₂₀(Ni,Co)₂₅ to Ti_34.6Zr₅₀(Ni,Co)_15.4 with Ni/Co ratio equal to 3/1, and in a sharply diminished region from Ti₄₈Zr₃₀(Ni,Co)₂₂ to Ti₄₂Zr₄₀(Ni,Co)₁₈ with decreased Ni/Co ratio equal to 2/2 or 1/3. Ti₄₈Zr₃₀Ni_16.5Co_5.5 is the optimal quasicrystal-forming composition, where a nearly pure bulk IQC phase is achieved. The stepwise substitutions of Co for Ni in this pseudo-ternary system contribute to the formation of Ti₂Ni, fcc-Zr₂Ni, C14-type Laves and β-(Ti, Zr) solid solution phases. However, these substitutions greatly reduce the forming ability of the IQC phase. It is suggested that Co is essentially unfavorable to Ti/Zr-based quasicrystal formation.     

A comparison of the structures of Ni---Zr amorphous alloys obtained by mechanical alloying and melt spinning

The structures of mechanically alloyed (MA) and melt-spun (MS) Ni_xZr_1-x (x = 0.28, 0.33, 0.40) amorphous alloys were investigated by X-ray diffraction. The interatomic distances, r_NiZr and r_ZrZr, were calculated from the deconvolution of the principal peak of the radial distribution function (RDF) using Gaussian functions. The distances were nearly equal for the amorphous alloys of the same composition prepared by the two methods, i.e., 2.69 and 2.67 Å and 3.28 and 3.22 Å on average for the MA and MS specimens, respectively.     

Crystallization of the amorphous phase and martensitic transformations in multicomponent (Ti,Hf,Zr)(Ni,Cu)-based alloys

Shape memory materials with specific microstructure can be obtained from amorphous precursors. Rapidly solidified amorphous and crystalline–amorphous ribbons have been produced by planar flow casting for a number of multicomponent TiNi-based alloys of pseudobinary 50:50 and 55:45 compositions with substitutions of Zr, Hf, Nb for Ti and Cu, Co, Pd, Ag, Al for Ni. The glass transition and crystallization behavior of the amorphous phase as well as the martensitic transformations in the crystallized materials have been studied by differential scanning calorimetry. To control the crystal structure at different stages of crystallization, X-ray diffractometry and transmission electron microscopy were used. The effects of crystallization conditions and the resulting microstructural state on the martensitic transformation characteristics have been investigated. The transformation temperature intervals are shown to depend on the mean size of the parent phase crystals.     

Effect of heat treatment on transport and magnetic properties of Co-based amorphous alloys

Magnetotransport and magnetic properties of Co₆₆Fe₄B₁₅Si₁₅ amorphous ribbons, annealed in vacuum and in open air for various times, are studied. The air-annealed samples exhibit the presence of the near-surface hard magnetic layer. The anomalous Hall effect coefficient and the resistivity change simultaneously under heat treatment with slightly non-linear correlation between them. Positive magnetoresistance in low fields changes to negative magnetoresistance with increase of magnetic field. We explain both positive and negative magnetoresistance in terms of spin-dependent scattering.     

Structural investigation of amorphous FeZr,CoZr and NiZr alloys with low zirconium concentration

A structural investigation has been made of alloy glasses a low concentration of zirconium: compositions M_100?xZr_x with M = Fe, Co and Ni, and x = 9 at.% using X-ray diffraction. The characteristic second peak splitting in the radial distribution function is found for all samples presently investigated. The partial radial distribution functions of amorphous FeZr and CoZr alloys were derived from the measured total distribution function data by applying the concentration method with the anomalous scattering technique. The amplitude in the oscillation of the radial distribution function for M-M and M-Zr pairs is more enhanced in comparison with that of glasses with high Zr content, x = 40–45 at.%. The estimated coordination number of nearest neighbor Fe atoms (11.6 ± 0.5) for iron in the Fe₈₄Zr₁₆ glass is larger than that (10.7 ± 0.5) found in the Fe₈₃B₁₇ glass. This is consistent with the measurements of magnetic properties of these glasses with low zirconium concentration. The possible structural features of intertransition metal alloy glasses with low zirconium concentration is also discussed based on the present experimental data.     

Thermal and magnetic behavior of a nanocrystalline Fe(Ni, Co) based alloy

A nanocrystalline alloy of Fe₅₁Co₂₀Ni₁₄Zr₆B₉, was produced by mechanical alloying. The main structural analysis was carried out via X-ray diffraction. The spectra were analyzed by Rietveld refinement using the MAUD program. The as-milled, 10, 20, 40 and 80 h alloys consisted primarily of metastable bcc-Fe(Co, Ni) nanocrystals reaching around 8 nm mean diameter, after 80 h. The morphology was examined via SEM and composition by ICP and EDX. Results show a slight (<1.5 at.%) contamination from milling tools. Differential scanning calorimetry data reveal broad hump between 100 and 450 °C associated with the relief of internal stresses. The exothermic peaks over 450 °C correspond to the crystallization related initially to the crystalline growth of the bcc-Fe(Co, Ni) phase and to the formation of fcc-(Fe, Ni, Co) and (Fe, Ni, Co)₂B phases at higher temperatures. Frequency-dependent AC-susceptibility on the 80 h alloy confirms the presence of nanoparticles displaying a wide distribution of sizes favoring the existence of disordered magnetic interactions among them through the nanoparticle boundaries.     

Effect of Nb in the nanocrystallization and magnetic properties of FeNbBCu amorphous alloys

The crystallization of melt-spun Fe_79?xNb_5+xB₁₅Cu₁ (x = 0, 2, 4) ribbons has been studied by differential scanning calorimetry and X-ray diffraction. A primary crystallization of bcc-Fe nanoparticles embedded in an amorphous matrix, followed by the precipitation of metastable borides from the residual matrix at higher temperatures is observed. The characteristic temperatures of crystallization events change with Nb concentration. The results obtained from thermal and structural characterization are related to the magnetic properties of the sample. A dependence of the magnetic behavior with the Fe/Nb content in the alloy is also unveiled. The decrease of Nb content in the alloy leads to an enhancement of both the saturation polarization and the Curie temperature due to variations in the exchange coupling between Fe atoms. However, the maximum values of magnetic entropy change do not vary appreciably among the three amorphous alloys. In nanocrystalline samples the amount of the nanocrystalline transformed fraction seems to be the main reason for the change in the saturation polarization of the sample.     

On the electronic transport properties of amorphous (Si,Ge) alloys: charged scattering centers and compositional disorder

The electronic transport properties of a-(Si,Ge):H alloys over the entire range of Ge content (0-100%) grown using low pressure, reactive ECR plasma deposition with high H dilution and subtle (sub-ppm) B-doping have been investigated in detail by employing the microwave photomixing technique. Strong evidence for the existence of long-range potential fluctuations in a-(Si,Ge):H alloys has been found from the measurements of electric field dependence of the drift mobility. It was observed for the first time that the film transport properties do not degrade monotonically with increasing Ge content; there exists a valley point near the middle of the composition range, where the strongest potential fluctuations occur as a result of a significant increase in the charged defect density. Beyond this point, the film quality increases again, but still much worse than that for the Si end. The effect of potential fluctuations can be enhanced by adding Ge or Si to the alloy system resulting in deterioration of the transport properties of a-(Si,Ge):H alloys. Our present results demonstrate that the increased charged scattering centers and compositional disorder upon adding Ge or Si to the alloys play an important role in the potential fluctuations.     

Effect of Al substitution on the magnetic properties of amorphous Fe73.5Cu1Mo3Si13.5−xAlxB9 alloy

We have synthesized FINEMET type amorphous Fe_73.5Cu₁Mo₃Si_13.5−xAl_xB₉ alloy by the single wheel melt spinning technique. The effect of Al substitution on the magnetic properties has been studied using a vibrating sample magnetometer, SQUID and Mössbauer spectroscopy. Magnetization and Curie temperature of the amorphous phase of the alloys were found to decrease with Al concentration. The results are attributed to the dilution effect of Al on the magnetic moment of Fe and to the increase in Fe–Fe interaction distance resulting in the weakening of exchange interaction.     

Magnetic susceptibility and atomic structure of paramagnetic Zr–(Co, Ni, Cu) amorphous alloys

The experimental results for the magnetic susceptibility and superconducting transition temperature of amorphous Zr–(Co, Ni, Cu) alloys extending over a wide composition range are analyzed in some detail. By combining the results of this analysis with the literature results for the electronic density of states at E_F for the same alloy systems we obtained a set of parameters associated with the electronic structure of the amorphous Zr. The comparison of these parameters with the results of the band structure calculations for different crystalline phases of Zr and with the results of the atomic structure and crystallization studies of the same alloy systems indicates an fcc-like local atomic structure for amorphous Zr.     

New AlDy(Ni,Co) glasses - the influence of pre-aging on decomposition and modelling of the crystallisation

The decomposition of AlDy₆Ni₁₀ and AlDy₈Ni₆Co₂ glasses prepared by melt spinning after different pre-aging regimes was investigated. The modelling of the transformation at the glass transition and the crystallisation was tested by a more general Johnson-Mehl-Avrami-Kolmogorov model and by the determination of the local kinetic parameter. The relaxation was tested with an lnt kinetic model after Gibbs.     

Effect of substitution of rare earth by mischmetal on the devitrification process of Al-X-Ni-Co (X = Y, Ce, Mm) alloys

The effect of substitution of Ce or Y by mischmetal (Mm) on the glass transition and the kinetics of crystallization of quaternary Al-Ni-Co-(Ce, Y, Mm) alloys was studied using isothermal and non-isothermal calorimetry and X-ray experiments. The alloy containing Mm shows more similarity to Ce containing alloy in all the studied properties than to Y containing alloy. The glass transition is less evident for Ce and Mm containing alloys than for the alloy containing Y. For the two former alloys, the first and the second crystallization stages are strongly overlapped and yield a fully crystallized alloy. Whereas for Y alloy, the first and the second stages are well resolved and the primary crystallization yields nanosized α-Al particles embedded in a residual amorphous matrix.     

Amorphization and crystallization processes of the ball-milled Al–Y–Fe–TM alloys (TM = Ni,Co, Cu,and Fe)

High-energy ball milling was used to synthesize aluminum-based alloys containing amorphous and nanocrystalline phases to investigate the compositional effects of transition metals (TM) on the amorphization and crystallization processes of the ball-milled Al₈₅Y₇Fe₅TM₃ alloys (TM = Ni, Co, Cu, and Fe) were investigated. The crystallization kinetics of the ball-milled Al–Y–Fe–TM nanocomposite powders were studied using differential scanning calorimetry (DSC). The DSC results of Al₈₃Y₇Fe₅Ni₅ show that the crystallization temperature and the activation energy of crystallization are 668 K and 310 kJ/mol, respectively. In-situ high-temperature X-ray diffraction showed that the crystallization was a complex process involving growth of the nanocrystalline phase along with crystallization of the amorphous matrix phase.     

Effects of Sn addition on the glass forming ability and crystallization behavior in Ni-Zr-Ti-Si alloys

The effect of Sn substitution for Si on the glass forming ability (GFA) and crystallization behavior has been studied in Ni₅₉Zr₂₀Ti₁₆Si_5 − xSn_x (x=0, 3, 5) alloys. A bulk amorphous Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ alloy with diameter up to 3 mm can be fabricated by injection casting. Partial substitution of Si by Sn in Ni₅₉Zr₂₀Ti₆Si_5 − xSn_x alloys improves the glass forming ability. The improved GFA of the Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ alloy is can be explained based on the lowering of liquidus temperature. The crystallization sequence becomes completely different with addition of Sn. The amorphous Ni₅₉Zr₂₀Ti₁₆Si₅ alloy crystallizes via precipitation of only a cubic NiTi phase in the first crystallization step, whereas the amorphous Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ alloy crystallizes via simultaneous precipitation of orthorhombic Ni₁₀(Zr,Ti)₇ and cubic NiTi phases. Addition of Sn in the Ni₅₉Zr₂₀Ti₁₆Si₅ alloy suppresses the formation of the primary cubic NiTi phase. The bulk amorphous Ni₅₉Zr₂₀Ti₁₆Si₂Sn₃ alloy exhibits high compressive fracture strength of about 2.7 GPa with a plastic strain of about 2%.     

Synthesis and characterization of nanocrystalline Fe60X20B10P10 (X = Co,Ni) alloys

In this work, two nanocrystalline alloys, Fe₆₀Ni₂₀P₁₀B₁₀ and Fe₆₀Co₂₀P₁₀B₁₀, were produced by mechanical alloying processing them at milling times of 5, 40 and 100 h. Structural properties were determined by X-ray diffraction and transmission Mössbauer spectroscopy. The 100-h milled alloys consisted primarily of metastable bcc Fe(Co, Ni) nanocrystals (11–17 nm) with different Fe-rich environments. Differential scanning calorimetry measurements were performed in order to study the thermal stability of the nanocrystalline phases and their crystalline growth. The values for the apparent activation energy of the main crystallization processes were 2.1 ± 0.1 and 2.4 ± 0.1 eV, respectively, which were associated with grain growth mechanisms.     

X-ray absorption analysis of core/shell magnetic (Fe, Co)–B nanoparticles of amorphous and crystalline structure obtained by chemical reduction

Core/shell nanoparticles of M–B (M = Fe, Co) with amorphous and mixed amorphous + bcc-Fe core have been synthesized by chemical reduction of metallic salts by sodium borohydride. The XANES spectra show the existence of a surface layer consisting of metallic hydroxides in a percentage around 15 at.%. The EXAFS reveals the existence of short-range order in the amorphous core with a contribution from the M–O pairs of the shell and a variation of the coordination numbers with respect to bulk alloys. The existence of Fe-bcc in the mixed alloys is observed in the Fourier transform of the EXAFS data.