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.     

Electrical resistivity evolution in the annealed amorphous Fe78Si9B13 ribbons

The electrical resistivity was measured at room temperature for the amorphous Fe₇₈Si₉B₁₃ ribbons annealed at various temperatures for different holding time. Although the annealed Fe₇₈Si₉B₁₃ ribbons are in full amorphous state, their electrical resistivity obviously varies with the annealing time. At every annealed temperature, the electrical resistivity evolution can be divided into regions I, II, and III, respectively. Using X-ray diffraction (XRD), scanning electron microscope (SEM), and differential scanning calorimetry (DSC), we investigated the ribbons overlapping regions I and II (called the focused ribbons, FRs). The results show that the change of electrical resistivity, fracture morphology, thermal effect in DSC analysis of the focused ribbons (FRs) can be ascribed to the evolution of the short range order (SRO) in the amorphous alloy.     

Liquid phase separation in immiscible Ag-Ni-Nb alloy and formation of crystalline/amorphous composite

Phase decomposition in liquid miscibility gap presents a unique opportunity for designing amorphous/crystalline composites with a multilayer structure. One of typical monotectic systems, Ag-Ni alloy, is selected, and element Nb is added in the Ag-Ni alloy. The new ternary Ag-Ni-Nb monotectic alloys are rapidly quenched by using melt spinning technique. It is revealed that the ternary alloy undergoes a liquid-liquid phase decomposition and forms two immiscible Ag-rich and NiNb-rich liquids. Subsequently, the immiscible liquids solidify into Ag-rich crystalline and NiNb-based amorphous phases, respectively. Self-assembled amorphous/crystalline composites with double layer and sandwich type structures have been developed. The structure of the alloy ribbons with a dependence on the composition changes from a particle-dispersed structure to double layer structure and to sandwich type structure. The phase formation and thermal property of the quenched alloys are investigated. The formation mechanism of the crystalline/amorphous composite structures has been discussed.     

Characterization of immiscible Ni78Ag2P20 alloy and formation of amorphous/crystalline composite

This paper presents characterization of the ternary Ni₇₈Ag₂P₂₀ alloy that was arc-melt in argon protective atmosphere and melt spun in helium. The microstructure of the arc-melt droplet was investigated in scanning electron microscope with EDS. The microstructural studies of the melt-spun alloy was made using light microscope, transmission electron microscope and X-ray diffraction. A tendency for separation of the alloy was found and the phases were identified. The mechanical properties were evaluated with use of micro-hardness measurements and the results compared with the available data.     

Mechanical behavior under nanoindentation of a new Ni-based glassy alloy produced by melt-spinning and copper mold casting

An investigation was made into the thermal stability and mechanical behavior under nanoindentation of a new glassy alloy with composition Ni₅₀Nb₂₈Zr₂₂, produced in the form of melt-spun ribbons and copper mold-cast wedges. The alloy composition was designed based on the lambda criterion combined with the electronegativity difference among the elements. X-ray diffraction and scanning electron microscopy confirmed that the ribbons and wedges (up to 200 μm in thickness) are amorphous. The thermal properties of these samples were evaluated by differential scanning calorimetry (DSC). Nanoindentation revealed that the hardness of this alloy, around 10 GPa, is among the highest reported for metallic glasses. Remarkably, the cast wedge exhibits greater hardness and higher elastic modulus than the ribbon. This correlates with the larger amount of frozen-in free volume in the ribbons than in the cast wedges, as evidenced by DSC. In addition, finite element simulations of nanoindentation curves were performed. The Mohr-Coulomb yield criterion allows for better adjustment of the experimental data than the pressure-independent Tresca yield criterion. The simulations also reveal that the cohesive stress in the ribbons is lower than in the wedges, which explains the difference in hardness and Young's modulus between the two samples.     

Phase transformation and crystallization kinetics of melt-spun Ni60Nb20Zr20 amorphous alloy

The glass transition and crystallization kinetics of melt-spun Ni₆₀Nb₂₀Zr₂₀ amorphous alloy ribbons have been studied under non-isothermal and isothermal conditions using differential scanning calorimetry (DSC). The dependence of glass transition and crystallization temperatures on heating rates was analyzed by Lasocka's relationship. The activation energies of crystallization, E_x, were determined to be 499.5 kJ/mol and 488.6 kJ/mol using the Kissinger and Ozawa equations, respectively. The Johnson–Mehl–Avrami equation has also been applied to the isothermal kinetics and the Avrami exponents are in the range of 1.92–2.47 indicating a diffusion-controlled three-dimensional growth mechanism. The activation energy obtained from the Arrhenius equation in the isothermal process was calculated to be E_x = 419.5 kJ/mol. The corresponding three dimensional (3D) time–temperature–transformation (TTT) diagram of crystallization for the alloy has been drawn which provides the information about transformation at a particular temperature. In addition, the intermetallic phases and morphology after thermal treatment have been identified by X-ray diffraction (XRD) and scanning electron microscope (SEM).     

Estimation of critical cooling rates for formation of amorphous alloys from critical sizes

The critical cooling rate for marginal glass formers, like Al-based alloys, is difficult to measure experimentally. In this work, we acquired the critical cooling rate for formation of amorphous alloys by estimating the cooling rates of the specimens with critical dimensions. Analytical solutions were given to estimate the cooling rates for the gas-atomized powders and melt-spun ribbons, and as an example the critical cooling rate to form an amorphous Al₈₂Ni₁₀Y₈ alloy was estimated to be ~ 1.0 × 10⁶ K s^? 1. The effect of melt temperature on the cooling rate was quantitatively evaluated and its effect on the glass forming ability was also discussed.     

The nature of phase separation in binary oxide melts and glasses. III. Borate and germanate systems

A review of immiscibility data in binary borate and germanate systems was performed in order to compare miscibility gap consolute temperatures with ionic potentials and radii of their associated cations. The trends obtained demonstrate that a selective solution mechanism similar to the one identified for the binary silicate systems is present in the borate and germanate binaries. More importantly, the borate and germanate immiscibility data permitted the identification of a new group of cations depicting an immiscibility behaviour different from the ones identified in binary silicate systems. The new group involves highly polarizable cations possessing a lone pair of electrons. This lone pair of electrons together with oxygen bonded by strong covalent bonds to modifier cations provides efficient shielding to the cations' nuclei which considerably reduces the coulombic repulsions and produces miscibility gaps with very low consolute temperatures. A new group of cations having an homogenizing effect on melts (i.e. a capacity to make immiscible melts single phase) is thus reported. Experimental and spectroscopic data suggest that miscibility gaps associated with cations having a lone pair of electrons exist in binary silicate systems such as TlO_1/2-SiO₂, PbO-SiO₂, SnO-SiO₂ and Bi₂O₃-SiO₂. The consolute temperature of their miscibility gaps is expected to be relatively low and metastable.     

Properties characterization of the Cu68.5Ni12P19.5 alloy at elevated temperatures

The Cu_68.5Ni₁₂P_19.5 alloy was cast into the ribbons using melt spinning (23 m/s). The amorphous ribbon in the as-cast state was investigated by differential thermal analysis (DTA), dynamic mechanical analysis (DMA), resistivity measurements and X-ray diffraction ‘in situ’ at different temperatures. The work presents attempts to find correlation between the changes of the mechanical properties presented by DMA cycles and during the other tests. The measurements of the relative resistivity R/R₀ versus temperature for repeated heating and cooling cycles to different temperatures show changes of the temperature coefficient of resistivity (TCR) indicating reversible and irreversible transformations in the studied alloy.     

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.     

Crystallization behavior of e-beam evaporated Ga5Ge15Te80 thin films

Ga₅Ge₁₅Te₈₀ thin films have been deposited by e-beam evaporation method. The chemical composition of the deposited films was identified using energy dispersive X-ray spectrometry. The electrical conductivity, σ of the deposited films during heating/cooling cycles was investigated in the temperatures 298–570 K. The conductivity curve showed two sudden upward trends during the first heating cycle. The first upward trend occurs in the temperature range 408–430 K and was attributed to the amorphous-to-crystalline phase transformation. While the second is in the temperature range 470–495 K, and can be attributed to the crystallization process. However, for second heating cycle the conductivity curve becomes reversible. The optical band gap of the as-deposited and annealed film at annealing temperature 423 K was determined from the recorded transmittance and reflectance spectra. The obtained results were confirmed throughout the X-ray and transmission electron microscope studies.     

Effects of sintering temperature on microstructure and property evolution of Fe81Cu2Nb3Si14 soft magnetic materials fabricated from amorphous melt-spun ribbons by spark plasma sintering technique

Bulk Fe₈₁Cu₂Nb₃Si₁₄ soft magnetic materials were fabricated by spark plasma sintering (SPS) of amorphous powder, which was prepared by ball milling of melt-spun ribbons. Effects of sintering temperature on the evolution of microstructure and related properties were systematically investigated. Results show that the as-milled powders exhibit similar microstructure and thermal property in comparison with the original melt-spun ribbons. With an increase in the sintering temperature, the relative density, microhardness and saturation magnetization of the sintered samples improved obviously, but the coercive force decreased at the beginning and then increased with the increase of sintering temperature. The sintered samples exhibit typical soft magnetic characteristic. The desirable soft magnetic property of the sintered samples was achieved by SPS at 630 °C. Meanwhile, microstructure and densification behaviors of the sintered samples were also investigated.     

Simulation of the spinodal phase separation dynamics of the Bi–Zn system

In the phase separation occurring at the miscibility gap (at the spinodal region) of an alloy a discrete symmetry is spontaneously broken and a domain wall network is formed. Field theory simulations are often used to study the dynamics of topological defects networks appearing in different physical contexts. In this work, we focus on the dynamics of the two immiscible liquids appearing on the phase diagram of the Bi–Zn system, one of the basic systems of lead free solders. We use phase field simulations to quantitatively simulate the dynamics of the two liquids separation in the Bi–Zn system, at different temperatures and for different concentrations. We obtain the miscibility gap curve and the domain morphologies of the system as a function of time, temperature and component concentrations using simulations.     

Effects of phase separation on crystallization behavior

The effects of phase separation on the kinetics of crystal growth have been investigated for three compositions in the Na₂OSiO₂ system, containing about 1.5, 10 and 15 mol% Na₂O. In the two cases for which reliable data could be obtained over the full range of temperature (10 and 15% Na₂O), the measured growth rates are independent of time, for crystallization temperatures within as well as outside the miscibility gap. In both cases, the growth rate versus temperature relations are continuous through the region of the immiscible boundary. These results are related to previous data on the effects of phase separation on viscosity in the Na₂OSiO₂ system and on nucleation and crystallization in the Li₂OSiO₂ system. It is suggested that transport at the crystal-liquid interface in a phase-separating system involves different types of atomic rearrangements than those involved in viscous flow.     

Effect of Co substitution for Ni on the microstructure and magnetic properties of (Fe, Ni)-based amorphous alloys produced by melt spinning

Transmission electron microscopy with selected area electron diffraction, and X-ray diffraction were applied to study the effect of Co substitution for Ni in (Fe, Ni)-based amorphous alloys. The investigation was performed to obtain information on correlations between microstructure and magnetic properties of the (Fe, Ni, Co)-based amorphous alloys. The examination of the microstructure reveals that there are small crystallized free surface regions because the actual quenching rate is distributed inhomogeneously over the cross-section of the ribbon. Since in the free surface region, the solidification rate is lower, a spontaneous annealing process occurs at the top surface of the ribbon. The crystallization degree of the free surface region is higher for alloy ribbons that contain Co up to a 15 at.% concentration. Magnetic domains pattern are sensitive to the surface crystallization and Co content of the (Fe, Ni)-based alloy ribbons. Fine-scaled stripe domains were evidenced on the free ribbon surface while on contact surface stress domain pattern appeared. With the increase of the Co content, the domain width became small and long stripes appeared. The striped domains are responsible for an increased coercivity of the ribbons. However, there is a critical Co content (x_Co = 10) for which spontaneous narrow stripe domains are no longer more energetically favourable for ribbons with specific magnetic applications.     

Intergrowth and stacking faults in the Sm5Co19 Phase

An electron microscope observation was made on the binary Sm-Co alloy. One-dimensional lattice images of Sm₅Co₁₉ taken with the incident beam parallel to the [010] and [110] directions of the hexagonal axis showed many stacking faults. The intergrowth with the Sm₂Co₇ phase was observed in the matrix of the Sm₅Co₁₉ phase, the analysis being based on the thickness of blockj layers estimated by the distance of lattice fringes.     

Growth and Electron Microscopical Investigations of (CaBa)F2 Epitaxial Layers on GaAs and InP Substrates

(CaBa)F₂ layers have been grown on (100)-oriented GaAs and InP substrates by flash evaporation technique. They were investigated by means of electron microscopical methods (RHEED, TED, TEM). Epitaxial growth was found at temperatures in the range of 725 to 825 K for GaAs substrates and 700 to 800 K for InP substrates, respectively. The films showed a fine-grained structure and consisted of two phases always with a composition of approximately Ca_0.99Ba_0.01F₂ and Ca_0.08Ba_0.92F₂. This is in accordance with the occurrence of a miscibility gap observed in the binary CaF₂ BaF₂ system.     

Nanostructural aspects, free volume and phase constitution of rapidly solidified Nd-Fe-B

A complement of experimental characterization techniques - positron annihilation spectroscopy (PAS), transmission electron microscopy, synchrotron X-ray diffraction and elevated temperature ac and dc magnetic measurements - were applied to the study of melt-quenched stoichiometric Nd₂Fe₁₄B ribbons modified by various amounts of the alloying additions Ti and C. These alloying additions are known to enhance the glass-forming ability in Nd₂Fe₁₄B melts, allowing for a wider processing window to produce homogeneous nanoscale materials with tailored magnetic properties. The experimental techniques used in this study reveal the complex multi-phase and multi-scale nature of the ribbons, a result that had escaped detection by lower-resolution techniques. The as-quenched ribbons were found to consist of at least three phases: α-Fe nanocrystals, poorly crystalline Nd₂Fe₁₄B and glass. The measured weight fraction of glass does not show a direct correspondence with quenching wheel speed, a result attributed to the complexity of the melt-spinning heat-transfer process. The Curie temperature of the glassy component of the ribbons varies in a non-systematic way with both Ti and C alloying addition content and wheel speed. PAS provides quantitative measurements of the S (or ‘shape’)-parameter which represents the size of a void or open volume in the material volume probed. The experimental results indicate that an excess of free volume in the glass is associated with increased glass stability, a counterintuitive conclusion. However, the results are consistent with the model of Sietsma and Thijsse [Phys. Rev. B 52 (5) (1995) 3248] who propose that thermal relaxation in glass causes the larger free volume regions in the amorphous structure to break up into smaller voids, which necessarily increases the total number of voids, but decreases the volume per void. This void breakup fosters the processes of cooperative diffusion and subsequent devitrification. It is concluded that the free volume concentration in the amorphous component of melt-spun Nd₂Fe₁₄B alloys produced by the highest quenching wheel speeds and Ti/C alloying addition content is thus not sufficient to allow cooperative diffusion to take place, resulting in an increased stability against devitrification.     

Microstructural evolution of slowly solidified Cu-Ti-Zr-Ni amorphous alloy

The microstructures of slowly solidified Cu-Ti-Zr-Ni bulk amorphous alloy were identified by X-ray diffractometry (XRD) and transmission electron microscope (TEM). XRD and TEM examinations show that the deep eutectic structures of the tested alloy consist of CuTi₂-Cu₁₀Zr₇、Cu₃Ti-CuZr、Cu₃Ti-Cu₁₀Zr₇-CuZr low-order eutectics. Moreover, short-range ordering clusters in the melt with configuration similar to that of Cu₁₀Zr₇ compound may contribute to the glass forming ability of bulk amorphous Cu-Ti-Zr-Ni alloy.