Crystallization of CoFeSiB metallic glass induced by long-time ball milling

Milling up to 800 h causes amorphous Co_70.3Fe_4.7Si₁₀B₁₅ alloy, prepared in the form of thin ribbon, to partially crystallize thus forming a powder material consisting of an amorphous phase and fcc-Co nanocrystals with an average grain size of about 10 nm. A gradual increase of the nanocrystalline fcc-Co fraction, produced by ball milling, was detected. Prolonged milling results in destabilization of the fcc-Co phase and oxidation of the powder material (presence of CoO phase after 1500 h of milling). The thermal stability studies of as-quenched and milled Co_70.3Fe_4.7Si₁₀B₁₅ alloy emphasized a two step crystallization behavior. During the first crystallization event, cobalt rich phases, i.e., fcc-Co and hcp-Co crystallize, whereas after the second crystallization event, Co₂B and Co₂Si are formed.     

Phase evolution and magnetic properties of Co/α-Fe2O3 powder mixtures with different molar ratios treated by mechanical alloying

In this work, we report the phase formation and magnetic properties of Co–hematite powder mixtures with two different molar ratios: Co/α-Fe₂O₃: 1/0.7 and 1/1.3 subjected to high-energy mechanical milling using metallic cobalt and hematite powder as the initial raw material in ambient air atmosphere. The samples were activated with a ball to powder weight ratio (BPR) of 10 and the milled powders were collected after 0, 1, 5, 15, 25 and 30 h. Various characterization techniques such as XRD, HRTEM, VSM and Mössbauer spectroscopy were utilized to study the prepared samples. For the samples with Co/α-Fe₂O₃: 1/0.7 milled for 1 and 5 h the formation of cobalt ferrite was confirmed. However, this was not the case for the samples milled above 5 h for whose both Mössbauer and XRD results confirmed the phase decomposition taken place for the previously formed cobalt ferrite phase. Further, the formation of superparamagnetic nanoclusters of iron oxide, a wustite-like Fe_1?XCo_XO phase and the existence of small amounts of metallic Fe/Fe_1?XCo_X phase/s were also detected for these samples. The presence of the latter phase is not believed to be solely related to contamination from the steel vial/balls used. A mechanochemical-reduction process is assumed to be also possibly responsible for the formation of the observed reduced phases. For the powder mixture with Co/α-Fe₂O₃: 1/1.3, however, increased formation of cobalt ferrite phase was observed by increasing the milling time. The highest maximum magnetization (53 e.m.u/g) and coercive field (500 Oe) was obtained for the sample milled for 25 h among various samples of this series of powder mixture. The lower magnetization obtained for this sample compared to that of the bulk is attributed to the size effect. Furthermore, the structural–magnetic properties relationship of the various powders prepared is discussed in detail.     

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.     

Effect of carbon on the microstructural evolution of Zr66.7−xNi33.3Cx (x = 0, 1, 3) alloys during mechanical alloying

In the present paper, the effect of carbon on the microstructural evolution of Zr_66.7−xNi_33.3C_x (x = 0, 1, 3) alloys during mechanical alloying has been investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results show that these three alloys undergo similar amorphization and crystallization processes, and the final milling product is a metastable fcc-Zr_66.7−xNi_33.3C_x phase. The carbon addition can shorten the milling time for the complete amorphization reaction and enhance the stability of the formed amorphous alloy, which can suppress the mechanically induced amorphous-crystalline phase transformation with further increasing milling time.     

Structure and properties of nanocrystalline Cu70Fe18Co12 obtained by mechanical alloying

Cu₇₀Fe₁₈Co₁₂ alloy is prepared by mechanical alloying of pure Cu, Fe, Co powder using a high energy ball mill, with increasing milling time ranging from 4 to 8, 24, 36 and 54 h. The variation of the morphology and the elemental distribution were measured at these different stages on various grains of the alloy using a scanning electron microscope with a dispersive energy analyzer. Atomic clusters of iron were observed on some grains after 8 h of milling, confirming the non-homogenisation of the powder at this stage. Beyond 12 h, the homogenisation is ensured over a volume of one cube micron. Microstructural changes during the mechanical alloying have been studied by X-ray diffractometry (XRD) and Mössbauer spectroscopy. X-ray diffraction measurements confirm the dissolution of iron and cobalt phases in the FCC matrix of copper after 24 h of milling with increase of the structural parameter. This same dissolution was also measured by Mössbauer spectroscopy, confirming that after 4 h of milling the CuFe phase begins to form and iron dissolution is incomplete with partial amount of alpha Fe phase surviving after 36 h of milling.     

Microstructure evolution and thermal properties in FeMoPCB alloy during mechanical alloying

Fe_79.3Mo_4.5P_8.1C_6.75B_1.35 amorphous alloy composite powder from respective element powders of Fe, Mo, C, B, and Fe–P intermediate compound, was synthesized by mechanical alloying. Microstructure evolution analysis indicates that the synthesized amorphous alloy composite powder after a milling time of 70 h encompasses predominately amorphous matrix embedded by nanocrystalline α-Fe with a grain size of about 5.5 nm. However, unlike other Fe-based amorphous alloys, the synthesized amorphous alloy composite powder exhibits no obvious supercooled liquid region with only crystallization temperature. The corresponding crystallization onset temperature and exothermic enthalpy measured from DSC curves are about 762 K and 15.86 J/g, respectively. The results obtained provide good candidate materials for fabricating bulk metallic glass composites and related bulk nanocrystalline materials.     

Ni59Zr20Ti16Sn5 amorphous alloy obtained by melt spinning and mechanical alloying

Amorphous Ni₅₉Zr₂₀Ti₁₆Sn₅ alloys were fabricated by melt spinning and by mechanical alloying (MA) techniques. Additionally the melt spun ribbon was powdered by ball milling. Differential scanning calorimetry (DSC) measurements revealed two or three-steps crystallization of the amorphous alloys. MA powders exhibited the lowest crystallization temperature of the first DSC peak, the melt spun ribbon – the highest one. On the other side, the lowest value of activation energy of crystallization was calculated for ball milled ribbon. Further studies are needed to clarify the differences in thermal stability and crystallization behavior of the alloys.     

Influence of outphase Cu50Ti50 amorphous alloy addition on microstructural evolution of mechanically alloyed Zr66.7Ni33.3 amorphous alloy

The influence of outphase Cu₅₀Ti₅₀ amorphous alloy addition on microstructural evolution of Zr_66.7Ni_33.3 amorphous alloy has been investigated using a mechanical alloying method. It has been found that the milling induced microstructural evolution is related to the change of peak positions of the first maximum on X-ray diffraction patterns of the as-obtained amorphous alloys. With increasing milling time, the 3 wt.% Cu₅₀Ti₅₀ addition can give rise to the cyclic amorphization transformation of the as-milled alloy. The mechanical stability of the mixing amorphous phase can be greatly enhanced with increasing Cu₅₀Ti₅₀ addition up to 10 wt.%. Moreover, the addition of outphase Cu₅₀Ti₅₀ amorphous alloy not only increases the onset crystallization temperature of Zr_66.7Ni_33.3 amorphous alloy but also alters its crystallization mode. The effect of outphase amorphous addition on the mechanical stability of the Zr_66.7Ni_33.3 amorphous phase has been discussed based upon the bond order theory.     

Preparation and mechanical properties of a bulk icosahedral quasicrystalline Ti45Zr35Ni17Cu3 alloy

A bulk Ti₄₅Zr₃₅Ni₁₇Cu₃ alloy, which consisted of the icosahedral quasicrystalline phase, was prepared by mechanical alloying(MA) and subsequent pulse discharge sintering. Ti₄₅Zr₃₅Ni₁₇Cu₃ amorphous powders (with particle size <50 μm) were obtained after mechanical alloying for more than 150 h from the mixture of the elemental powder. The transformation temperature range from amorphous phase to the quasicrystalline phase was from 400 K to 900 K. The mechanical properties of the bulk quasicrystalline alloy have been examined at room temperature. The Vickers hardness and compressive fracture strength were 620 ± 40 and 1030 ± 60 MPa, respectively. The bulk quasicrystalline alloy exhibited the elastic deformation by the compressive test. The fracture mode was brittle cleavage fracture.     

Comparison of glass formation by mechanical alloying and solid-state interdiffusion in Ni---Zr composites

The amorphization process in mechanically alloyed Ni---Zr powders has been investigated by optical microscopy, scanning and transmission electron microscopy, X-ray diffraction, differential scanning calorimetry and saturation magnetization measurements. Starting from elemental crystalline Ni and Zr powders, ball milling first produces a characteristically layered microstructure. Further milling leads to an ultrafine composite in which amorphization by solid-state reaction sets in between 4 and 16 h milling time. Longer milling results in fully homogeneous amorphous material. The obtained results corroborate the similarity of the amorphization process during mechanical alloying with the solid-state interdiffusion reaction in artificially modulated multilayer composites. In particular, mechanical alloying prevents intermetallic phase formation during the interdiffusion reaction because of extremely thin starting layers, thus resulting in a wider glass-forming range than obtained by other preparation techniques based on solid-state interdiffusion.     

Mössbauer study of nanocrystalline Co60Fe30Ni10 and Co50Fe35Ni15 alloys obtained during mechanical synthesis

The mechanical alloying method was used to prepare Co₆₀Fe₃₀Ni₁₀ and Co₅₀Fe₃₅Ni₁₅ alloys from the elemental powders. As X-ray diffraction studies proved the final products of milling were the solid solutions with b.c.c. lattice and the average grain size of about 60 nm. After heating of the alloys up to 993 K, the mixtures of two solid solutions with a b.c.c. and f.c.c. lattices were formed. Annealing of the alloys at 1173 K for 1 h resulted in the formation of solid solutions with an f.c.c. lattice. Mössbauer spectroscopy revealed hyperfine magnetic field distributions which reflected the different surroundings of ⁵⁷Fe isotopes by Co, Fe and Ni atoms, depending on the chemical composition of the alloy. The most probable atomic configurations were determined on the basis of a local environment model. The results of the macroscopic magnetic measurements testified that Co₆₀Fe₃₀Ni₁₀ and Co₅₀Fe₃₅Ni₁₅ alloys exhibited relatively good soft magnetic properties.     

Structure and Electronic Properties of La(Ni,Al)5 Alloys

Nanocrystalline and polycrystalline La(Ni,Al)₅ alloys were prepared by mechanical alloying (MA) followed by annealing and arc melting method, respectively. The amorphous phase of MA samples forms directly from the starting mixture of the elements, without other phase formation. Heating the MA powders at 800 °C for 1 h resulted in the creation of hexagonal CaCu₅‐type nanocrystalline compound with mean crystallite size less than 80 nm. XPS studies showed that the shape of the valence band measured for the arc melted (polycrystalline) LaNi₅ is practically the same compared to that reported earlier for the single crystalline sample. The substitution of Ni in LaNi₅ by Al leads to significant modifications of the electronic structure of the polycrystalline sample. On the other hand, the XPS valence band of the MA nanocrystalline LaNi_4.2Al_0.8 alloy is considerably broader compared to that measured for the polycrystalline sample. The strong modifications of the electronic structure of the nanocrystalline LaNi_4.2Al_0.8 alloy could significantly influence on its hydrogenation properties.     

Thermal stability of amorphous Mg50Ni50 alloy produced by mechanical alloying

Amorphous Mg₅₀Ni₅₀ alloy was produced by mechanical alloying (MA) of the elemental powders Mg and Ni using a SPEX 8000D mill. The alloyed powders were microstructurally characterized by X-ray diffraction (XRD). The thermal transformation of amorphous Mg₅₀Ni₅₀ into stable intermetallics (Mg₅₀Ni₅₀ → remaining amorphous + Mg₂Ni → Mg₂Ni + MgNi₂) was analyzed using the Kissinger and isoconversional methods based on the non-isothermal differential scanning calorimetry (DSC) experiments. The apparent activation energies (E_a) and the transformation diagrams, temperature-time-transformation (T-T-T) and temperature-heating rate-transformation (T-HR-T), were obtained for both processes. A good agreement was observed between the calculated transformation curves and the experimental data, which verifies the reliability of the method utilized.     

On the bindings in the crystal phases of Mn

The heterotypism of Mn may be interpreted energetically when a temperature dependent valence electron concentration is supposed which takes on values between 2.2 and 0.6 electrons per atom. The room temperature phase Mn.r (= αMn) belongs to a series of structural types: Cr₃Si, U.h₁ (= βU), Mn.r, which occur in alloy systems such as MoRe_M (M = undetermined mole number) at certain values of the averaged group number (AGN) of the perodic system of chemical elements (rule of Raub). An interpretation of the series by means of the plural-correlations model becomes possible when instead of the (Ekmanian) AGN count another (non-Ekmanian) electron count is used. The phase Mo₃Re (Cr₃Si-type) yields a simple bonding type (binding) which undergoes moderate transformations to form the phases Mo₂Re₃(U.h₁) and MoRe₃(Mn.r) and the binding of MoRe₃ may be taken to be valid also for Mn.r; it corresponds to the valence electron concentration N′^A_b = 2.16. For the high temperature phases Mn.h₁ (= βMn) and Mn.h₂ (= γMn) the values N′^A_b = 1.6 and 1.0 are probable and allow the brass-like structures Mn.h₁ (C20) and Cu(Fl). The binding of Mn.h₂, incidentally, explains the occurrence of the tetragonal metastable phase Mn.m. finally Mn.h₃ (= δMn) crystallizing in the W-type is isodesmic to Fe.h₂ (= δFe), i.e. of the same binding.     

Determination of trace metal release during corrosion characterization of FeCo-based amorphous metallic materials by stripping voltammetry. New materials for GMI biosensors

The objective of this investigation has been the study of metal release during corrosion characterization of Fe_2.5Co_64.5Cr₃Si₁₅B₁₅, Fe₃Co₆₇Cr₃Si₁₅B₁₂ and Fe₅Co₇₀Si₁₅B₁₀ amorphous metallic alloys, used as based materials to create new type for giant magnetoimpedance (GMI) biosensors. Alloy samples were tested in phosphate buffered saline solutions (PBS, artificial biological solutions) at pH 7.4 and 22 °C. Individual alloy constituents (Fe, Co, Cr, B) released during the electrochemical corrosion tests, were analyzed by differential pulse stripping voltammetry (DP) using a hanging mercury drop electrode technique (HMDE). In this work, the experimental results obtained are discussed in order to study their corrosion behavior in artificial biological solutions and thus determine their possible use as GMI biosensor prototype materials.     

On the solidification of the Fe50Cr15Mo14C15B6 bulk-amorphized alloy

Investigations of the effect of the initial liquid phase state on the processes of solidification in Fe₅₀Cr₁₅Mo₁₄C₁₅B₆ bulk-amorphized melts have been carried out by differential thermal analysis, X-ray structural analysis, metallography and viscosimetry. The anomalies caused by changing the relation of the atom microgroups of boride and carbide types have been discovered for the first time in polytherms of the melt viscosity in the vicinity of 1653 K and 1793 K. The structural changes observed in the liquid phase result in anomalies in the Fe₅₀Cr₁₅Mo₁₄C₁₅B₆ melt supercooling behaviour and a change in the crystallization mechanism. Structural transformation temperatures, can be considered to be an additional factor in the search for promising alloys with optimized glass forming abilities and enhanced service properties.     

Ball milling induced abnormal crystallization behavior of an amorphous Fe78Si9B13 alloy

This paper investigates the effect of milling atmospheres on mechanical crystallization of an amorphous Fe₇₈Si₉B₁₃ alloy during ball milling. Under an air atmosphere, the amorphous alloy completely transforms into a single α-Fe(Si) phase after milling of 30 h. The crystallization process and products are different from those of thermal crystallization and milling induced crystallization under an argon atmosphere. Moreover, the milling atmosphere has a significant influence on the thermal crystallization of the amorphous phase in the as-milled alloy.     

Structure and thermomagnetic properties of powders produced from melt spun FeNbBCu ribbons

Amorphous ribbons of Fe₇₇Nb₇B₁₅Cu₁ prepared by melt-spinning and powders produced from them by ball-milling were characterized by means of calorimetry, X-ray diffraction, scanning electron microscopy and vibrating sample magnetometry. Upon thermal treatment the amorphous alloy experiences a primary crystallization that leads to bcc-Fe nanocrystals dispersed in an amorphous matrix. Magnetic measurements indicate that this alloy in the amorphous and nanocrystalline state is a good soft magnetic material. Values of saturation magnetization and coercivity are 120 Am²/kg and 5 A/m respectively, for the alloy in the nanocrystalline state. Pre-annealing, post-relaxation and nanocrystallization as well as various milling parameters were explored and the structural and magnetic changes induced have been studied. The analysis of the particle size distribution and morphology of the powders show that the brittleness resulting from pre-annealing of the ribbons is very effective in reducing the particle’s size. Recovery of the high coercitive field induced by milling is achieved by post-annealing to an extent that depends mostly on the milling conditions.     

The evolution of structure induced by intensive milling in the system 2Bi2O3 · 3TiO2

Concurrent mechanochemical treatment of either a stoichiometric 2Bi₂O₃ · 3TiO₂ powder mixture or (pulverized) Bi₄Ti₃O₁₂ compound was performed in a planetary ball mill. Relevant structural parameters: crystallite size, amount of amorphous phase and the transformed fraction (as a result of chemical reaction between Bi₂O₃ and TiO₂) of the powders milled for various milling times and intensities were derived from X-ray powder diffraction data. The obtained structural parameters were used to follow the kinetics of the reduction of crystallite size, amorphous phase formation and chemical reaction. In the powder mixture, during the early stage of mechanochemical treatment the Bi₂(CO₃)O₂ phase was found as an intermediate product which transformed into the highly amorphized Bi₄Ti₃O₁₂ phase as the milling progressed. On the other hand, mechanochemical treatment of Bi₄Ti₃O₁₂ powder induced a gradual deformation of the crystal lattice and destruction of the perovskite-type structure. However, in both cases, after a certain milling time, a very disordered, amorphous/nanocrystalline structure was obtained. It was demonstrated that in the steady state, the amorphous/nanocrystalline phase ratio depends on the milling conditions. Higher milling intensities induce ‘nanocrystallization’ of the amorphous phase, i.e., precipitation of crystallites with an average size below 20 nm. A kinetic model involving a nanocrystalline ↔ amorphous reaction, in which the forward and reverse reaction were first-order was postulated and used to analyze the formation of an amorphous phase.     

Preparation and optical properties of nanoscale MgAl2O4 powders doped with Co ions

Co²⁺-doped MgAl₂O₄ nanocrystalline powders were prepared by co-precipitation method. The gels and/or calcined samples were characterized by means of thermogravimetry and differential scanning calorimetry (TG/DSC), X-ray diffraction (XRD), transmission electron microscope (TEM), Fourier transform infrared (FTIR) spectrum and near-infrared absorption spectrum. MgAl₂O₄ nanocrystals were produced by calcining the gel above 800 °C, with the crystallite size of 10-30 nm in the temperature range of 800-1100 °C. The influence of pH value of precipitant solution on the dispersing of powders was studied and the result showed that Co:MgAl₂O₄ nanocrystalline powders exhibited good dispersion when pH = 11. The absorption spectrum of Co²⁺-doped MgAl₂O₄ exhibited a broad absorption band in the wavelength range of 1200-1600 nm, which indicated that Co²⁺ ions substituted for the tetrahedrally coordinated Mg²⁺ ions in the MgAl₂O₄ lattice.