Annealing behaviour of amorphous Fe80B20 on continuous heating

Resistance measurements during direct heating of Fe₈₀B₂₀ amorphous alloys indicate phase changes occur at 395, 500, 720 and 840° C. Samples heated to these temperatures, and maintained for five minutes in a neutral atmosphere, show that a hardness maximum occurs at the crystallization temperature of 395° C and that annealing at 500° C produces a material with the same hardness. Above 500° C the microhardness is seen to drop below that of the amorphous alloy. Saturation magnetization measurements show a steady increase following each anneal, up to a temperature of 720° C, and the rate of increase is seen to drop in the range of 720 to 840° C. X-ray diffraction studies show that only a small fraction of the matrix is crystallized following the anneal at 395° C and the transformed phases are -Fe and Fe₃B. Following annealing at 500° C, an increased proportion of -Fe and Fe₃B are observed with complete crystallinity while samples heattreated at 720° C are seen to consist of a three-phase mixture of -Fe, Fe₂₃B₆ and Fe₂B. Annealing at 840° C is seen to produce an equilibrium phase mixture of -Fe and Fe₂B phases. Only in the sample annealed at 395° C is a fraction of the amorphous phase seen to persist, indicating that a 5 min anneal is not sufficient, at this temperature, to induce complete crystallization. These structural features are corroborated by field ion microscope analyses, made at liquid nitrogen temperature in a medium of pure neon, and scanning electron microscopy, and are also consistent with our earlier study involving the isothermal annealing, for various times, of Fe₈₀B₂₀ alloy at 780° C.     

Structural analysis of amorphous (Fe1-x Mn x )78B22 alloys by X-ray diffraction and electrical resistance

The structure of amorphous (Fe_1–x Mn _x ) alloys prepared by a single roller technique has been investigated in terms of X-ray diffraction and electrical resistance. The lattice parameter of the crystalline precipitates, which were-Fe and b c t (FeMn)₃B, was determined under different heat treatments. On heating up to 440° C where a mixture of amorphous and crystalline phases exists and up to 550° C corresponding to the completion of crystallization, the lattice parameter of the-Fe phase rises to that of pure-Fe with increasing manganese concentration. In samples annealed at 660° C for 5 h, the opposite behaviour is observed. These results can be explained on the basis of the position of the boron atom occupying the-Fe lattice, the pressure effect exerted by the environment, and the enhancement of the chemical short-range ordering between manganese and boron atoms with manganese concentration. In the b c t phase, which shows a reduction in lattice parameter with manganese concentration independent of heat treatment, the effect of redistribution of the atoms in the unit cell should be also taken into account.     

High-temperature crystallization behaviour of amorphous Fe80B20

Samples of 0.003 in. round Fe₈₀B₂₀ amorphous wires were annealed in vacuo for 1 sec to 8 h periods at 780° C and the crystallinity induced in these wires from this heat treatment was studied through X-ray diffraction and field-ion microscopy. X-ray diffraction studies indicate that complete crystallinity is produced following 1 sec anneal at 780° C. However, the initial product is a primitive-tetragonal Fe₃B phase unlike the body-centred tetragonal Fe₃B observed in low-temperature isothermal transformation studies with this alloy. The Fe₃B phase is seen to persist in the diffraction patterns for annealing durations of up to 15 min. Upon annealing for periods of up to 1 h, an intermediate three-phase structure consisting of -Fe, Fe₃B, and Fe₂B is seen to result with a gradual decrease in the Fe₃B phase corresponding to longer annealing durations. Anneals of more than 1 h at 780° C are seen to result in the disappearance of the Fe₃B phase producing a two-phase microstructure consisting of -Fe(b c c) and Fe₂B (b c t). Field-ion-microscopy with a pure neon imaging gas at 78 K likewise indicates that existence of a three-stage phase structural change during the isothermal anneals, and the atomic arrangement of the various species are quite readily discernible because of the different symmetries contained in the three distinct phases.     

In situ X-ray diffraction of the early stages of the crystallization of Fe80B20

A new technique has been used for the early stages of crystallization of amorphous materials, like metallic alloys. In situ X-ray diffraction has been performed during the early stages of crystallization of Fe₈₀B₂₀. The samples are resistively heated to 600°C in a customized vacuum chamber. A programmable charge-coupled device detector records simultaneously the evolution of the three phases: -Fe, Fe₃B and Fe₂B in the minute scale. This is the first in situ X-ray diffraction study of this system in these temperature and time scales. Interesting behaviours have been seen: appearance and disappearance of phases, -Fe supersaturation solution in boron (found for the first time in this compound), and migration of B out of the -Fe matrix. The two-dimensional diffraction pictures show topography irregularities indicating crystallite inhomogeneties.     

Crystallization and structure of high boron content iron-boron ultrafine amorphous alloy particles

Amorphous to crystalline transformation of chemically prepared Fe₆₄B₃₆ ultrafine amorphous alloy particles has been investigated by Mössbauer spectroscopy, Brunauer-Emmett-Teller surface area measurements and transmission electron microscopy. Structural relaxation was observed below 350°C, which resulted in narrowing the full width at half maximum for the hyperfine field distribution from 13.0 to 10.6 T, while the average hyperfine field kept unchanged, to be about 20.3 T. Crystallization started on the surface at about 300°C and proceeded into the bulk at about 400°C. Partial crystallization between 400 and 450°C resulted in increasing the average hyperfine field for the remaining Fe-B amorphous matrix to 21.6 T. -Fe and Fe₂B were the only iron containing phases related to bulk crystallization, with the latter as a predominant component, accompanied by the segregation of about 19% boron atoms. Above 500°C, sintering of the particles became very remarkable and a solid state reaction between diffusing iron and boron atoms to form Fe₂B took place making the spectral area ratio for Fe₂B to -Fe components increase accordingly. A locally distorted non-stoichiometric Fe₂B quausicrystalline structure for the high boron content sample was proposed.     

Nanocrystalline microstructures in Fe93 − x Zr7B x alloys

The crystallization behaviour of amorphous Fe_{93 – x} Zr₇B _x (x = 3, 6, 12 at.%) alloys, the microstructures of the primary crystallization products of stable and metastable phases and the subsequent transformations, have been studied using a combination of differential scanning calorimetry, differential thermal analysis, X-ray diffraction and transmission electron microscopy, including microdiffraction. It has been found that, for x = 3 and 6 at.%, the sole product of primary crystallization is the bcc -Fe phase and the average grain sizes of the crystalline phase were 14 nm and 12 nm for the two alloys, respectively. However, when x = 12 at.%, primary crystallization results in more than one crystalline phase, and a metastable phase with the cubic Fe₁₂Si₂ZrB structure is the major crystallization product after the primary crystallization reaction, accompanied by the -Fe phase. The average grain size of this metastable phase was 35 nm for the alloy heated to 883 K at 20 K/min. Isothermal heat treatments at 873 K and 973 K confirm that after being heated for 240 h, this metastable phase transforms into equilibrium phases: bcc -Fe, hcp ZrB₂ and probably hcp Fe₂Zr. The apparent activation energies for the primary crystallization reaction during continuous heating for these three alloys are 4.4 ± 0.2 eV, 3.5 ± 0.2 eV and 6.9 ± 0.3 eV, respectively.     

Crystallization of amorphous Fe78B13Si9

Crystallization of amorphous Fe₇₈B₁₃Si₉ has been investigated using a combination of differential scanning calorimetry (DSC) and conventional and high-resolution transmission electron microscopy. The crystallization mechanisms and crystalline products are sensitive to the annealing temperature. At 450C, crystallization takes place by the growth of b c c -Fe (Si) dendrites, while at 510 and 515C there are three simultaneous reactions to form dendritic b c c -Fe (Si), elliptical crystals of b c t Fe₃B and lamellar eutectic spherulites of b c c -Fe (Si) and b c t Fe₃B. Quantitative TEM shows that the b c c -Fe (Si) dendrites and b c c -Fe (Si)-b c t Fe₃B spherulites both form with constant nucleation and growth rates, in agreement with previous. DSC measurements of an Avrami exponent of 4.     

Crystallization process and magnetic properties of Fe100−x,B x (10 ≦ × ≦ 35) amorphous alloys and supersaturated state of boron inα-Fe

Amorphous specimens of Fe_100–x B _x were prepared in the range 10 × 35 at % B by a single-roller method. The crystallization process and the boron concentration dependence of the Curie temperature were examined by differential scanning calorimetry, X-ray diffraction, Mössbauer spectroscopy and magnetic measurements. Two-step crystallization was observed in specimens with× < 17: amorphous amorphous + boron-supersaturated b c c phase (-Fe(B)) t-Fe₃B +-Fe. A single-Fe(B) phase was not observed. The transition temperature from t-Fe₃B to stable (-Fe + t-Fe₂B) sensitively depends on the boron content in the alloys. The crystallization temperature (T _x) of the amorphous alloys was almost unchanged for 17 × 31, but increased remarkably at high boron concentrations of× 33, where the decomposition products consisted of t-Fe₂B and o-FeB. The Curie temperature (T _c) of the amorphous phase was as low as 480 K at× = 10, increased with increasing boron content up to 820 K and then decreased in the high boron concentration alloys of× > 28. A single-Fe(B) phase was not detected in the as-quenched specimens of× = 8 and 10. The phase coexisted with the o-Fe₃B and amorphous phases. The lattice parameter of the phase was 0.2861₀ nm which was smaller than that of pure iron by 2/1000, indicating the substitutional occupation of boron atoms in the b c c lattice.     

Crystallization of Fe-Si-B metallic glasses studied by X-ray synchrotron radiation

We have studied the crystallization kinetics of Fe_90-x Si _x B₁₀ amorphous alloys withx ranging from 7 to 21, by synchrotron X-ray radiation. Using energy- dispersive X-ray diffraction, the kinetics of the different crystalline phases evolving during isothermal annealing were followed. These crystalline phases were identified as precipitation of-Fe(Si) and/or Fe₃Si in the amorphous matrix. At a later time or at a higher temperature, Fe₂B starts to crystallize (x < 21 ). Only at low iron concentration (x = 21) was the second phase different, namely Fe₅SiB₂ The hypo- and hyper-eutectic Fe-Si-B glasses were found to crystallize differently. The crystallization processes are discussed in some detail.     

Mössbauer spectroscopic studies of the crystallization of amorphous Fe80B20−x Si x (x=0, 2, and 8) alloys

The crystallization process of amorphous Fe₈₀B_20–x Si _x (x=0, 2, and 8) ferromagnetic alloys has been studied by using ⁵⁷Fe Mössbauer spectroscopy and X-ray diffraction studies. Results for samples heat treated at different temperatures for different times show that the crystallization of Fe₈₀B_20–x Si _x samples having x=0 and 2 leads to -Fe and t-Fe₃B, while for x=8, it leads to -Fe, t-Fe₂B, and perhaps Fe-Si. It is further observed that the addition of silicon to the Fe-B system improves the thermal stability of the system.     

The morphology of α-Fe crystals which grow in the amorphous Fe80(C1−xBx)20 alloys

Crystallization behaviour of amorphous Fe₈₀(C_1–x B _x )₂₀ alloys, obtained by splat-cooling technique, for x values ranging from zero to unity has been investigated mainly by transmission electron microscopy. The crystalline phase which first appeared in the amorphous matrix was -Fe for all alloys studied. However, the morphology of -Fe phase changed from a spherical shape for low x values to a watch-glass shape for intermediate x values and to dendritic for large x values. The nucleation of -Fe crystals was homogeneous for low x samples while preferred nucleation on edges and surfaces was noted for samples with higher x values. The final volume fraction of -Fe phase before the appearance of the second crystalline phase increased with the increase in x.     

The effect of Fe on the crystallization behavior of Al–Mm–Ni–Fe amorphous alloys

The crystallization behavior and thermal stability of Al₈₆Mm₄Ni_10–x Fe _x alloys were investigated as a function of Fe content. Alloys, produced by a single roll melt-spinner at a circumferential speed of 52 m/s, revealed fully amorphous structures. The thermal stability of the present amorphous alloys increased with the increase of Fe content. The activation energy for crystallization of -Al increased as the Fe content increased. This increase of activation energy resulted in the simultaneous precipitation of -Al and intermetallic phase observed especially in Al₈₆Mm₄Ni₅Fe₅ and Al₈₆Mm₄Ni₂Fe₈ alloys. The glass transition was observed in DSC thermogram only after proper annealing treatment. The effect of alloy composition on the thermal stability could be explained in terms of the atomic structure of the amorphous alloy.     

Crystallization of the metallic glass Fe78B13Si9

The microstructures and kinetics with heating for an amorphous Fe₇₈B₁₃Si₉ alloy were studied by X-ray diffraction, transmission electron microscopy, differential thermal analysis and differential scanning calorimetry. The first crystallization takes place by the simultaneous formation of -(Fe,Si) and Fe₃B having the shapes of dendrite and spherulite, respectively. Metastable Fe₃B then transformed into a stable phase of Fe₂B at a higher temperature. The activation energy for crystallization and the Avrami exponent were determined. It was found that crystallization behaviour in Fe₇₈B₁₃Si₉ is controlled by nucleation rather than growth.     

Mössbauer study on the magnetic state of iron particles in Fe-N and Fe-Zr-N soft magnetic thin films

The magnetic state of -Fe particles and the behaviour of nitrogen and zirconium during annealing in Fe₉₆N₄ and Fe_85.6Zr_7.6N_6.8 magnetic thin films have been studied by conversion electron Mössbauer spectroscopy for ⁵⁷Fe. The crystalline phases present in the Fe-N annealed films were -Fe and -Fe₄N, and those in the Fe-Zr-N annealed films were -Fe and ZrN. In the Fe-N films annealed below 300°C, about 60% nitrogen is incorporated interstitially into -Fe and the rest is used for the formation of -Fe₄N. In the Fe-N film annealed at 500°C, almost all nitrogen participates in the formation of -Fe₄N, leading to the grain growth of -Fe particles and an increase in coercive force. The values (291–325 kOe) of internal magnetic field of iron sites in -Fe in the Fe-Zr-N films are much smaller than that (333 kOe) of the iron site in pure -Fe. Even if the Fe-Zr-N films were annealed at 500–700°C, some zirconium and nitrogen is still incorporated substitutionally and interstitially into -Fe, respectively. In particular, the substitutional zirconium depresses the grain growth of -Fe particles, perhaps due to a chemical interaction between zirconium and iron.     

Conversion electron Mössbauer spectroscopy of the surface crystallization of the Fe75Co9B16 amorphous alloy

Using a new design of helium-methane gas-flow detector of conversion electrons for Mössbauer spectroscopy, non-uniform nucleation of the primary -Fe-Co phase on both contact and free surfaces of the Ar(+H₂) annealed amorphous Fe₇₅Co₉B₁₆ alloy was observed in its early crystallization stage. In this state the amount of crystalline phase on the contact ribbon side surpasses that on the free one by a factor of three, whereas no traces of volume crystallization were observed in the transmission spectra. By applying ion implantation to both ribbon surfaces, a slight reduction of the crystalline phase contribution was found. Magnetic domain structure observations were performed in order to evaluate the influence of surface crystallization on magnetic properties.     

A study of Nd2Fe14B and a neodymium-rich phase in sintered NdFeB magnets

The microstructure and properties of NdFeB sintered permanent magnets were analysed by different methods. Samples analysed were sintered and thermally treated. The hard magnetic Nd₂Fe₁₄B phase and amorphous neodymium-rich phase were observed by TEM. The neodymium-rich phase contained iron and boron, in elemental and in B₂O₃ form, which is known as a glass former. At the sintering temperature, Nd₂Fe₁₄B and the neodymium-rich phase are supersaturated with iron, which should be dissolved at the annealing temperature to react with neodymium and boron and form additional Nd₂Fe₁₄B phase. Iron precipitates of size up to 2 nm were detected in the Nd₂Fe₁₄B phase. These superparamagnetic precipitates of -Fe could affect the hard magnetic properties of NdFeB magnets.     

Thermal evolution of Fe62.5Co6Ni7.5Zr6Nb2Cu1B15 metallic glass

Thermal evolution of Fe_62.5Co₆Ni_7.5Zr₆Nb₂Cu₁B₁₅ amorphous alloy prepared by one-roll melt-spinning technique was studied by XRD and DTA. The crystallisation process, occurring in several steps, can be summarised as follows: a a + -Fe a + -Fe + -Fe -Fe + -Fe + ZrB₁₂, where a and a are amorphous phases, and a can be indexed as a -Fe (fcc) structure, with a crystalline order on an average distance of 8 Å. The metallic glass demixed on quenching, but component phases tended to mix by exchanging Fe atoms in a temperature range overlapped with the first crystallisation, which yields -Fe nanocrystals (27 Å). Higher temperature exo-peaks correspond mainly to a recrystallisation of the phases formed at lower temperature. It was found that this alloy has nanocrystalline structure also after heating at a well higher temperature than first crystallization. Even after the last exo-peak, the average crystallite size (D) was considerably smaller than that found in the literature for analogous metallic glasses; D values for our alloy were comparable to those of nanocrystalline phases of other systems heat treated below the temperature of exothermal DTA peaks. Extensive oxidation above 600°C, even at a low oxygen content (c _{o 2} 2 ppm), led to a marked modification of the surface layer: two zirconia polymorphs were identified on the surface of the ribbons, and the ratio of -Fe to -Fe content increased with respect to the bulk. Differences in thermal evolution between outer layer and bulk are attributed to a different phase composition and non-uniform distribution since the as-quenched stage.     

Iron dispersed carbon composites formed from iron-polyvinylalcohol complexes

Iron-polyvinylalcohol (Fe-PVA) complexes have been pyrolysed at the temperatures up to 1000 K, and the iron-carbon composites formed have been characterized. The yield of carbon was much higher for the complexes than for PVA alone. The degree of carbon graphitization and the chemical form of iron species were dependent on the pyrolysis temperature. About 30 wt% fine particles of Fe₃O₄ or -Fe were dispersed in the matrix of amorphous carbon at 800 or 900 K, respectively. At 1000 K, -Fe was partly transformed to Fe₃C, and the agglomeration of -Fe was not so significant. At this temperature the carbon was graphitized, which resulted in a lowering of the surface area of the composite. It is suggested that the graphitization proceeds through the mechanism involving the formation and subsequent decomposition of Fe₃C. Thus, the use of Fe-PVA complexes achieves a high yield of carbon and a high dispersion of a large amount of iron species throughout the carbon matrix.     

High-resolution electron microscopy characterization of partially crystallized Fe78Si12B10 alloy as a catalyst

As a catalyst, partially crystallized Fe₇₈Si₁₂B₁₀ alloy shows a three times higher activity than totally crystallized Fe₇₈Si₁₂B₁₀ in the hydrogenation of CO. High-resolution electron microscopy (HREM) reveals that the catalyst contains a great number of minute (less than 10 nm), highly dispersed -Fe particles which act as the major active component. Many tiny B and Si oxide clusters also exist in the amorphous matrix. After being rinsed by a NaOH solution, the catalyst exhibits a decreasing selectivity to methane due to the dissolution or aggregation of B and Si species. After reaction, the sizes of the active particles increase and an overlayer of B or Si species is found over the surface of some -Fe particles. This coverage is thought to be the primary origin of the deactivation of the catalyst.     

Laser glazing of an Fe-C-Sn alloy

Morphology and geometry of melted zones, cooling rates, microstructure and microhardness in the laser-glazed Fe-4%C-10%Sn alloy have been investigated. The computer simulation on the basis of the moving gaussian source model was used successfully to predict the maximum width and depth of the melted zone and the cooling rate. The microstructure from the surface to the bottom of the laser-melted zone is a non-crystalline phase, dendritic grains and a microcrystalline zone successively. Values of the averaged-spacing of the non-crystalline phase are 0.205⁶ and 0.121⁹nm, respectively; twinned martensites, having an axial ratioc/a of 1.128, existed in dendritic grains, and carbides of Fe₃ C at the interdendritic regions; the microcrystalline zone was composed of -Fe and a new bet (a=0.415 nm,c=0.955 nm) phase. The different microstructure in the melted zone can be explained by the results of the heat flow calculation. A fine eutectic structure (-Fe + Fe₃C) was observed in heat-affected zones. Microhardness of the eutectic structure can be predicted by the empirical relation of fracture stress to the interlamellar spacing of pearlite.