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.     

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.     

Effect of high pressure on the crystallization of an amorphous Fe83 B17 alloy

Amorphous Fe-17 at %B alloy prepared by a splat-quenching technique was annealed at temperatures ranging from 250 to 500° C for different periods. A time-temperature transformation diagram has been constructed from X-ray diffraction examination of annealed samples. On annealing the alloy at a pressure of 50 kbar, an appreciable retardation of crystallization was observed. The crystalline phase precipitated first from the amorphous matrix at 1 bar. This was -Fe containing a small amount of boron, but at 50 kbar this was a mixture of -Fe(B) and intermetallic phase Fe₃B. Under the increased pressure of 100 kbar the mode of the crystallization was further changed and Fe₃B became the first precipitating phase. Preferential formation of Fe₃B under pressure can be explained assuming a modified dense-random packing model for the amorphous structure.On leave from Institute of Physics, Academia Sinica, Beijing, China.     

Phase-precipitation studies of Fe-B-Si metallic glasses using Mössbauer spectroscopy

Phase-precipitation studies have been performed on samples of the metallic glasses Fe₇₉B₁₆Si₅ and Fe₇₈B₁₃Si₉, heated in the range 300–475 °C for various times (1–16 h) using ⁵⁷Fe Mössbauer transmission spectroscopy and X-ray diffraction methods. These measurements have helped in identifying the temperature ranges and annealing durations in which the amorphous structure of these metallic glasses is retained. The results revealed that the thermal stability increases as boron is replaced by silicon in the Fe-B-Si metallic glasses and that these alloys remain amorphous below 450 °C. The various phases precipitated above this temperature were identified as -Fe, -(Fe, Si), Fe₃B, and Fe₂B. The direction of magnetization in the two metallic glasses appears to change upon annealing.     

Characterization of crystallization in Metglas 2826 MB alloy

The crystallization behaviour of the Metglas 2826 MB alloy (Fe₄₀Ni₃₈Mo₄B₁₈) has been studied using resistance measurements and X-ray diffraction techniques. Three annealing sequences were used to follow the process. Samples were annealed isothermally (a) at 780° C in a vacuum of 2×10^–5 torr for times in the range 1 sec to 4 h, (b) for 2 h in an argon atmosphere at temperatures where the resistance curve indicated phase changes to occur, and (c) for 300 h in 100 torr of helium at 400, 600, 700 and 850° C. From these annealing sequences it was found that the alloy did not crystallize below 410° C and followed a crystallization process of: amorphous Fe₄₀Ni₃₈Mo₄B₁₈ Fe _x Ni_23–x B₆ (cubic)+glassy matrix Fe _x Ni_23–x B₆+(Fe, Ni) (FCC) (Fe, Ni)₃B(bct). This series of transformations was followed for Sequences (a) and (c) above, but was slightly different for Sequence (b). An orthorhombic (Fe, Ni)₃ B phase was found in the samples annealed in a vacuum of 2×10^–5 torr.Trademark of Allied Chemical Co.     

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.     

Magnetic and structural study of mechanochemical reactions in the Al-Fe3O4 system

The solid state reaction between Al and Fe₃O₄ (magnetite) using mechanochemical activation of powder mixtures under Ar atmosphere is studied. The phase evolution during the reaction is analyzed by X-ray diffraction (XRD), vibrating sample magnetometry (VSM), differential thermal analysis (DTA) and scanning electron microscopy (SEM). At 37 minutes of high-energy ball-milling the disappearance of reactive phases and the production of -Fe, FeAl₂O₄ and -Al₂O₃ is observed, together with significant changes in the magnetic behavior of the system. The composition and properties of samples heated up to 1200°C are also investigated. The behavior of the saturation magnetization M _s is interpreted on the basis of the formation of a variable composition spinel phase Fe [Al _x Fe_2–x ] O₄ with 0 x 2 and a canting effect due to the presence of Al³⁺ ions in the spinel structure.     

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.     

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.     

Mössbauer study of nanocrystallization in amorphous Fe-P-Si-V alloys

Nanocrystallization of amorphous Fe-P-Si-V alloys during annealing is studied by Mössbauer spectroscopy and x-ray diffraction. The resulting nanostructured materials are shown to contain not only equilibrium phases ( -Fe and Fe₃P) but also a significant amount of the metastable paramagnetic phase Fe₂P and trace levels of the and D phases. Nanocrystallization markedly enhances the strength of the alloys but has an adverse effect on their magnetic properties.__________Translated from Neorganicheskie Materialy, Vol. 41, No. 4, 2005, pp. 427–433.Original Russian Text Copyright © 2005 by Baldokhin, Vavilova, Kovneristyi, Kolotyrkin, Palii, Perfilev.     

Crystallization of stable and metastable eutectics in FeSiB metallic glasses

Two iron-based metallic glasses, Fe₇₈Si₉B₁₃ and Fe₇₈Si₁₀B₁₂, have been examined after isothermal annealing at different temperatures and it has been found that both stable and metastable eutectics can crystallize simultaneously during the annealing process. At low temperatures, the majority of the eutectic cells in the structure consist of -iron and the stable Fe₂B phase with a lesser amount of the -iron and metastable Fe₃B eutectic, whereas at higher temperatures the metastable eutectic predominates. It is suggested that these observations may be explained in terms of the presence of overlapping coupled zones of both the stable and metastable eutectics in the phase diagram.     

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.     

Characterization of crystallization in amorphous 2605 SC alloy

The annealing characteristics, the crystallization sequence of precipitating phases and their geometry have been studied for the amorphous alloy 2605 SC through X-ray and reflection electron diffraction, scanning electron microscopy and field ion microscopy.The as-received samples were found to contain small domains of crystalline Fe₃Si, Fe₂₃(B, C)₆ and an oxide of iron (maghemite, Fe₃O₄) on the ribbon surfaces.Samples annealed at 673 K for 10, 50, 100 and 1000 min reveal that there is a sequential difference in the crystallization of the ribbon surfaces and the bulk, the latter exhibiting the nucleation of α-Fe first. Moreover, in situ (in an electron microscope) continuous annealing of the samples indicates that, although formation of ferrite crystals occurs in the bulk, the initial crystallizing phase on the surfaces is the f.c.c. Fe₂₃(B, C)₆. Heat treatment at temperatures above 773 K, however, is seen to produce similar phase consistency, and thus diffraction patterns, from both regions. These structures primarily are seen to be composed of a dendritic α-Fe and the Fe₂₃(B, C)₆ phases along with the as yet non-crystallized amorphous domains. The equilibrium Fe₂B phase is not observed at the annealing temperatures (above 923 K) used.     

Iron precipitation in Cu-Au-Fe alloys

Interest is focused on the segregation and clustering behaviour of iron atoms in fcc Cu₁₀₀–x–yAu_xFe_y, alloys (x=6 to 50.7 at%, y=0.2 to 1.0 at%) of three different metallurgical states: as-rolled, fast-quenched and melt-spun. The gold concentration was varied to assess the effect of change in lattice parameter. Mössbauer spectroscopy has been used to examine iron clusters and phases in samples as functions of annealing temperature and time. The development of f c c -Fe, segregates with increasing annealing time at temperatures around 410° C has been monitored and the Néel temperature of the antiferromagnetic -Fe precipitates as well as their particle size determined, the latter by transmission electron microscopy. The increase in isomer shift with increasing gold concentration is accounted for mainly by changes in atomic volume.     

Structural and optical study of GaMnAs/GaAs

GaMnAs/GaAs was obtained with mass-analyzed low energy dual ion beam depostion technique with Mn ion energy of 1000 eV and a dose of 1.5 × 10¹⁸ Mn⁺/cm² at the substrate temperature of 400°C and was annealed at 840°C. X-ray diffraction spectra showed that Ga_5.2Mn, Ga₅Mn₈, -Mn and Mn₃Ga were obtained in the as-grown sample. After annealing Mn₃Ga and -Mn disappeared, Ga₅Mn₈ tended to disappear, Ga_5.2Mn crystallized better and new phase of Mn₂As was generated. The photoluminescence spectra of the as-grown sample showed that the 1.5042 eV GaAs exciton peak, 1.4875 eV peak involving a carbon acceptor and a broad band near 1.35 eV. After annealing at 840°C, the 1.5042 eV peak and 1.4875 eV shifted to 1.5065 and 1.4894 eV, respectively, and the photoluminescence intensity of the 1.35 eV band increased greatly.     

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.     

Synthesis and alteration of α-LiFeO2 by mechanochemical processes

The effects of grinding on a stoichiometric mixture of LiOH · H₂O and -FeOOH were studied. It was found that, in the course of grinding, losses of structural water occurred and a phase structurally related to disordered -LiFeO₂ was formed. X-ray diffraction data suggest the occurrence of an ordered phase as intermediate and both -Fe₂O₃ and -Fe₂O₃ were undetected during the comminution process. A prolonged mechanical treatment of this mixture originated an elimination of Li⁺ from the -LiFeO₂ structure and the appearance of the spinel phase, -LiFe₅O₈. Additionally, the mechanical activation of a sample of -LiFeO₂ prepared at high temperatures also leads to a similar rearrangement of cations. The structural transformation is explained with the help of a model in which the vacancies of Li⁺ created during grinding promote the migration of the Fe³⁺ ions from octahedral to tetrahedral sites.     

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.     

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.     

Morphology of ordering Fe-Si alloys

The microstructure of some Fe-Si alloys water-quenched from 700–1200°C and aged at 550–675°C for various times has been studied by means of transmission electron microscopy (TEM). It has been shown that at temperatures of 700°C and above the microstructure of the alloy is a solid solution with two-dimensional particles of a B2 ordered phase. These particles have a FeSi composition and lie along (100) planes of the matrix. At temperatures of 675°C and below a modulated microstructure is formed; in its enriched modulations the composition of the Fe₃Si phase is attained and D0₃ ordering occurs. Phase composition of some high-silicon alloys has been determined by X-ray diffraction phase analysis. It corresponds to + B2 or + D0₃. These results are plotted on the accepted Fe-Si phase diagram.