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.     

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.     

Preparation of ultrafine Fe-Si-C powders in a radio-frequency thermal plasma and their catalytic properties

Ultrafine Fe-Si-C powders were prepared from mixed gases of SiH₄-CH₄-Fe(CO)₅ using a newly developed thermal-plasma apparatus with a dual-radio-frequency-coupled-plasma-torch system. The phases of the powders prepared were classified into two groups; one composed of -FeSi₂ which was prepared by rapid quenching, while the other was composed of -Fe, Fe₃Si, Fe₅Si₃, FeSi, -SiC and amorphous Si, which were prepared by slow quenching. The diameters of these powders were in the range 5–50 nm. The catalytic activities of these powders for Fischer-Tropsch synthesis were examined. The ultrafine -FeSi₂ powder was very active for the selective formation of olefin. On the other hand, powders of -Fe, Fe₃Si and Fe₅Si₃ were low in their selectivity but gave high CO conversion. Relations between the plasma conditions for the preparation of ultrafine Fe-Si-C powders and their catalytic properties are discussed.     

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.     

Analytical electron microscopy of Al2O3 implanted with iron ions

Single crystals of -Al₂O₃ were implanted with iron ions at room temperature to fluences ranging from 4×10¹⁶ Fe cm^–2 to 1×10¹⁷ Fe cm^–2. The microstructure and composition in the implanted region were examined using analytical electron microscopy techniques. Special emphasis was placed on monitoring the microstructural changes which take place during post-implantation annealing. Clusters of metallic -Fe were identified in the specimen after implantation to a dose of 1×10¹⁷ Fe cm^–2. Analytical electron microscopy of implanted specimens annealed in oxygen revealed the redistribution of the implanted iron and the formation of surface precipitates of -Fe₂O₃, subsurface precipitates of various forms of spinel, and, in some cases, subsurface precipitates of iron, depending on the annealing temperature. Examination of implanted specimens annealed under reducing conditions revealed the presence of precipitates of -Fe.     

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.     

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.     

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.     

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 behaviour of amorphous Sc100−x Fe x alloys near x=25 composition

A detailed study of the process of crystallization in amorphous Fe₂₅Sc₇₅, Fe₂₈ Sc₇₂ and Fe₂₅Sc₄₅Zr₃₀ alloys has been carried out using differential scanning calorimetry, X-ray diffraction and Mössbauer spectroscopic techniques. The complex multi-stage transformation process in Fe₂₅Sc₇₅ and Fe₂₈Sc₇₂ is understood in terms of polymorphous formation of an intermediate metastable crystalline phase of orthorhombic structure (a = 0.521 nm, b = 0.648 nm and c = 1.212 nm) and of likely Sc₃Fe stoichiometry, before final eutectic crystallization to stable Fe₂Sc and -Sc.     

Stability of the interphase surface in the freezing of moist ground

It is suggested that the formation of ice layers should be regarded as a consequence of a loss of stability of the motion of the freezing front. The kinetics of the freezing process is investigated and a stability criterion is obtained.Notation s(t) coordinate of the moving front - L length of the specimen - k moisture conductivity - W moisture content - heat of phase transition - W_H amount of unfrozen water - q flow of moisture from the melted zone into the frozen zone - v velocity of motion of the front - T_i temperature - Q_i heat flux - _i thermal conductivity - a_i thermal diffusivity (i=1 is the frozen zone and i=2 is the melted zone) - mass transfer coefficient - T_H the initial temperature Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 39, No. 1, pp. 96–101, July, 1980.     

High temperature and pressure preparation and properties of iron carbides Fe7C3 and Fe3C

The formation regions of Fe₇C₃ and Fe₃C were determined at high temperature and high pressure in the iron-graphite system. Fe₇C₃ formed at relatively higher pressures and Fe₃C at lower pressures. Both Fe₇C₃ and Fe₃C were isolated from coexisting excess carbon powders by a magnetic method. Fe₇C₃ had a Curie point of 250° C and a saturation magnetization of 120 emu g^–1 at room temperature and Fe₃C had those of 210° C and 125 emu g^–1. Fe₇C₃ decomposed to Fe₃C and carbon at 600° C, but to -Fe and carbon at 700° C at atmospheric pressure, and Fe₃C to -Fe and carbon at 700° C. The substitution of other metals (Cr, Mo and W) for iron in these carbides leads to changes in the thermal stabilities and the magnetic properties.     

Corrosion Behavior of α-Fe and 20kh13 Steel in Contact with Oxygen-Containing Lead Melts

We study the structure and chemical composition of the reaction products formed in the process of contact of -Fe and 20Kh13 steel with oxygen-containing stagnant lead melts (600–700°C, 3000 h, C _O[Pb] (1–4) · 10^–5 wt.%). It is shown that a heterogeneous structure is formed in the interaction zone. This structure consists of an external intermediate layer (with low hardness, the same composition as the matrix, and lead accumulated on the grain boundaries) and a thin oxide film (Fe₃O₄ for -Fe and FeCr₂O₄ and Cr₂O₃ for 20Kh13 steel), which separates the intermediate layer from the internal porous suboxide layer of the matrix and blocks the process of penetration of lead into the matrix.     

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.     

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.     

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.     

Microstructure and topological analysis of Co : Al2O3 nanocermets in new FCC and BCC metastable Co-structures

Co : Al₂O₃nanocermets are synthesized by co-reducing Co²⁺-cations dispersed in a mesoporous AlO(OH) · H₂O matrix (amorphous) in a pure H₂atmosphere at 850–1150 K. The dispersed Co²⁺-cations in pores co-reduce to separated Co-particles of controlled size, as small as 50 nm, encapsulated in thin Al₂O₃films, which are formed in-situ by molecular decomposition of the matrix, 2AlO(OH) · H₂O Al₂O₃+ (2 + 1)H₂O. The Al₂O₃film which coats Co-particles has an amorphous structure. This is possible only if it is extremely thin limited to a thickness of t< 2r _c, with r _c 1.9 nm the critical size of its nucleation and growth into a stable crystallite. The thin Al₂O₃surface film supports the formation and existence of Co-particles in a modified FCC or BCC crystal structure. As a result, unusually, large crystallites of an average 100 nm diameter could be observed. Normally, such large particles of pure Co-metal exist in an HCP structure which undergoes a reversible martensitic transformation to FCC structure at 695 K. The results are analyzed and discussed in terms of microstructure, x-ray diffraction and XPS studies of nanocermets prepared under selected conditions.     

A Mössbauer study of the oxidation of Fe-Ni alloys at 535 and 635°C

Fe⁵⁷ transmission Mössbauer spectroscopy, supported by metallography, SEM and X-ray diffraction analysis, has been employed to study the oxidation of Fe-Ni alloys at 535 and 635° C in 1 atm. of air. With increasing Ni content of the alloy, the composition of the scale changed and the oxidation rate decreased. For an alloy containing 0.9% Ni, the oxide scale produced at 535° C was Fe₃O₄ covered by a thin outer layer of-Fe₂O₃, while at 635° C FeO was additionally present as a major phase. The scale formed on a 10% Ni alloy at both 535 and 635° C was similar to that observed for the 0.9% Ni alloy oxidized at 535° C (i.e. of Fe₃O₄ and-Fe₂O₃), although the-Fe₂O₃ layer tended to be relatively thicker. For a 49% Ni alloy, the scale at both 535 and 635° C comprised an inner layer of Ni _x Fe_3–x O₄ (withx0.5, on average) and an outer layer of-Fe₂O₃, of similar thickness. Finally, on an 83% Ni alloy oxidized at 635° C, the scale consisted of roughly equally thick layers of NiO (next to the metal) and NiFe₂O₄, and a thin outer covering of-Fe₂O₃. The decrease in oxidation rate with increasing Ni content of the alloy is discussed briefly in relation to the changing composition of the scale and diffusion in the alloy.     

The magnetic properties of the glassy ferromagnets Fe81.5B14.5Si4 and Fe40Ni40B20

The galssy ferromagnets Fe_81.5B_14.5Si₄ and Fe₄₀Ni₄₀B₂₀ have been studied by Mössbauer spectroucopy from 77.5 up to 650 and 680K, respectively, In the glassy state, the average magnetic hyperfine field decreases with increasing temperature due to a distribution of exchange inter-actions. At low temperatureH(T)/H(0) has a temperature dependence (1 -BT ^/32 -CT ^/52) whereB andC are constants, indicative of spin-wave excitations in glassy ferromagnets. The value of B_/32) = 0.40 = 0.05 is almost four times larger than those of crystalline iron and nickel. On the other hand, fluctuations of the exchange interaction constant are found to decrease with increasing temperature. The Curie temperaturesT _c - 608 K for the glassy Fe_81.5B_14.5Si₄ andT _c = 583K for the glassy Fe₄₀Ni₄₀B₂₀ are found to be well defined. AtT close toT _c H(T) behaves as a power law with critical exponent = 0.3. The crystallizationT _cr glassy Fe_81.5B_14.5Si₄ was found to be 645K. The crystalline material obtained contains three different phases, namely -Fe, Fe₂B and Fe-8% Si.     

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.