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.     

Crystallization kinetics of the amorphous alloy Fe80B20 studied using a thermomagnetic balance

The kinetics of isothermal crystallization of the amorphous metal alloy Fe₈₀B₂₀ have been studied using a Perkin-Elmer thermomagnetic balance. Well-defined and reproducible incubation periods were observed on the degree of crystallization against time curves, in agreement with the theory of non-steady state nuclear ions in glasses. Values from 1.4 to 2.4 were obtained for the exponentn in the Johnson-Mehl-Kolmogorov-Avrami equation. The activation energy of viscous flow in the glass was found from the incubation period against temperature dependences at temperature near the crystallization point. The activation energy of crystallization was determined from the temperature dependence of the 50% crystallization times. Metallographic observations show unambiguously that two crystallization reactions proceed simultaneously: the growth of ready athermal centres located on the more slowly quenched surface of the amorphous metal ribbon along two directions, and growth in the bulk of the ribbon.     

Effects of non-steady state nucleation in the kinetics of crystallization of the amorphous alloy Fe80B20

The existence of non-steady state nucleation in the isothermal crystallization of the amorphous alloy Fe₈₀B₂₀ is shown. The incubation time ₀ of isothermal volume crystallization of the alloy is investigated in a wide range as a function of temperature. From these data an equation for the temperature dependence of the viscosity = (T) is derived: = 6.62 exp (2526.97T) × exp [836.52/(T – 530)].[/p]     

Isochronal crystallization kinetics of Fe40Ni40B20 amorphous alloy

The kinetics of crystallization of amorphous Fe₄₀Ni₄₀B₂₀ alloy upon isochronal annealing was investigated applying power-compensating differential scanning calorimetry with heating rates of (5, 10, 20, 30, 40) K/min. The corresponding microstructural evolution was studied by means of X-ray diffraction, focused ion beam and scanning electron microscopy, and transmission electron microscopy. Crystallization of Fe₄₀Ni₄₀B₂₀ alloy upon isochronal annealing takes place by formation of nano-scaled grains consisting of a face-centered cubic solid solution phase (Fe,Ni) and an orthorhombic compound phase (Fe,Ni)₃B. Kinetic analysis was performed by application of a modular model of phase transformation kinetics, fitted to all experimental transformation-rate curves simultaneously. The crystallization reaction can be described by nucleation with a continuous nucleation rate incorporating a nucleation index a and by growth in three dimensions according to a linear growth law. The kinetics of transformation and the resulting microstructure observed upon isochronal annealing clearly differ from those upon isothermal annealing investigated in a previous study, reflecting different mechanisms operating upon isochronal and isothermal crystallization.     

FMR study of surface and isothermal annealing effects in amorphous Fe80B20−x C x and Fe82 B18−y Ge y alloys

Alterations in the bulk (surface) magnetic properties of amorphous Fe₈₀ B_20?x C _x (0 5x ? 10) and Fe₈₂ B_18?y Ge _y (0 ?y ? 6) alloys, caused by isothermal annealing (mechanical polishing) of the alloy ribbons at a temperature T _A = 0.5T _cr (whereT _cr is the crystallization temperature) for various lengths of time,t _a, ranging from 0 to 240 min, have been studied by measuring the corresponding changes in the ferromagnetic resonance (FMR) linewidth, AHPP, and resonance field,H _res. Polishing induces significant changes in bothH _res and ΔH _pp but leaves their composition dependence practically unaltered. By comparison, isothermal annealing has no influence onH _res for ali the investigated alloy compositions whereas it affects ΔH _pp to different extents depending on the alloy composition. Physical implications of these results are discussed in terms of the effect of surface and annealing treatments on various contributions toH _res and AHPP in the glassy alloys in question.     

Crystallization kinetics of Fe40Ni38Mo4B18 and Fe80B20 metallic glasses

Experimental results for magnetic susceptibility, calorimetry and electrical resistivity for the metallic glasses 2826 MB (Fe₄₀Ni₃₈Mo₄Bi₁₈) and 2605 (Fe₈₀B₂₀) are reported. The crystallization kinetics of these alloys is investigated and activation energies are estimated. Time variation of crystallized fraction derived from isothermal electrical runs is interpreted to give the preponderant mechanisms involved in the crystallization process.     

Electrochemical passivation behaviour of nanocrystalline Fe80Si20 coating in borate buffer solution

Passivation behaviour of nanocrystalline coating (Fe₈₀Si₂₀) obtained by in situ mechanical alloying route is studied and compared with that of the commercial pure iron and cast Fe₈₀Si₂₀ in sodium borate buffer solution at two different pH values (7·7 and 8·4). The coating reveals single passivation at a pH of 7·7 and double stage passivity at a pH of 8·4. The first passive layer is due to the dissolution mechanism and second passivity is related to stable passivation. The cast sample shows single stage passivity in the solution of pH 8·4. The difference in the passivation behaviour for the cast alloy (Fe₈₀Si₂₀) and the coating is related to the presence of highly iron-enriched localized regions, formed during the processing stage of coating.     

Crystallization study of amorphous Fe40Ni40B20 by electrical resistivity measurement

The crystallization in amorphous Fe₄₀Ni₄₀B₂₀ alloy has been studied by electrical resistivity measurement. It shows a three stage compared to the more conventional two-stage crystallization behaviour in many metallic glasses. The Johnson-Mehl-Avrami equation was found to be operative only for 40% transformed crystallized volume for the highest measured isotherm (740 K). The Avrami exponent and activation energy were found to be 1.75 and 53 kcal mol^–1, respectively. The low activation energy in the amorphous alloy has been explained by the structural relaxation model.     

Viscous behaviour and glass formation in Fe78B13Si9 amorphous alloy

Using a rotational viscometer, the viscosity of molten Fe₇₈B₁₃Si₉ alloy was measured over the temperature range 1165–1350 °C. The temperature dependence of the viscosity of liquid alloy is adequately described by the Arrhenius equation. However, this expression cannot be directly extended to the undercooled liquid temperature region. In this paper a proper equation derived from the free volume theory is proposed to describe the viscosity-temperature behaviour of Fe₇₈B₁₃Si₉ alloy in the temperature range of the undercooled liquid and above its melting point. Furthermore, the isothermal time-temperature-transformation curve for crystallization is constructed based on the assumption of homogeneous nucleation and crystal growth, together with the temperature dependence of the viscosity. The critical cooling rate required to form a glass of Fe₇₈B₁₃Si₉ alloy was calculated to be approximately 10⁵° Cs^–1.     

Effects of Cu on the crystallization behaviour of amorphous Fe71Cr15Mo4B10

Following the approach of earlier studies which have demonstrated that nanocrystalline microstructures can be produced in soft magnetic alloys through the addition of one percent of Cu to Fe based amorphous precursor alloys, it is demonstrated that substantial grain refinement can be produced in alloys of interest for structural applications. Grain sizes of approximately 50 nm were produced in the alloy Fe₇₀Cr₁₅Mo₄B₁₀Cu₁. Such grain sizes should permit superplastic consolidation of powder or flake at reasonable rates with moderate die pressures at temperatures in the vicinity of 650 °C.     

Studies of crystallization kinetics of Fe40Ni40P14B6 and Fe80B20 metallic glasses under non-isothermal conditions

A model for the glass crystallization at constant rate heating is presented. Based on the model a technique for determination of the constants involved in the classical equations for the rates of homogeneous nucleation and linear crystal growth is derived. The effect of the heating rate (in the wide range from 2×10-2 to 16 K s-1) on the temperature of crystallization as well as on the average grain size in fully crystallized specimens of Fe40Ni40P14B6 and Fe80B20 metallic glasses has been studied. The values of the interface diffusion coefficient, the rates of nucleation and growth and the volume density of quenched-in nuclei deduced in the present study are in good agreement with those derived from direct observations. It has been confirmed that crystallization of Fe80B20 occurs mainly by the three-dimensional growth of the pre-existing crystallites while the Avrami exponent for the Fe40Ni40P14B6 glass exceeds 4 implying non-steady-state nucleation. It has been demonstrated that the proposed model allows one to generalize the isothermal and non-isothermal kinetic crystallization curves. This revised version was published online in November 2006 with corrections to the Cover Date.