Crystallization Of Ribbons Research Articles

Self-Diffusion in Nanocrystalline Fe and Fe-Rich Alloys

The present work aims at a comparison of the self-diffusion behaviour of nanocrystalline (n-)Fe produced by cluster condensation and compaction with that of Fe-rich n-alloys made by crystallization of melt-spun amorphous ribbons. In cluster-synthesized Fe (relative density higher than 91 %), a decrease of the (59)Fe tracer diffusivity upon annealing indicates interface relaxation. The diffusion coefficients in the relaxed grain boundaries are similar to those extrapolated from high-temperature data of conventional grain boundaries. Substantially lower interface diffusivities in crystallized n-Fe(90)Zr(10) and n-Fe(90)Zr(7)B(3) presumably arise from residual intergranular amorphous layers. Due to the reduced amorphous fraction, in n-Fe(90)Zr(10) additional fast diffusion paths exist like in conventional grain boundaries.

On the crystallization of amorphous Fe85B15 ribbons produced with different heat treatments of the liquid alloy before ejection

The effect of the melt heat treatment on the crystallization of the hypoeutectic amorphous Fe85B15 ribbons is studied. The samples were produced by melt spinning with different ejection temperatures and heat treatment of the melt. Viscosity measurements on Fe85B15 molten alloys show that changes take place in the liquid when heated above a critical temperature. Crystallization was investigated in these samples by differential thermal analysis, on-line electrical measurements and X-ray diffraction. Kissinger analysis was used for the determination of the activation energies of phase transformations. We found that mainly the decomposition reactions, that take place after two steps of crystallization, are affected by the heat treatment of the liquid before solidification of these amorphous FeB ribbons.

Nanocrystalline Fe and Fe-riched Fe-Ni through electrodeposition

There is a constant need for new softer magnetic materials, especially with high saturation moments. A number of studies have shown that by decreasing the average crystal size of a material below 20 nm, it is possible to reduce its magnetic losses. This was clearly demonstrated in nanostructured magnetic alloys based on the thermal crystallization of amorphous ribbons. However, this demonstration has been very difficult for other nanostructured materials, mainly because most of them are based on powder processing. This study presents the synthesis and properties of nanostructured magnetic materials produced by pulsed electrodeposition. Surprisingly, compared to other synthesis techniques, electrodeposition has received little attention for producing large quantities of fully dense nanostructured materials. In this work we describe some of the electrodeposition parameters leading to the production of nanostructured Fe and Fe-riched Fe-Ni alloys. Some magnetic properties obtained using a vibrating sample magnetometer are also presented.

Preannealing effect and soft magnetic properties of the nanocrystalline Fe74CulNb3Si12B10 alloy

The magnetic susceptibility, its disaccommodation and microstructure of the nanocrystalline Fe74Cu1Nb3Si12B10alloy obtained by the crystallization of amorphous ribbons (after different preannealing) are investigated. It is stated that the preannealing leads to the increase of the magnetic susceptibility for the nanocrystalline samples. It is due to the narrowed size distribution ofα-FeSi grains in these samples. However, the disaccommodation intensity of the nanocrystalline samples is independent of preannealing conditions.

Fe diffusion in Nanocrystalline alloys and the influence of Amorphous Intergranular Layers

59Fe tracer diffusion is used as a highly structure-sensitive tool for probing the interface structure and residual amorphous intergranular phases in nanocrystalline (n-) Fe 90Zr 10 produced by crystallization of melt-spun amorphous ribbons. Analyses of 59Fe-diffusion profiles show the existence of two types of interfaces. In the one type the diffusivities are low; the other type acts as fast diffusion paths like conventional grain boundaries in α-Fe. In accordance with positron-annihilation and hydrogen-site spectroscopy of the interfacial structure on n-Fe 90Zr 10, the slow diffusion process may indicate the presence of amorphous intergranular layers similar as recently found for n-Fe 73.5Si 13.5B 9Nb 3Cu 1.

Magnetic phase transitions in Fe-(Nb, Zr)-B-Al amorphous and nanocrystalline alloys

Nanocrystalline Fe±(Nb, Zr)±B alloys, prepared by primary crystallization of melt-spun amorphous ribbons, are new soft magnetic materials with a low coercivity and a high saturation magnetization [1]. The microstructure consists of a large proportion of body-centred cubic (b.c.c.) a-Fe nanograins embedded in a residual amorphous matrix. It was reported that both the reduced magnetocrystalline and magnetoelastic anisotropies provide the basis for the magnetic softness of these materials [2]. The nanocrystallization mechanism was described by the nucleation-and-growth and grain-growth models [3, 4]. In this letter we report the magnetic phase transitions in Fe83:5Nb7B8:5Al1 and Fe85:5Nb4Zr4 B5:5Al1 (compositions given in atomic per cent) amorphous and nanocrystalline alloys. The amorphous alloys were prepared by the single-roller melt-spinning method. The thickness and width of the resulting ribbons were about 30 im and 1.5 mm, respectively. The amorphous ribbons were annealed in a conventional furnace under an argon atmosphere. The magnetic phase transitions were followed by thermogravimetric analysis (TGA) (Perkin±Elmer 7) by heating from room temperature to about 800 8C at a heating rate of 10 8C miny1 under a small applied magnetic ®eld. The microstructure was observed by transmission electron microscopy (TEM) using a JEM-2010 operating at 200 kV. Fig. 1a, b and c shows the TGA curves of Fe83:5Nb7B8:5Al1 alloy as quenched, annealed for 1 h at 460 8C and annealed for 1 h at 655 8C, respectively.

Dilatometric measurements on metallic glass ribbons with a wide glass transition range

A special dilatometric device which avoids tensile stress to the specimen is used to examine structural relaxation, glass transition and crystallization of metallic glass ribbons of Al 7.5Cu 17.5Ni 10Zr 65 and Pd 40Ni 40P 20. The relaxation kinetics can be well described either in the free volume model or by the Kohlrausch-Williams-Watts formula or by an activation energy spectrum, all indicating co-operative atomic rearrangements. The first model yields for the product of the critical free volume and overlap factor, initiating a structural relaxation event, values of 0.17 (Al 7.5Cu 17.5Ni 10Zr 65) and 0.14 (Pd 40Ni 40P 20) of an atomic volume. The coefficient of thermal expansion (and its temperature dependence) is measured and found not to change with crystallization in the case of Al 7.5Cu 17.5Ni 10Zr 65, unlike that of Pd 40Ni 40P 20.

Small-angle neutron scattering of the system Fe 80− xNi xB 12Si 8 in the amorphous state with 0 ⩽ x ⩽ 80: comparison with the evolution of crystallization studied by the controlled Joule effect analysis

The evolution of crystallization of amorphous ribbons has previously been studied by the controlled Joule effect analysis (CJEA) in the system Fe 80− x Ni x B 12Si 8. The sign and magnitude of resistivity changes due to the relaxation depend strongly on the Ni/Fe ratio. The anomalous behavior observed during crystallization was explained by large fluctuations of local environments of Ni and Fe atoms which have a high mobility in these glasses. Depending on x, small amorphous local units with different atomic compositions would be present in the glassy matrix. To confirm these results, the small-angle neutron scattering (SANS) of: these amorphous alloys has been measured at room temperature. A significant contribution to the SANS intensity is observed for all the alloys. The radii of gyration of the scattering particles depend on x and vary between 230 and 300 Å. Except for 50 < x < 65, an interference peak present in the intensity curves confirms the presence of numerous other aggregates for which the radii of gyration are distributed between 65 and 90 Å and their mean distances between 300 and 400 Å. Sizes and distances have been determined with an accuracy equal to ±10% using more than one approximation method. Their rough volume has been evaluated from the normalized intensities assuming spherical form and particular compositions defined from the X-ray patterns. The presence of aggregates in the glasses partially explains the anomalous behavior of resistivity but it is not possible to confirm the real composition of these aggregates for all x-values.

Nanocomposite R 2Fe 14B/α-Fe and Sm 2 (Fe-Ga) 17C x/α-Fe magnets

We have studied the crystallization, crystal structure, microstructure, and magnetic properties of melt-spun ribbons of R 6(Fe-Nb) 88B 6 with RNd, Pr, Dy, Tb and Nd 3.5(Fe-Nb) 93B 3.5 consisting of a mixture of exchange-coupled magnetically hard R 2Fe 14B and soft α-Fe phases. The as-spun ribbons of R-rich composition crystallize in two steps; for RNd, Pr, and Dy at first the Y 3Fe 62B 14-type + α-Fe phases and subsequently they transform to 2:14:1 and α-Fe upon heating above 700 °C. The intermediate phase in the case of Tb 6(Fe-Nb) 88B 6 is of the TbCu 7-type. Very high remanences up to 145 emu/g with reduced remanences ranging from 0.6 to 0.7 were observed. The coercivity of the samples was found to vary with the R element and the hard phase content. The highest room temperature coercivity of 4.5 kOe was obtained in a Nd 4Tb 2(Fe-Nb) 88B 6 sample. The observed reversible demagnetization curves are characteristic of exchange-spring magnets. A two-phase microstructure was also obtained in Sm-Fe-Ga-C ribbons consisting of exchange-coupled 2:17 and α-Fe phases after annealing in the temperature range of 700–800 °C. The highest coercive force (12.8 kOe) was obtained in samples annealed at 800 °C. Annealing at 700 °C led to finer grains with lower coercivities (5kOe) but higher remanences.

Structure and magnetic properties of rapidly quenched SmFeTi alloys

In the study of rapidly quenched SmFeTi alloys, it was found that crystallites with metastable TbCu7-type structure were first formed during crystallization of the melt-spun ribbons. A transformation from Sm(Fe,Ti)7 with TbCu7 structure to SmFe11Ti having ThMn12 structure takes place at annealing temperature higher than 740°C. The presence of plenty of α-Fe and the imperfect transformation may impede exploiting the potential coercivity of SmFe11Ti-based alloys.

Diffusion of hydrogen in nanocrystalline transition-metal alloys

Nanocrystalline Fe 90Zr 10, Fe 603oZr 10, and Fe 60Co 30Zr 10 alloys with average grain sizes of 15 nm to 200 nm were produced by crystallization of melt-spun amorphous ribbons. After hydrogen charging, the diffusion of hydrogen was studied by magnetic after-effect measurements and by thermal desorption experiments. Nanocrystalline Fe 60Zr 10and Fe 60Co 30Zr 10 alloys show a strong concentration dependence of the short-range and long-range diffusion of hydrogen, which is characteristic for a statistical distribution of interstitial site and saddle-point enthalpies. The origin of these statistical diffusion potentials can be assigned to the interfacial regions between the grains possessing a high hydrogen solubility in these alloys. In contrast. Nanocrystalline Co 90Zr 10 alloys show no concentration dependence of hydrogen diffusion. The desorption kinetics observed over a wide range of grain sizes indicate that hydrogen is mainly dissolved within the grains and diffuses very fast in the interfacial regions. Therefore the degassing rate depends on the grain size and follows Einstein's diffusion law.

Stability of (Fe-Tm-B) amorphous alloys: relaxation and crystallization phenomena

Fe-Tm-B base (TM=transition metal) amorphous alloys (metallic glasses) are thermodynamically metastable. This limits their use as otherwise favourable materials, e.g. magnetically soft, corrosion resistant and mechanically firm. By analogy of the mechanical strain-stress dependence, at a certain degree of thermal activation the amorphous structure reaches its limiting state where it changes its character and physical properties. Relaxation and early crystallization processes in amorphous alloys, starting already around 100°C, are reviewed involving subsequently stress relief, free volume shrinking, topological and chemical ordering, pre-crystallization phenomena up to partial (primary) crystallization. Two diametrically different examples are demonstrated from among the soft magnetic materials: relaxation and early crystallization processes in the Fe-Co-B metallic glasses and controlled crystallization of amorphous ribbons yielding rather modern nanocrystalline “Finemet” alloys where late relaxation and pre-crystallization phenomena overlap when forming extremely dispersive and fine-grained nanocrystals-in-amorphous-sauce structure. Mossbauer spectroscopy seems to be unique for magnetic and phase analysis of such complicated systems.

Thermal and structural investigation of the devitrification of the melt-spun FeNdB alloys

FeNdB alloys of three different boron contents were rapidly solidified to an overquenched (amorphous) state by chill block melt spinning. The structure of the ribbons in the as-cast state and after isothermal and dynamic annealing in a differential scanning calorimeter were investigated by X-ray diffraction and transmission electron microscopy. The changes in Curie temperature were monitored by DSC. Evidence was obtained for the formation of an intermediate metastable phase during the transformation from the amorphous to the hard magnetic Fe 14Nd 2B phase. The mechanism of microstructure development is briefly discussed in relation to the alloy composition, notably boron content and metastable phase equilibria. The Curie temperature was found to vary during the course of the transformation in dynamic annealing suggesting structural changes within the complex tetragonal unit cell as temperature is increased. The effect was found to vary with the overall boron concentration in the alloy and the results are discussed. The mechanism of crystallization of amorphous FeNdB alloy ribbons is a function of composition.

Magnetic properties of the Sm/sub 20/Fe/sub 70/Ti/sub 10/- omega -phase

The magnetic properties of the hard magnetic 20:70:10 ( omega ) phase in melt-spun Sm/sub 20/Fe/sub 70+x/Ti/sub 10-x/ (x=-2, 0, and 2) and Sm/sub 25/Fe/sub 65.6/Ti/sub 9.4/ were investigated. The intrinsic coercivity, H/sub ci/, and the remanence were measured from room temperature up to the Curie temperature. The maximum coercivities, H/sub ci/, at room temperature for the compositions Sm/sub 20/Fe/sub 70/Ti/sub 10/ and Sm/sub 25/Fe/sub 65.6/Ti/sub 9.4/ are 35.6 and 46.4 kA/cm, respectively. The temperature dependence of the coercive field is compared with that of melt-spun Dy/sub 2/Fe/sub 14/B, and the anisotropy field of omega at room temperature is roughly estimated to be greater than 200 kA/cm. The Curie temperature of the omega phase is between 293 and 320 degrees C as revealed by magnetic and calorimetric measurements. The formation of omega and related phases by crystallization of amorphous ribbons is discussed. >

High-Resolution Electron Microscopy Study of Iron Nanocrystals Prepared by High-Energy Mechanical Crystallization

ABSTRACTRecently, there has been a growing interest on materials called “nanocrystalline solids”. These materials are made of crystals with sizes ranging typically from 1 to 10 nm. Several methods have been used to produce these solids. One of them is the high energy mechanical deformation (HEMD) process. Indeed, we have shown recently that some iron-based amorphous alloys crystallize completely into a nanocrystalline structure when they are subjected to HEMD. In this work, we present a TEM study of the nanocrystals formed by the mechanical crystallization of Fe66Co18si1B15 amorphous metallic ribbons.

Structural relaxation, glass transition and crystallization of Cu-Zr amorphous ribbons studied by dilatometry and differential scanning calorimetry

The structural relaxation, glass transition and crystallization of Cu 33Zr 67, Cu 40Zr 60 and Cu 60Zr 40 amorphous ribbons were studied by dilatometry and differential scanning calorimetry (DSC). A highly sensitive dilatometer (built by us) was used which allowed samples in the form of thin ribbon to be heated at a slow heating rate (1.7°C min −1) in an ultrahigh vacuum (10 −9 Torr). Thermal expansion coefficients were determined on heating and cooling. Isothermal annealings were also performed in situ in the dilatometer. The mean coefficients of linear thermal expansion (α × 10 6) measured for the relaxed amorphous alloys and for the crystalline alloys were the same between room temperature and the crystallization temperature: 9.2 ± 0.2 for Cu 33Zr 67, 9.9 ± 0.2 for Cu 40Zr 60, and 11.8±0.2 for Cu 60Zr 40. The increase in the coefficient of thermal expansion is thus strongly correlated with the increase in copper content. These values are very close to those calculated assuming a linear relationship between the mean composition and the values of α m of the individual components. This suggests that the cohesion varies smoothly with composition without any substantial change in the nature of bonding. After the initial heating of the as-quenched ribbons below the glass transition, structural relaxation resulted in shrinkage of the samples after cooling to room temperature. The glass transition was indicated by the creep of the specimen. Crystallization resulted in an important shrinkage. DSC was used to aid in the interpretation of the dilatometric events, in particular for the characterization of the enthalpy relaxation associated with structural relaxation and of the enthalpy increase at the glass transition. The irreversibility of the structural relaxation below the glass transition was clearly demonstrated by DSC as were the reversible enthalpy changes at the glass transition.

Joule heating method coupled with Mössbauer spectroscopy for the study of the crystallization of amorphous ribbons

The interesting steps of the crystallization process of two types of amorphous ribbons Fe50Ni30B12Sie and Fe72Cr9Si0·3P9C10 have been detected and locked by the Joule heating method. This method reveals instantaneously with high reproductibility the various steps but Mossbauer spectroscopy seems the most adequate technique to evaluate the crystallinity rate during the crystallization process.

The effect of picosecond laser treatment on the stability, kinetics and the mode of crystallization of metallic glassy ribbons

We have studied the stability and crystallization properties of Fe 82B 18, Pd 85Si 15 and Zr 76Fe 24 amorphous ribbons treated with 100 ps pulses from a neodymiumdoped glass laser. X-ray diffractometry, differential scanning calorimetry (DSC) and metallography were used to characterize the samples prior to and after laser irradiation. The isothermal DSC data were analysed using a novel concept of the local value of the Avrami exponent. For every composition studied, the thermal stability was improved by laser treatment. Furthermore, both the crystalline phases formed on crystallization and the kinetics of crystallization were different from those of as-quenched and/or aged ribbons. In general, the laser-treated ribbons did not exhibit separate surface modes of crystallization, usually associated with the precipitation on the surface of a textured form of the more abundant element. The details of the crystallization process deduced from X-ray diffractometry and the isothermal DSC data are in very good agreement.

Coercivity after heat treatment of overquenched and optimally quenched Nd-Fe-B

Rapidly quenched Nd-Fe-B magnets show a pronounced composition dependence of the intrinsic coercivity HcI which increases with increasing Nd content to values of 23.5 kOe. The magnetic hardening is ascribed to a Nd-rich phase located at the phase boundaries of the Nd2Fe14B grains. Samples prepared by crystallization of amorphous ribbons have slightly lower coercivities but are less sensitive to prolonged heat treatment at higher temperatures. The microstructure obviously differs for the two types of material.

The influence of quenching procedures on the kinetics of embrittlement in a Fe40Ni40B20 metallic glass

The kinetics of mechanical stability (embrittlement), stress relief, and crystallization of Fe40Ni40B20 metallic glassy ribbons obtained by different melt spin quenching processes were investigated. Ribbons made with higher rate of quenching exhibit better mechanical stability over those made with lower quenching rate. The thermal stability, temperatures, and activation energies for crystallization of all ribbons, however, are identical. The rates of stress relief are higher for ribbons made with higher quenching rate. Values of activation energy for stress relief are about the same for all ribbons. These activation energies for stress relief are also close to those values reported for structural relaxation.