A. Zaluska

Amorphous and nanocrystalline Fe-Ti prepared by ball milling

Nanocrystalline FeTi has been prepared in two ways: by ball milling the intermetallic compound and mechanically alloying a mixture of the elemental powders. The materials obtained in each case are identical. The reaction proceeds via the formation of interfacial β–Ti(Fe) which then grows to include all of the material present. Oxygen levels above 3 at. % suppress this reaction and lead to the formation of amorphous Fe–Ti.

The formation of single-phase equiatomic MnBi by rapid solidification

The low temperature phase (LTP) of MnBi, which is of interest because of its large magnetic anisotropy, has been obtained in almost single-phase form (>95%) by rapid solidification, followed by thermal annealing. X-ray and electron microscope studies indicate that the melt-spun ribbons are amorphous, contrary to the accepted rules for glass formation. The transformation of the amorphous phase is a complex process. The amorphous phase crystallizes first into Bi, Mn 3 Bi, and ferrimagnetic MnBi. The formation of the LTP begins immediately after the eutectic melting of these phases around 540 K.

The properties of sub-20-micron Permalloy fiber formed by melt extraction

Fibers of Permalloy of different compositions have been manufactured in diameters from 5-20 mu using a technique of melt-extraction. The molten material is raised on to the edge of a sharpened rotating molybdenum wheel whose tangential velocity is typically about 60 ms/sup -1/. The surface of fibers is free formed and is almost without defects. The radial cooling and high quenching rates result in a unique crystalline and magnetic microstructure. The fibers show outstanding soft magnetic properties and require no annealing to optimize performance. They also exhibit strong resistance to degradation under mechanical stress and are especially attractive for sensor applications. >

Ultra fine, ultra soft metallic fibres

Fine metallic and ceramic fibres with diameters ranging from 5 to 25 microns and lengths up to several meters have been produced by a simple single step fabrication process based on the melt extraction technique. The technology developed permits the production of fine fibres of amorphous or crystalline material from liquids with extremely low viscosity and high surface tension, parameters normally inhibiting fibre production by drawing and similar technologies. We believe that magnetic and mechanical properties of the fibres are linked to complex magnetic and crystallographic structures reflecting the formation of a free surface and frozen stress introduced during the radial cooling from the surface to the line of contact. The quenched-in stress strongly affects the crystallographic and magnetic domain structure, and the easy axis of magnetization is usually perpendicular to the fibre length. Examples of practical applications are presented, such as: anti-theft devices, proximity, stress, identification and magnetoresistive sensors. >

Effects of quench rate on the microstructure in melt-spun Nd–Fe–B alloys

Electron microscope studies have shown significant changes in the microstructure of the Nd{sub 14}Fe{sub 80}B{sub 6} melt-spun ribbons, depending on the quenching conditions. The average grain size of about 20 nm near the wheel side, increasing to 500--700 nm near the free side, can be observed for ribbons of the thickness of about 25--30 {mu}m, indicating strong internal variations in quench rate across the ribbon thickness. For low quenching rates different morphologies of the crystalline phase Nd{sub 2}Fe{sub 14}B are observed. Equiaxed grains near the wheel side of the ribbon become columnar grains or even dendritic grains near the free side. Texturing effects in the equiaxed grains are observed only for grains larger than 30 nm and disappear for very large equiaxed grains (about 500 nm), as well as for dendritic structures. Most evident texture is observed in columnar grains, which crystallized with the c-axis perpendicular to the ribbon plane.

Structural changes and physical properties of Fe-Ni based metallic glasses rapidly heated by pulsed electrical currents

Fe–Ni–Si–B metallic glasses have been annealed and crystallized using short electrical current pulses. Two types of electrical heat treatment have been used. The first one is an isothermal annealing treatment using a very high initial heating rate while the second one is a thermal spike applied on an amorphous sample held at various initial temperatures. The microstructure of the alloys after heat treatment has been characterized by x-ray diffraction, transmission electron microscopy, and Mossbauer spectroscopy. The thermal and magnetic properties of the samples measured by DSC and hysteresis loop tracer have been studied after the various heat treatments and correlated with the microstructure of the alloys. The crystallization at high temperatures produces the gamma phase only, while at low temperatures, a mixture of the gamma and alpha phases (the alpha phase being predominant) is usually observed. The samples initially held at liquid nitrogen temperature and heat treated with a thermal spike remain amorphous and show improved magnetic properties (lower coercive field and higher induction at saturation) without loss of ductility.

Microstructure studies of melt-extracted Nd-Fe-B fibers

Abstract The correlation between quenching conditions and microstructure of Nd-Fe-B fibers obtained by melt-extraction method has been studied. The structure of as-quenched fibers was observed by Scanning Electron Microscopy (SEM) on fracture cross section and by Transmission Electron Microscopy (TEM). Different microstructures of the Nd-Fe-B fibers were found, depending on the melt-extraction parameters and ranged from amorphous to fully crystalline. The following microstructures of the Nd 2 Fe 14 B phase were observed: equiaxed grains with a size from 10–20 to 200–900 nm; strongly textured columnar grains with the easy axis of magnetization perpendicular to the fiber axis; dendrites. Besides the main hard-magnetic Nd 2 Fe 14 B phase, two grain-boundary Nd-rich phases were observed by TEM: a platelet-shaped phase with a distorted bcc structure ( a ≈ 0.40 nm) and an fcc phase with the unit cell of about 0.56 nm.

Electron microscope studies of phase transformations in rapidly solidified MnBi alloys

Abstract As-made Mnue5f8Bi ribbons produced by the melt-spinning method are amorphous, as shown by X-ray diffraction and TEM studies. The subsequent transformations (induced by heat treatment) involve complex processes. The amorphous phase crystallizes first into Mn 3 Bi, Bi and ferrimagnetic MnBi (NP). The formation of the low-temperature MnBi phase (LTP) immediately follows the melting of the Bi phase. It is possible to obtain the LTP in the amount exceeding 95 wt.%. At higher temperature, transformation of the LTP into the high-temperature MnBi phase (HTP) occurs.

Rapid crystallization of amorphous CoZr and FeB close to eutectic compositions

We present in situ time-resolved X-ray scattering data and transmission electron microscopy studies on ribbons of Co 92 Zr 8 , and Fe 81.5 Bi 18.5 . Both systems exhibit a profound change as the crystallization temperature, and hence transformation rate, is raised. At low temperatures (half-time t 1/2 for crystallization of the order of minutes), crystallization occurs by eutectic growth of two crystalline phases; at high rates (t 1/2 of the order of seconds) the mode changes to simultaneous polymorphic crystallization of the two phases, with no eutectic growth. It is suggested that the change of mode occurs when the temperature exceeds the glass transition temperature

Bi-Based High Tc Superconducting Fibers by Melt Extraction

Bismuth-based high T c superconductors have been prepared as fibers by a technique of melt extraction. As-made, the fibers are amorphous with diameters ranging from 0.7 μm to 100 μm and lengths of up to 5 cm. The fibers were subsequently transformed into high T c superconductors by heat treatment in air. Superconducting transitions at 105 K and 82 K were measured in annealed fibers of initial composition Bi 1.8 Pb 0.2 Sr 2 Ca 3 Cu 4 O x by SQUID magnetometry. The volume fractions of superconducting phases were estimated to have lower bounds of 30% for 2212 and 5% for 2223. The crystallization process has been studied by differential scanning calorimetry, electron microscopy, and x-ray diffraction. Crystallization involves first the formation of the Bi-2201 phase and a bcc phase with lattice parameter a = 0.425 nm before finally significant fractions of both the Bi-2212 and Bi-2223 phases are formed.