T. Kulik

Nanocrystallization of metallic glasses

The paper summarizes briefly the current status of research in the field of nanocrystallization of metallic glasses especially highlighting the influence of glass composition and conditions of devitrification process on size, morphology and composition of crystallization products. Conventional crystallization creates a nanocrystalline structure only in glasses with particular compositions. Any metallic glass, decomposing in a primary crystallization process, can be converted into partially nanocrystalline material using non-conventional methods of heat treatment, e.g. high-temperature or low-temperature nanocrystallization. Temperature of devitrification process influences sizes and compositions of crystallization products for any volume fraction of crystalline phase. The change of crystallites sizes can change their morphologies. The change of a crystallite composition usually affects the lattice parameter but also can result in a change of crystallographic structure of the same phase or in formation of another phase. Composition of primary crystallites is dependent on temperature as well as on time of devitrification process. The lower the annealing temperature and the shorter the annealing time (smaller crystallites) the more the crystallites composition differs from the equilibrium state.

A high-performance hysteresis loop tracer

A high‐performance and inexpensive hysteresis loop tracer has been developed to measure quasistatic (0.02 Hz or less) hysteresis loops of soft ferromagnetic materials. It was applied very successfully to measure straight pieces of amorphous and nanocrystalline ribbons and amorphous wires. Especially high‐magnetic‐field resolution is required when nanocrystalline ferromagnets and amorphous wires are measured. Nanocrystalline materials exhibit very low coercivity (Hc=0.1–0.5 A/m). The error of Hc measurement using this tracer does not exceed 0.05 A/m even though the amorphous wires have very small cross section (0.008 mm2). The examples of hysteresis loops measured at low (50 A/m) and high magnetic field (14 kA/m) are presented. The apparatus consists of an IBM‐compatible computer equipped with 12 bit analog‐to‐digital and digital‐to‐analog converters, bipolar power supply, fluxmeter, solenoid and a pickup coil connected to a compensation coil. This equipment is free of 50 Hz noise, a significant problem in...

Formation of nickel aluminides by mechanical alloying and thermodynamics of interaction

Abstract Structure and temperature stability of Ni–Al alloys obtained by mechanical alloying (MA) were studied using X-ray diffraction and differential scanning calorimetry (DSC). MA of Ni–Al powders produces the B2 phase for the composition range 40–61 at.% Ni, and a nanocrystalline supersaturated solid solution Ni(Al) in the range 65–85 at.% Ni. MA of Ni 62.5 Al 37.5 leads to a nanocrystalline phase, probably with L1 0 structure. The NiAl (B2) phase is formed through direct rapid exothermic solid state reaction without formation of intermediate solid solutions. Formation of the ordered tetragonal Ni 5 Al 3 phase occurs after prolonged deformation of the nonstoichiometric B2 phase with 62.5 at.% Ni. The results of MA were compared with calculated Gibbs energies of principal phases namely FCC, L1 2 and B2 using the results of computer assessment of the system. The final product of MA is always single phase which has minimal Gibbs energy of the competing phases at alloy composition. This indicates a leading role of the thermodynamic factors on phase formation by MA.

The influence of copper, niobium and tantalum additions on the crystallization of FeSiB-based glasses

Abstract The influence of alloying elements (copper, niobium and tantalum) on the crystallization process of FeSiB-based amorphous alloys and their structure after the first stage of crystallization was studied. The suitability of continuous heating for the creation of nanocrystalline materials was examined. The crystallization process was investigated using differential scanning calorimetry, X-ray diffractometry and transmission electron microscopy. It was found that controlled crystallization of metallic glasses using continuous heating (10 K min −1 ) only enables the formation of a nanocrystalline structure in alloys with specific composition, e.g. in FeSiB alloys containing at least two alloying elements: copper and niobium or copper and tantalum. The continuous heating combined with knowledge of the thermal properties of the glass makes possible the precise fabrication of a nanocrystalline microstructure (different grain size and volume fraction of crystalline phase) by controlled partial crystallization of the appropriate alloy.

Flash annealing nanocrystallization of FeSiB-based glasses

Abstract The possibility of reducing the role of alloy composition in the creation of nanocrystalline structure in FeSiB-based amorphous alloys (with copper, niobium and tantalum additions) by application of flash annealing was studied. Crystallization products were studied using X-ray diffractometry and transmission electron microscopy. It was found that flash annealing remarkably decreases the influence of composition on alloy microstructure after crystallization of the glasses studied. Ultrafine structure with an average grain diameter of less than 25 nm was created in all the iron-based glasses studied in this work. Flash annealing appeared to be a very suitable method for the creation of nanocrystalline materials from metallic glasses. It was revealed that the crystallization product of the first stage of crystallization of Fe 73.5 Cu 1 Ta 3 Si 13.5 B 9 glass depends on the heat treatment method. Primary crystals of α-Fe(Si) and Fe 3 B were observed after conventional and flash annealing respectively.

Correlation between structure and the magnetic properties of amorphous and nanocrystalline Fe73.5Cu1Nb3Si22.5−xBx alloys

Abstract The effects of the metalloid composition and annealing temperature (400–720°C) on the magnetic properties of Fe73.5Cu1Nb3Si22.5−xBx (x = 5−10 at%) nanocrystalline alloys have been investigated. Crystallization products were studied using X-ray diffractometry. The coercive field, remanence and out of ribbon axis anisotropy were determined from quasi-static hysteresis loops. It was found that the beginning of nanocrystallization causes some magnetic hardening, which is manifested in a remarkable increase in coercive field for all the alloys studied. This effect is ascribed to the increase in magnetostriction. The softest magnetic behavior was observed for alloys with nanocrystalline structure composed of α-Fe(Si), Fe3Si and an amorphous matrix. The very strong magnetic hardening coincides with the second stage of crystallization and the appearance of borides (e.g. Fe2B). The Si-rich alloys also exhibit minor hardening after annealing at temperatures well below the onset temperature of the second stage of crystallization. This effect is attributed to the increase in grain size and the precipitation of copper crystals.

Solid state reactions in Ni–Al–Ti–C system by mechanical alloying

Abstract Several Ni–Al–Ti–C compositions from different areas of the Ni–Al phase diagram containing various amounts of Ti and C were prepared by mechanical alloying. Synthesized samples were investigated by X-ray diffraction and differential scanning calorimetry methods. Solid state reactions in Ni–Al–Ti–C powder mixtures with an equiatomic Ni/Al ratio lead to the formation of NiAl and TiC equilibrium phases. Metastable solid solution of Ni(Al,Ti,C) and non-stoichiometric TiC x are formed in the mixtures with C Ni /C Al ratio of 1.7–3. The grain size of metastable intermetallic phases is less than 5 nm, if Ni and Al content in the mixtures corresponds to the dual-phase and Ni 3 Al-areas of the Ni–Al phase diagram.

Effect of flash- and furnace annealing on the magnetic and mechanical properties of metallic glasses

Abstract Furnace- and flash annealing followed by cooling in air or quenching in water was applied to improve the magnetic properties of the magnetostrictive Fe 78 Si 9 B 13 and the non-magnetostrictive Co 71.5 Fe 2.5 Mn 2 Mo 1 Si 9 B 14 glases. Flash annealing in air was found to be the most effective method of heat treatment for both glasses, as far as reduction of coercive field H c is concerned. Also much higher remanence and fracture strain correspond to the smaller value of H c obtained by flash- than furnace annealing. The results are interpreted in terms of structural relaxation and magnetoelastic- and induced magnetic anisotropy.

Correlation between microstructure and temperature dependence of magnetic properties in Fe60Co18(Nb,Zr)6B15Cu1 alloy series

Temperature dependence of magnetic properties of nanocrystalline Fe60Co18Cu1B15Nb6−xZrx (x=0, 3, 6) alloys has been studied at different stages of devitrification. Transmission electron microscopy shows nanocrystals of the size ∼5 nm, which remains almost constant along the nanocrystallization process. Curie temperature of the residual amorphous phase decreases as nanocrystallization progresses for all the studied alloys. Thermal dependence of the exchange stiffness constant is obtained from the measurement of specific magnetization and coercivity as a function of crystalline fraction and temperature for the three studied alloys.

Tailoring soft and hard magnets by annealing Co-based metallic glass

Abstract Co 67 Fe 4 Mo 2 Si 17 B 11 metallic glass ribbon has been subjected to the isothermal annealing at temperatures in the range 250–600°C so as to produce a series of samples with gradually coarser microstructure. For this series of samples a giant increase of the coercivity, exceeding five orders of magnitude, is observed. It shows a possibility to tailor soft or hard magnets using the same parent material. An abrupt increase of the coercivity occurs in a relatively small range of annealing temperatures between 480 and 520°C, and is mainly due to a strengthening of the pinning effect of the precipitates (fine crystalline structure) on the domain walls. Samples annealed at higher temperatures become fully crystallized. First, the metastable phase(s) is created which decomposes to the stable phases at still higher temperature. Coercivity for fully crystallized samples shows first a narrow plateau and afterwards a gradual decrease of its value with increasing temperature of annealing. Magnetic and microstructural properties of the samples, annealed at various temperatures, were investigated applying a number of complementary techniques including DSC and TGM methods, X-ray diffraction, TEM, strain-modulated FMR spectroscopy as well as conventional magnetic measurements.