Igor Matko

Crystallization characterisics in the FeSiB glassy ribbon system

Abstract The kinetics of crystallization of Fe 80 Si x B 20 − x (with x = 2, 4, 6, 8, 10) and Fe 75 Si 15 B 10 glassy ribbons was examined using differential scanning calorimetry and high precision electrical resistivity measurements. The main crystallization parameters are presented. The corresponding structure of the materials was observed using transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis. The mechanism of crystallization is described in terms of the local ordering (cluster) model.

Formation of nuclei of metastable phases in nanocrystalline materials

Abstract We have studied the role of Cu and Nb in the nucleation of nanocrystalline phase in the system FeCuNbSiB. Experimental facts prove that only a combination of Cu and Nb ensures fine-grained phase formation in this system. The role of Cu and Nb in the formation of chemically heterogeneous regions in the alloy is discussed.

Annealing behaviour of structural and magnetic properties of evaporated Co thin films

Cobalt thin films of 50 nm nominal thickness were e-beam evaporated on silicon substrates covered with thermal oxide. Two series of independent and cumulative vacuum annealings up to 600 ◦ C and 650 ◦ C, respectively, were performed. The x-ray diffraction, specular and non-specular x-ray reflectivity and longitudinal magneto-optical Kerr effect measurements were applied to probe the annealing behaviour of the film structure and magnetic properties. A gradual transition from the hexagonal close-packed (hcp) to the face-centred cubic (fcc) structure was observed. Evolution of the in-plane magnetic anisotropy is dominated by residual stresses which relax during the structural transformation. The coercivity follows the stress behaviour in the hcp phase up to 300 ◦ C and increases abruptly above 400 ◦ C due to improving the magneto-crystalline anisotropy in the growing fcc crystallites and enhanced surface/interface roughness. (Some figures in this article are in colour only in the electronic version)

Local ordering model in Fe-Si-B amorphous alloys

Abstract The main kinetics parameters of the crystallization of Fe 80 Si x B 20− x (with x = 2, 4, 6, 8, 10) and Fe 75 Si 15 B 10 glassy ribbons have been determined using differential scanning calorimetry measurements. The structure of the materials at various stages has been observed using transmission electron microscopy, electron and X-ray diffraction analysis. The crystallization process in this system is found to consist of two distinct steps; individual steps represent complex processes from a structural point of view. The mechanism of crystallization is described in terms of a local ordering (cluster) model, based on the role of the Si atoms.

Interface study of a high‐performance W/B4C X‐ray mirror

A high-performance W/B4C multilayer mirror with 80 periods of nominally 1.37 nm was measured by grazing-incidence small-angle X ray scattering (GISAXS) in order to analyse the lateral and vertical correlations of the interface roughness within the framework of a scaling concept of multilayer growth. A dynamic growth exponent z = 2.19 (7) was derived, which is close to the value predicted by the Edwards–Wilkinson growth model. The effective number of correlated periods indicates a partial replication of the low interface roughness frequencies. A simulation of the GISAXS pattern based on the Born approximation suggests a zero Hurst fractal parameter H and a logarithmic type of autocorrelation function. The as-deposited mirror layers are amorphous and exhibit excellent thermal stability up to 1248 K in a 120 s rapid thermal vacuum annealing process. At higher temperatures, the B4C layers decompose and poorly developed crystallites of a boron-rich W–B hexagonal phase are formed, and yet multilayer collapse is not complete even at 1273 K. Ozone treatment for 3000 s in a reactor with an ozone concentration of 150 mg m−3 results in the formation of an oxidized near-surface region of a thickness approaching ∼10% of the total multilayer thickness, with a tendency to saturation.

Activation energy distribution in nanocrystallization kinetics of amorphous Fe73.5Cu1Nb3Si13.5B9 alloy

Nanocrystalline phase produced by suitable annealing of iron-based amorphous alloys known as Finemets shows extremely good soft magnetic properties. It is desirable to understand the mechanisms by which these materials crystallize in order to prepare relative stable materials for prospective applications. The presence of crystalline grains in the nanometer range in Finemet alloys is considered to be a consequence of processes of compositional and structural ordering. Therefore, the electrical resistivity variations occurring in amorphous alloys during the isothermal annealing are usually exploited to obtain information on the kinetics of the nanocrystallization process. Assuming inhomogeneities in the nanoscale range the authors applied the idea of Krueger and Woldt on isothermal resistivity transformation curves during the nanocrystallization of Fe{sub 73.5}Cu{sub 1}Nb{sub 3}Si{sub 13.5}B{sub 9}. It will be presented that characterization by a rather simple distribution of activation energies is possible.

Phase Transformations in Amorphous Bilayer Ribbons

Bilayer ribbons were prepared by rapid quenching from the melt using a double-nozzle technique. The composition of the layers was selected from the Fe/Co-Si-B and Fe-Cu-Nb-Si-B systems, respectively. Ribbons with typical thickness of 45-50 microns and width of 6 mm and 10 mm exhibited amorphous structure of both layers in as-quenched state. Temperature dependencies of electrical resistivity, dilatation and magnetization have been investigated in the amorphous state and during crystallization of both layers, which take place at different temperatures. The results combined with investigation of the structures formed in each layer and at the layer interface were compared to those of single-layer ribbons having the compositions of each layer, respectively.

Influence of preannealing on the crystallization of Fe75Si15B10 metallic glass

The thermal stability of Fe75Si15B10 metallic ribbon was analyzed. The kinetics of the two-step crystallization of as-quenched and isothermally preannealed samples was studied by differential scanning calorimetry using various heating rates and the Kissinger, isoconversional and Surinach methods. Both crystallization stages are characterized by the Johnson-Mehl-Avrami kinetic law. The first step is dependent on both the heating rate and the preannealing. After sufficient heat treatment, the Avrami exponent decreases from 2.5 to 1.5, and the activation energy decreases to the value characteristic for the crystal growth mode. The second-step Avrami exponent n2 = 3, and the activation energy E2* = 395 kJ (g-atom)-−1, remain constant. The first step was related to the crystallization of αFe(Si) and Fe3Si, during which some of these primary crystals nucleate heterogeneously on the Fe3B cores. The eutectic decomposition of Fe3B into stable Fe2B and αFe was observed during the second step.

The effect of a particle–matrix interface on the Young’s modulus of Al–SiC composites

In order to improve the Young’s modulus of Al–SiC composites, the matrix-reinforcement interface was modified via chemical reactions between the Al matrix and SiC particles. To prepare diverse interfaces, various types of composites were fabricated using the direct hot extrusion of Al-based powder mixtures containing 30 vol% untreated or oxidized SiC particles. The extruded composites were subjected to different annealing treatments. A detailed microstructural characterization of the Al–SiC interfacial regions was performed. The effect of the interface on the Young’s modulus and on the other mechanical properties of the composites was systematically investigated. Depending on the interface quality, the Young’s modulus of the composites can be varied over the range of 88–121 GPa. The results proved the importance of a stiff phase—comparable to SiC stiffness at the interface, which leads to the SiC particles contributing more effectively to the increase in the composite Young’s modulus. Conversely, the segregation of liberated Si at SiC interface led to decrease of composites Young’s modulus.

Magnetic and Surface Properties of High-Induction Nanocrystalline Fe-Nb-Cu-B/P-Si Ribbons

High-induction Si-poor Fe-Nb-Cu-B-Si Finemet ribbons annealed in inert gas ambience are known to build oxides and prefer surface crystallization. Phosphorus-3 at% of P instead of B was substituted to see how it can influence these surface effects. The off-axis magnetic anisotropy caused by surfaces squeezing the ribbon interior was reduced and it was found out that the major source of the squeeze is the surface crystallization and not the oxides. Phosphorus appears to hamper preferentially the surface crystallization. The substitution also improves the magnetic softness of vacuum-annealed ribbons by grain size reduction. Slight reduction of saturation induction is a minor sacrifice to the improvements.