Pavol Duhaj

The study of phase transitions in amorphous bilayers prepared by rapid quenching

Abstract Amorphous bilayers with layers of Feue5f8Niue5f8B and Coue5f8Feue5f8Crue5f8Siue5f8B have been prepared by planar flow casting from a single crucible with two nozzles close to each other and with a partition between them forming two separate vessels. Such an arrangement has allowed us to obtain ribbons with two homogeneous layers, one on top of the other, along the whole ribbon length with high quality surface and with contact interlayer having submicron thickness. The character of the interlayer has been investigated by SEM, EMA, CS-TEM, AES and resistometry in the as-quenched state and after annealing below and after crystallization. From the results it seems evident that the process of connection of the two layers takes place below the crystallization temperature by mutual interdiffusion of component atoms, thus giving rise to mechanically solid connection.

Correlation between saturation magnetization, magnetostriction and creep-induced anisotropy in amorphous (FeCo)/sub 85/B/sub 15/ alloys

Compositional dependence of saturation magnetization, magnetostriction and creep-induced magnetic anisotropy is investigated in amorphous (Fe/sub 1-x/Co/sub x/)/sub 85/B/sub 15/ alloys. A close correlation of the three quantities was found. An anomaly centred at x=0.25 may indicate chemical short-range order of the Fe/sub 3/Co type in this concentration range. An explanation of the creep-induced anisotropy behaviour based on the bond-orientational anisotropy model is presented. It takes into account both the short-range ordering in the shear centres and medium-range deformations of their surroundings. >

Structure and magnetic properties of the Finemet alloy Fe73Cu1Nb3Si13.5B9.5

Abstract Structural changes taking place in the course of crystallization in nanocrystalline Fe 73 Cu 1 Nb 3 Si 13.5 B 9.5 and their influence on the initial permeability of the material have been studied as a function of ribbon thickness and thermal treatment. To obtain good magnetic properties it is necessary, as well as keeping the α(FeSi) grains to a suitable size, to ensure the appropriate Cu and Nb content. Inhomogeneous Cu distribution and eventual decrease in its content has been found to cause formation of borides or silicides in regions of Cu depletion, influencing the magnetic properties of the Finemets. It has further been determined that the content of Si in α-Fe is of great importance for the permeability of the Finemets.

The study of phase transformations in nanocrystalline materials

Abstract The structure of ferromagnetic amorphous alloys Fe 73.5 X 1 Nb 3 Si 13.5 B 9 , with X = Cu, Au, Pt, Pd, has been investigated by TEM, resistometry, dilatometry and magnetic measurements. Nanocrystalline grain formation is observed in these alloys upon crystallization. Ultrafine grains are preserved in alloys with additives which are insoluble in b.c.c.-Fe (Cu, Au), thus forming, together with niobium, a relatively stable interfacial barrier around the grains preventing their growth. In alloys with additives soluble in b.c.c.-Fe, grain sizes increase upon prolonged annealing.

Structural investigation of Fe(Cu)ZrB amorphous alloy

The crystallization process in Fes6(Cu~)ZrTB6 and FesvZr7B6 is .investigated using the methods of transmission electron microscopy, electron and X-ray diffraction and resistrometry. Two crystallization reactions take place during thermal annealing of amorphous Fe86(Cul)ZrvBe and Fes7Zr7B6 alloys. In both alloys the first crystallization begins with the formation of nanocrystalline c~-Fe at temperature to approximately 800 K. The second crystallization starts above 1000 K; the nanocrystalline phase dissolves and together with the remaining amorphous matrix form rough grains of a-Fe and dispersed Fe23Zr6 phases. From M6ssbauer spectroscopy it seems that there exist two neighbourhoods of Fe atoms in the amorphous structure. One of them is characterized by low Zr content and is responsible for the high-field component of the hyperfine field distribution p(H). The second one is rich in Zr and B and is responsible for the low-field component of p(H). This is in accord with the observation of two crystallization steps separated by a large interval of temperatures due to the existence of two chemically different regions or clusters.

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 Feue5f8Cuue5f8Nbue5f8Siue5f8B. 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.

The relation between the bulk and ribbon Zr55Ni25Al20 metallic glasses

Abstract The planar-flow-casting technique and quenching in flowing water were used to produce the glassy Zr 55 Ni 25 Al 20 ribbon and bulk samples. The thermodynamic states of these two types of samples were compared using differential scanning calorimetry and analysis of the structural relaxation kinetics. The relaxation enthalpy is 3 times larger and the glass transition temperature is higher by 67.5 K in the case of the ribbon sample. These differences were theoretically confirmed by a kinetic model and were caused by the different quenching rates for the ribbon and bulk samples, namely 10 5 and 10 2 K s −1 , respectively. X-ray analysis has shown, that the main difference between the studied glassy structures consists in the different degree of the short-range order in the samples.

Direct spectrum analysis of anelastic deformation response during structural relaxation of amorphous metals

Direct relaxation time spectrum analysis method has been successfully used to observe and to study changes in the kinetics of isothermal anelastic deformation response of soft magnetic metallic glass Fe/sub 40/Ni/sub 40/B/sub 20/ during structural relaxation. Computed relaxation time spectra contain three or four quite well separated peaks. >

Crystallization behavior of amorphous (Fe100−xCox)85B15

Abstract Fe–Co–B amorphous alloys exhibit anomaly in their magnetic properties which has been attributed to a short-range order of Fe 3 Co or FeCo around specific Co concentrations. In the present work, crystallization behavior of amorphous (Fe 100− x Co x ) 85 B 15 has been studied in order to further elucidate the structural changes in the amorphous phase as a function of the Co concentration. The primary phase precipitating out is a bcc Fe–Co alloy while in the second crystallization step (Fe 100− y Co y ) 2 B compound is formed. An anomaly in the first crystallization temperature obtained from DSC measurements for composition x =24.7 is observed. During the initial stages of crystallization the Co concentration in the primary phase is close to the nominal Co concentration. However at later stages, in specimens with x >27, the concentration of Co increases and exceeds the nominal composition. Mossbauer measurements during annealing suggest a decrease in the inhomogenity of the remaining amorphous phase.

Magnetic properties and surface roughness of Fe64Co21B15 amorphous ribbons quenched from different melt temperatures

Abstract The effect of different melt temperature and of surface roughness on the magnetic properties of the amorphous phase in Fe 64 Co 21 B 15 alloy prepared by Planar Flow Casting (PFC) has been investigated. The obtained ribbons have been magnetically characterized analyzing hysteresis loops and coercivity in the as-cast condition, after relaxation induced by field-annealing treatment and after thinning by chemical etching. Differential Scanning Calorimeter (DSC) has been used to quantitatively characterize structural relaxation and crystallization of the amorphous phase. Optical and Scanning Electron Microscopy (SEM) have been used to evaluate surface roughness. The average size of the surface grooves on the wheel side was found to increase with melt temperature and thickness. Magnetic properties have been observed to be mainly influenced by the roughness on the wheel side of the ribbons and surface frozen-in stresses are considered to be the main source for the magnetic anisotropy. Crystallization process doesnt vary significantly among the samples, while relaxation heat was found to increase with thickness reduction. Melt temperature was related to previous remarks as controlling viscosity and thus modifying the wetting grade of the melt onto the cooling substrate.