D. Janičkovič

A constant magnetocaloric response in FeMoCuB amorphous alloys with different Fe∕B ratios

The magnetocaloric effect of Fe91−xMo8Cu1Bx (x=15,17,20) amorphous alloys has been studied. The temperature of the peak of magnetic entropy change can be tuned by altering the Fe∕B ratio in the alloy, without changing its magnitude, ∣ΔSMpk∣. The average contribution of the Fe atoms to ∣ΔSMpk∣ increases with increasing B content. This is correlated with the increase in the low temperature mean magnetic moment of Fe. A recently proposed master curve behavior for the magnetic entropy change is also followed by these alloys and is common for all of them.

The rapidly quenched Ag-Cu-Ti ribbons for active joining of ceramics

The technique of rapid quenching from the melt (planar flow casting method) was used to prepare active brazing Ag-Cu-Ti alloys with varying Ag and increased Ti content for joining alumina ceramics in form of ribbons 0.1mm thick and 40 mm wide. Compositional optimization has been performed for high wetting and good spreading of the alumina substrate and for high strength of the joint. Microchemical and structural investigations of the reaction interlayer (alumina/Ag-Cu-Ti/alumina sandwich) have been performed using X-ray spectroscopy, EDX analysis, scanning electron microscopy (SEM) transmission electron microscopy (TEM) and electron diffraction (ED). The structure and local chemical composition profile of the reaction interlayer has been determined with respect to the distribution of Ag and Ti in the interface area. By TEM, it has been found that the reaction zone near alumina consists of TiO while the zone further from the alumina, rich in Ag-Cu contains mainly the mixture of Ti2O3 and Cu2O.

Magnetic properties of Ni-Mn-Ga ribbon prepared by rapid solidification

Magnetic shape memory Ni/sub 49.7/Mn/sub 28.7/Ga/sub 21.6/ alloy was prepared in the form of the thin ribbon by a planar-flow casting method. Optical microscopy and X-ray diffraction were used to check the microstructure of as-quenched and annealed ribbons (800C/24 h and 72 h). The as-quenched ribbon was microcrystalline with grain size about 1.5-3 /spl mu/m which after annealing increased to about 40 /spl mu/m. Annealing also increased the martensitic transformation and Curie temperatures. The annealed ribbon transformed to martensite at 299 K with a reverse transformation to austenite at about 308 K. Room-temperature martensite had a five-layered modulated structure similar to that observed in bulk material. Magnetization at room temperature, 63 Am/sup 2//kg and Curie temperature, 367 K, were close to that of the master alloy. The magnetization loop of the as-quenched ribbon was very flat due to the high level of quenched-in stresses. The loops of annealed ribbon were round indicating a random distribution of easy magnetization axes. The magnetocrystalline anisotropy constant of the annealed ribbon was K/sub u/=1.6/spl times/10/sup 5/ J/m/sup 3/. Thermoelastic strain due to martensitic transformation was about 0.3%.

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.

Development and characterisation of Ag-Cu-Ti brazes prepared with planar flow casting

Abstract Using the technique of rapid quenching from the melt, it becomes possible to prepare active brazes of Ag–Cu–Ti composition with varying silver content for joining alumina ceramics, involving a single technological operation in the form of ribbons up to 40 mm wide and 50–100 μm thick. The melting temperature of the alloys increased with decreasing silver content, in agreement with the phase diagram. The wetting of the alumina by the brazes, determined by the sessile drop method, has been satisfactory for all alloys. The ceramic/ceramic joints attain high joint strengths above 150 MPa. Microchemical and structural investigations of the reaction interlayer (alumina/Ag–Cu–Ti/alumina joint) have been performed using x-ray spectroscopy, EDX analysis and scanning electron microscopy. By transmission electron microscopy and electron diffraction the structures formed in the reaction zone of the joint have been determined. It has been found that the reaction zone near alumina consists of TiO while the zone further from alumina, rich in Ag–Cu, contains mainly the mixture of Ti 2 O 3 and Cu 2 O.

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 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.

Laser-induced structural modifications of FeMoCuB metallic glasses before and after transformation into a nanocrystalline state

The effects of laser treatments on the structural and magnetic properties of metallic ribbons have been studied using the melt-spun Fe76Mo8Cu1B15 alloy in as-quenched and nanocrystalline states. 57Fe Mossbauer effect techniques, comprising transmission geometry measurements (TM) and detection of conversion electrons (CEMS), have been employed in addition to magnetization measurements, differential scanning calorimetry and x-ray diffraction. The Curie temperature of the as-quenched alloy was about 70 °C. The distributions of hyperfine magnetic fields as well as quadrupole splitting obtained from TM and CEM spectra have revealed the possibility of observing laser-induced structural modifications even at room temperature when the system is only weakly magnetic. Consequently, both types of hyperfine interactions have been detected and they are nearly in equilibrium (having the same strength or occurring to the same extent). After treatments with a pulsed XeCl excimer laser (with a homogeneous beam of 5×5 mm2, 308 nm, 55 ns, 1 Hz), the significance of magnetic dipole interactions rises as a function of the number of laser pulses (up to 64) and the laser beam fluence (up to 3 J cm-2). No traces of laser-induced crystallization have been found. In the nanocrystalline Fe76Mo8Cu1B15 alloy, surface crystallization was already completely removed after the first pulse of 1 J cm-2.

Evolution of Structure and Magnetic Properties of Rapidly Quenched Fe–B-Based Systems With Addition of Cu

Rapidly quenched Fe85B15 and Fe64Co21B15 with addition of 1 at.% Cu have been prepared by planar flow casting. Selected magnetic properties were measured in as-cast state and after annealing targeted to produce fine-grain structure of body-centered cubic (bcc)-Fe in amorphous matrix. The transformation process has been followed by electrical resistivity and magnetization measurements. The evolution of structure has been compared with alloys without the Cu addition. Suitable processing conditions are analyzed in order to obtain the control of size and content of crystalline phase. The results are discussed with respect to the possible enhancement of properties related to the microstructure and leading to potential application for power electronics or alternatively to convenient tailoring of magnetic characteristics using thermal treatment in external magnetic fields.