P. Švec

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%.

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.

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

Abstract Amorphous bilayers with layers of FeNiB and CoFeCrSiB 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.

The influence of microstructure on magnetic properties of nanocrystalline Fe-Pt-Nb-B permanent magnet ribbons

A FePt-based hard-magnetic nanocomposite of exchange spring type was prepared by isothermal annealing of melt-spun Fe52Pt28Nb2B18 (atomic percent) ribbons. The relationship between microstructure and magnetic properties was investigated by qualitative and quantitative structural analysis based on the x-ray diffraction, transmission electron microscopy, and F57e Mossbauer spectrometry on one hand and the superconducting quantum interference device magnetometry on the other hand. The microstructure consists of L10-FePt hard-magnetic grains (15–45 nm in diameter) dispersed in a soft magnetic medium composed by A1 FePt, Fe2B, and boron-rich (FeB)PtNb remainder phase. The ribbons annealed at 700 °C for 1 h exhibit promising hard-magnetic properties at room temperature: Mr/Ms=0.69; Hc=820 kA/m and (BH)max=70 kJ/m3. Strong exchange coupling between hard and soft magnetic phases was demonstrated by a smooth demagnetizing curve and positive δM-peak in the Henkel plot. The magnetic properties measured from 5 to 750...

Strain Rate Sensitivity, Work Hardening, and Fracture Behavior of an Al-Mg TiO 2 Nanocomposite Prepared by Friction Stir Processing

Annealed and wrought AA5052 aluminum alloy was subjected to friction stir processing (FSP) without and with 3 vol pct TiO2 nanoparticles. Microstructural studies by electron backscattered diffraction and transmission electron microscopy showed the formation of an ultra-fine-grained structure with fine distribution of TiO2 nanoparticles in the metal matrix. Nanometric Al3Ti and MgO particles were also observed, revealing in-situ solid-state reactions between Al and Mg with TiO2. Tensile testing at different strain rates determined that FSP decreased the strain rate sensitivity and work hardening of annealed Al-Mg alloy without and with TiO2 nanoparticles, while opposite results were obtained for the wrought alloy. Fractographic studies exhibited that the presence of hard reinforcement particles changed the fracture mode from ductile rupture to ductile-brittle fracture. Notably, the failure mechanism was also altered from shear to tensile rupture as the strain rate increased. Consequently, the fracture surface contained hemispherical equiaxed dimples instead of parabolic ones.

Continuous stress annealing of amorphous ribbons for strain sensing applications

Abstract Stress-annealed amorphous Co 69 Fe 2 Cr 7 Si 8 B 14 ribbons are suitable materials for strain sensors required in civil engineering applications. The equipments for the continuous stress-annealing of ribbons and the automatic homogeneity test of magnetic properties are described. The magnetic properties of stress-annealed ribbons are discussed. The use of ribbon in an inductive strain sensor is also illustrated.

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.