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A mixture of 50.0 wt% Fe, 47.0 wt% BaTiO3, and 3.0 wt% TiO2 powders is milled and then pressed and sintered. Samples sintered from ground powders have a structure consisting of plates. They consist of the crystalline phase BaFe12O19 and crystalline and amorphous phases Fe, BaTiO3, and TiO2. Samples sintered from powders ground for more than 180 min are compact and composed of BaFe12O19 crystals embedded in an amorphous matrix consisting of amorphous BaTiO3 and TiO2 phases. Magnetization of a sample ground for 220 min gradually decreases with increasing temperature up to 440°C. This decrease result from the transition of ordered domains into chaotically directed domains. Above 440°C, the thermal energy is sufficiently large to destroy ordered paramagnetic ferromagnetic and weakly ferromagnetic domains. During sample cooling in the absence of an external magnetic field, the temperature dependence of magnetization is the same as that obtained during heating in the magnetic field. However, during the sample cooling in the magnetic field, substantially higher values of magnetization are obtained. Considerably higher magnetization of the cooled sample remains even after switching off the external magnetic field. The sintered sample, previously pressed in the magnetic field, has a higher dielectric constant than the sample pressed in the absence of the magnetic field. The dielectric constant decreases with increasing frequency.

Refractory devices composed of ceramics are employed wherever there is contact with molten metals as in crucibles filters, furnaces or systems for flow control. The mechanical properties of these materials are very different from metals. The presentation focuses on basic equations related to thermal and mechanical transmission conditions across an interface between two different materials. These equations are linked to the development of material instabilities and crack propagation within the ceramics, in the view of application to refractories operating at high-temperature conditions in steel plants.

Kinetics and mechanism of hydrogen adsorption in as-obtained and ground nuclear graphite Wendelstein 7-X are examined. In the first time interval the adsorption process is determined by dissociation of the hydrogen molecule, occurring at the outer surface and in open micropores of nuclear graphite particles. However, in the second time interval, the slowest step in the hydrogen adsorption is inter-granular and inter-crystallite diffusion in nanopores of graphite. The X-ray analysis shows, that grinding of as-obtained nuclear graphite results in finer particles with finer nanocrystals and larger density of opened pores and carbon reactive sites. The capacity and rate of adsorption increase with comminution of nuclear graphite particles and adsorbed hydrogen does not sub stantially alter the microstructure of nuclear graphite.

A nanostructured powder of the Ni86,0Fe9,8W1,3Cu2,9 alloy is deposited on a titanium cathode by electrodeposition from the citrate bath, at the current density of 400 mA cm−2. The powder particles are of a cauliflower and dendritic shape and composed of nanocrystals of the FCC phase of the solid solution of iron, tungsten and copper in nickel, captive in an amorphous matrix. The freshly deposited powder is thermally stable up to 160 °C. The annealing of the powder in the temperature range from 160 °C to 460 °C results in its structure relaxation. At temperatures higher than 460 °C, the amorphous phase in the powder is crystalizing and crystal grains of the FCC solid solution are growing. The structural changes caused by annealing, affect magnetic properties of the alloy. The cooled powders after the structure relaxation have higher magnetization, a lower loss of the active power and a lower coercive field. Crystallization of the amorphous phase and crystalline grain growth of the FCC phase causes a decrease in magnetization, an increase in the power loss and the power of coercive field.

Nanostructured Ni-Fe-W-Cu alloy powders were electrodeposited from an alkaline ammonium citrate solution on a titanium cathode. Powder particles were dendrite- and cauliflower-shaped. The dendritic particles had a high density of branches made up of interconnected globules. XRD analysis showed that the powder contained an amorphous matrix and FCC nanocrystals of the solid solution of Fe, W and Cu in Ni. As the deposition current density increased, the mean nanocrystal size decreased, and the mean value of internal microstrain and the total weight percent of Fe and Ni in the alloy increased. The powders deposited at higher current densities exhibited higher magnetization. During annealing at temperatures up to 460 °C, the powders underwent short-range ordering, which caused an increase in magnetization, whereas at temperatures above 460 °C, the magnetization decreased due to the formation of large FCC crystalline grains.

Ni-Fe-W-Cu alloy powders were obtained by electrodeposition from an ammonium citrate bath at current densities ranging between 70 and 600 mA cm-2. As the deposition current density increased, the contents of Fe and W in the alloy increased, and those of Ni and Cu decreased. The total cathodic polarization curve was recorded, and partial polarization curves for Ni, Fe and W deposition and hydrogen evolution were determined. The current efficiency of alloy deposition was measured. The powders contained an amorphous matrix and FCC nanocrystals of the solid solution of Fe, W and Cu in Ni. At high current densities, small-sized nanocrystals exhibiting high internal microstrain values were formed. Powder particles were dendrite-and cauliflower-shaped. The dendrites had a large number of secondary branches and higher-order branches containing interconnected globules. The density of branches was higher in particles formed at high current densities. The powders formed at high current densities exhibited higher magnetization. Annealing at temperatures up to 460 °C resulted in structural relaxation, accompanied by an increase in magnetization. At temperatures above 460 °C, amorphous matrix crystallization and FCC crystal growth took place, accompanied by a decrease in magnetization.

REZIME: Predmet istraživanja u ovoj doktorskoj disertaciji su uzorci amorfne masivne metalne legure sastava Fe65,5Cr4Mo4Ga4P12C5B5,5. Ispitivani su uzorci u obliku valjka prečnika 1,5 mm i 1,8 mm dobijeni tehnologijom livenja rastopa u bakarne kalupe. Amorfne metalne legure nalaze se u termički neravnotežnom stanju što omogućava da se različitim postupcima odgrevanja vrši optimizacija njihovih funkcionalnih svojstava. Uticaj toplotnog dejstva na termičku stabilnost legure ispitan je postupkom višestrukih termičkih tretmana uzoraka na temperaturama ispod i iznad temperature kristalizacije. DTA analizom pri različitim brzinama grejanja definisan je temperaturni prelaz iz amorfnog u kristalno stanje. Pokazano je da u temperaturnom intervalu od 293 K do 993 K egzistira samo jedan egzotermi pik uzrokovan procesom kristalizacije legure, a čiji se oblik, visina i položaj menjaju u zavisnosti od brzine zagrevanja uzorka. Potvrđeno je da su rezultati dobijeni DTA analizom u korelaciji sa dilatometrijskim praćenjem promena dimenzija uzoraka. Strukturne transformacije legure pre i nakon termičkih tretmana ispitane su XRD analizom. Utvrđeno je da relaksirana amorfna struktura poseduje znatno bolja magnetna svojstva u odnosu na polaznu i iskristalisalu leguru. Pokazano je da rezultati ispitivanja termičkog tretmana na mehanička, magnetna i strukturna svojstva ne zavise samo od temperature odgrevanja i brzine zagrevanja, već da krucijalni uticaj ima stepen amorfnosti polazne strukture, tj. prečnik uzorka. Nakon sprovedenih višestrukih termičkih tretmana ispod i iznad temperature kristalizacije, DTA i XRD analize, merenja magnetne permeabilnosti, koercitivne sile, Curie-eve temperature, dilatacije, mikrostrukturne i SEM analize, tvrdoće i mikrotvrdoće, detaljno su ispitane strukturne promene tokom procesa relaksacije i kristalizacije. Utvrđena je međusobna korelacija parametara sinteze i strukturnih promena sa poboljšanim funkcionalnim svojstvima ispitivane legure. S obzirom da le

Barium hexaferrite ceramics were prepared using mechanically activated mixtures of iron and barium titanate. The 60:40 mass% Fe:BaTiO3 powder mixtures were mechanically activated for different times (100–240 min) and sintered at 1100 and 1200 °C in order to determine the influence of mechanical activation of the precursor on the magnetic and dielectric properties of the resulting barium hexaferrite ceramics. The final product contained 84–89 mass% of Ba2Fe22.46O38Ti1.54 phase, with higher content corresponding to longer mechanical activation of the precursor. XRD and Raman measurements indicated that the remainder of the sample consists of leftover BaTiO3 and hematite, which was formed by the oxidation of iron during mechanical activation and sintering in air. Magnetic properties of samples sintered at 1200 °C are superior to those sintered at 1100 °C, which can be attributed to higher Ba2Fe22.46O38Ti1.54 phase content. The position of the Curie temperature in 350–420 °C temperature region is consistent with 0.8:1 ratio of Ti to Ba. Maximum magnetization was observed for samples activated for 120 min. Dielectric properties of samples sintered at 1200 °C showed a dependence on frequency, with a significant drop in relative permittivity with an increase in frequency in the low-frequency region, and relatively constant values of relative permittivity in the high-frequency region. The tangent loss showed a decrease with increase in frequency, where peaks corresponding to the resonance of the electron hopping frequency with the external field were observed in the samples corresponding to the longer mechanical activation. Dielectric properties showed relatively small changes for samples activated longer than 150 min. © 2018 Elsevier Ltd and Techna Group S.r.l.

Rietveld analysis of XRD patterns of mechanically activated Fe:BaTiO3 powder mixtures sintered at 1200°C

The thermal electromotive force (TEMF) and the thermal electromotive force coefficient (TEMFC) of the thermocouple consisting of a copper wire and an (X5CrNi1810) steel wire plastically deformed under tension or bending conditions were found to increase with increasing degree of plastic deformation. The increase in the degree of deformation disturbs the microstructure of steel due to increases in the density of chaotically distributed dislocations and internal microstress, resulting in a decrease in the electron density of states near the Fermi level. Through the effect of thermal energy, annealing at elevated temperatures up to 300°C leads to microstructural ordering along with simultaneous increases in the free electron density of states, TEMF and TEMFC. Based on the temporal change of the TEMF, the kinetics of microstructural ordering was determined. During the initial time interval, the process is a kinetically controlled first-order reaction. In the second time interval, the process is controlled by the diffusion of reactant species.

Nanostructured Ni-Fe-W-Cu alloy powders were electrodeposited from an alkaline ammonium citrate solution on a titanium cathode. Powder particles were dendrite- and cauliflower-shaped. The dendritic particles had a high density of branches made up of interconnected globules. XRD analysis showed that the powder contained an amorphous matrix and FCC nanocrystals of the solid solution of Fe, W and Cu in Ni. As the deposition current density increased, the mean nanocrystal size decreased, and the mean value of internal microstrain and the total weight percent of Fe and Ni in the alloy increased. The powders deposited at higher current densities exhibited higher magnetization. During annealing at temperatures up to 460°C, the powders underwent short-range ordering, which caused an increase in magnetization, whereas at temperatures above 460°C, the magnetization decreased due to the formation of large FCC crystalline grains.

Electromagnetic casting (EMC) is the technology developed as by combining the magnetic hydrodynamics and casting technique. Electromagnetic forces, arising from the interaction of Eddy currents induced in the metal by inductor magnetic field, cause an increased flow of the fluid, forced convection, more uniform temperature field and weak gravitation influence thus changing the conditions of solidification. The advantage of EMC reflects in obtaining a better quality of ingots compared to convent ional continuous casting process. Namely, the structure obtained is finer and more uniform through the cross section, with reduced segregation of alloy element and porosity. Apart from that, due to the reduced contact pressure (result of electromagnetic field effect) between the mould and the metal, the quality of ingot surface is improved, having no need for additional machine processing. The investigations conducted in the world were aimed to investigate the effect of electromagnetic, magnetic and hydrodynamic phenomena on Al ingots, but very little attention was given to the characterization of microstructure and mechanical properties. This work, should contribute to better knowledge of the effect of electromagnetic field on the obtained microstructure (morphology, size, volume fraction and distribution of phases) and properties of Al alloys. The chosen alloy was EN AW 7075 heat treatable, intended for forge with wide industry use. It is characterized by a number of defects that occur during the solidific ation process: porosity, hot cracks, non -uniformal grain size and crystal segregation. Since the quality of final product is directly affected by these defects it is necessary to prevent or reduce their appearance by the choice of the appropriate process and optimal parameters of casting. To compare the results of electromagnetic casting, (with different operating parameters), the process of vertical continual casting without the presence of low frequency electromagnetic, Elektromagnetni postupak livenja zasniva se na teoriji metalurških procesa i magnetohidrodinamici. Prilikom livenja u prisustvu elektromagnetnog polja naizmenična struja generiše vremenski promenljivo mognetno polje u istopljenoj masi što povećava indukovanu struju u rastopu. Rastop metala je pod uticajem elektromagnetne sile koja je prouzrokovana interakcijom indukovane struje i magnetnog polja. Elektromagnetna sila izaziva veći protok fluida, prinudnu konvekciju, ravnomernije temperaturno polje i slabiji uticaj gravitacije. Na ovaj način se menjaju uslovi očvršćavanja. Dobijeni odlivci su kvalitetniji, struktura je finija i uniformnija po preseku. Zbog smanjenog kontaktnog pritiska, (što je takođe posledica delovanja elektromagnetnog polja) između kalupa i metala, kvalitet površine je poboljšan tako da nije potrebna veća dodatna mašinska obrada. U svetu postoje određena istraživanja u oblasti elektromagnetnog postupka livenja, ali je malo pažnje posvećeno karakterizaciji mikrostrukture i mehaničkih svojstava tako dobijenih odlivaka. Ovaj rad treba da doprinese boljem poznavanju uticaja elektromagnetnog polja na dobijenu mikrostrukturu (morfologiju, veličinu zrna, raspodelu i udeo dobijenih faza) i ostale karakteristike dobijenih odlivaka. Izabrana legura EN AW 7075 ima široku primenu u industriji, termički je obradiva i namenjena je za plastičnu preradu odnosno kovanje. Proizvodnja ove legure je dugotrajna i skupa, a prate je nedostaci tipa poroznosti, toplih pukotina i neujednačenosti u veličini zrna. Ove metalurške greške utiču na pogoršanje mehaničkih svojstava i kvaliteta dobijenih odlivaka. U cilju smanjenja ovih grešaka još u livenom stanju neophodno je primeniti odgovarajući postupak livenja sa optimalnim radnim parametrima. Da bi se ispitao uticaj elektromagnetnog polja prilikom livenja upoređeni su uzorci odliveni bez dejstva polja i pod dejstvom polja različite frekvencije 10, 15, 20 i 30 Hz. Rezultati ispitivanja mikrostrukture i mehaničkih svojstava po

Crystal structure and morphology of mechanically activated nanocrystalline Fe/BaTiO3 was investigated using a combination of spectroscopic and microscopic methods. These show that mechanical activation led to the creation of new surfaces and the comminution of the initial powder particles. Prolonged milling resulted in formation of larger agglomerates of BaTiO3 and bimodal particle size distribution, where BaTiO3 particles were significantly larger than those of iron-containing phases. Milling times of 210 min and above lead to a significant decrease in temperature of the oxidation of iron in the sample, indicating abrupt change in reactivity. Raman spectroscopy analysis has revealed that activation had a pronounced influence on Fe/BaTiO3 lattice, thereby affecting both the stability of the crystal structure and the phase transition phenomena.

The hydrogenation of a crystalline Ni-Fe (80 wt.% Ni, 20 wt.% Fe) powder mixture leads to the formation of a mixture of Face Centered Cubic (FCC)-Ni and FCC-Fe phase nanocrystals embedded in an amorphous matrix. The magnetic susceptibility of the nanostructured powder is 2.1 times higher than that of the as-produced crystalline mixture. Heating in the temperature range 420-590 K causes structural relaxation in the hydrogenated powder, resulting in an increase of the magnetic susceptibility and a decrease of the electrical resistivity. During the heating procedure, the reorientation of magnetic domains in nickel and iron takes place in the temperature range 580-650 K and 790-850 K, respectively. In the pressed sample from the powder mixture, the crystallization of the amorphous phase of nickel and its FCC lattice crystalline grain growth occurs in the temperature range 620-873 K causing a decrease in the magnetic susceptibility of the nickel FCC phase and a sudden drop in the electrical resistivity. Prolonged heating of the mixed powders at 873K results in the formation of a Ni-Fe solid solution with higher magnetic susceptibility than the starting mixture.