Dariusz Oleszak

X-ray diffraction, magnetization and Mössbauer studies of nanocrystalline Fe–Ni alloys prepared by low- and high-energy ball milling

Abstract Fe–Ni alloys were prepared both by low- and high-energy ball milling processes. Structure and magnetic properties were studied by using X-ray diffraction, differential scanning calorimetry, Mossbauer spectroscopy and magnetization measurements. Mechanical treatment influenced the magnetic properties of Fe–Ni alloys as compared with the equilibrium alloys. Reduction of grain size resulted in the increase of magnetization. Invar anomaly for 35 at% Ni was not detected.

Magnetic properties and structure of nanocrystalline Fe-Al and Fe-Ni alloys

Abstract Magnetic and structural properties of nanocrystalline Fe-Al and Fe-Ni alloys were investigated. Alloys were prepared by the mechanical alloying process. The standard X-ray diffraction, Mossbauer and magnetic studies were carried out on the powder samples. The structural investigations proved that in the low-energy mechanical alloying process of Fe and Al the disordered bcc solid solution was formed up to 50 at.% Al. Alloys were amorphous for Al content between 50 and 90 at.%. The final products of milling exhibited interesting magnetic properties different as compared with microcrystalline alloys. In the case of Fe-Ni alloys the bcc solid solution was formed during high- energy ball milling for the composition with 20 at.% Ni, while above 35 at.% Ni the fcc solid solutions were obtained. All mechanically synthesized Fe-Ni alloys were ferromagnetic and no Invar effect was observed for 35 at.% Ni.

Mechanical alloying in the FeAl system

Abstract The mechanical alloying technique has been applied to alloy synthesis from FeAl powder mixtures containing 10, 20, 30, 40 and 50 at.% Al. An evolution of the powder morphology from flaky in the first stage to spherical in the final stage has been observed. The particle size has been estimated as 50 and 5 μm in the first and final stages of milling respectively. For all alloys studied a disordered b.c.c. solid solution has been formed by ball milling. A grain size of several nanometres and a maximum r.m.s. strain level up to 1% have been observed. The change in lattice parameter has been calculated to be up to 2.4%.

Hydrogen evolution on hot and cold consolidated Ni-Mo alloys produced by mechanical alloying

Ni80Mo20 and Ni57Mo43 alloys produced by mechanical alloying (MA) were used as the electrode materials for hydrogen evolution from NaOH within a limited temperature range of 20–60°C. To form the electrodes, the Ni–Mo alloys were consolidated in two different ways. One was ‘cold pressing’, which retained the original nanocrystalline structure of Ni80Mo20, and the amorphous structure of Ni57Mo43 and another one was ‘hot pressing’, which produced multiphase systems (Ni4Mo+MoO2) and improved the mechanical properties of the resulting electrodes too. Appreciable cathodic current densities of ∼100 mA cm−2 were measured at these electrodes. The estimated values of the apparent activation energy for cold consolidated materials were much lower, whereas those for the exchange current were much higher than the apparent activation energy values for a smooth, polycrystalline Ni plate.

Hyperfine interactions in nanocrystalline Fe-Al alloys

Nanocrystalline powder samples of Fe-30 at.% Al, Fe-40 at.% Al and Fe-50 at.% Al alloys were prepared by the mechanical alloying method. X-ray diffraction studies indicated that the solid solution with bcc structure was formed with increasing milling time for all investigated compositions. The magnetic ordering temperature of the nanocrystalline mechanically synthesized alloys was larger than that of the corresponding alloys on a micrometric scale. The magnetization curves as well as the Mossbauer spectra revealed that the Fe-Al alloys formed during the low energy ball milling process contained different magnetic phases.

Nickel-molybdenum catalysts fabricated by mechanical alloying and spark plasma sintering

Abstract The low energy ball milling process has been used to fabricate Ni–43at.%Mo based electro-active powder. Nanometre-sized Mo particles have been embedded into an agglomerated amorphous matrix. The spark plasma sintering (SPS) process has been used to densify the powder in

NiAl-Al2O3 intermetallic matrix composite prepared by reactive milling and consolidation of powders

Reactive milling of nickel oxide and aluminium powders corresponding to the stoichiometric reaction 3NiO + 5Al resulted in the formation of intermetallic matrix composite NiAl-Al2O3, with 28 wt% of alumina. Prolongation of the milling process allowed obtaining the microstructure with nanosize range of crystallites of both phases, as showed XRD measurements and TEM observations. The refinement of microstructure was accompanied with an increase of lattice strain as a result of ball milling. The particles size and morphology changed from several tens of micrometers and polyhedron shape observed immediately after the reaction took place, to several micrometers and spherulitic shape after long-term milling.Two consolidation techniques of nanocomposite powders were applied: explosive compaction and hot-pressing under high pressure. Both methods allowed obtaining the samples of high density (up to 99% of theoretical one) and microhardness above 13 GPa. Simultaneously, a nanocrystalline structure of the material was preserved.

Structural and magnetic study of mechanically alloyed Fe-Ni

Abstract The Fe x Ni 100-x (x = 50, 65 and 80) alloys were synthesized in a conventional horizontal low energy ball mill. The X-ray diffraction was used to identify and characterise various phases during the milling process of the Fe 80 Ni 20 alloy exhibiting a bcc structure whereas for x = 65 and 50 the fcc structures are found. The steady state grain size is about 10 nm. Magnetisation measurements after various milling periods allow to monitor a rate at which Ni atoms dissolve in the iron lattice. The room temperature values of the effective magnetic moment raise with the increasing milling period. All the alloys studied exhibit the ferromagnetic ordering. The magnitude of the magnetic interactions is moderately suppressed at prolonged milling as revealed by the Curie temperatures reduced down to 950 K. Such variations are caused by the deviations in the interatomic arrangements of atoms especially in the intergrain regions. The Moessbauer spectroscopy confirmed the ferromagnetic ordering and was used to calculate the distribution of hyperfine magnetic fields. The mean hyperfine fields are 33.8 T for Fe 80 Ni 20 and correspond to the one to two Ni atoms in nearest neighbourhood. In the remaining alloys, at most, five Ni atoms are located in a neighbourhood of the Fe atom.

Structural and magnetic study of crystalline Fe80Ni20 alloys with nanometer-sized grains

Abstract Samples of the Fe80Ni20 alloys were synthesized in the low energy conventional horizontal and high energy planetary ball mills. Both in the low and high energy processes, the solid bcc Fe(Ni) solutions with a similar structure are formed. The lattice constants are slightly larger than for the pure iron. Thermal stability of alloys was measured upto 1100 K. The magnetization and ferromagnetic Curie temperatures confirm a ferromagnetic ordering. The magnetic moment equal to 2.07 μB for the longest milling periods proves that the electronic structure of the crystalline alloy with nanometer-sized grains does not differ from that of polycrystalline samples. The distributions of the hyperfine magnetic fields revealed that an iron atom has one to two Ni atoms in the nearest neighborhood.

Phase transformations in the Fe(Co, Ni)ZrB alloys induced by ball milling

Abstract Phase transformations in the FeZrB-based powders induced by high- and low-energy ball milling are investigated by X-ray diffraction and transmission Mossbauer spectroscopy. The structural and magnetic properties of the processed powders are studied as a function of milling time. The influence of Co and Ni additions on the phase transformations is also investigated. X-ray diffraction measurements of the high-energy ball milled powders indicate the formation of the b.c.c. Fe solid solution. Mossbauer results, however, reveal more complex structure of the milled powders, in particular, the formation of an FeZr amorphous phase at the early stages of milling as well as formation of Fe–B phases. The low-energy ball milling is found to be poorly effective in the alloying of the elemental FeZrB-based powders.