Structural and magnetic properties of nanocrystalline NiFeCuMo powders produced by wet mechanical alloying

The NiFeCuMo nanocrystalline soft magnetic powders were successfully obtained by wet mechanical alloying route in a planetary ball mill using benzene (C₆H₆) as process control agent (PCA). The milling time used was ranging from 2 up to 20 h. The synthesis conditions and alloy formation have been investigated by X-ray and neutron diffraction as well as their influence on the intrinsic physical properties. Nanometer scale (≈10 nm) crystallites were obtained. A decrease of the samples magnetization has been observed and attributed to the stresses induced during the milling and to the benzene adsorbed on the powders surface. Differential scanning calorimetry investigation shows the presence of an exothermic peak related to the presence of benzene. The adsorbed benzene, internal stresses and crystalline defects removal took place during the heat treatment at 350 °C for 4 h, leading to an improvement of the powders magnetization.     

Bulk amorphous Ni59Zr20Ti16Sn5 alloy fabricated by powder compaction

The possibility of fabrication of bulk amorphous Ni₅₉Zr₂₀Ti₁₆Sn₅ alloy by hot isostatic pressing of powders was investigated. The amorphous powders were obtained by ball milling of amorphous melt spun ribbon and by mechanical alloying of a mixture of powders of pure crystalline elements. Fully amorphous bulk samples were successfully obtained by hot isostatic pressing of both types of powders. However, at least 10% porosity of the sample fabricated from the ball milled ribbon was observed. Further optimisation of the compaction process needs to be performed.     

Microstructural characterization and amorphous phase formation in Co40Fe22Ta8B30 powders produced by mechanical alloying

In this work, microstructural evolution and amorphous phase formation in Co₄₀Fe₂₂Ta₈B₃₀ alloy produced by mechanical alloying (MA) of the elemental powder mixture under argon gas atmosphere was investigated. Milling time had a profound effect on the phase transformation, microstructure, morphological evolution and thermal behavior of the powders. These effects were studied by the X-ray powder diffraction (XRD) in reflection mode using Cu Kα and in transmission configuration using synchrotron radiation, transmission electron microscopy (TEM), scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). The results showed that at the early stage of the milling, microstructure consisted of nanocrystalline bcc-(Fe, Co) phases and unreacted tantalum.Further milling, produced an amorphous phase, which became a dominant phase with a fraction of 96 wt% after 200 h milling. The DSC profile of 200 h milled powders demonstrated a huge and broad exothermic hump due to the structural relaxation, followed by a single exothermic peak, indicating the crystallization of the amorphous phase. Further XRD studies in transmission mode by synchrotron radiation revealed that the crystalline products were (Co, Fe)_20.82Ta_2.18B₆, (Co, Fe)₂₁ Ta₂ B₆, and (Co, Fe)₃B₂. The amorphization mechanisms were discussed in terms of severe grain refinement, atomic size effect, the concept of local topological instability and the heat of mixing of the reactants.     

Microstructural, thermal and magnetic properties of amorphous/nanocrystalline FeCrMnN alloys prepared by mechanical alloying and subsequent heat treatment

Amorphous FeCrMnN alloys were synthesized by mechanical alloying (MA) of the elemental powder mixtures under a nitrogen gas atmosphere. The phase identification and structural properties, morphological evolution, thermal behavior and magnetic properties of the mechanically alloyed powders were evaluated by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC) and vibrating sample magnetometer (VSM), respectively. According to the results, at the low milling times the structure consists of the nanocrystalline ferrite and austenite phases. By progression of the MA process, the quantity and homogeneity of the amorphous phase increase. At sufficiently high milling times (>120 h), the XRD pattern becomes halo, indicating complete amorphization. The results also show that the amorphous powders exhibit a wide supercooled liquid region. The crystallization of the amorphous phase occurs during the heating cycle in the DSC equipment and the amorphous phase is transformed into the crystalline compounds containing ferrite, CrN and Cr₂N. The magnetic studies reveal that the magnetic coercivity increases and then decreases. Also, the saturation magnetization decreases with the milling time and after the completion of the amorphization process (>120 h), the material shows a paramagnetic behavior. Although the magnetic behavior does not considerably change by heating the amorphous powders up to the crystallization temperature via DSC equipment, the material depicts a considerable saturation magnetization after the transformation of the amorphous phase to the nanocrystalline compounds.     

The synthesis of nanocrystalline Ni75Nb12B13 alloys by high energy ball milling of elemental components

Ni₇₅Nb₁₂B₁₃ alloys were synthesized by mechanical alloying (MA) of individual Ni, Nb and B components. X-ray investigation showed the formation of Ni (Nb, B) solid solution and amorphous phase at the intermediate stage of milling. Metastable phases formed by MA turned into Ni (Nb), Ni₂₁Nb₂B₆ and Ni₃Nb stable phases during heating up to 720 °C. The exothermal effects on DSC curves were caused with these processes. The disintegration of Ni (Nb, B) solid solution and crystallization of an amorphous phase resulted in the stable phases formation during the milling prolongation as well as after thermal treatment.     

Milling and subsequent thermal annealing effects on the microstructural and magnetic properties of nanostructured Fe90Co10 and Fe65Co35 powders

The influence of milling and subsequent annealing on the microstructural and magnetic properties of Fe₉₀Co₁₀ and Fe₆₅Co₃₅ alloys is investigated. After milling for 8 h a body-centred cubic nanostructured Fe–Co alloy forms with an average crystallite size of about 12 nm. The magnetization saturation (M_S) increases 16% for Fe₆₅Co₃₅ and 5% for Fe₉₀Co₁₀ alloys by milling for 8 h. Subsequent annealing of Fe₉₀Co₁₀ and Fe₆₅Co₃₅ powders for 105 min at 550 °C improves the M_S about 6 and 11%, respectively. Before annealing, the coercivity increases (up to 60 Oe) by milling for 3 h, followed by a reduction on milling for longer periods (45 h). At the initial stage of the heating, a sharp decrease in H_C to 8–10 Oe occurs due to the relief of internal strain. Further heating leads to an increase in the coercivity (intermediate times) followed by a slight diminution on heating for final stage.     

Magnetic and structural studies of mechanically alloyed (Fe50Co50)62Nb8B30 powder mixtures

Amorphous (Fe₅₀Co₅₀)₆₂Nb₈B₃₀ powder mixture was prepared by mechanical alloying from elemental Fe, Co, B and Nb powders in a planetary ball mill under argon atmosphere. Structural, thermal and magnetic properties were performed on the milled powders by means of X-ray diffraction, differential scanning calorimetry and magnetic measurements. The amorphous state is reached after 125 h of milling. The excess enthalpy due to the high density of defects is released at temperature below 300 °C. Crystallisation and growth of crystal domains are the dominating processes at high temperatures. The saturation magnetisation decreases rapidly during the first 25 h of milling to about 15.24 A m²/kg and remains nearly constant on further milling. Coercivity, H_c, value of about 160 Oe is obtained after 125 h of milling.     

Glass-forming ability and magnetic properties of mechanically solid-state reacted Co100−xTix alloy powders

A high-energy ball milling technique using the mechanical alloying method has been employed for fabrication of glassy Co_100−xTi_x (25≤x≤67) alloy powders at room temperature. The fabricated glassy alloys in the Co-rich (33≥x) side exhibit good soft magnetic properties. The binary glassy alloys for which the glass transition temperatures (T_g) have rather high temperatures (above 800 K), show large supercooled liquid regions before crystallization (ΔT_x larger than 50 K). The reduced glass transition temperature (ratio between T_g and liquidus temperatures, T_l (T_g/T_l)) was found to be larger than 0.56. We have also performed post-annealing experiments on the mechanically deformed Co/Ti multilayered composite powders. The results show that annealing of the powders at 710 K leads to the formation of a glassy phase (thermally enhanced glass formation reaction), of which the heat of formation was measured directly. The similarity in the crystallization and magnetization behaviors between the two classes of as-annealed and as-mechanically alloyed glassy powders implies the formation of the same glass state.     

Microstructural and mechanical properties of Al-Mg/Al2O3 nanocomposite prepared by mechanical alloying

The effect of milling time on the microstructure and mechanical properties of Al and Al-10 wt.% Mg matrix nanocomposites reinforced with 5 wt.% Al₂O₃ during mechanical alloying was investigated. Steady-state situation was occurred in Al-10Mg/5Al₂O₃ nanocomposite after 20 h, due to solution of Mg into Al matrix, while the situation was not observed in Al/5Al₂O₃ nanocomposite at the same time. For the binary Al-Mg matrix, after 10 h, the predominant phase was an Al-Mg solid solution with an average crystallite size 34 nm. Up to 10 h, the lattice strain increased to about 0.4 and 0.66% for Al and Al-Mg matrix, respectively. The increasing of lattice parameter due to dissolution of Mg atom into Al lattice during milling was significant. By milling for 10 h the dramatic increase in microhardness (155 HV) for Al-Mg matrix nanocomposite was caused by grain refinement and solid solution formation. From 10 to 20 h, slower rate of increasing in microhardness may be attributed to the completion of alloying process, and dynamic and static recovery of powders.     

Structure, hyperfine interactions and magnetization studies of mechanically alloyed Fe50Ge50 and Fe62Ge38

X-ray diffraction, Mössbauer spectroscopy and magnetization measurements were used to study the structure and some magnetic properties of Fe₅₀Ge₅₀ and Fe₆₂Ge₃₈ prepared by mechanical alloying from the elemental powders. In both cases in the early stages of milling the intermediate paramagnetic FeGe₂ phase was formed. The mechanical alloying process of Fe₅₀Ge₅₀ resulted in the formation of the paramagnetic FeGe (B20) phase with an average crystallite size of about 15 nm. In the case of the Fe₆₂Ge₃₈, the ferromagnetic Fe₅Ge₃ (β) phase with a Curie temperature of about 430 K was obtained. The average crystallite size was about 9 nm. The average hyperfine magnetic field of about 16 T allowed it to determine that more than four germanium atoms exist in the nearest environment of the ⁵⁷Fe isotopes in the Fe₅Ge₃ phase.     

On the general outline of physical properties of amorphous-nanocrystalline Fe-Cr-Mn-N alloy powders prepared by mechanical alloying under nitrogen

This paper reviews last findings about physical properties of Fe-Cr-Mn-N powders synthesized by mechanical alloying under nitrogen. Their thermal, magnetic, indentation, and grain growth behaviors and nitrogen distribution in their amorphous-nanocrystalline structure are regarded as a function of milling time. Particularly, the role of nitrogen in the aforementioned phenomena is reviewed in detail.     

Application of controlled and electrical discharge assisted mechanical alloying for the synthesis of nanocrystalline MgB2 superconducting compound

In this paper we present the results of our efforts to synthesize the nanocrystalline MgB₂ superconducting compound from elemental Mg and B powders by combination of controlled mechanical pre-alloying in a magneto-mill Uni-Ball-Mill 5 under shearing mode followed by electrical discharge (ED) assisted mechanical alloying (MA). There is no conclusive evidence of MgB₂ formation in the Mg-2B mixture using crystalline boron after controlled mechanical alloying (CMA) under protective argon or helium atmosphere as well as subsequent ED assisted alloying. There seems to be some XRD evidence of the strongest (1 0 1) MgB₂ peak presence in the Mg-2B mixture processed using both crystalline and amorphous boron after CMA under hydrogen as well as subsequent ED assisted alloying but this evidence is rather ambiguous. We postulate here that it is highly likely that a certain critical Mg nanograin size must be achieved before a successful reaction to form nanocrystalline MgB₂ is going to be completed. Following recent report by Gümbel et al. [Appl. Phys. Lett. 80 (2002) 2725] this critical value can be roughly estimated at 15 nm or less. Calculations of the Mg nanograin size in the present work show that only three Mg-2B powders ball milled under hydrogen meet this critical nanograin size criterion for the Mg phase. However, a massive formation of the β-MgH₂ hydride in these powders consumes the available Mg in the reaction with hydrogen which may leave inadequate concentration of Mg to form MgB₂ even though the nanograin size of Mg is sufficiently refined, say below 15 nm.     

Structure and magnetic properties of Fe-based powders prepared by mechanical alloying

FeSiAlCr alloy powders were prepared by mechanical alloying, the milling time were 20 h, 40 h, 60 h and 80~h, respectively. Powders morphology was studied by SEM. Microstructure of powders milled for various times were analyzed by XRD. The complex permittivity and complex permeability of four powders were tested in the frequency range from 0.5 to 18 GHz, and their microwave absorption properties were analyzed. It was found that the particle size of powders milled for 80~h was less than 2μm. Silicon and aluminum atoms were dissolved into the crystal lattice of iron, and chromium atoms can form alloy with iron atoms. The minimum peak value of reflectivity can reach to -11.3 dB at the frequency of 4.3 GHz for 80 h milling powders, and the other one was -6 dB at 16.5 GHz.     

Phase formation during mechanical alloying and subsequent low-temperature heat treatment of Al–27.4at%Fe–28.7at%C powders

Phase formation during high energy ball milling of a ternary elemental powder mixture with a composition of Al–27.4at%Fe–28.7at%C and during low temperature heat treatment of the milled powder was studied. It was found that an amorphous phase formed during prolonged milling. During heating the shorter time milled powder, Al and Fe reacted first, forming the AlFe phase and then at a higher temperature, AlFe reacts with Fe and C, forming the AlFe₃C_0.5 phase. During heating the longer time milled powder which contains a substantial amount of amorphous phase, the amorphous phase partially crystallizes first, forming the AlFe and AlFe₃C_0.5 phases, and then AlFe reacts with the remaining amorphous phase, forming the AlFe₃C_0.5 phase. Overall, mechanical alloying of Al, Fe and C elemental phases enables formation of an amorphous phase, while low temperature heat treatment of mechanically milled powder facilitates formation of AlFe and AlFe₃C_0.5 phases.     

Direct synthesis of MgCNi3 by mechanical alloying

MgCNi₃, an intermetallic compound with superconductivity, was synthesized from the Mg (or Mg₂Ni), Ni and graphite powders by mechanical alloying (MA). It is shown that the preliminary condition for the formation of MgCNi₃ is that Mg₂Ni must form in advance of MgCNi₃ in the MA process or be the starting component.     

Effect of sintering on microstructure and mechanical properties of nano-TiO2 dispersed Al65Cu20Ti15 amorphous/nanocrystalline matrix composite

Mechanically alloyed Al₆₅Cu₂₀Ti₁₅ amorphous alloy powder with or without 10 wt% nano-TiO₂ dispersion was consolidated by isothermal spark plasma sintering in the range 200–500 °C with pressure up to 50 MPa. Selected samples were separately cold compacted with 50 MPa pressure and sintered at 500 °C using controlled atmosphere resistance and microwave heating furnaces. Phase and microstructural evolution at appropriate stages of mechanical alloying/blending and sintering was monitored by X-ray diffraction and scanning and transmission electron microscopy. Measurement and comparison of relevant properties (density/porosity, microhardness and yield strength) of the sintered compacts suggest that spark plasma sintering is the most appropriate technique for developing nano-TiO₂ dispersed amorphous/nanocrystalline Al₆₅Cu₂₀Ti₁₅ matrix composite for structural application.     

Synthesis and functional properties of the Ni1−xMnxFe2O4 ferrites

Nanocrystalline Ni_1−xMn_xFe₂O₄ (x = 0; 0.17; 0.34; 0.5) ferrite powders were successfully synthesized using the sol-gel combustion method, by using nitrates as cations source and citric acid (C₆H₈O₇) as combustion/chelating agent. The reaction advancement was observed by means of IR absorption spectroscopy, by monitoring two characteristic bands for the spinel compounds at about 600 cm⁻¹ and 400 cm⁻¹, respectively. The as-synthesized powders were characterized by IR spectroscopy, X-ray diffraction (XRD) and scanning electronic microscopy (SEM). The magnetic study shows that the saturation magnetization decreases with increasing the Mn addition, as result of the particle size reduction. The dielectric properties were measured as a function of frequency in the range of 10 Hz to 1 MHz. The real part of permittivity has values of ∼88 at 1 kHz and ∼7 at 1 Hz for x = 0. An increasing dielectric permittivity with increasing the amount of Mn is observed. For all the investigated compositions, both the real and imaginary parts of permittivity decrease with frequency.     

Characterization and formation mechanism of nanocrystalline (Fe,Ti)3Al intermetallic compound prepared by mechanical alloying

The nanocrystalline (Fe,Ti)₃Al intermetallic compound was synthesized by mechanical alloying (MA) of elemental powder with composition Fe₅₀Al₂₅Ti₂₅. The structural changes of powder particles during mechanical alloying were studied by X-ray diffractometry and microhardness measurements. Morphology and cross-sectional microstructure of powder particles were characterized by scanning electron microscopy. It was found that a Fe/Al/Ti layered structure was formed at the early stages of milling followed by the formation of Fe(Ti,Al) solid solution. This structure transformed to (Fe,Ti)₃Al intermetallic compound at longer milling times. Upon heat treatment of (Fe,Ti)₃Al phase the degree of DO₃ ordering was increased. The (Fe,Ti)₃Al compound exhibited high microhardness value of about 1050 Hv.     

Structural investigation of the alluaudite-like mixed-valence iron phosphate: Na1.25Mg1.10Fe1.90(PO4)3

A new mixed-valence iron phosphate Na_1.25Mg_1.10Fe_1.90(PO₄)₃ has been synthesized as single crystals by a flux technique and its structure has been refined from X-ray data to a residual R₁ = 0.032. The compound crystallizes in the monoclinic space group C2/c with the parameters: a = 11.7831(3) Å, b = 12.4740(3) Å, c = 6.3761(2) Å, β = 113.643(2)° and Z = 4. The structure belongs to the alluaudite structural type, and thus it obeys to the X(2)X(1)M(1)M(2)₂(PO₄)₃ general formula. The X(2) and X(1) sites are occupied by sodium while the M(1) and M(2) sites feature a statistical distribution of iron and magnesium.

Additional information about the cation distribution has been extracted from a Mössbauer spectroscopy study which confirmed the mixed valency of the compound. A magnetic susceptibility study has also been undertaken and has shown the compound to be antiferromagnetic with a Neel temperature of about 35 K.     

Fabrication of Ti-Cu-Ni-Al amorphous alloys by mechanical alloying and mechanical milling

Ti-based amorphous alloy powders were synthesized by the mechanical alloying (MA) of pure elements and the mechanical milling (MM) of intermetallic compounds. The amorphous alloy powders were examined by X-ray diffraction (XRD), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM). Scanning electron micrographs revealed that the vein morphology of these alloy powders shows deformation during the milling. The energy-dispersive X-ray spectral maps confirm that each constituent is uniformly dispersed, including Fe and Cr. The XRD and DSC results showed that the milling time required for amorphization for the MA of pure elements was longer than that of the MM for intermetallic compounds. The activation energy and crystallization temperature of the MA powder are different from those of the MM powder.