S.D. Kaloshkin

Formation of iron-nickel nanocrystalline alloy by mechanical alloying*

Abstract Fe1-xNix alloys were prepared by mechanical alloying of elemental powders in high-energy planetary ball mill in a wide concentration range of components (10 ≤x ≤0). The structure was studied by X-ray diffractometry. It is shown that concentration ranges of single-phase solid solution of MA samples are markedly wider than that of thermodynamically stable alloys. Character of X-ray peaks indicates that there is a high density of crystalline lattice defects including stacking faults. Block sizes, calculated without considering stacking faults presence, was found to be 8 – 15 nm, and calculated with considering of stacking faults 30 – 80 nm. The results were discussed on the basis of thermodynamic model of MA.

Crystallochemical aspects of solid state reactions in mechanically alloyed Al–Cu–Fe quasicrystalline powders

Abstract A number of elemental mixtures having initial compositions close to Al65Cu23Fe12 were mechanically alloyed at different energy intensities using a planetary mill. Laboratory X-ray diffraction analysis, differential scanning calorimetry and Mossbauer spectroscopy were used for characterization of the phase and structural state of mechanically alloyed powders after different periods of milling and annealing. Several exothermic effects were found, and these were ascribed to specific solid-state reactions. Quantitative phase analysis was applied in order to identify the mechanism of solid-state reactions taking place in the vicinity of the quasicrystalline phase domain in the Al–Cu–Fe system as a result of mechanical and thermal excitation and homogenization. Metastable intermetallics were identified which possess certain structural and topological elements identical to those found in quasicrystals.

Phase transformations in Fe–Ni system at mechanical alloying and consequent annealing of elemental powder mixtures

Fe 100-x Ni x alloys (10<x<90at%) were prepared by mechanical alloying (MA) of elemental powders in a high-energy planetary ball mill and studied by X-ray diffractometry and Mossbauer spectroscopy. It is shown that the concentration ranges of single-phase solid solutions of MA samples extend significantly as compared with those obtained by conventional techniques. In our case, the BCC phase exists in the range from 0 to 20 at% Ni and FCC phase from 30 to 100at% Ni. Block size was 10-15 nm. Consequent annealing of MA samples resulted in further extension of FCC single-phase concentration range to the relatively low Ni content (20at%). This was caused by considerable retardation of austenite-martensite transformation in MA alloys. The FCC alloys with 20-28 at% Ni were found to be non-ferromagnetic at room temperature; only the paramagnetic component was observed in the corresponding Mossbauer spectra. However, the treatments of low-nickel austenite alloys like cooling in liquid nitrogen or mechanical deformation provoked austenite-martensite transformation and led to the rise of ferromagnetic properties.

Mechanically alloyed low-nickel austenite Fe-Ni phase: evidence of single-phase paramagnetic state

Abstract Fe 100− x Ni x alloys were obtained by a mechanical alloying technique (MA) from elemental metals. The alloys consist of: single body-centered cubic phase (bcc) at nickel concentrations ⩽22 at.%, single face-centered cubic phase (fcc) – at x >28 at.% and two of these phases – at 22⩽ x ⩽28 at.%. Annealing results in formation of single fcc phase structure in the samples with x ⩾22 at.%. According to the Mossbauer spectrometry data these annealed alloys with 22–28 at.% Ni were not ferromagnetic at room temperature. Cooling austenitic samples in liquid nitrogen as well as mechanical deformation stimulated austenite–martensite transformation accompanied by the appearance of ferromagnetism.

Magnetostriction of Co–Fe-Based Amorphous Soft Magnetic Microwires

We studied the correlation between magnetic softness and magnetostriction coefficient for as-prepared and annealed Co–Fe-rich microwires. We found that the hysteresis loops and magnetostriction coefficients of Co and Fe-rich microwires depend not only on the chemical composition of the metal but also on internal stress. Consequently, both hysteresis loop and magnetostriction coefficient can be adjusted by annealing. We varied the time and temperature of annealing and observed changes of the character of the hysteresis loops. These changes correlated with evolution of the magnetostriction coefficient. Drastic changes of the hysteresis loop for Co-rich microwires were attributed to changes of the sign and value of the magnetostriction coefficient.

Quasicrystalline phase formation by heating a mechanically alloyed Al65Cu23Fe12 powder mixture

Abstract Elemental powder mixtures of a Al65Cu23Fe12composition milled for two and four hours in a planetary ball mill were used to form quasicrystals. Annealing of the as-milled samples led to complex solid-state transformations. During the heat-up a sequence of solid-state reactions takes place in the as-milled powder. These reactions were studied both by differential scanning calorimetry and X-ray diffraction methods. An analysis of the phase formation shows the effect of the difference in the thermodynamic driving forces, such as the positive heats of mixing for the Cu–Fe system and the negative ones for the Al–Fe and Al–Cu systems, on the phase transformation consequence.

Mechanochemical synthesis and hydrogen sorption properties of nanocrystalline TiFe

The equiatomic intermetallic compound TiFe has been prepared by elemental mechanochemical synthesis in a planetary ball mill from Fe and Ti powders. The structural and phase transformations during synthesis were followed using X-ray diffraction. The reaction of the synthesized compound with hydrogen was studied volumetrically. The results demonstrate that the hydrogen capacity of the mechanochemical TiFe is 1.2 wt % at 2.5 MPa. Its absorption isotherm has an extended plateau in the range 1.6–1.7 MPa at room temperature.

Fe–Mn mechanically alloyed powders characterised by local probes

Abstract High-energy ball milling of Fe–Mn elemental powder mixtures has been carried out for Mn atomic concentrations in the range 10–90%. X-ray diffraction (XRD), Mossbauer spectroscopy (MS) and perturbed angular correlations (PAC) have been used to investigate the crystalline structure in the milled samples. It is found that ball milling gives rise to concentration ranges of existence of terminal solid solutions more extended than in the equilibrium phase diagram. Particular attention has been paid to the environment of 57Fe (MS) and 111In (PAC) probe atoms: 57Fe atoms are a constituent of the system, while 111In atoms have been implanted at 400 keV into pills made from the milled powders. The PAC spectra of milled Fe show the ferromagnetic α-phase with a high fraction of a single vacancy next to the probe. Accordingly, the Mossbauer spectrum is a sextet with the characteristic splitting of α-Fe and a broadening which indicates a certain degree of atomic structure disorder. With an amount of Mn up to 15% PAC probes with one and two Mn next neighbours are identified by their smaller magnetic hyperfine field, corresponding to two distinct magnetic components in the Mossbauer spectra. At 20 and 30% of Mn the PAC magnetic signal of the a-phase disappears in favour of a distorted cubic apparently non-magnetic signal. From 40 to 70% of Mn a broad distribution of hyperfine magnetic fields is observed by PAC. Mossbauer spectra in the 20–70% of Mn range can be fitted with an unresolved low field magnetic sextet. Finally, the PAC spectra at 80 and 90% are quite similar to the one which is obtained after milling pure Mn, while the corresponding Mossbauer data can be roughly approximated to those of the α-Mn(Fe) solid solution.

Influence of the defects on magnetic properties of glass-coated microwires

We observed different kind of defects, such as gas bubbles, glass coating thickness inhomogeneities, indications of the chemical interaction between the glass shell and the metallic nucleus. We showed that the existence of such bubbles might be the origin of the spontaneous fluctuations of the local nucleation field.

The Evolution of Crystalline Precursors During the Formation of Al-Cu-Fe Quasicrystalline Intermetallics in Mechanically Alloyed Powders

Mechanical alloying (MA) of elemental powders in the Al-Cu-Fe system was carried out using two types of laboratory equipment (vibratory and planetary mills) to produce single phase, stable icosahedral quasicrystals. The sequence and nature of the solid state reactions during milling and subsequent annealing were studied in detail using qualitative and quantitative laboratory X-ray analysis and differential scanning calorimetry. The effect of milling parameters (energy intensity, temperature, milling time) on the course of transformations was elucidated. Specific reactions were identified which took place during annealing in different temperature ranges. The evolution of crystalline precursors of the quasicrystalline phase was established.