Mössbauer study of mechanically alloyed powders

Using the⁵⁷Fe Mössbauer spectroscopy, the microscopic behavior of Fe powders has been investigated during and after the mechanical alloying (MA) process for Al?Fe and Ag?Fe systems. A repeated rolling method as well as a conventional ball-milling method are employed in order to understand the microscopic process of the kneading of Al and Fe powders during their MA and the resultant powders show quite similar Mössbauer spectra suggesting that the kneading by an impact between colliding balls is the same process as that of the thickness reduction by cold rolling. Mössbauer spectra show clearly the occurrence of the mutual atomic dispersion for the thermodynamically immiscible Ag?Fe system.     

Mössbauer study on mixing and kneading of metallic powders

Utilizing⁵⁷Fe and¹¹⁹Sn Mössbauer spectroscopy, the microscopic behavior of metallic powders has been investigated during and after the mechanical alloying (MA) process. A conventional ball-milling method is employed and the following combinations of metallic powders have been studied for their MA; Al?Fe, Fe?Sn and Ag?Fe. Mössbauer spectra show the occurrence of mutual atomic dispersion during the MA process even for the Ag?Fe system which is known as mutually immissible even in the liquid state. Formation of an amorphous phase in the Al?Fe system has been detected.     

Mössbauer study of incompletely alloyed nanocrystalline Fe100 − x Al x prepared by high energy ball milling

Mechanically alloyed Fe_100???x Al _x powders, with 20≤?x?≤90, have been studied by X-ray diffraction and room temperature ⁵⁷Fe Mössbauer spectroscopy. The milling time was chosen such that complete alloying does not take place. For a fixed milling time of 10 h, the rate of alloying was seen to increase exponentially with increase in Fe content. Mössbauer spectra of all the samples consist of a broad magnetic sextet and a quadrupole doublet. The isomer shifts and quadrupole splitting of the doublets are typical of Al-rich, Fe–Al alloys. The area under the quadrupole doublet is a maximum for x?=?66. Analysis of the Mössbauer spectra indicates the formation off- stoichiometric Fe₃Al phase for x?<?66, while the formation of Fe clusters is largely responsible for the magnetic hyperfine component in x?≥?66 compositions.     

Protection against corrosion of iron alloys by aluminized coatings produced using two different processes

Aluminized coatings were produced on iron by means of two different processes: electron beam deposition under UHV of Al on iron samples previously covered with ⁵⁷Fe films, and hot-dipping of iron samples in molten aluminium. Aluminized samples were submitted to thermal treatments in order to promote interdiffusion at the Fe–Al interface and favour the formation of Fe–Al intermetallic compounds of composition suitable to protect the underlying iron from oxidation. Phase composition, structure and morphology of both as deposited and thermally treated coatings were characterized by means of X-ray diffraction, Mössbauer spectroscopy and metallographic techniques. Significant differences among the effects of the Fe–Al interdiffusion occurring for Al layers produced with the two processes are pointed out and discussed.     

Application of 57Fe Mössbauer spectroscopy as a tool for mining exploration of bornite (Cu5FeS4) copper ore

Nuclear resonance methods, including Mössbauer spectroscopy,are considered as unique techniques suitable for remote on-line mineralogical analysis. The employment of these methods provides potentially significant commercial benefits for mining industry. As applied to copper sulfide ores, Mössbauer spectroscopy method is suitable for the analysis noted. Bornite (formally Cu₅FeS₄) is a significant part of copper ore and identification of its properties is important for economic exploitation of commercial copper ore deposits. A series of natural bornite samples was studied by ⁵⁷Fe Mössbauer spectroscopy. Two aspects were considered: reexamination of ⁵⁷Fe Mössbauer properties of natural bornite samples and their stability irrespective of origin and potential use of miniaturized Mössbauer spectrometers MIMOS II for in-situ bornite identification. The results obtained show a number of potential benefits of introducing the available portative Mössbauer equipment into the mining industry for express mineralogical analysis. In addition, results of some preliminary ^63,65Cu nuclear quadrupole resonance (NQR) studies of bornite are reported and their merits with Mössbauer techniques for bornite detection discussed.     

Mössbauer comparative study of Fe–Si (3.5 wt%) alloys produced by melting and by mechanical alloying

Mössbauer spectroscopy and X-ray diffraction measurements were done on Fe–Si (3.5 wt%) alloys produced by melting and by mechanical alloying during 15, 30, 50 and 75 milling hours from over 99% purity powders. The Mössbauer spectra were fitted using hyperfine field distribution and it was obtained for all the samples in three ferromagnetic sites with fields of 27, 30 and 33 T for the mechanical alloyed samples and 26.8, 30.13 and 32.83 T for the commercial sample. These three sites are attributed to the pure Fe, Fe with one Si in the next near neighbor (nnn) and Fe with two Si in the nnn. As the milling time increases, the mean field increases too. X-ray diffraction measurement shows that all the samples are BCC, with a lattice parameter that increases with the milling time. These lattice parameters are bigger than that of the commercial alloy.     

Hendrik de waard: A personal view

Hyperfine Interactions - A series of powders of Fe0.2Mn0.4Al0.4 system were produced by mechanical alloying for 2, 4, 6, 8, 12, 24 and 48 hours milling and then characterized by 57Fe Mössbauer...     

Mechanochemical Reactions in Fe2O3–M (M: Al, Ti)

The production of metal–ceramic nanodispersion by mechanical milling of powders through the displacement reaction Fe₂O₃+M→Fe+M-oxide (with M: Al, Ti) was studied. The reaction progress with milling time was followed by recording the temperature and pressure during the process. The samples were characterized by X-ray diffraction and Mössbauer spectroscopy at the intermediate and final stages. In both cases self-sustained reactions were observed with different activation times. The results confirm that mechanical work at room temperature yields the reduction of hematite by Ti and Al. The final oxides were identified as Ti₂O₃ and Al₂O₃, respectively. The dependence of the intermediate and final stages on the milling conditions and the starting composition will be discussed.     

Mechanochemical Transformations in the Zinc-Magnetite System

Mixtures of magnetite and zinc powders were milled for up to 540 minutes and the development of the system was followed using X-ray diffraction, Mössbauer spectroscopy, and magnetic measurements. The process takes place in two overlapping steps. During the first hour of milling, a nonmagnetic intermediate mixed oxide phase forms which decomposes into ZnO and Fe upon further milling. The freshly formed iron particles are supersaturated with Zn.     

Mechanical alloying of Fe and Cr: Structural and magnetic studies

Mechanical milling of a mixture of pure iron and chromium powders with a nominal composition of 28 at. % of Fe and 72 at. % of Cr was performed using two kinds of tools. The alloying process was followed by X ray diffraction and⁵⁷Fe Mössbauer spectroscopy. The room temperature Mössbauer spectra revealed the formation of a broad single line at the expense of the area of α-Fe sextet. At 4.2 °K most of the singlet still remains unsplit. This behavior may be due to some metastable structure induced by the preparation method.     

Observation of iron impurity diffusion in silicon under bending stress by Mössbauer spectroscopy

In the present work, the formation of the Al₇₀Cu₂₀Fe₁₀ icosahedral phase by mechanical alloying the elemental powders in a high-energy planetary mill was investigated by X-ray diffraction and Mössbauer spectroscopy. It was verified that the sample milled for 80 h produces an icosahedral phase besides Al(Cu, Fe) solid solution (β-phase) and Al₂Cu intermetallic phase. The Mössbauer spectrum for this sample was fitted with a distribution of quadrupole splitting, a doublet and a sextet, revealing the presence of the icosahedral phase, β-phase and α-Fe, respectively. This compound is not a good hydrogen storage. The results of the X-ray diffraction and Mössbauer spectroscopy of the sample milled for 40 h and annealed at 623°C for 16 h shows essentially single i-phase and tetragonal Al₇Cu₂ Fe phase.     

Mössbauer study of Mg–Ni(Fe) alloys processed as materials for solid state hydrogen storage

Mg–Ni–Fe magnesium-rich intermetallic compounds were prepared following two distinct routes. A Mg₈₈Ni₁₁Fe₁ sample (A) was prepared by melt spinning Mg–Ni–Fe pellets and then by high-energy ball milling for 6 h the obtained ribbons. A (MgH₂)₈₈Ni₁₁Fe₁ sample (B) was obtained by high-energy ball milling for 20 h a mixture of Ni, Fe and MgH₂ powders in the due proportions. A SPEX8000 shaker mill with a 10:1 ball to powder ratio was used for milling in argon atmosphere. The samples were submitted to repeated hydrogen absorption/desorption cycles in a Sievert type gas–solid reaction controller at temperatures in the range 520?÷?590 K and a maximum pressure of 2.5 MPa during absorption. The samples were analysed before and after the hydrogen absorption/desorption cycles by X-ray diffraction and Mössbauer spectroscopy. The results concerning the hydrogen storage properties of the studied compounds are discussed in connection with the micro-structural characteristics found by means of the used analytical techniques. The improved kinetics of hydrogen desorption for sample A, in comparison to sample B, has been ascribed to the different behaviour of iron atoms in the two cases, as proved by Mössbauer spectroscopy. In fact, iron results homogeneously distributed in sample A, partly at the Mg₂Ni grain boundaries, with catalytic effect on the gas–solid reaction; in sample B, instead, iron is dispersed inside the hydride powder as metallic iron or superparamagnetic iron.     

Mössbauer spectroscopic studies of Fe-20 wt.% Cr ball milled alloy

Interesting differences were noticed in the alloying process during ball milling of Fe-10 wt.% Cr and Fe-20 wt.% Cr alloys by ⁵⁷Fe Mössbauer spectroscopic studies. In both cases, there is almost no diffusion of Fe in Cr or vice versa up to 20 h of milling time. As the powders are milled for another 20 h substantive changes occur in the Mössbauer spectra showing atomic level mixing. But the two compositions behave differently with respect to alloying. Fe-20 wt.% Cr sample does not differ much in the hyperfine field distribution as it is milled from 40 to 100 h. On the other hand, the hyperfine field distribution keeps on changing with milling time for Fe-10 wt.% Cr sample even up to 100 h of milling. The average crystallite size is found to be 7.5 nm for Fe-10 wt.% Cr and 6.5 nm in Fe-20 wt.% Cr after milling.     

Investigation of magnetic properties in 57Fe/Al multilayers

Fe/Al multilayer thin films prepared by ion beam sputtering, with an overall atomic concentration ratio of Fe/Al = 1:2 have been studied by x-ray diffraction spectroscopy (XRD), X-ray reflectivity (XRR) and D.C. Magnetization. These studies show the formation of Fe–Al intermetallic layers. Two magnetic regions and transition temperatures of 473 and 533 K are evident from magnetization studies. Conversion Electron Mössbauer Spectroscopy (CEMS) shows formation of off-stoichiometric Fe₃Al like phase and phases consisting of pure Fe and Fe-rich extended Fe–Al solutions.     

Interfacial phenomena and magnetic properties of Fe/Al multilayers of different thickness

Fe/Al thin film multilayers, differing in the thickness of Fe films (30÷10 Å, were electron-beam evaporated in ultra-high vacuum. Interdiffusion and reaction phenomena occurring during deposition at interfaces were studied by means of conversion electron Mössbauer spectroscopy and Auger electron spectroscopy depth-profiling. Magnetic behavior was investigated by alternating force gradient magnetometry. The formation of Fe–Al solid solution and intermetallic compound is observed. Multilayers are ferromagnetic with magnetization in the film plane for iron film thickness ?15 Å, while exhibiting a superparamagnetic behavior at 10 Å.     

Mössbauer studies of the re-entrant spin-glass behaviour of Fe–Al alloys

Fe–Al alloys around the concentration of 30 at. % Al present re-entrant spin-glass behaviour at low temperatures. This behaviour is not completely understood and Mössbauer spectroscopy, combined with other experimental techniques, is useful to describe and explain this behaviour. Results show that the Mössbauer spectra coincide with the magnetic behaviour showed in literature and they can be explained as a magnetic cluster system whose magnetic clusters are getting smaller when the temperature is decreasing. When the temperature is reaching to the spin-glass transition at 92 K the spins in the paramagnetic matrix are moving slower and below this transition the spins are completely frozen.     

Mössbauer spectroscopy of disordered magnetic systems

In the given summarizing article, we present the most important aspects of the technique of studying disordered magnetic systems by the Mössbauer effect being developed by us for the broad circle of researchers in magnetism and Mössbauer spectroscopy. The potentialities of this technique in investigating local magnetic characteristics and spin structures of disordered magnetics are assessed by the interpretation of Mössbauer spectroscopy data of various classes of well-studied magnetics (Fe?Al, Fe-V). In the concluding part, we present original results of Mössbauer investigations of binary (Fe?Cr, Fe?Pd and Fe?Au) and quasi-binary (Fe?Pd?Au) alloys characterized by non-trivial macroscopic magnetic properties and complex magnetic phase diagrams.     

Hyperfine Study on Mechanically Alloyed Fe–Mn Systems

Hyperfine Interactions - High-energy ball milling of Fe–Mn elemental powder mixtures has been carried out for different Mn concentrations, ranging from 0 to 90%. Mössbauer spectroscopy...     

Mössbauer study of mechanical alloying in a Mo<Subscript>80</Subscript>Fe<Subscript>20</Subscript> system

The sequence of solid state reactions upon the mechanical alloying of Mo and Fe powders with an 80: 20 atomic ratio was established by means of Mössbauer spectroscopy and X-ray diffraction. At the first stage, a nanostructure and Mo₆₃Fe₃₇ hexagonal close packed (HCP) phase are formed in Mo body-centered cubic lattice (BCC) particles. At the second stage, a body-centered cubic lattice of Mo-Fe solid solution is formed. The process is accompanied by the formation of a minor amount (about 20%) of X-ray amorphous phase.     

Ion beam mixing of bimetallic systems: CEMS studies

The effect of irradiation with argon, krypton and xenon energetic ions of bimetallic systems containing a Mössbauer active element is reviewed to show the contribution of conversion electron Mössbauer spectroscopy (CEMS) to the studies of the ion-beam mixing mechanisms and to the structural characterization of mixed interfaces. In the field of ion beam mixing, CEMS is often used to complement the more used RBS, TEM, XRD, XPS, SIMS, AES and SEM analytical techniques. Recent results concerning the Fe/Pd, Fe/Ni and Fe/Al couples mixed with argon and krypton ions are presented and discussed.