Spin- and charge density oscillations around Ir impurity in α-Fe studied by Fe Mössbauer spectroscopy

Random solution of iridium in ferromagnetic α-Fe of the BCC structure has been investigated by means of the 14.4 keV Mössbauer transition in ⁵⁷Fe at room temperature. Iridium has been randomly substituted on the iron sites with the concentration up to 11 at.%. Contributions to the hyperfine field and isomer shift on the iron nuclei have been determined as the function of the distance between iron nucleus and iridium impurity up to the third co-ordination shell. Iridium atom as the nearest iron neighbor changes iron hyperfine field by +0.66 T, as the second neighbor makes change by −1.88 T and finally as the third neighbor changes the field by +0.70 T. Corresponding changes in the isomer shift are as follows: −0.031, +0.003 and +0.033 mm/s. Hence, it was found that iridium atoms lead to the very strong oscillations in the spin- and charge density around themselves in the α-Fe BCC structure.     

Structural and Mössbauer spectroscopic investigation of Fe substituted Ti–Ni shape memory alloys

In the present investigation the effect of Fe substitution in Ti₅₁Ni₄₉ alloy has been studied. The alloys were synthesized through radio frequency induction melting. The alloy was characterized through X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Mössbauer spectroscopy and positron annihilation techniques. It was found that the Fe substitution stabilized the TiNi type cubic (a = 2.998 Å) phase. The microstructure and presence of the oxide phase in Ti₅₁Ni₄₅Fe₄ alloy have been investigated by scanning electron microscopy. The positron annihilation measurements indicated a similar bulk electron density in both the as-cast and annealed (1000 °C for 30 h) alloys, typically like that of bulk Ti. Mössbauer spectroscopy studies of as-cast and annealed iron substituted samples showed regions in the samples where nuclear Zeeman splitting of Fe levels occurred and an oxide phase was found to be present in as cast Ti₅₁Ni₄₅Fe₄ alloy, while annealed sample indicated the presence of bcc iron phase.     

Hyperfine interactions on iron nuclei in the BCC and fractally decomposed BCC/FCC mixed phase iron–gold alloys

Iron–gold alloys for the gold concentration ranging from 1 at.% till 70 at.% were investigated by means of the ⁵⁷Fe-14.4 keV Mössbauer spectroscopy, X-ray diffraction and scanning electron microscopy. Samples were prepared by arc melting of the elements, and investigated as cast and after annealing. A single BCC phase is obtained for the gold concentration up to about 3 at.%, while for the higher gold concentration one obtains mixed phase samples containing BCC and FCC phases both. The BCC phase is ferromagnetically ordered at room temperature. Contributions to the charge and spin density on iron atoms in the BCC phase due to the gold impurities were determined up to the second neighbors. The FCC phase is either magnetically ordered at room temperature or it is paramagnetic at the above temperature depending upon iron concentration, as the magnetic transition temperature is increasing with the increasing iron concentration. BCC/FCC mixed phase samples are characterized by very small crystallites (nanoparticles) of both phases. These nanoparticles form hierarchical fractal structures on the scale ranging from more than 1 mm till less than 30 nm.     

Characterization of Delhi iron pillar rust by X-ray diffraction, Fourier transform infrared spectroscopy and Mössbauer spectroscopy

Rust samples obtained from the region just below the decorative bell capital of the Delhi iron pillar (DIP) have been analyzed by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and Mössbauer spectroscopy. The identification of iron hydrogen phosphate hydrate in the crystalline form by XRD was unambiguous. Very weak diffraction from the oxyhydroxides/oxides of iron was observed indicating that these phases are most likely to be present in the amorphous form in the rust. The present XRD analysis of rust obtained from an inaccessible area of the DIP has also been compared with earlier analyses of DIP rust obtained from regions accessible to the public. FTIR indicated that the constituents of the scale were γ-, α-, δ-FeOOH, Fe_3−xO₄ and phosphate, and that the scale was hydrated. The unambiguous identification of the iron oxides/oxyhydroxides in the FTIR spectrum implied that they are present in the amorphous state, as XRD did not reveal these phases. The FTIR results have also been compared with earlier FTIR spectroscopic results of atmospheric rust formation. Mössbauer spectroscopy indicated that the rusts contained γ-FeOOH, superparamagnetic α-FeOOH, δ-FeOOH and magnetite, all in the amorphous form. The Mössbauer spectrum also confirmed that iron in the crystalline iron hydrogen phosphate hydrate, whose presence was confirmed by XRD, was in the ferric state indicating that it was a stable end corrosion product.     

A Mössbauer effect investigation of the formation of MnZn nanoferrite phase

In this paper, Mn_0.5Zn_0.5Fe₂O₄ nanopowders were prepared by mechanochemical processing of the mixture of two single phase ferrites, MnFe₂O₄ and ZnFe₂O₄. Room-temperature ⁵⁷Fe Mössbauer spectroscopy was used to study the mechanically induced evolution of the ZnFe₂O₄/MnFe₂O₄ mixture submitted to the high-energy milling process. The Mössbauer spectrum of the ZnFe₂O₄/MnFe₂O₄ sample milled for 30 h revealed the presence of Mn_0.5Zn_0.5Fe₂O₄. The mean crystallite size of the mechanosynthesized mixed ferrite, estimated using Scherrer's formula, was found to be 14 nm.     

Mössbauer studies of phase separation in nanocrystalline Fe0.55−xCr0.45Snx alloys prepared by mechanical alloying

Mössbauer spectrometry (⁵⁷Fe and ¹¹⁹Sn) was used to investigate phase separation in coarse-grained Fe_0.55Cr_0.45 and in mechanically-alloyed nanocrystalline Fe_0.55Cr_0.45, Fe_0.52Cr_0.45Sn_0.03 and Fe_0.49Cr_0.45Sn_0.06 alloys during isothermal annealing at 748 K. Phase separation occurs faster in nanocrystalline Fe–Cr than in cold-rolled coarse-grained alloys. The effect of the interconnected microstructure on room-temperature hyperfine magnetic field distributions of alloys aged for hundreds of hours is qualitatively discussed. Tin hinders grain growth of nanocrystalline alloys.     

Development and characterization of composite Ni–Cr–C–CaF2 laser cladding on γ-TiAl intermetallic alloy

Mössbauer spectroscopy has been used to study the crystallization behaviour in paramagnetic bulk amorphous steels Fe₅₀Cr₁₄Mo₁₄C₁₄B₆X₂ (X = Y and Dy), prepared by arc-melting and Cu mould casting techniques. Room temperature Mössbauer spectra of as-cast samples exhibit a broadened and asymmetric doublet-like structure that indicates the paramagnetic and amorphous nature of the as-cast alloys. During heat-treatment alloys crystallized into γ-Fe and η-Fe₃Mo₃C iron phases and crystallization completed at 1123 K. Alloy containing Dy shows better performance against crystallization at low temperature as compared to the Y-containing alloy.     

Hyperfine interactions studied by Sn Mössbauer spectroscopy in TbAuSn and TmAuSn compounds

X-ray diffraction, magnetic susceptibility measurements and ¹¹⁹Sn Mössbauer spectroscopy were used to study structure and hyperfine interactions in intermetallic TbAuSn and TmAuSn compounds. It was shown that the first compound orders antiferromagnetically at low temperatures with Tb magnetic moments inclined at an angle of about 25° to the crystallographic c-axis. The second compound stays non-magnetic in the whole measured region. The occurrence of a magnetic hyperfine field distribution in TbAuSn suggests that this compound crystallizes in the disordered CaIn₂-type of structure.     

Positron annihilation and Mössbauer spectroscopy applied to WWER-1000 RPV steels in the frame of IAEA High Ni Co-ordinated Research Programme

Positron annihilation spectroscopy (PAS) and Mössbauer spectroscopy (MS) were applied in the evaluation of the microstructure parameters and degradation processes of nuclear reactor pressure vessel (RPV) steel surveillance specimens. Study was oriented to the material investigation of Russian WWER-1000 steels (15Kh2MNFAA and 12Kh2N2MAA) with high Ni content (1.26 wt% in base metal and 1.7 wt% in weld). For comparison, the WWER-440 weld metal (Sv10KhMFT) without Ni was also measured. Specimens were studied in as-received form, after irradiation in LVR-15 experimental reactor to the neutron fluence F_(E>0.5MeV)=1.47×10²⁴ m⁻² s⁻¹ and after annealing process in vacuum at 475 °C per 2 h. Changes due to different chemical composition and irradiation were registered using MS. Post-irradiation thermal treatment and annealing of defects was well detected by different PAS techniques. Results confirm the hypothesis that Ni affects size (decrease) and distribution (more homogeneous) of the Cu- and P-rich clusters and M_xC_x carbides.     

Sn Mössbauer spectroscopy study of the magnetic properties of the HfFe6Ge6-type compounds: SmMn6Sn4Ge2 and GdMn6Sn4Ge2

The HfFe₆Ge₆-type compound SmMn₆Sn₄Ge₂ has been studied by single-crystal magnetisation and ¹¹⁹Sn Mössbauer spectroscopy. The compound orders ferromagnetically at T_c = 420 K and displays an easy-axis anisotropy from T_c to T_SR = 130 K. Below T_SR, both magnetisation and ¹¹⁹Sn Mössbauer spectroscopy measurements indicate a deviation from the [0 0 1] direction and the presence of easy-cone anisotropy. The angle of the moments with respect to the [0 0 1] direction is estimated to 26-31° from Mössbauer spectroscopy results, in good accordance with the magnetisation results. The isotypic compounds GdMn₆Sn₄Ge₂ and GdMn₆Sn₆ studied by ¹¹⁹Sn Mössbauer spectroscopy display easy plane anisotropy in the whole temperature range 300-4.2 K. The anisotropy behaviours of the LMn₆Sn₄Ge₂ compounds are discussed and the coexistence of easy cone anisotropy for both the SmMn₆Sn₄Ge₂ and HoMn₆Sn₄Ge₂ suggests the play of a positive second-order anisotropy constant of the Mn sublattice.     

Influence of the borohydride concentration on the composition of the amorphous Fe–B alloy produced by chemical reduction of synthetic, nano-sized iron-oxide particles: Part I: Hematite

Amorphous Fe–B alloys can be prepared at room temperature by reduction with borohydride of iron-oxide particles in suspension. By varying the borohydride concentration, amorphous Fe–B alloys with boron contents between 2 and 13 at.% have been produced by reduction of synthetic (nano-sized particles) and natural (micro-sized) hematite (α-Fe₂O₃) using sodium borohydride (NaBH₄). The results presented in this paper were obtained from a systematic study of the effect of borohydride concentration on the resulting reaction products using a variety of experimental techniques, such as X-ray diffraction, wet chemical analyses, thermal analyses, scanning electron microscopy, transmission Mössbauer spectroscopy (TMS) and integral low-energy electron Mössbauer spectroscopy (ILEEMS). Three distinct NaBH₄ concentrations have been applied. Beside unreacted hematite, amorphous Fe_1−xB_x alloys have been identified from the TMS spectra recorded at various temperatures between 15 K and room temperature. The amount of Fe_1−xB_x increases strongly with increasing NaBH₄ concentration, and for a given concentration with increasing specific surface area (SSA). Thermal analyses have suggested that for any given reduction condition, the boron content x in the formed amorphous alloy has a bimodal distribution. This is found to be consistent with the finding that the contribution of the Fe_1−xB_x phase to the total Mössbauer spectra consists of a superposition of a broad sextet and doublet. ILEEMS has further revealed that especially the surface layers of the hematite grains are affected by the reduction processes.     

Determination of the thickness of hematite and magnetite layers on oxidized iron by electron reemission Mössbauer spectroscopy

A reanalysis of earlier electron reemission Mössbauer (ERM) data obtained from iron oxidized in pure oxygen at 150 to 500°C is presented. The new data analysis utilized the same theoretical method used previously, but with significantly altered values for the electron attenuation coefficients determined from recent experimental work by Graham, Mitchell, and Channing. The recalculated oxide-thickness values were found to be in much better agreement with the volumetrically measured oxygen uptake.     

Structure and phase transformations in Fe–Ni–Mn alloys nanostructured by mechanical alloying

Ternary Fe₈₆Ni_xMn_14−x alloys, where x = 0, 2, 4, 6, 8, 10, 12, 14, 16 at.%, were prepared by the mechanical alloying (MA) of elemental powders in a high-energy planetary ball mill. X-ray diffraction analysis and Mössbauer spectroscopy were used to investigate the structure and phase composition of samples. Thermo-magnetic measurements were used to study the phase transformation temperatures. The MA results in the formation of bcc α-Fe and fcc γ-Fe based solid solutions, the hcp phase was not observed after MA. As-milled alloys were annealed with further cooling to ambient or liquid nitrogen temperatures. A significant decrease in martensitic points for the MA alloys was observed that was attributed to the nanocrystalline structure formation.     

A Mössbauer effect investigation of site preferences and Fe environments in Ga-substituted Sm2Fe17 compounds

Room temperature ⁵⁷Fe Mössbauer studies of the 2:17 compounds Sm₂Fe_17−xGa_x with x=0, 1, 2, 3, 4 and 5 are reported. These measurements yield values of the Fe hyperfine field and relative concentration of Fe atoms at the four inequivalent transition metal sites, 6c, 9d, 18f and 18h, in the rhombohedral Th₂Zn₁₇ structure. Changes in the Fe hyperfine field as a function of Ga content can be correlated to changes in lattice parameter and Curie temperature and are the result of changes in the transition metal-transition metal exchange coupling. The relative intensity of the various spectra components shows that for small concentrations, Ga preferentially enters into the 18h sites. For x & 1 a substantial quantity of Ga also enters into the 18f sites. It is the Ga which enters into the 18f sites which is responsible for the formation of a uniaxial anisotropy in these materials for higher Ga content.     

Influence of the borohydride concentration on the composition of the amorphous Fe–B alloy produced by chemical reduction of synthetic, nano-sized iron oxide particles: Part II. Goethite

A systematic study of the effect of the NaBH₄ concentration upon the composites produced by chemical reduction of nano-sized goethite (-FeOOH) particles suspended in water is presented. Amorphous Fe–B alloys with boron contents between 2 and 8 at.% have been produced. The obtained samples were characterized by X-ray diffraction, wet chemical analyses, thermal analyses, scanning electron microscopy, transmission Mössbauer spectroscopy (TMS) and integral low-energy electron Mössbauer spectroscopy (ILEEMS). It was observed that the boron content of this amorphous composite sensitively depends on the borohydride concentration. The TMS analyses have shown that the contribution of the Fe_1−xB_x phase, which is present as a superposition of a broad sextet and a doublet, accounts for about 38% of the total spectral area. ILEEMS has further revealed that the reduction process mainly affects the surface layers of the goethite grains, without causing any changes in both particle shape and sizes.     

Influence of composition and annealing conditions on the site-occupation in the σ-phase of Fe–Cr and Fe–V systems

Neutron diffraction technique was used to study the site-occupation in the σ-phase in the Fe–Cr and Fe–V systems. It was found that all five sites A, B, C, D and E are “mixed”, i.e. occupied by both elements. The occupation is neither random – the degree of randomness increases with Fe content – nor regular, i.e. sites A and D are predominantly occupied by Fe atoms while B, C and E sites are occupied preferentially by Cr or V atoms. For all five sites the increase of Cr(V) concentration results in an irregular decrease of the number of Fe atoms on each site, the rate of decrease being the smallest for the sites A and D. In the Fe–Cr system the population of A with Fe atoms, N_A, is similar to that of D, N_D, while for the system Fe–V N_D > N_A and the difference increases with V content. The lowest Fe population in both investigated systems has B, but N_B(V) < N_B(Cr). N_C = N_E for Fe–V, while N_C > N_E for Fe–Cr. The influence of sample preparation conditions (plastic deformation prior to phase transformation, annealing time, t_a, and temperature of annealing, T_a) were also tested on a σ-Fe_53.8Cr_46.2 sample. In general the influence of these conditions on the site population is small. The largest effect had the plastic deformation, and the smallest one the annealing time in case of non-deformed samples. The most insensitive sites were revealed to be D and E, and the most sensitive ones were B and A.     

Corrosion rate of iron and iron–chromium alloys in CO2 medium

In the present paper corrosion of iron (99.95%) and iron–chromium alloys (1–5% Cr) were investigated by mass loss and ac impedance. These low-chromium content alloys have been proposed as an intermediate corrosion-performance material between C-steel and stainless steels to be used in oil plants. It was observed that small contents of chromium in the alloy decrease the corrosion attack only in very specific pH range 4 < pH < 5.In CO₂ medium, mass loss showed a linear behaviour with respect to time whereas in the presence of acetate the results were not monotonic. By ac impedance, it was found that and vary proportionally with respect to mass loss. A discussion concerning the online monitoring of the corrosion rate is presented.     

Ball-milling in vacuum of α and σ phases of near-equiatomic FeCr alloys

The structural changes induced by ball-milling of near-equiatomic σ-FeCr and α-FeCr in vacuum were followed. Besides the α-phase, an amorphous phase appears when milling the σ-phase for times longer than 20 h. An amorphous phase forms too, but at a slower rate than the latter, when milling the α-phase. The partial amorphisation of σ-FeCr and α-FeCr ball-milled in vacuum is concluded to be a phenomenon of intrinsic origin. The amorphous phase crystallizes into a bcc Cr-rich phase and a bcc Fe-rich phase during short annealing steps.     

Development of cube texture in pure Ni, Ni–W and Ni–Mo alloys prepared by the powder metallurgy route

Development of textures after heavy cold rolling (95%) and annealing were studied in powder metallurgically prepared pure Ni, Ni–5at.%W and Ni–5at.%Mo alloys. It has been found that W and Mo additions to Ni are beneficial for the development of sharp cube texture, although W has a much more pronounced effect than Mo.     

Synthesis of HCP, FCC and BCC structure alloys in the Mg–Ti binary system by means of ball milling

Mg_xTi_100−x (35 ≤ x ≤ 80) alloys with hexagonal close packed (HCP), face centered cubic (FCC) and body centered cubic (BCC) structures were successfully synthesized by means of ball milling. Mg_xTi_100−x alloys with a BCC structure at x = 35 and 50 and with a HCP structure at x = 80 were synthesized by milling of Mg and Ti powder using stainless steel milling balls and pots. At x = 65, the BCC and HCP phases were synthesized. Mg_xTi_100−x alloys with a FCC structure were synthesized at x = 35 and 50 by milling using zirconia milling balls and pots. The FCC and HCP phases were synthesized at x = 65 and 80 using zirconia milling balls and pots. The crystal structure of Mg_xTi_100−x alloys synthesized by the ball milling method depended on the materials of milling balls and pots. That indicates that milling products are determined by the dynamic energy given by the milling setup. The lattice parameters of Mg_xTi_100−x in the HCP, FCC and BCC phases increased with increase of the Mg content, x.