J. Desimoni

Formation of carbides by electro-discharge machining of alpha iron

Abstract The surface of pure iron sheets have been submitted to spark-planing erosion using kerosene as dielectric under a wide range of machining conditions: discharge duration times between 2 and 3072 μs, and spark currents between 3.3 and 25 A. The carbides formed by the electro-discharge machining process have been identified by X-ray diffraction and Mossbauer spectroscopy. Two different regimes of carbide formation were established: for sparks of energy below ≈0.5 J only e -carbide, austenite and martensite were observed; for sparks of higher energy mainly cementite, austenite and traces of martensite, Fe 7 C 3 , or Fe 5 C 2 were detected.

Assessment of natural radioactivity levels and their relationship with soil characteristics in undisturbed soils of the northeast of Buenos Aires province, Argentina

Surface and depth profile concentrations (down to 50 cm) of ²³²Th chain, ²²⁶Ra, and ⁴⁰K radionuclides were determined in undisturbed coastal and inland soils of La Plata city region, Argentina, through their gamma-ray activity using a high-purity Ge detector spectrometer. These results were compared with superficial activities determined in soils from the surroundings of the Centro Atómico Ezeiza (Ezeiza Atomic Center) located in Ezeiza, Buenos Aires Province, Argentina. The hyperfine and magnetic Fe phases properties of soil profiles were characterized by Mössbauer spectroscopy, magnetic hysteresis loops and AC magnetic susceptibility. No dependence of the activity of the ²³²Th natural chain on depth was found, whereas variations for ²²⁶Ra and ⁴⁰K activities were observed. Positive correlations, determined by the Pearson correlation coefficients, were established between ⁴⁰K, ²²⁶Ra and ²³²Th activity concentrations for the whole set of soil samples. The annual external equivalent dose for adults was similar for La Plata and Ezeiza regions, with average values of 0.08 ± 0.01 mSv and 0.06 ± 0.02 mSv, respectively. The thermal dependence of the AC magnetic susceptibility revealed the existence of magnetite and hematite. The Mössbauer spectra of all soils were made up of signals associated with α-Fe₂O₃, a paramagnetic relaxation component, and Fe³⁺ and Fe²⁺ doublets. In addition, the spectra of inland soils revealed the presence of Fe₃O₄. A negative correlation was found between the activity concentrations and the α-Fe₂O₃ and Fe₃O₄ relative fractions, whereas a positive correlation was found between the Fe³⁺ relative fraction and the ⁴⁰K activity.

Characterization and corrosion studies of laser-melted carbon steel surfaces

Abstract We have observed by conversion electron Mossbauer spectroscopy (CEMS) that the irradiation of carbon steel surfaces with an industrial CO 2 laser, under different experimental conditions, brings about the formation of α-Fe 2 O 3 , Fe 3 O 4 and Fe 1− x O. The larger beam-surface interaction times favour the formation of greater amounts of Fe 3+ oxides (mainly α-Fe 2 O 3 ) within the depth that can be probed by CEMS (≈300 nm). A model based on the numerical solution of the heat-diffusion equation gives evidence that the oxidation processes are mainly dictated by the time at which the metal remains at temperatures higher than the melting point. In addition, the samples have been subjected to wet–dry corrosion cycles in SO 2 -polluted atmospheres and monitored by CEMS and weight gain. The oxide layer reduces noticeably the corrosion rate of steel against corrosion. This protection is related to the composition of the oxidation layer.

Phase Transformations in Fe–Mn Alloys Induced by Ball Milling

We have studied the dependence on the milling time of the amounts of metastable fcc and hcp phases produced by ball milling on Fe-13.7 wt% Mn alloys by Mössbauer spectroscopy and X-ray diffraction. The relative proportions of γ-Fe(Mn) fcc and ε-Fe(Mn) hcp increase with milling times up to 9 h and decline afterwards. A random solid solution of Mn in the α-Fe matrix is found for milling times up to 15 h. Mn segregation is observed after 25 h. The evolution of the relative percentages suggests that ball milling brings about processes that on the one hand induce the fcc/hcp martensitic transformation, and on the other favour transformations towards the phases dictated by the thermodynamic phase diagram.

Hyperfine characterization of tin-doped indium sesquioxide

The hyperfine interactions at In and Sn sites of In2O3∶Sn (ITO) were measured through time-differential perturbed angular correlations and Mössbauer spectroscopy, respectively. Polycrystalline samples prepared by co-precipitation with nominal 0.025, 1 and 5 at.% Sn were studied. They all showed the cubic bixbyite structure characteristic of In2O3 after annealings at 200 °C. The quadrupole interaction at In sites appears nearly independent of Sn concentration being the main result of the presence of Sn in the lattice, the gradual disappearence of the dynamic perturbation caused by after-effects. The Mössbauer data demonstrate that Sn ions are in a 4+ state with ΔQ=0.606 mm/s andδ=0.226 mm/s relative to SnO3Ca at room temperature.

CEMS studies of Sn−O thin films prepared by thermal evaporation

Thermal evaporated thin Sn−O films subjected to annealing treatments in air in the range 473–1173 K and in Ar in the range 473–773 K followed by annealings in air up to 1373 K were studied by CEMS (Conversion Electron Mössbauer Spectroscopy). Complementary Mössbauer and X-ray measurements were also performed on SnO powder that underwent the same series of annealings. The presence of the intermediate oxide Sn3O4 was detected. A temptative hyperfine characterization for the Sn2+ site in Sn3O4 is given.

Inventories and concentration profiles of 137Cs in undisturbed soils in the northeast of Buenos Aires Province, Argentina.

Inventories and vertical distribution of (137)Cs were determined in La Plata region undisturbed soils, Argentina. A mean inventory value of 891 ± 220 Bq/m(2) was established, which is compatible with the values expected from atmospheric weapon tests fallout. The study was complemented with pH, organic carbon fraction, texture and mineralogical soil analyses. Putting together Southern Hemisphere (137)Cs inventory data, it is possible to correlate these data with the mean annual precipitations. The large differences in (137)Cs concentration profiles were attributed to soil properties, especially the clay content and the pH values. A convection-dispersion model with irreversible retention was used to fit the activity concentration profiles. The obtained effective diffusion coefficient and effective convection velocity parameters values were in the range from 0.2 cm(2)/y to 0.4 cm(2)/y and from 0.23 cm/y to 0.43 cm/y, respectively. These data are in agreement with values reported in literature. In general, with the growth of clay content in the soil, there was an increase in the transfer rate from free to bound state. Finally, the highest transfer rate from free to bound state was obtained for soil pH value equal to 8.

TDPAC characterization of tin oxides using181Ta

In connection with a general study of the evolution of tin-oxygen thin films, we report here on the hyperfine interactions of181Ta substitutionally replacting tin in the isolated phases SnO and SnO2. For this purpose, pure SnO pressed powder and a thin SnO2 film were implanted with181Hf. In both cases, unique quadrupole frequencies were found after thermal annealing treatments. The results indicate that the following hyperfine parameters: υQ= 740.6(2.1) MHz, η=0.07(2) and υQ= 971.5(1.9) MHz, η=0.72(1) characterize181Ta in SnO and SnO2, respectively.

Morphology and Phase Composition of Particles Produced by Electro-Discharge-Machining of Iron

Towards producing metallic particles of controlled size and spherical shape, which are of technological importance, we have collected in the filters of an electro-discharge-machine (EDM) the material ejected from the surface of EDM iron pieces. The conditions of machining were varied for kerosene and water as dielectrics, using a discharge current of 25 A and duration times of 16 and 3072 μs for kerosene and of 32, 384 and 768 μs for water, respectively. Scanning electron microscopy was used to assess the effect of the time of discharge on the size of the particles. Mössbauer spectroscopy and X-ray diffraction revealed that for kerosene EDM particles only cementite-like carbides of diverse stoichiometry were formed. While no oxide was found for kerosene spheres, the analyses showed that besides the main fraction of α-Fe, a small percentage of wüstite (and traces of hematite for the 384 μs sample) formed on the water EDM ones.

Austempering Transformation Kinetics of Compacted Graphite Cast Irons Obtained by Mössbauer Spectroscopy

Mössbauer spectroscopy has been applied systematically to study the processes occurring during the Stage I of the austempering transformation of compacted graphite cast irons at temperatures from 573 to 673 K for two Mn concentrations. The kinetics of transformation (γ→αFe+γhc) was followed determining the dependence of the high-carbon austenite percentage on austempering time for different austempering temperatures and Mn contents. The evolution of the C concentration and the total amount of C incorporated into high-carbon austenite were also monitored. The results are compared with those of other morphologies and discussed in the frame of Johnson-Mehls and diffusion models.