Program: K3 Metals and Alloys
K3 Metals and Alloys

K3.1 Mössbauer Investigation of Surface Processing by Pulsed Laser Irradiation in Reactive Atmospheres
E. Carpene, M. Kahle and P. Schaaf
Universität Göttingen, Zweites Physikalisches Institut, Göttingen, Germany

Due to its high sensitivity to the local atomic environment, Mössbauer Spectroscopy has become a standard tool to investigate iron and iron alloys. Conversion Electron Mössbauer Spectroscopy (CEMS) in particular, thanks to its information depth of the order of 150 nanometers, is largely employed for the analysis of surface treatments and modifications. In addition, also the conversion X-rays can be used (CXMS) in combination with CEMS having a larger information depth of about 10 µm.

Irradiations of iron (Armco, Fe > 99.85%) and stainless steel (1.4401, X5CrNiMo18.10) substrates in controlled gaseous atmospheres (nitrogen and methane) have been performed with nanosecond pulses of an Excimer laser, leading to the formation of nitride and carbide surface layers, respectively, with different stoichiometry and crystallographic structures. The evolution and abundance of each phase is correlated to the experimental parameters such as the number of laser pulses, the laser fluence and the ambient gas pressure [1,2]. The capability of CEMS to distinguish the atomic surrounding in combination with its sensitivity to the surface makes it an indispensable tool for a proper investigation and optimisation of such surface treatments.

Additional analytical techniques have been used in order to have complementary information. Ion beam analysis (Rutherford Backscattering Spectrometry and Resonant Nuclear Reaction Analysis) was performed to measure the depth profiles of nitrogen and carbon, while X-ray Diffraction and Rietveld refinement were employed to characterize the crystallographic morphology of the irradiated samples.

The results revealed that under proper experimental conditions, the laser treatments can produce almost homogeneous layers of nitride/carbide, where the composition and the structure are determined only by the parameters of irradiations.

[1] P. Schaaf, Prog. Mat. Sci. 47 (2002) 1.
[2] E. Carpene and P. Schaaf, Appl. Phys. Lett. 80 (2002) 891.


K3.2 Hyperfine Interactions in Epitaxial Fe1-xSi Synthesized by Pulsed Laser Annealing: Experiment and Theory
A. Vantomme1, A. Falepin1, S. Cottenier1 and C.M. Comrie2
1) Instituut voor Kern- en Stralingsfysica, K.U. Leuven, Leuven, Belgium
2) University of Cape Town, Rondebosch, South Africa

Polycrystalline e-FeSi/Si(111) layers have been subjected to pulsed laser annealing to induce the formation of epitaxial metastable Fe-silicide phases. The applied laser energy densities vary from 150 to 450 mJ/cm2. Increasing the energy density or the number of pulses, results in an interfacial layer with a more Si rich composition (Fe/Si~1/2) and an improved epitaxial ordering, as revealed by Rutherford backscattering and channelling spectrometry (RSB). However, due to the fast cooling rate, the Si is not homogeneously distributed throughout the silicide layer - a decrease in Si concentration towards the surface is observed with RBS. It is proposed that the phase produced by the laser annealing corresponds to the metastable [CsCl]Fe1-xSi phase.

The conversion electron Mössbauer spectra from the films displaying significant channelling could not be fitted with the hyperfine parameters of any (mixture) of the stable Fe-silicide phases. They are interpreted as a combination of the e-FeSi doublet and a distribution of quadrupole doublets assigned to the metastable phase. With increasing number of pulses, the fraction of e-FeSi decreases, in agreement with improved epitaxial ordering.

This CsCl structure, from which a fraction x of Fe atoms have been removed in a random way, results in a wide variety of local configurations around the 57Fe probe nucleus, each of which results in a specific quadrupole splitting and isomer shift. Therefore, quadrupole splittings and isomer shifts in the disordered Fe1-xSi phase where calculated with the FLAPW-method. The electric-field gradient is found to be drastically influenced by the Fe-distribution in relatively far-lying coordination shells, making a few-site model to study the Fe-distribution impossible. Conversely, the calculated isomer shift depends smoothly on the number of Fe-atoms in the neighbourhood. This allowed to interpret the experimental variation of the isomer shift with increasing number of pulses in terms of characteristics of the local Fe-distribution.


K3.3 On the Structural and Magnetic Properties of Fe-Mn-Si Alloys
S.H. Mahmood1, A-F. D. Lehlooh1, A.S. Saleh1 and F.E. Wagner2
1) Physics Department, Yarmouk University, Irbid, Jordan
2) Technical University of Munich, Garching, Germany

Both alloys Fe3Si and Mn3Si are known to have a D03 crystallographic structure. Fe3Si orders ferromagnetically below 850 K, where as Mn3Si orders antiferomagnetically below 22 K. The two alloys combine to form the solid solution Fe3-xMnxSi with Mn atoms preferentially substituting Fe atoms in the B site of the D03 structure for x values up to 0.75, and then partially occupying A, C sites for higher concentrations.

For x £ 0.75, the alloys exhibit ferromagnetic ordering, and onset of antiferromagnetic ordering is observed for x > 1.8. The alloys in the concentration region 0.75 £ x £ 1.75 exhibit complex magnetic properties, showing a paramagnetic to ferromagnetic transition at Tc followed by a re-entrant transition at a lower temperature Tf. This re-entrant spin-glass behaviour is a result of the competition between the ferromagnetic ordering and the antiferromagnetic ordering.

This work involves a comprehensive study of the alloy system Fe3-xMnxSi at temperatures down to 4.2 K using Mössbauer spectroscopy. The complex Mössbauer spectra at 4.2 K consisting of several magnetic sextets result from the presence of distinct local environments of Fe atoms in the various sites of the alloys with x £ 1.15. For x > 1.15, the B site is completely filled with Mn atoms, and the spectra show simpler patterns consisting of broad distributions of hyperfine fields with appreciably lower field values for x up to 1.5. This is associated with the spin-glass behaviour of the alloys with Fe atoms residing in equivalent A, C sites. For x > 2, the spectra show an appreciably broadened central singlet associated with the antiferromagnetic phase. Mössbauer spectra at temperatures ranging from room temperature to 4.2 K for alloys in the spin-glass region show a critical dependence of the transition temperature on the concentration of Mn.


K3.4 Application of Mössbauer Spectroscopy and Positron Annihilation Spectroscopy for Testing of Neutron-Irradiated Reactor Steels
V. Slugeò, J. Lipka, J. Hašèík, R. Gröne, I. Tóth, A. Zeman and K. Vitázek
Department of Nuclear Physics and Technology, Slovak University of Technology, Bratislava, Slovakia

The work is focused on the application of Mössbauer spectroscopy (MS) and positron annihilation spectroscopy (PAS) in the evaluation of the microstructure parameters of materials used in nuclear industry. Usefulness of these methods is documented on the evaluation of degradation processes going on in the NPP reactor pressure vessel steels.

The samples originated from the Russian 15Kh2MFA and Sv10KhMFT steels, commercially used at WWER-440 reactors, were irradiated near the core at NPP Bohunice (Slovakia) to neutron fluences in the range from 7.8x1023 m-2 to 2.5x1024 m-2. The systematic changes in the MS and PAS spectra were observed mainly during the first period (1-year stay in irradiation containers in operating conditions by "speed factor" of about 10). It could be explained due to changes cased by precipitation of elements like Cu or Cr mainly in carbides to the surface. These MS results confirm that the close environment of Fe atoms in bcc lattice of RPV steels stay after initial changes almost stable and perhaps could be correlated with the ductile-brittle transition temperature curve from mechanical tests.

Positron annihilation lifetime measurements using the Pulsed Low Energy Positron System (PLEPS) were applied for first time for the investigation of defects of irradiated and thermally treated reactor pressure vessel (RPV) steels. PLEPS results showed that the changes in the microstructure of the RPV-steel properties caused by neutron irradiation and post-irradiation thermal treatment can be well detected.


O3.1 Mössbauer and Atomic Force Microscopy Observations of Modified Surfaces of Fe-Si Steel
Y. Jirásková1, O. Schneeweiss1, P. Schaaf2 and C. Blawert3
1) Institute of Physics of Materials, Academy of Sciences of the Czech Republic, Brno, Czech Republic
2) Universität Göttingen, II. Physikalisches Institut, Göttingen, Germany
3) Zentrum für Funktionswerkstoffe gGmbH Clausthal, Clausthal - Zellerfeld, Germany

The morphologic, phase and surface changes associated with the various surface modifications of Fe-6at.%Si steel samples are investigated by Conversion Electron Mössbauer spectroscopy (CEMS), X-ray diffraction (XRD), Rutherford Backscattering Spectroscopy (RBS), Atomic Force Microscopy (AFM) and Lateral Force Microscopy (LFR) completed by microhardness measurements. The mechanically and heat treated samples were modified by nitrogen using plasma immersion ion implantation at 300o C for 3 h with and without the presence of Si in implantation chamber.

The a-Fe(Si) solid solution influenced by defects is detected in the mechanically treated surface. A reduced content of Si atoms, as indicated by CEMS in the surface layers of heat-treated Fe-Si samples, causes a decrease in the surface microhardness. The different surfaces are reflected in the varied nitrogen distributions and surface phase compositions of samples after an implantation process resulting also in different microhardness depth profiles. While the sharp peaks of a paramagnetic e-Fe2N in the XRD pattern and a pronounced doublet spectrum in the Mössbauer investigations dominate the surface of annealed samples, the markedly broadened XRD peaks and magnetically distributed components in the CEMS spectra reflect the increased contribution of e-nitrides with lower nitrogen content in addition to the a-Fe(Si) phase in mechanically treated surfaces. A presence of Si during implantation influences the nitrogen concentration in the top layers of the surface influencing also the relative amounts of the phases. AFM and LFM observations show various morphologies of the mechanically and heat-treated surfaces which remains preserved also after the surface nitriding.


O3.2 Mössbauer Spectroscopy of Commercial Galvannealed Zinc Coatings
M. Zmrzlý1, O. Schneeweiss2 and J. Fiala1
1) Brno University ofTtechnology, Faculty of Chemistry, Institute of Chemistry of Materials, Brno, Czech Republic
2) Institute of Physics of Materials, Academy of Sciences of the Czech Republic, Brno, Czech Republic

Phase composition and structure of commercial galvannealed zinc coatings on steel substrate were studied using optical microscopy, SEM, EDX and Mössbauer spectroscopy. The optical microscopy yielded thickness of the coating and grain size of the Fe-Zn intermetallic phases. Then the samples were etched in inorganic acids and the influence of the etching time on the thickness decrease was evaluated. Etched samples were analyzed by EDX and Mössbauer spectroscopy and content of the intermetallic phases was determined. Subsequently changes of the intermetallic phases were investigated in dependence on isochronal annealing at various temperatures in the range of 300 - 600° C.