A Hanc, J Kansy, G Dercz - Point defect concentrations in fe-al systems determined from mossbauer spectroscopy data - страница 1
ВІСНИК ЛЬВІВ. УН-ТУ
Серія фіз. 2009. Bun. 43. С. 103-109
VISNYKLVIV UNIV. Ser. Physics. 2009. Is. 43. P. 103-109
PACS number(s): 75.50.Bb 61.72.Ji 76.80.+y
POINT DEFECT CONCENTRATIONS IN Fe-Al SYSTEMS DETERMINED FROM MOSSBAUER SPECTROSCOPY DATA
A. Hanc, J. Kansy, G. Dercz, L. Pajak
Institute of Materials Science, University of Silesia 12 Bankowa, 40-007 Katowice, Poland email@example.com
In this work, we employed the Mossbauer spectroscopy and X-ray powder diffraction (XRD) in a study of point defect formation in intermetallic phases of the B2 structure from the Fe-Al system as a function of Al concentration. In the Mossbauer effect, two types of samples are investigated: Fe-Al alloys with small additives obtained by induction melting and Al-rich metallic powders produced by the self-decomposition method and intensive grinding. We present the values of the 57Fe isomer shift and quadruple splitting for the components describing the point defect in the local environment of a Mossbauer nuclide. The concentration of the Fe vacancies and Fe atoms substituting Al (Fe-AS) are determined. The results shown that an increase in Al content causes an increase in vacancy and Fe-AS concentration.
Key words: point defects, Mossbauer spectroscopy, Fe-Al.
Iron aluminides represent an intriguing class of new materials: they offer a good combination of mechanical properties, specific weight/strength ratio, corrosion and oxidation resistance and low raw material cost [1-3], which makes them potential candidates for the substitution of stainless steel in applications at moderate to high temperatures. The extensive technological application of iron aluminides, however, is impaired by their low room temperature tensile ductility. This is attributed to extrinsic (environmental embitterment) or intrinsic (low grain boundary cohesion) mechanisms, with the dominant mechanism depending on the aluminum content of the alloy. The development of new, more ductile Fe-Al alloys depends on a thorough understanding of their properties, implicating a better comprehension of the properties and behavior of defect in these materials. Experimental as well as theoretical studies [1-6, 9-10, 12-19], suggest that iron aluminides present complex point defect, especially triple defect. It is well known that upon rapid quenching from elevated temperatures, iron aluminides retain a high concentration of thermal vacancies, which frozen, increase their yield strength and hardness at room temperature [1-3]. It is expected that the concentration of point defect can be strongly changed in the aluminides with the variation of Al content and the composition modification of the aluminides by transition metal ternary additives [3-4].
© Hanc A., Kansy J., Dercz G., Pajak L., 2009
In this paper, we employ the Mossbauer spectroscopy and X-ray powder diffraction (XRD) in a study of point defect formation in intermetallic phases of the B2 structure from the Fe-Al system. Two types of samples are investigated: Fe-Al (of Al content below 50 at. %) with small additives and metallic powders of Fe-Al (of Al above 50 at. %) obtained by the self-decomposition method and intensive grinding [7-8, 15-18]. Mossbauer spectra are analyzed with a model  according to which the vacancies and Fe atoms substituting Al (Fe-AS) in atomic shells, close to probe atom, influence the isomer shift and quadrupole splitting of particular spectrum components. The concentrations of the point defects are determined from the intensities of these components and they are correlated with the variation of Al content.
The chemical compositions of the investigated samples are presented in table 1.
Chemical compositions of the investigated materials [at.%]
Mo-0,20; Zr-0,05; C-0,1; B-0,01
a) material prepared using the gravity casting technique;
b) material after self-decomposition process and milling;
c) material after self-decomposition process.
The Fe-Al (Fe-rich) samples were obtained from Armco iron, aluminum of 99,99% purity, and a small amount of other additives added in order to improve the thermal and mechanical properties of alloys. The samples were prepared by melting in spinel Al2O3xMgO crucibles in an induction furnace at vacuum of 10-2 Torr. The ingots were re-melted three times to insure homogeneity and annealed in a vacuum furnace for 48 h, and then cooled down slowly with the furnace. The Fe-Al (Al-rich) samples were produced by the self-decomposition method  and intensive grinding in the high-energy electro-magneto-mechanical mill . Phase analysis was carried out by applying X-ray diffraction, using X-ray Philips diffractometer equipped with graphite monochromator on diffracted beam. The copper radiation was applied. During X-ray data collection, the samples were rotated. Selected X-ray diffraction patterns Fe62Al38 and Fe55Al45 samples are presented in fig. 1.
The FeAl (B2 type structure) phase is the main of the studied samples. Lattice constant parameters and long order parameters determined by Rietveld refinement method show the tendency to increase with the increase in aluminum content in the samples.
Fig. 1. X-ray diffraction patterns of Fe62Al38 and Fe55Al45 samples annealed at 1000°C for 48 hours
The 57Fe Mossbauer spectra were measured in transmission geometry at room temperature by means of a constant spectrometer of the standard design. The 14.4 keV gamma rays were provided by a 50mCi source of 57Co/Rh. Hyperfine parameters of the investigated spectra were related to the a-Fe standard. Experimental spectrum shape was described with a transmission integral calculated according to the numerical Gauss-Legandre's procedure  which made it possible to determine the real intensities of the fitted components.
The selected Mossbauer spectra are presented in fig. 2.
-6-4. -2 О 2 4. S
Fig. 2. The 57Fe Mossbauer transmission spectra for Fe62Al38 (a), Fe55Al45 (b) alloys annealed for 48 h at 1 000°C and Fe48Al52 obtained by self-decomposition method and intensive grinding
All Mossbauer spectra were fitted with the model proposed by Bogner et al. . The Mossbauer spectrum of Fe55Al45 sample fitted with the model according to the spectrum contains four components describing different local environments of a 57Fe nuclide. The first component (I) - a single line - represents an ordered B2 structure. The second component (II) - a single line (which approximates an unresolved quadruple doublet) -relates to the case when the Mossbauer Fe nuclide is located in a corner of the cubic centered unit, and an Fe-AS atom is situated in the center of this unit. The third component (III) - a doublet of lines (which approximates an unresolved Zeeman sextet) - corresponds to an Fe atom located in the Fe-AS position. The fourth component (IV)
also a quadruple doublet of lines - represents the case where there is a vacancy in the near Fe environment.
In table 2, we present the evolution of the values of the isomer shift (IS) and quadruple splitting (QS) of the spectra components depending on the aluminum concentration. Similar values of the IS and QS for the components describing the ordered B2 structure and the point defect were observed in theoretical calculations  and experimental research [9, 12, 16-17].
Values of hyperfine parameters (IS, QS); W- line width and A - subspectral area
The values of vacancy and anti-site atoms concentrations found using the described model are shown in table 3.
To estimate the value for the concentration of vacancies in the Fe sublattice, the intensity of a subspectrum was divided by 26 . The obtained values of concentrations of vacancies and anti-site atoms (Fe-AS) show an increase with Al content, which confirms the results of theoretical calculations  and some experimental data [9-10, 16-17, 19]. According to the literature [3-6, 9-10, 12, 14], vacancies in the Fe sublattice VFe are the dominant type of defects in Fe-Al system (maybe organized in triple defects, i.e. two vacancies and an anti-side atom [5, 10]). Vacancy formation in Fe-rich Fe-Al alloys has been studied by Schaefer et al.  using the positron lifetime technique. They found that the thermal vacancy concentration is about 2,3T0-5 at 600°C, increasing to about 1,3T0-3 at 900°C. We also carried out a study of vacancies formation in alloys I and II (see table 1) as a function of Al concentration . We observe that the
total concentration of defects is so high that the positrons are exclusively trapped by defects. Above 38 at % Al, vacancies in Al sublattice (VAl) are formed in a minor amount, in comparison to the vacancies in Fe sublattice (VFe).
Values of vacancy and anti-site atom Fe-AS concentrations in the samples of Fe-Al determined with Mossbauer spectroscopy investigations
Estimated phase composition
V Fe [%]*
Concentration Fe-As [%]**
The values of vacancies concentrations in the samples containing micro-additions (I and II - see table 1), estimated in this work, are slightly lower than vacancies concentrations in FexAl1-x (x>0,5) alloys, calculated in theoretical papers  and than some experimental data [9-10]. We connect this result with a defect and electron structure modification in the examined materials by the micro-additions. Such a character of the defect structure, mainly the lowered concentration of vacancies in the alloys modified with micro-additions, confirms the advisability of their introduction in order to improve plasticity of these materials.The values of point defect concentrations in the Fe-Al metallic powders (samples III-V see table 1) obtained by milling after self-decomposition process shown much higher concentration of vacancies than that in the samples Fe62Al38 and Fe55Al45 (samples I and II). That suggests a decrease in energy of vacancies formation in Al-rich alloys. However, the determined concentration of point defects in the samples III - V is even higher than those described in literature [3, 9-10, 12, 14]. This very high concentration of vacancies in the metallic powders can be related to their preparation by self-decomposition method and milling process.
In this work, point defect concentrations for the series of intermetallic compound samples Fe-Al were determined applying Mossbauer spectroscopy. It was found that the investigated materials contain high concentrations of point defects, which significantly increases with the increase in aluminum content. The values of vacancies concentrations in the FexAl1-x (x>0,5) samples containing micro-additions, estimated in this work, are slightly lower than vacancies concentrations in FexAl1-x (x>0,5) alloys determined by other authors. Such a character of the defect structure - mainly the lowered concentration of vacancies - in the alloys modified with micro-additions confirms the advisability of their introduction in order to improve plasticity of these materials.
A comparison of point defects concentrations in materials obtained by various methods (melting, self-decomposition and milling) indicates a complexity of the factors influencing the defect structure of the materials.
The higher concentrations of point defect in the Al-rich region samples (III-V) in comparison to the Fe62Al38 and Fe55Al45 samples, suggest that a decrease in vacancies
A. Hanc, J. Kansy, G. Dercz, L. Pajak
concentrations in the metallic powders can be related to their preparation by self-decomposition method and milling process.
The work was supported by the State Committee of Scientific Research, Grant N PB-581/T/2006.
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ВИЗНАЧЕННЯ КОНЦЕНТРАЦІЇ ТОЧКОВИХ ДЕФЕКТІВ В СИСТЕМІ Fe-Al ЗА ДАНИМИ МЕСБАУЕРІВСЬКОЇ СПЕКТРОСКОПІЇ
A. Ханк, Й. Канси, Г. Дерц, Л. Пайонк
Інститут матеріалознавства, Університет Сілеція вул. Банкова, 12, 40-007 Катовіце, Республіка Польща
Використано месбауерівську спектроскопію та рентгенівську порошкову дифракцію з метою утворення точкових дефектів в міжметалічних фазах структури В2 як функцію концентрації Al. Досліджено два типи зразків: сплави Fe-Al з незначними домішками, отримані методом індукційного плавлення, та збагачені Al металічні порошки, одержані методом саморозкладу та інтенсивного помолу. Визначено концентрацію вакансій та атомів Fe, які заміщені Al (Fe-AS). Показано, що збільшення вмісту А1 зумовлює збільшення вакансій та концентрації
Ключові слова: точкові дефекти, месбауерівська спектроскопія, Fe-Al.
ОПРЕДЕЛЕНИЕ КОНЦЕНТРАЦИИ ТОЧЕЧНЫХ ДЕФЕКТОВ В СИСТЕМЕ Fe-Al ПО ДАННЫМ МЕСБАУЭРОВСКОЙ
A. Ханк, Й. Канси, Г. Дерц, Л. Пайонк
Институт материаловедения, Университет Силеция ул. Банковая, 12 40-007 Катовице, Республика Польша
Использована месбауэровская спектроскопия и рентгеновская порошковая дифракция с целью исследования образования точечных дефектов в междуметаллических фазах структуры В2 как функции концентрации Al. Исследовано два типа образцов: сплавы Fe-Al с незначительными примесями, полученные методом индукционного плавления, и обогащенные Al металлические порошки, полученные методом саморазложения и интенсивного помола. Определена концентрация вакансий и атомов Fe, замещенных атомами Al ^е-AS). Показано, что увеличение содержания Al приводит к увеличению вакансий и концентрации Fе-AS.
Ключевые слова: точечные дефекты, месбауэровская спектроскопия,
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