In this work, we employed the Mössbauer spectroscopy and X-ray powder diffraction in a study of point defect formation in intermetallic phases of the B2 structure of the Fe{Al system as a function of Al concentration. The
results are compared with the concentrations of point defect determined from positron annihilation data. In the MÄossbauer effect, two types of samples are investigated: Fe{Al alloys with few additives obtained by induction melting and Al-rich metallic powders produced by the self-decomposition method and intensive grinding of high energy in the electro-magneto-mechanical mill.
We present the values of the 57Fe isomer shift and quadrupole splitting for the components describing the point defect in the local environment of a MÄossbauer nuclide. The concentration of the Fe vacancies and Fe atoms
substituting Al (Fe{AS) are determined. The results showed that an increase in Al content causes an increase in vacancy and Fe{AS concentration

Drecz, Grzegorz

Hanc, Aneta

Kansy, Jerzy

Mrzigod, J.

Oleszak, D.

Pająk, Lucjan

Acta Physica Polonica A

English

Repozytorium Uniwersytetu Śląskiego RE-BUŚ

             Title: A Study of Point Defects in the B2-Phase Region of the Fe-Al System by Mossbauer Spectroscopy  Author: Aneta Hanc, Jerzy Kansy, Grzegorz Dercz, Lucjan Pająk, D. Oleszak, J. Mrzigod  Citation style: Hanc Aneta, Kansy Jerzy, Dercz Grzegorz, Pająk Lucjan, Oleszak D., Mrzigod J. (2008). A Study of Point Defects in the B2-Phase Region of the Fe-Al System by Mossbauer Spectroscopy. "Acta Physica Polonica A" (Vol. 114, nr 6 (2008), s. 1555-1562).  Vol. 114 (2008) ACTA PHYSICA POLONICA A No. 6Proceedings of the Polish Mo¨ssbauer Community Meeting 2008A Study of Point Defects in theB2-Phase Region of the Fe–Al Systemby Mo¨ssbauer SpectroscopyA. Hanca,∗, J. Kansya, G. Dercza, L. Paja¸ka , D. Oleszakband J. MrzigodcaInstitute of Materials Science, Silesian UniversityBankowa 12, 40-007 Katowice, PolandbFaculty of Materials Science and EngineeringWarsaw University of TechnologyWoÃloska 141, Warsaw, PolandcInstitute of Chemistry, Silesian UniversitySzkolna 9, 40-007 Katowice, PolandIn this work, we employed the Mo¨ssbauer spectroscopy and X-ray pow-der diffraction in a study of point defect formation in intermetallic phases ofthe B2 structure of the Fe–Al system as a function of Al concentration. Theresults are compared with the concentrations of point defect determined frompositron annihilation data. In the Mo¨ssbauer effect, two types of samplesare investigated: Fe–Al alloys with few additives obtained by induction melt-ing and Al-rich metallic powders produced by the self-decomposition methodand intensive grinding of high energy in the electro-magneto-mechanical mill.We present the values of the 57Fe isomer shift and quadrupole splitting forthe components describing the point defect in the local environment of aMo¨ssbauer nuclide. The concentration of the Fe vacancies and Fe atomssubstituting Al (Fe–AS) are determined. The results showed that an in-crease in Al content causes an increase in vacancy and Fe–AS concentration.PACS numbers: 75.50.Bb, 61.72.J–, 76.80.+y1. IntroductionIron aluminides represent an intriguing class of new materials: they offera good combination of mechanical properties, specific weight/strength ratio, cor-rosion and oxidation resistance and low raw material cost [1–3], which makes∗corresponding author; e-mail: ahanc@us.edu.pl(1555)1556 A. Hanc et al.them potential candidates for the substitution of stainless steel in applications atmoderate to high temperatures. The extensive technological application of ironaluminides, however, is impaired by their low room temperature tensile ductil-ity. This is attributed to extrinsic (environmental embitterment) or intrinsic (lowgrain boundary cohesion) mechanisms, with the dominant mechanism dependingon the aluminum content of the alloy. The development of new, more ductile Fe–Al alloys depends on a thorough understanding of their properties, implicating abetter comprehension of the properties and behavior of defect in these materials.Experimental as well as theoretical studies [1–16] suggest that point defects in ironaluminides present complex, especially triple defect structure. It is well known thatupon rapid quenching from elevated temperatures, iron aluminides retain a highconcentration of thermal vacancies, which frozen, increase their yield strength andhardness at room temperature [1–3]. It is expected that the concentration of pointdefect can be strongly affected by the variation of Al content and the compositionmodification of the aluminides by transition metal ternary additives [3, 4].In this paper, we employ the Mo¨ssbauer spectroscopy and X-ray powderdiffraction (XRD) in a study of point defect formation in intermetallic phases of theB2 structure of the Fe–Al system. Two types of samples are investigated: Fe–Al(Al content below 50 at.%) with small additives and metallic powders of Fe–Al (Alabove 50 at.%) obtained by the self-decomposition method and intensive grinding[17, 18, 12–15]. Mo¨ssbauer spectra are analyzed with a model [7] according towhich the vacancies and Fe atoms substituting Al (Fe–AS) in atomic shells closeto probe atom, influence the isomer shift and quadrupole splitting of particularspectrum components. The concentrations of point defects are determined fromthe intensities of these components and they are correlated with the changes of Alcontent.2. Experimental detailsThe chemical compositions of the investigated samples are presented in Ta-ble I. The Fe–Al (Fe-rich) samples were obtained from Armco iron, aluminum of99.99% purity, and a small amount of other additives added in order to improvethe thermal and mechanical properties of alloys. The samples were prepared bymelting in spinel Al2O3 × MgO crucibles in an induction furnace at vacuum of10−2 Torr. The ingots were re-melted three times to insure homogeneity and an-nealed 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 [17]and intensive grinding in the high-energy electro-magneto-mechanical mill [18].Phase analysis was carried out by applying X-ray diffraction using X-rayPhilips diffractometer equipped with graphite monochromator. The Cu Kα radi-ation was used. The samples were rotated during X-ray data collection. SelectedX-ray diffraction patterns of Fe62Al38 and Fe55Al45 samples are presented in Fig. 1.The features of B2-type structure prevail in the diffraction spectra. Lattice con-stant parameters and long-range order parameters determined by the RietveldA Study of Point Defects in the B2-Phase Region . . . 1557TABLE IChemical compositions of the investigated mate-rials [at.%].Contents [at.%]Ia IIa IIIb IVc VcFe 61.64 54.64 48 46 44Al 38 45 52 54 56Additions Mo-0.20; Zr-0.05; –C-0.1; B-0.01amaterial prepared using the gravity casting tech-nique; bmaterial after self-decomposition processand milling; cmaterial after self-decompositionprocessFig. 1. X-ray diffraction patterns of Fe62Al38 and Fe55Al45 samples annealed at 1000◦Cfor 48 h.refinement method show the tendency to increase with the increase in aluminumcontent in the samples.The 57Fe Mo¨ssbauer spectra were measured in transmission geometry atroom temperature by means of a constant-acceleration spectrometer of the stan-dard design. The 14.4 keV gamma rays were provided by a 50 mCi source of57Co/Rh. Hyperfine parameters of the investigated spectra were related to the α-Fe standard. Experimental spectrum shape was described with a transmission in-tegral calculated according to the numerical Gauss–Legendre procedure [19] whichenables determination of real intensities of fitted components.3. Results and discussionThe selected Mo¨ssbauer spectra are presented in Fig. 2. All Mo¨ssbauerspectra were fitted with the model proposed by Bogner et al. [7]. The Mo¨ssbauerspectrum of Fe55Al45 sample fitted with this model is shown in Fig. 3. Accordingto this model, the spectrum contains four components describing different local1558 A. Hanc et al.Fig. 2. The 57Fe Mo¨ssbauer transmission spectra for Fe62Al38, Fe55Al45 alloys annealedfor 48 h at 1000◦C and Fe48Al52 obtained by self-decomposition method and intensivegrinding.Fig. 3. The Mo¨ssbauer transmission spectrum for the Fe65Al45 alloys. The best fit andcomponents are drawn with the lines, points present the experimental data.environments of a 57Fe nuclide. The first component (I) — a single line — repre-sents an ordered B2 structure. The second component (II) — a single line (whichapproximates an unresolved quadrupole doublet) — relates to the case when theMo¨ssbauer Fe nuclide is located in a corner of the cubic centered unit, and anFe–AS atom is situated in the center of this unit. The third component (III) — adoublet of lines (which approximates an unresolved Zeeman sextet) — correspondsto a Fe atom located in the Fe–AS position. The fourth component (IV) — also aA Study of Point Defects in the B2-Phase Region . . . 1559quadrupole doublet of lines — represents the case where there is a vacancy in thenear Fe surrounding.TABLE IIValues of hyperfine parameters (IS, QS); W — line widthand A — subspectral area.Samples Component IS∗,∗∗ QS∗ W A[mm/s] [mm/s] [%]L-I 0.19 0.27 83L-II 0.06 0.34 14Fe63Al38a Q-III 0.04 0.25 0.36 1.9Q-IV 0.23 0.13 0.29 1.1L-I 0.24 0.25 85L-II 0.07 0.32 11.7Fe55Al45a Q-III 0.05 0.22 0.34 2.1Q-IV 0.21 0.17 0.26 1.2L-I 0.25 0.31 84L-II 0.09 0.29 12Fe48Al52b Q-III 0.06 0.31 0.35 2.3Q-IV 0.29 0.19 0.31 1.7L-I 0.27 0.26 78L-II 0.17 0.29 16.6Fe46Al54c Q-III 0.09 0.33 0.33 3.1Q-IV 0.30 0.21 0.33 2.3L-I 0.33 0.26 75L-II 0.18 0.28 19Fe44Al56c Q- III 0.11 0.37 0.32 3.3Q-IV 0.34 0.23 0.32 2.7∗Error estimated from the fitting procedure is equal to±0.005.∗∗Relative to the α-Fe foil at room temperature.amaterial prepared using the gravity casting technique;bmaterial after self-decomposition process and milling;cmaterial after self-decomposition process.In Table II, we present the evolution of the values of the isomer shift (IS) andquadrupole splitting (QS) of the spectra components depending on the aluminumconcentration. Similar values of the IS and QS for the components describing theordered B2 structure and the point defect were observed in theoretical calculations[20] and experimental research [7, 9, 13, 14]. The values of vacancy and anti-siteatoms concentrations found using the described model are shown in Table III.1560 A. Hanc et al.TABLE IIIValues of vacancy and anti-site atom Fe–AS concentrations inthe samples of Fe–Al determined with the Mo¨ssbauer spec-troscopy investigations.Estimated Vacancy concentration Concentrationphase composition VFe [%]∗ Fe–As [%]∗∗Fe62Al38a 0.04 1.9Fe55Al45a 0.05 2.1Fe48Al52b 0.07 2.3Fe46Al54c 0.09 3.1Fe43Al56c 0.10 3.3∗Error estimated is equal to ±0.006.∗∗Error estimated is equal to ±0.05.amaterial prepared using the gravity casting technique;bmaterial after self-decomposition process and milling;cmaterial after self-decomposition process.To estimate the value for the concentration of vacancies on the Fe sublattice,the intensity of a subspectrum was divided by 26 [7]. The obtained values ofconcentrations of vacancies and anti-site atoms (Fe–AS) show an increase withAl content, which confirms the results of theoretical calculations [10] and someexperimental data [7, 8, 13, 14, 16]. According to the literature [3–8, 9, 11],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-site atom [5, 8]).Vacancy formation in Fe-rich Fe–Al alloys has been studied by Schaefer et al.[21] using the positron lifetime technique. They found that the thermal vacancyconcentration is about 2.3×10−5 at 600◦C, increasing to about 1.3×10−3 at 900◦C.We also carried out a study of vacancies formation in alloys I and II (see Table I)as a function of Al concentration [16]. We observe that the total concentrationof defects is so high that the positrons are exclusively trapped by defects. Above38 at.% Al, vacancies in Al sublattice (VAl) are formed in a minor amount, incomparison to the vacancies in Fe sublattice (VFe).The values of vacancies concentrations in the samples containing micro--additions (I and II — see Table I), estimated in this work, are slightly lowerthan vacancies concentrations in FexAl1−x (x > 0.5) alloys, calculated in theoret-ical papers [10] and then some experimental data [7, 8]. We connect this resultwith a defect and electron structure modification in the examined materials by themicro-additions. Such a character of the defect structure, mainly the lowered con-centration of vacancies in the alloys modified with micro-additions, confirms theadvisability of their introduction in order to improve plasticity of these materials.The values of point defect concentrations in the Fe–Al metallic powders (sam-ples III–V, see Table I) obtained by milling after self-decomposition process haveA Study of Point Defects in the B2-Phase Region . . . 1561shown much higher concentration of vacancies than that in the samples Fe62Al38and Fe55Al45 (samples I and II). That suggests a decrease in energy of vacan-cies formation in Al-rich alloys. However, the determined concentration of pointdefects in the samples III–V is even higher than those described in literature[3, 7–9, 11]. This very high concentration of vacancies in the metallic powders canbe related to their preparation by self-decomposition method and milling process.4. ConclusionsIn this work point defect concentrations for the series of intermetallic com-pound samples Fe–Al were determined applying the Mo¨ssbauer spectroscopy. Itwas found that the investigated materials contain high concentrations of pointdefects, which significantly increases with the increase in aluminum content. Thevalues of vacancies concentrations in the FexAl1−x (x > 0.5) samples containingmicro-additions, estimated in this work, are slightly lower than vacancies con-centrations in FexAl1−x (x > 0.5) alloys determined by other authors. Such acharacter of the defect structure — mainly the lowered concentration of vacanciesin the alloys modified with micro-additions — confirms the advisability of theirintroduction in order to improve plasticity of these materials.A comparison of point defects concentrations in materials obtained by var-ious methods (melting, self-decomposition, and milling) indicates a complexity ofthe 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 de-crease in vacancies concentrations in the metallic powders can be related to theirpreparation by self-decomposition method and milling process.AcknowledgmentsThe work was supported by the State Committee for Scientific Research,grant no. PB-581/T/2006.References[1] C.G. McKamey, Physical Metallurgy and Processing of Intermetallic Compounds,Chapman&Hall, New York 1996, p. 351.[2] P.R. Munroe, Processing, Properties and Applications Proceedings of ASM, Eds.S.H. Whang, C.T. Liu, D.P. Pope, J.D. Stiegle, Materials Society, Warrenale 1996,p. 96.[3] J.L. Jordan, S.C. Deevi, Intermetallics 11, 507 (2003).[4] D.G. Morris, M.A. Morris-Mun˜oz, Intermetallics 7, 1121 (1999).[5] M. Kogachi, T. Haraguchi, S.M. Kim, Intermetallics 6, 499 (1998).[6] M. Hillert, M. Selleby, J. Alloys Comp. 329, 208 (2001).[7] J. Bogner, W. Steiner, M. Reissner, P. Mohn, P. Blaha, K. Schwarz, R. Krachler,H. Ipser, B. Sepiol, Phys. Rev. B 58, 14922 (1998).1562 A. Hanc et al.[8] A. Broska, J. Wolff, M. Franz, Th. Hehenkamp, Intermetallics 7, 259 (1999).[9] S. Gianella, R.S. Bursa, W. Deng, F. Marino, T. Spataru, G. Principi, J. AlloysComp. 317-318, 485 (2001).[10] X. Ren, K. Otsuka, Philos. Mag. A 80, 467 (2000).[11] T. Haraguchi, M. Kogachi, S.M. Kim, Intermetallics 7, 981 (1999).[12] A. Hanc, G. Dercz, J.E. Fra¸ckowiak, L. Paja¸k, F. Binczyk, in: Proc. of the XIXConf. on Applied Crystallography, Eds. H. Morawiec, D. Stro´z˙, World scientific,Singapore 2004, p. 312.[13] A. Hanc, J.E. Fra¸ckowiak, Nukleonika 49, S3, 7 (2004).[14] A. Hanc, J.E. Fra¸ckowiak, Nukleonika 52, S3, 24 (2006).[15] A. Hanc, J.E. Fra¸ckowiak, G. Dercz, L. Paja¸k, Solid State Phenom. 130, 181(2006).[16] J. Kansy, A. Hanc, D. Giebel, M. JabÃlon´ska, Acta Phys. Pol. A 113, 1409 (2008).[17] F. Binczyk, Inz˙ynieria MateriaÃlowa 4, 51 (1989).[18] F. Binczyk, W. Polechon´ski, S.J. Skrzypek, Powder Technology 114, 237 (2001).[19] A. Ralston, A first course In numerical analysis, McGraw-Hill Book Company,London 1975.[20] T. Michalecki, J. Deniszczyk, J.E. Fra¸ckowiak, Nukleonika 49, S3, 3 (2004).[21] H.E. Schaefer, R. Wurschum, M. Sob, T. Zak, W.Z. Yu, W. Eckert, F. Banhart,Phys. Rev. B 41, 11869 (1990).

A Study of Point Defects in the B2-Phase Region of the Fe-Al System by Mossbauer Spectroscopy

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A Study of Point Defects in the B2-Phase Region of the Fe-Al System by Mossbauer Spectroscopy

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