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Dependence of electrical resistivity of Fe–Al alloys on composition
Intermetallics 18 (2010) 1303–1305
Contents lists available at ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
Dependence of electrical resistivity of Fe–Al alloys on composition A. Pazourek a, *, W. Pfeiler b, V. Sˇı´ma c a
´ 2, 46117 Liberec, Czech Republic Department of Material Science, Technical University of Liberec, Studentska Dynamics of Condensed Systems, Faculty of Physics, University of Vienna, Strudlhofgasse 4, A-1090 Vienna, Austria c Department of Physics of Materials, Charles University in Prague, Ke Karlovu 5, 121 16 Praha 2, Czech Republic b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 October 2009 Received in revised form 22 December 2009 Accepted 21 January 2010 Available online 16 February 2010
A series of binary Fe–Al alloys containing 20–40 at.% Al, having ordered D03 (Fe3Al) or B2 (FeAl) structure, was cast from pure elements. For a comparison, specimens were also prepared from corresponding complex Fe–Al alloys with additional alloying elements of C, Cr, Ti, Zr and B. All specimens were investigated in two states, as-prepared and heat treated (723 K/120h). Electrical resistivity was measured at 291 K (RT) and 77 K (liquid N2), respectively, by means of usual potentiometric method. Electrical resistivity data of these series of Fe–Al alloys with and without additional alloying elements are presented and discussed. Differences between resistivity of binary and complex alloys are mainly due to substitution of Fe by Cr in the D03 matrix. The addition of C in alloys with B2 structure lowers the resistivity of the matrix due to k-phase Fe3AlCx precipitation. A strong correlation was observed between the temperature dependent part of electrical resistivity and long-range order in pure binary Fe–Al alloys. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: A. Iron aluminides B. Electrical resistance and other electrical properties
1. Introduction Iron aluminides have good oxidation, corrosion and wear resistance, relatively low density and material cost. These materials exceed in some properties even stainless steels; moreover, they offer a possibility of substituting strategic elements such as nickel and chromium. Iron aluminides in spite of long-range order exhibit high electrical resistivity, which is higher than for most commercial heating elements. Thus, the optimal combination of oxidation resistance and high electrical resistivity of iron aluminides make them ideally suited for resistive-heating applications [1]. The structural phase diagram of pure Fe–Al system is rather complex [2]. In the Fe-rich side, the two ordered phases B2 (FeAl) and D03 (Fe3Al) and disordered body-centered a – phase are mutually separated by two-phase regions a þ B2 and a þ D03 [2]. At room temperature the Fe–Al alloy with 20 at.% Al belongs to the a þ D03 region, around 23 at.% Al the structure becomes single phase D03, at about 37 at.% Al a line of second order transition separates the D03 region and B2 region, persisting above 40 at.% Al. The presence of magnetic order makes the situation in Fe–Al system even more complicated. The magnetic phase diagram shows paramagnetic, ferromagnetic and spin-glass regions, which meet at a multicritical point near 30 at.% Al at low temperatures
* Corresponding author. Tel.: þ420 605483777; fax. þ420 485353631. E-mail address: [email protected] (A. Pazourek). 0966-9795/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2010.01.019
below 200 K [3]. The Curie temperature decreases from about 1000 K at 20 at.% Al to the multicritical point, and above 30 at.% Al ferromagnetic order disappears due to competition between nearest-neighbor Fe–Fe ferromagnetic exchange and an indirect Fe–Al–Fe antiferromagnetic superexchange [3]. The purpose of this paper is to present the influence of often used alloying elements (C, Cr, Ti, Zr, B) on electrical resistivity of complex alloys based on iron aluminides with 20–40 at.% Al, with a potential for future use in technical applications. 2. Experimental methods All Fe–Al alloys with additional alloying elements are complex alloys for future technical use. These alloys were prepared by vacuum chill casting in induction furnace, the castings were hotrolled at 1423 K and slowly cooled in air to room temperature. Binary Fe–Al alloys were prepared from pure elements (Fe (3N), Al (5N)) by repeated melting in laboratory arc furnace in argon atmosphere prior to drop casting them in water cooled copper crucibles. Results of the chemical analysis of all alloys are summarized in Table 1. Specimens for electrical resistivity measurement were cut from as-cast material by diamond saw or spark erosion. The final form of specimens was obtained by mechanical polishing. From each material three specimens were prepared and then contacted with four Pt-wires (0.3 mm diameter) by spot welding. Each specimen
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A. Pazourek et al. / Intermetallics 18 (2010) 1303–1305
Table 1 Chemical composition of alloys (at.%). Al 40.0 40.0 40.0 39.9 39.2 39.1 37.0 35.0 33.0 30.0 28.0 27.8 27.7 26.5 25.0 24.5 20.0 19.9
C
Cr
Ti
3.0 1.00 0.95 0.99 0.93
Zr
B
0.20
0.007 0.009
0.03 0.77 0.48
3.5 2.7 3.6
0.425 0.009
0.49 0.18 0.27
1.09 0.01
4.74
2.7
0.008
0.22
0.01
0.06
4.2
0.074
0.49
0.28
Fe Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance
was subsequently controlled for surface cracks by optical microscopy. Resistivity measurement was carried out for a well defined room temperature in acetone (291 K) (RT) and in liquid nitrogen (77 K) (LNT). Each specimen was measured twice in both current directions. Measurements in the as-prepared (AP) state were repeated for all specimens after their heat treatment (723 K/120h) (HT) in vacuum to remove excess vacancies and enable full equilibrium long-range order. A similar 5 day anneal is often used for Fe–Al alloys according to Nagpal and Baker [4]. 3. Results and discussion 3.1. Electrical resistivity at RT Fig. 1 shows the electrical resistivity, measured at RT, in AP (open symbols) and HT state (full symbols), respectively, as a function of Al content. 3.1.1. Binary Fe–Al alloys The values of electrical resistivity of binary Fe–Al alloys are higher in AP state than in HT state, mainly for 25, 28 and 30 at.% Al.
Fig. 1. Electrical resistivity at RT as a function of Al content for Fe–Al-based materials binary Fe–Al (B, C), first group with additions of C, Ti (6, :), second group with additions of C, Ti, B, Zr and Cr (,,-). The open and full symbols indicate data measured in AP state and HT state, respectively. The full line connecting points serves as a guide for the eye, the dashed line represents literature data [5]. The error bar (right) shows the absolute error of plotted data.
The data for HT state are in quite good agreement with literature data (dashed curve) for long-range ordered Fe–Al alloys [5], the higher value for 20 at.% Al is due to disordering in the two-phase (a þ D03) matrix [6]. This corresponds also to the fact that for 20 at.% Al there is no difference between AP and HT values, indicating negligible contribution of the ordered D03 phase to the resistivity of this alloy. On the other hand the highest difference between AP and HT values was observed for stoichiometric Fe3Al, above 25 at.% Al the effect of the HT decreases with decreasing long-range order in the D03 structure. In the B2 structure practically no effect of the HT was observed, the resistivity is controlled by structural order, and not by the vacancy concentration. The maximum resistivity near 37 at.% Al is in agreement with the previous resistivity data published in [1,5,6], it corresponds with the lowest structural long-range order in the matrix (the D03 structure becomes B2 at room temperature for this composition). The further increase of Al content increases the long-range order and lowers the resistivity in B2 structure. 3.1.2. Fe–Al alloys with additional alloying elements Two groups of technical alloys with additional alloying elements were measured – the first group with addition of C and Ti in B2 structure and the second group with additions of C, Ti, B, Zr and relatively high concentration of Cr in D03 or a structure. Additionally to the second group one material was prepared from carbon free iron with 40 at.% Al and addition of Cr and Zr. The addition of C and Ti in B2 structure surprisingly lowers the resistivity below the corresponding value for the binary alloy. This effect was not observed, when the material was prepared from carbon free iron (see Fig. 1). The reason for this can be found in ternary Fe–Al–C diagram [7]. The real concentration of Al in the Fe– Al matrix increases due to precipitation of the k-phase Fe3AlCx (x w 0.5). Supposing the resistivity is determined mainly by the matrix with negligible content of carbon, the points marked in Fig. 1 by triangles should be shifted by about 1.5 at.% Al to higher concentrations. This is in good agreement with data for binary alloys. The additions in D03 or a structure raise the resistivity of the alloy significantly. The addition of chromium seems to have the decisive effect among used additions, because most of the Cr atoms replace Fe atoms in the matrix [6], whereas most of the other alloying elements tend to form precipitates, which have a minor influence on the resulting resistivity of the alloy. The highest value of electrical resistivity was obtained for the alloy with 24.5 at.% Al
Fig. 2. Electrical resistivity at LNT as a function of Al content for Fe–Al-based materials, the symbols are the same as in Fig. 1. The error bar (right) shows the absolute error of plotted data.
A. Pazourek et al. / Intermetallics 18 (2010) 1303–1305
1305
order and temperature dependent contribution to resistivity in Fe–Al very probably reflects a complex scattering process of conduction electrons in the Fe–Al matrix, where the resistivity depends on structural, spin and thermal disorder (see ref. [6]) and on the nature of density of states at the Fermi level (see ref. [8]). These contributions are not independent and cannot be separated. 3.2.2. Fe–Al alloys with additional alloying elements The results at LNT are qualitatively very similar to that at room temperature. In the case of complex alloys, the difference between resistivity at RT and LNT (Fig. 3) in contrast to binary alloys does not show the above described correlation with long-range order. The values of this difference for all measured alloys lie in the range of values determined for binary alloys. 4. Conclusions Fig. 3. Difference of electrical resistivity at RT and LNT as a function of Al content for Fe–Al-based materials, the symbols are the same as in Fig. 1. The error bar (right) shows the absolute error of plotted data.
(see Table 1 for the composition). The high resistivity value can be seen as a combined effect of the precipitation of the k-phase Fe3AlCx, shifting the Al-concentration near ideal stoichiometry, and solute effect of Cr in the Fe-sublattice. The solute effect is not related to the structural disorder in the D03 matrix only, the addition of transition elements such as Cr and mainly V causes that the resistivity increases rapidly at low temperatures, forming a maximum near the Curie temperature (well above RT for Fe3Al with addition of Cr) and a negative resistivity slope at higher temperatures [6]. The electrical resistivity of these alloys is influenced by complex changes in the electronic structure of the matrix with atomic and magnetic long-range order. 3.2. Electrical resistivity at LNT Fig. 2 shows the electrical resistivity, measured at LNT, in AP (open symbols) and HT state (full symbols), respectively, as a function of Al content. 3.2.1. Binary Fe–Al alloys The results at LNT are qualitatively very similar to that at room temperature. Plotting the difference between resistivity as measured at RT and LNT, respectively, versus concentration (Fig. 3), the dependence is qualitatively opposite to that shown in Fig. 1 (RT) and 2 (LNT). Where local minima are obtained in Figs. 1 and 2 (25 at.% Al and 40 at.% Al), local maxima result for the difference of both curves and vice versa. It is concluded that for high degrees of order (25 at.% Al and 40 at.% Al) the temperature-dependent contribution to resistivity is high, whereas for low degree of order (20 at.% Al and 35 at.% Al) it is low. We presume that these findings are not simply related to an interaction between structural and magnetic order, because for concentrations higher than 30 at.% Al the Fe–Al alloys should not be magnetically ordered [3], in spite of some theoretical predictions for ferromagnetic ground state in FeAl [8,9], strain-induced and disorder-induced ferromagnetism in B2 structure [10,11]. The observed correlation between degree of
Comparison of resistivity data of technical Fe–Al-based materials with that of pure binary alloys has shown that additions responsible for formation of precipitates have only a minor influence on the resistivity of the material. A significant influence, however, arises from the addition of chromium in the D03 structure, the substitution of iron atoms by transition elements in the D03 matrix seems to be the easiest way to increase the resistivity of technical Fe–Al-based materials. The addition of C in alloys with B2 structure lowers the resistivity of the matrix due to k-phase Fe3AlCx precipitation. A strong correlation was observed between the temperature dependent part of electrical resistivity and long-range order in pure binary Fe–Al alloys. Acknowledgement This research has been partly supported by the Ministry of Education, Youth and Sports of the Czech Republic (Project MSM0021620834) and the Czech Science Foundation (Project GACR 106/06/0019). One of authors (A. P.) would like to thank to the people from University of Vienna (Prof. Dr. Bogdan Sepiol, Dipl.-Ing. Witold Wroczewski, Mag. David Geist, Ing. Andreas Berger and Mag. Dr. Erhard Schafler) and to Ing. David Pospı´sˇil from Technical University of Liberec for kind assistance and help with experiments and preparation of specimens. References [1] Lilly AC, Deevi SC, Gibbs ZP. Mater Sci Eng A 1998;258:42–9. [2] Massalski TB, editor. Binary alloy phase diagrams. Metals Park, USA: ASM International; 1986. p. 111. [3] Shukla P, Wortis M. Phys Rev B 1980;21:159–64. [4] Nagpal P, Baker I. Metall Trans 1990;21A:2281–2. [5] Kass M, Brooks CR, Falcon D, Basak D. Intermetallics 2002;10:951–66. [6] Nishino Y. Mater Sci Eng A 1998;258:50–8. [7] Palm M, Inden G. Intermetallics 1995;3:443–54. [8] Das GP, Rao BK, Jena P, Deevi SC. Phys Rev B 2002;66. 184203–1–13. [9] Arzhnikov AK, Dobysheva LV, Timirgazin MA. J Magn Magn Mater 2008;320:11904–8. [10] Wu D, Munroe PR, Baker I. Phil Mag 2003;83:295–313. [11] Menendez E, Liedke MO, Fassbender J, Gemming T, Weber A, Heyderman LJ, et al. Small 2009;5:229–34.
Contents lists available at ScienceDirect
Intermetallics journal homepage: www.elsevier.com/locate/intermet
Dependence of electrical resistivity of Fe–Al alloys on composition A. Pazourek a, *, W. Pfeiler b, V. Sˇı´ma c a
´ 2, 46117 Liberec, Czech Republic Department of Material Science, Technical University of Liberec, Studentska Dynamics of Condensed Systems, Faculty of Physics, University of Vienna, Strudlhofgasse 4, A-1090 Vienna, Austria c Department of Physics of Materials, Charles University in Prague, Ke Karlovu 5, 121 16 Praha 2, Czech Republic b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 October 2009 Received in revised form 22 December 2009 Accepted 21 January 2010 Available online 16 February 2010
A series of binary Fe–Al alloys containing 20–40 at.% Al, having ordered D03 (Fe3Al) or B2 (FeAl) structure, was cast from pure elements. For a comparison, specimens were also prepared from corresponding complex Fe–Al alloys with additional alloying elements of C, Cr, Ti, Zr and B. All specimens were investigated in two states, as-prepared and heat treated (723 K/120h). Electrical resistivity was measured at 291 K (RT) and 77 K (liquid N2), respectively, by means of usual potentiometric method. Electrical resistivity data of these series of Fe–Al alloys with and without additional alloying elements are presented and discussed. Differences between resistivity of binary and complex alloys are mainly due to substitution of Fe by Cr in the D03 matrix. The addition of C in alloys with B2 structure lowers the resistivity of the matrix due to k-phase Fe3AlCx precipitation. A strong correlation was observed between the temperature dependent part of electrical resistivity and long-range order in pure binary Fe–Al alloys. Ó 2010 Elsevier Ltd. All rights reserved.
Keywords: A. Iron aluminides B. Electrical resistance and other electrical properties
1. Introduction Iron aluminides have good oxidation, corrosion and wear resistance, relatively low density and material cost. These materials exceed in some properties even stainless steels; moreover, they offer a possibility of substituting strategic elements such as nickel and chromium. Iron aluminides in spite of long-range order exhibit high electrical resistivity, which is higher than for most commercial heating elements. Thus, the optimal combination of oxidation resistance and high electrical resistivity of iron aluminides make them ideally suited for resistive-heating applications [1]. The structural phase diagram of pure Fe–Al system is rather complex [2]. In the Fe-rich side, the two ordered phases B2 (FeAl) and D03 (Fe3Al) and disordered body-centered a – phase are mutually separated by two-phase regions a þ B2 and a þ D03 [2]. At room temperature the Fe–Al alloy with 20 at.% Al belongs to the a þ D03 region, around 23 at.% Al the structure becomes single phase D03, at about 37 at.% Al a line of second order transition separates the D03 region and B2 region, persisting above 40 at.% Al. The presence of magnetic order makes the situation in Fe–Al system even more complicated. The magnetic phase diagram shows paramagnetic, ferromagnetic and spin-glass regions, which meet at a multicritical point near 30 at.% Al at low temperatures
* Corresponding author. Tel.: þ420 605483777; fax. þ420 485353631. E-mail address: [email protected] (A. Pazourek). 0966-9795/$ – see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2010.01.019
below 200 K [3]. The Curie temperature decreases from about 1000 K at 20 at.% Al to the multicritical point, and above 30 at.% Al ferromagnetic order disappears due to competition between nearest-neighbor Fe–Fe ferromagnetic exchange and an indirect Fe–Al–Fe antiferromagnetic superexchange [3]. The purpose of this paper is to present the influence of often used alloying elements (C, Cr, Ti, Zr, B) on electrical resistivity of complex alloys based on iron aluminides with 20–40 at.% Al, with a potential for future use in technical applications. 2. Experimental methods All Fe–Al alloys with additional alloying elements are complex alloys for future technical use. These alloys were prepared by vacuum chill casting in induction furnace, the castings were hotrolled at 1423 K and slowly cooled in air to room temperature. Binary Fe–Al alloys were prepared from pure elements (Fe (3N), Al (5N)) by repeated melting in laboratory arc furnace in argon atmosphere prior to drop casting them in water cooled copper crucibles. Results of the chemical analysis of all alloys are summarized in Table 1. Specimens for electrical resistivity measurement were cut from as-cast material by diamond saw or spark erosion. The final form of specimens was obtained by mechanical polishing. From each material three specimens were prepared and then contacted with four Pt-wires (0.3 mm diameter) by spot welding. Each specimen
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A. Pazourek et al. / Intermetallics 18 (2010) 1303–1305
Table 1 Chemical composition of alloys (at.%). Al 40.0 40.0 40.0 39.9 39.2 39.1 37.0 35.0 33.0 30.0 28.0 27.8 27.7 26.5 25.0 24.5 20.0 19.9
C
Cr
Ti
3.0 1.00 0.95 0.99 0.93
Zr
B
0.20
0.007 0.009
0.03 0.77 0.48
3.5 2.7 3.6
0.425 0.009
0.49 0.18 0.27
1.09 0.01
4.74
2.7
0.008
0.22
0.01
0.06
4.2
0.074
0.49
0.28
Fe Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance Balance
was subsequently controlled for surface cracks by optical microscopy. Resistivity measurement was carried out for a well defined room temperature in acetone (291 K) (RT) and in liquid nitrogen (77 K) (LNT). Each specimen was measured twice in both current directions. Measurements in the as-prepared (AP) state were repeated for all specimens after their heat treatment (723 K/120h) (HT) in vacuum to remove excess vacancies and enable full equilibrium long-range order. A similar 5 day anneal is often used for Fe–Al alloys according to Nagpal and Baker [4]. 3. Results and discussion 3.1. Electrical resistivity at RT Fig. 1 shows the electrical resistivity, measured at RT, in AP (open symbols) and HT state (full symbols), respectively, as a function of Al content. 3.1.1. Binary Fe–Al alloys The values of electrical resistivity of binary Fe–Al alloys are higher in AP state than in HT state, mainly for 25, 28 and 30 at.% Al.
Fig. 1. Electrical resistivity at RT as a function of Al content for Fe–Al-based materials binary Fe–Al (B, C), first group with additions of C, Ti (6, :), second group with additions of C, Ti, B, Zr and Cr (,,-). The open and full symbols indicate data measured in AP state and HT state, respectively. The full line connecting points serves as a guide for the eye, the dashed line represents literature data [5]. The error bar (right) shows the absolute error of plotted data.
The data for HT state are in quite good agreement with literature data (dashed curve) for long-range ordered Fe–Al alloys [5], the higher value for 20 at.% Al is due to disordering in the two-phase (a þ D03) matrix [6]. This corresponds also to the fact that for 20 at.% Al there is no difference between AP and HT values, indicating negligible contribution of the ordered D03 phase to the resistivity of this alloy. On the other hand the highest difference between AP and HT values was observed for stoichiometric Fe3Al, above 25 at.% Al the effect of the HT decreases with decreasing long-range order in the D03 structure. In the B2 structure practically no effect of the HT was observed, the resistivity is controlled by structural order, and not by the vacancy concentration. The maximum resistivity near 37 at.% Al is in agreement with the previous resistivity data published in [1,5,6], it corresponds with the lowest structural long-range order in the matrix (the D03 structure becomes B2 at room temperature for this composition). The further increase of Al content increases the long-range order and lowers the resistivity in B2 structure. 3.1.2. Fe–Al alloys with additional alloying elements Two groups of technical alloys with additional alloying elements were measured – the first group with addition of C and Ti in B2 structure and the second group with additions of C, Ti, B, Zr and relatively high concentration of Cr in D03 or a structure. Additionally to the second group one material was prepared from carbon free iron with 40 at.% Al and addition of Cr and Zr. The addition of C and Ti in B2 structure surprisingly lowers the resistivity below the corresponding value for the binary alloy. This effect was not observed, when the material was prepared from carbon free iron (see Fig. 1). The reason for this can be found in ternary Fe–Al–C diagram [7]. The real concentration of Al in the Fe– Al matrix increases due to precipitation of the k-phase Fe3AlCx (x w 0.5). Supposing the resistivity is determined mainly by the matrix with negligible content of carbon, the points marked in Fig. 1 by triangles should be shifted by about 1.5 at.% Al to higher concentrations. This is in good agreement with data for binary alloys. The additions in D03 or a structure raise the resistivity of the alloy significantly. The addition of chromium seems to have the decisive effect among used additions, because most of the Cr atoms replace Fe atoms in the matrix [6], whereas most of the other alloying elements tend to form precipitates, which have a minor influence on the resulting resistivity of the alloy. The highest value of electrical resistivity was obtained for the alloy with 24.5 at.% Al
Fig. 2. Electrical resistivity at LNT as a function of Al content for Fe–Al-based materials, the symbols are the same as in Fig. 1. The error bar (right) shows the absolute error of plotted data.
A. Pazourek et al. / Intermetallics 18 (2010) 1303–1305
1305
order and temperature dependent contribution to resistivity in Fe–Al very probably reflects a complex scattering process of conduction electrons in the Fe–Al matrix, where the resistivity depends on structural, spin and thermal disorder (see ref. [6]) and on the nature of density of states at the Fermi level (see ref. [8]). These contributions are not independent and cannot be separated. 3.2.2. Fe–Al alloys with additional alloying elements The results at LNT are qualitatively very similar to that at room temperature. In the case of complex alloys, the difference between resistivity at RT and LNT (Fig. 3) in contrast to binary alloys does not show the above described correlation with long-range order. The values of this difference for all measured alloys lie in the range of values determined for binary alloys. 4. Conclusions Fig. 3. Difference of electrical resistivity at RT and LNT as a function of Al content for Fe–Al-based materials, the symbols are the same as in Fig. 1. The error bar (right) shows the absolute error of plotted data.
(see Table 1 for the composition). The high resistivity value can be seen as a combined effect of the precipitation of the k-phase Fe3AlCx, shifting the Al-concentration near ideal stoichiometry, and solute effect of Cr in the Fe-sublattice. The solute effect is not related to the structural disorder in the D03 matrix only, the addition of transition elements such as Cr and mainly V causes that the resistivity increases rapidly at low temperatures, forming a maximum near the Curie temperature (well above RT for Fe3Al with addition of Cr) and a negative resistivity slope at higher temperatures [6]. The electrical resistivity of these alloys is influenced by complex changes in the electronic structure of the matrix with atomic and magnetic long-range order. 3.2. Electrical resistivity at LNT Fig. 2 shows the electrical resistivity, measured at LNT, in AP (open symbols) and HT state (full symbols), respectively, as a function of Al content. 3.2.1. Binary Fe–Al alloys The results at LNT are qualitatively very similar to that at room temperature. Plotting the difference between resistivity as measured at RT and LNT, respectively, versus concentration (Fig. 3), the dependence is qualitatively opposite to that shown in Fig. 1 (RT) and 2 (LNT). Where local minima are obtained in Figs. 1 and 2 (25 at.% Al and 40 at.% Al), local maxima result for the difference of both curves and vice versa. It is concluded that for high degrees of order (25 at.% Al and 40 at.% Al) the temperature-dependent contribution to resistivity is high, whereas for low degree of order (20 at.% Al and 35 at.% Al) it is low. We presume that these findings are not simply related to an interaction between structural and magnetic order, because for concentrations higher than 30 at.% Al the Fe–Al alloys should not be magnetically ordered [3], in spite of some theoretical predictions for ferromagnetic ground state in FeAl [8,9], strain-induced and disorder-induced ferromagnetism in B2 structure [10,11]. The observed correlation between degree of
Comparison of resistivity data of technical Fe–Al-based materials with that of pure binary alloys has shown that additions responsible for formation of precipitates have only a minor influence on the resistivity of the material. A significant influence, however, arises from the addition of chromium in the D03 structure, the substitution of iron atoms by transition elements in the D03 matrix seems to be the easiest way to increase the resistivity of technical Fe–Al-based materials. The addition of C in alloys with B2 structure lowers the resistivity of the matrix due to k-phase Fe3AlCx precipitation. A strong correlation was observed between the temperature dependent part of electrical resistivity and long-range order in pure binary Fe–Al alloys. Acknowledgement This research has been partly supported by the Ministry of Education, Youth and Sports of the Czech Republic (Project MSM0021620834) and the Czech Science Foundation (Project GACR 106/06/0019). One of authors (A. P.) would like to thank to the people from University of Vienna (Prof. Dr. Bogdan Sepiol, Dipl.-Ing. Witold Wroczewski, Mag. David Geist, Ing. Andreas Berger and Mag. Dr. Erhard Schafler) and to Ing. David Pospı´sˇil from Technical University of Liberec for kind assistance and help with experiments and preparation of specimens. References [1] Lilly AC, Deevi SC, Gibbs ZP. Mater Sci Eng A 1998;258:42–9. [2] Massalski TB, editor. Binary alloy phase diagrams. Metals Park, USA: ASM International; 1986. p. 111. [3] Shukla P, Wortis M. Phys Rev B 1980;21:159–64. [4] Nagpal P, Baker I. Metall Trans 1990;21A:2281–2. [5] Kass M, Brooks CR, Falcon D, Basak D. Intermetallics 2002;10:951–66. [6] Nishino Y. Mater Sci Eng A 1998;258:50–8. [7] Palm M, Inden G. Intermetallics 1995;3:443–54. [8] Das GP, Rao BK, Jena P, Deevi SC. Phys Rev B 2002;66. 184203–1–13. [9] Arzhnikov AK, Dobysheva LV, Timirgazin MA. J Magn Magn Mater 2008;320:11904–8. [10] Wu D, Munroe PR, Baker I. Phil Mag 2003;83:295–313. [11] Menendez E, Liedke MO, Fassbender J, Gemming T, Weber A, Heyderman LJ, et al. Small 2009;5:229–34.