Communication pubs.acs.org/IC
Synthesis, Crystal Structure, Magnetic Properties, and Stability of the Manganese-Rich “Mn3AlC” κ Phase Hannes Dierkes, Jan van Leusen, Dimitri Bogdanovski, and Richard Dronskowski* Institute of Inorganic Chemistry, RWTH Aachen University, Landoltweg 1, D-52056 Aachen, Germany S Supporting Information *
ABSTRACT: κ-carbides of varying composition, seemingly responsible for age hardening in high-Al steel alloys, have been detected to precipitate both at grain boundaries and in the bulk grain of steels. Herein we report the bulkphase synthesis of “Mn3AlC” by arc plasma sintering and rapid solidification. Single crystals have been found suitable for X-ray diffraction using Mo radiation and yield a lattice parameter of a = 3.875(2) Å. We find a mixed occupation of the 1a position by Al and Mn, which, together with the C position being fully occupied, leads to the actual composition Mn3.1Al0.9C. Additional energydispersive X-ray−scanning electron microscopy measurements support the composition and corroborate the homogeneity. SQUID data collected on the polycrystalline ferromagnetic sample exhibit a Curie temperature of about 295 ± 13 K and a soft magnetic behavior. The small but significant nonstoichiometry on 1a leads to a slightly larger lattice parameter, a higher electron count, and, thus, a lowered density of states at the Fermi level, indicative of increased phase stability.
Figure 1. Crystal structure of Mn3.1Al0.9C with octahedrally coordinated Mn atoms (black) and cuboctahedrally coordinated Al/Mn atoms (white/black at the corner positions) and C atoms (yellow).
carbides form in austenitic alloys with at least 6.2 wt % Al and 1.0 wt % C, while intergranular κ-carbides precipitate for at least 5.5 and 0.7 wt %, respectively.9 The formation of κ-carbides in a ferritic matrix through decomposition from metastable austenite has been shown to occur as well, with stoichiometries both different from the austenitic case and further dependent on the annealing temperatures.10 Recent studies suggested that the κ phase not only plays an important role in controlling the mechanical strength and deformation behavior5 but also functions as a H trap to counteract H embrittlement. This hypothesis is supported by the observation that delayed fracture, after H treatment, mainly occurs at the grain boundaries and especially at three-point junctions, exactly where the κ-carbides precipitate.11 Because of the aforementioned phenomena, these carbides have to be fully understood because profound knowledge of their properties is essential to improving rational alloy design. The available studies mostly deal with κ-carbides at the grain boundaries, except a few bulk-material contributions.12,13 There are also theoretical studies targeting the formation energy14,15 or the electronic structure in general16 but no single-crystal diffraction studies. We now succeeded in synthesizing a highpurity bulk material in both powder and single-crystal forms,
he κ-phase carbides MI3MIIC (MI = Fe, Mn; MII = Al, Gd) have recently attained interest in the development of highAl steels (e.g., Fe−Mn−Al−C), which are industrially looked for because of their combined lightweight and extraordinary mechanical properties. It has been suggested that these steels may play an important role in the future of the automotive industry, allowing for a significant reduction in fuel consumption in addition to excellent mechanical performance. Additionally, such materials may function as a substitute for expensive Nicontaining stainless steels.1 In these mainly austenitic Fe−Mn− Al−C alloys, Mn and C increase the austenite stability, while Al works as an agent against corrosion and further improves the oxidation resistance.2 The κ-carbides were first observed and described by Everest et al. and follow a cubic system similar to the austenitic matrix in high-Mn steels.3 Recent electronic-structure calculations of κ-phase cells suggest a thermobarically induced epitaxial diamond growth in high-C Fe−Al alloys along the [200] plane of the κ crystals.4 The crystal structure depicted in Figure 1 belongs to space group Pm3̅m (Strukturbericht L′12) with a = 3.87 Å. Inside the steel matrix, the κ-carbides form nanoscale precipitates during short-time aging and arrange into rods preferably at the grain boundaries and, to a lesser extent, inside grains during longer heat treatment.5−8 This behavior has been studied extensively, and it is clear that the intragranular κ-
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© XXXX American Chemical Society
Received: November 23, 2016
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DOI: 10.1021/acs.inorgchem.6b02816 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry thereby allowing for structural and magnetic investigations. “Mn3AlC” was prepared by the direct reaction of stoichiometric amounts of the powderous elements (Mn, Chempur, 99.99%; Al, Alfa Aesar, 99.97%; graphite, Alfa Aesar, 99.8%) via arc melting. The powders were thoroughly mixed and pressed into pellets of 5 mm diameter involving three pressing/releasing steps (1, 1.5, and 2.5 tons). The obtained silver lustrous reguli were pulverized in an agate mortar, and single crystals suitable for X-ray diffraction (XRD) were found. As expected, the phase crystallizes in space group Pm3̅m with a = 3.8750 ± 0.0024 Å. Further syntheses were carried out to also obtain partially Fe-substituted analogues of the title compound, and, indeed, products in high yields were obtained. Nonetheless, no suitable single crystals have been found so far. Thus, in the following we will concentrate on “Mn3AlC”. A suitable single crystal was measured on a Bruker D8 goniometer with an APEX area detector equipped with an Incoatec microsource (Mo Kα1 radiation, λ = 0.71073 Å, multilayer optics). Intensity data were integrated with SAINT17 and corrected for absorption by multiscan methods.18 The structure is known, so full-matrix least-squares refinements were accomplished with SHELXL97.19 Puzzlingly, the occupancy of the 1a position had to be refined with a mixed occupation by Mn and Al, arriving at a 11:89 ratio [Mn, 0.109(14); Al, 0.891(14)], although Wyckoff position 3c is exclusively occupied by Mn. A similar behavior has been described for the pure Fe phase;12 however, it has not been considered in newer CALPHAD calculations. Somewhat surprisingly, the 1b site is f ully occupied by C, 0.97(4), so the new result differs from earlier findings where the κ-carbides at grain boundaries of steel sheets showed a more variable occupation of C as calculated20 and also suggested using powder XRD.21 Further details on the structure determination may be obtained from Fachinformationszentrum Karlsruhe, Eggenstein-Leopoldshafen, Germany, upon quoting the deposition number CSD-432260. Magnetic measurements were carried out on a polycrystalline sample using a Quantum Design MPMS-5XL superconducting quantum interference device (SQUID) magnetometer. The magnetic susceptibility data were corrected for the diamagnetic contributions of the sample holder and compound. Temperature-dependent molar susceptibility χm data were collected between 2 and 400 K at fields of 0.01 and 0.1 T. Because of the nonlinearity of the χm−1 versus T data up to 400 K (Figure S1), the Curie temperature is derived from the minimum of the first derivative of the magnetic moment (per formula unit, inset). It is roughly estimated to be 295 ± 13 K and is, thus, in agreement with the expected region of about 288 K.22 In addition, hysteretic data were measured at 5 K, as depicted in Figure 2. The saturation magnetic moment per formula unit is 3.22 μB (Figure S2), the remanent magnetic moment 0.24 μB, and the coercive field 1.9 mT. Therefore, Mn3.1Al0.9C can be safely characterized as being a soft magnetic material. Energy-dispersive X-ray (EDX)−scanning electron microscopy (SEM) measurements were carried out for analytical information. While EDX provides rather rough compositional data, there exist more accurate methods such as our own XRD analysis or atom-probe tomography as employed in other studies.23 Our goal, however, was to evidence that the general composition agrees well with the expected values and is homogeneous. Elemental mapping additionally showed the distribution of Mn, Al, and C, which is depicted in Figure 3. The chemical composition indeed appears as homogeneous over the entire scanned region. In particular, EDX analysis resulted in a
Figure 2. Hysteretic loop (extract) of Mn3.1Al0.9C at 5 K. Inset: First derivative of magnetic moment versus T at 0.1 T.
Figure 3. SEM picture (top) and elemental mapping (bottom) of a polycrystalline sample of Mn3.1Al0.9C.
composition of 25.2 ± 2.5 atom % C (20% expected), 14.8 ± 1.5 atom % Al (17.8%), and 60 ± 3 atom % Mn (62%). A two-step electronic-structure analysis of Mn3.1Al0.9C was performed to elucidate the reason for the mixed Mn/Al occupation of the 1a position. First, structural optimization of a ferromagnetic daltonide Mn3AlC system with full Al, Mn, and C occupation of the 1a, 3c, and 1b Wyckoff sites, respectively, was performed by using the density-functional-theory-based VASP code,24,25 employing a projector-augmented-wave approach,26 and the generalized gradient approximation (GGA) parametrization of the Perdew−Burke−Ernzerhof functional.27 This arrived at a theoretical aopt = 3.805 Å, a bit smaller than the experimental result in which the 1a position is partially occupied with the slightly larger Mn atom. The electronic structure of this system was investigated using tight-binding linear muffin-tin orbital atomic-sphere approximation theory28 with the GGA-based PW91 functional,29 yielding the band structure and electronic density of states (DOS) in terms of local orbitals. Then, the electronic stabilities of Mn3AlC and B
DOI: 10.1021/acs.inorgchem.6b02816 Inorg. Chem. XXXX, XXX, XXX−XXX
Communication
Inorganic Chemistry ORCID
Mn3.1Al0.9C were compared by introducing a rigid-band electroncount shift from 28 electrons (Mn3AlC) to 28.44 electrons (Mn3.1Al0.9C), which corresponds to an energy shift of ΔE = 0.118 eV to the DOS of Mn3AlC. This immediately demonstrates the higher stability of Mn3.1Al0.9C because its DOS shows a welldefined local minimum at the Fermi level (Figure 4). Hence, the
Richard Dronskowski: 0000-0002-1925-9624 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
We thank the Collaborative Research Center (SFB 761) “Steel ab initio” of the Deutsche Forschungsgemeinschaft for funding and Tobias Storp for collecting X-ray data. Additionally, we thank Oliver Schmidt for EDX measurements, Christina Houben for SQUID measurements, Prof. Ulli Englert for crystallographic help, and Dr. Rachid Touzani for discussions concerning the linear muffin-tin orbital theory.
(1) Frommeyer, G.; Brüx, U. Microstructures and mechanical properties of high-strength Fe-Mn-Al-C light-weight TRIPLEX steels. Steel Res. Int. 2006, 77, 627−633. (2) Banerji, S. K. An austenitic stainless steel without nickel and chromium. Metal Prog. 1978, 113, 59−62. (3) Everest, A. B. The Influence of Aluminum on Iron Carbon Alloys. Foundry Trade J. 1927, 36, 169−173. (4) Mekhed, A. A.; Andrushenko, V. A. Calculation of electronic structure of the K-phase cells which are involved in thermobarically induced epitaxial growth of diamond crystals in high-carbon Fe-Alalloys. Solid State Phenom. 2013, 203-204, 402−405. (5) Gutierrez-Urrutia, I.; Raabe, D. High strength and ductile low austenitic FeMnAlC steels: Simplex and alloys strengthened by nanoscale ordered carbides. Mater. Sci. Technol. 2014, 30, 1099−1104. (6) Chao, C. Y.; Hwang, C. N.; Liu, T. F. Grain boundary precipitation in an Fe-7.8Al-31.7Mn-0.54C alloy. Scr. Metall. Mater. 1993, 28, 109− 114. (7) Hwang, C. N.; Chao, C. Y.; Liu, T. F. Grain boundary precipitation in an Fe-8.0Al-31.5Mn-1.05C alloy. Scr. Metall. Mater. 1993, 28, 263− 268. (8) Raabe, D.; Springer, H.; Gutierrez-Urrutia, I.; Roters, F.; Bausch, M.; Seol, J.-B.; Koyama, M.; Choi, P.-P.; Tsuzaki, K. Alloy design, combinatorial synthesis, and microstructure-property relations for lowdensity Fe-Mn-Al-C austenitic steels. JOM 2014, 66, 1845−1856. (9) Chu, C. M.; Huang, H.; Kao, P. W.; Gan, D. Effect of alloying chemistry on the lattice constant of austenitic Fe-Mn-Al-C alloys. Scr. Metall. Mater. 1994, 30, 505−508. (10) Seol, J.-B.; Raabe, D.; Choi, P.; Park, H.-S.; Kwak, J.-H.; Park, C.G. Direct evidence for the formation of ordered carbides in a ferritebased low-density Fe-Mn-Al-C alloy studied by transmission electron microscopy and atom probe tomography. Scr. Mater. 2013, 68, 348− 353. (11) Koyama, M.; Springer, H.; Merzlikin, S. V.; Tsuzaki, K.; Akiyama, E.; Raabe, D. Hydrogen embrittlement associated with strain localization in a precipitation-hardened Fe−Mn−Al−C light weight austenitic steel. Int. J. Hydrogen Energy 2014, 39, 4634−4646. (12) Choo, W. K.; Han, K. H. Phase constitution and lattice parameter relationships in rapidly solidified (Fe0.65Mn0.35)0.83Al0.17−xC and Fe3AlC pseudo-binary alloys. Metall. Trans. A 1985, 16, 5−10. (13) Wen, Y.; Wang, C.; Sun, Y.; Liu, G.; Nie, M.; Chu, L. Lattice, magnetic and electronic transport behaviors of Ge-doped Mn3XC (X= Al, Zn, Ga). J. Alloys Compd. 2010, 489, 289−292. (14) Han, S. Y.; Shin, S. Y.; Lee, B. J.; Lee, S.; Kim, N. J.; Kwak, J. H. Effect of Tempering Time on Microstructure, Tensile Properties, and Deformation Behavior of a Ferritic Light-Weight Steel. Metall. Mater. Trans. A 2013, 44, 235−247. (15) Ohtani, H.; Yamano, M.; Hasebe, M. Thermodynamic analysis of the Fe-Al-C ternary system by incorporating ab initio energetic calculations into the CALPHAD approach. ISIJ Int. 2004, 44, 1738− 1747.
Figure 4. Section of the DOS plot of Mn3.1Al0.9C and Mn3AlC showing the Fermi levels (EF) as solid/dashed lines. The lighter-gray-scale region signifies unoccupied states.
intrinsic Mn enrichment is clearly caused by electronic reasons, and the pseudogap might indicate a phase width such as Mn3+xAl1−xC. In addition, the total magnetic moment of Mn3.1Al0.9C as well as the individual moments of each atomic species were obtained from a VASP simulation. A ferromagnetic 3 × 3 × 1 supercell of Mn3AlC containing nine Al atoms, one of which was then replaced by Mn, was used to model the mixed occupancy with the required ratio. The total magnetic moment of Mn3.1Al0.9C per formula unit is μtot ≈ 3.56 μB, in good agreement with the experimental value of the saturation magnetization determined here with μsat = 3.22 μB. The average magnetic moments of the atoms arrive at −0.05 μB for Al, − 0.10 μB for C, and 1.32 μB for Mn. In summary, the κ-phase “Mn3AlC” has been synthesized in bulk form, with the exact chemical composition being Mn3.1Al0.9C. For the first time, single-crystal X-ray data were collected and corroborate the crystal structure with a lattice parameter of a = 3.8750(2) Å. The new study clearly yields a mixed occupation of the corner position by Al and Mn, which, as clarified by the electronic-structure theory, establishes a greater stability, as mirrored by a lowered DOS at the Fermi level. In addition, the C position is fully occupied. EDX data confirm a homogeneous elemental distribution throughout the sample. Finally, SQUID magnetic measurements suggest a Curie temperature of about 295 ± 13 K and a soft magnetic behavior.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.6b02816. SQUID magnetic susceptibility data (PDF) X-ray crystallographic information in CIF format (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. C
DOI: 10.1021/acs.inorgchem.6b02816 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry (16) Fujii, S.; Okada, M.; Ishida, S.; Asano, S. High Spin Polarizaion of Ferrimagnetic Heusler-type Alloys in Mn−Cr−Z System (Z= IIIb, IVb, Vb Elements). J. Phys. Soc. Jpn. 2008, 77, 074702. (17) SAINT, Program for Reduction of Data collected on Bruker CCD Area Detector Diffractometer, version 6.02; Bruker AXS Inc.: Madison, WI, 1999. (18) SADABS, Program for Empirical Absorption Correction of Area Detector Data, version 2004/1; Bruker AXS Inc.: Madison, WI, 2004. (19) Sheldrick, G. M. A short history of SHELX. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122. (20) Kumar, K. C. H.; Raghavan, V. A thermodynamic analysis of the Al-C-Fe system. J. Phase Equilib. 1991, 12, 275−286. (21) Rao, V. S. Fe3Al-Fe3AlC intermetallics for high temperature applications: An assessment. J. Mater. Sci. 2004, 39, 4193−4198. (22) Butters, R. G.; Myers, H. P. The structure and magnetic properties of the alloy Mn3AlC. London Edinb. Dubl. Philos. Mag. 1955, 46, 895− 902. (23) Yao, M. J.; Dey, P.; Seol, J.-B.; Choi, P.; Herbig, M.; Marceau, R. K. W.; Hickel, T.; Neugebauer, J.; Raabe, D. Combined atom probe tomography and density functional theory investigation of the Al offstoichiometry of κ-carbides in an austenitic Fe-Mn-Al-C low-density steel. Acta Mater. 2016, 106, 229−238. (24) Kresse, G.; Hafner, J. Ab initio molecular dynamics for liquid metals. Phys. Rev. B: Condens. Matter Mater. Phys. 1993, 47, 558−561. (25) Kresse, G.; Furthmüller, J. Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set. Comput. Mater. Sci. 1996, 6, 15−50. (26) Kresse, G.; Joubert, D. From ultrasoft pseudopotentials to the projector augmented-wave method. Phys. Rev. B: Condens. Matter Mater. Phys. 1999, 59, 1758−1775. (27) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized gradient approximation made simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (28) Andersen, O. K.; Jepsen, O. Explicit, first-principles tight-binding theory. Phys. Rev. Lett. 1984, 53, 2571−2574. (29) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Atoms, molecules, solids, and surfaces: applications of the generalized gradient approximation for exchange and correlation. Phys. Rev. B: Condens. Matter Mater. Phys. 1992, 46, 6671−6687.
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DOI: 10.1021/acs.inorgchem.6b02816 Inorg. Chem. XXXX, XXX, XXX−XXX
