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Creep of Heusler-Type Alloy Fe-25Al-25Co

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Creep of Heusler-Type Alloy Fe-25Al-25Co crystals Article Creep of Heusler-Type Alloy Fe-25Al-25Co Ferdinand Dobeš 1, Petr Dymáˇcek 1,2 and Martin Friák 1,2,* 1 Institute of Physics of Materials, Academy of Sciences of the Czech Republic, Žižkova 22, CZ-61662 Brno, Czech Republic; [email protected] (F.D.); [email protected] (P.D.) 2 CEITEC IPM, Institute of Physics of Materials, Academy of Sciences of the Czech Republic, Žižkova 22, CZ-61662 Brno, Czech Republic * Correspondence: [email protected]; Tel.: +420-532-290-400 Received: 20 December 2019; Accepted: 16 January 2020; Published: 20 January 2020 Abstract: Creep of an alloy based on the intermetallic compound Fe2AlCo was studied by compressive creep tests in the temperature range from 873 to 1073 K. The stress exponent n and the activation energy of creep Q were determined using the multivariable regression of the creep-rate data and their description by means of sinh equation (Garofalo equation). The evaluated stress exponents indicate that the dislocation climb controls creep deformation. The estimated apparent activation energies for creep are higher than the activation enthalpy for the diffusion of Fe in Fe3Al. This can be ascribed to the changes in crystal lattice and changing microstructure of the alloy. Keywords: iron aluminides; Heusler alloy; creep; stress exponent; activation energy 1. Introduction Fe–Al-based alloys are potential candidates for new structural materials due to their outstanding corrosion resistance in various hostile environments, relatively low density and the low cost of both the basic constituting elements [1–3]. Insufficient creep resistance has been identified as an obstacle for their extensive application at high temperatures [4,5]. A number of methods have been proposed to overcome this problem. These include solid-solution hardening, strengthening by second-phase particles or ordering [6,7]. A promising method of strengthening is the addition of an element that leads to the formation of the Heusler compound [8,9]. These compounds are currently attracting research attention due the many interesting properties they possess. In the Fe–Al system, the compound can be formed by the addition of 3d-transition elements: Sc, Ti, V, Cr, Mn, Co, Ni, Cu, and Zn were considered theoretically by Gilleßen and Dronskowski [10]. Among these elements, Co leads to the largest increase of Young’s modulus, and Voigt shear modulus [11]. A very high Curie temperature of 857 ◦C was also found in the Fe2CoAl compound [12]. In the previous study, quantum-mechanical calculations were used to investigate properties of four different polymorphs of Fe2AlCo and the occurrence of structural multiplicity at elevated temperatures was predicted [13]. Very few studies have experimentally investigated mechanical properties of Heusler compounds [14]. As far as we know, only Fe2AlTi-containing alloys were subjected to mechanical testing at elevated temperatures within the Fe–Al system [15–17]. The present study examines the creep behavior of a Fe2AlCo-based alloy and describes it in terms of the Garofalo equation [18]. The results are interpreted in terms of the available ternary phase diagrams and compared with the results of testing Fe–Al-Ti alloys. The Fe2AlCo has, under 1000 K, a two-phase superalloy nanostructure with cuboids of a partly ordered phase coherently co-existing in a differently (or less) ordered matrix. The ordered phase has not the full-Heusler lattice (the L21 structure as a ternary analogue of D03) because quantum-mechanical calculations showed that it is mechanically unstable [13] and the inverse-Heusler structure, cf., Figure1a, is both mechanically and thermodynamically more stable [10,13]. The inverse-Heusler is also in a better Crystals 2020, 10, 52; doi:10.3390/cryst10010052 www.mdpi.com/journal/crystals Crystals 2020, 10, x FOR PEER REVIEW 2 of 9 Crystals 2020, 10, 52 2 of 9 high-temperature range, the arrangement of atoms may become even less ordered, exhibiting B2 and Crystals 2020, 10, x FOR PEER REVIEW 2 of 9 A2agreement lattices (Figure with experiments 1b,c). (see, e.g., Ref. [19]). In the high-temperature range, the arrangement of atomshigh-temperature may become even range, less the ordered, arrangement exhibiting of atoms B2 ma andy become A2 lattices even (Figureless ordered,1b,c). exhibiting B2 and A2 lattices (Figure 1b,c). 2 FigureFigure 1.1. SchematicSchematic 1. Schematic visualizations visualizations visualizations of atomof of atom atom configurations configurationsconfigurations in Fe in2inAlCo: FeFe2AlCo:AlCo: an inverse-Heusler an an inverse-Heusler inverse-Heusler (a), a(a partially), a(a ), a partiallyorderedpartially B2ordered (b ordered) and B2 a ( disorderedB2b) (andb) and a disordered a A2disordered (c). A A2 schematic A2 ( c(c).). A A schematicschematic of the used of of the coloringthe used used coloring ofcoloring atoms of atoms (ofd ).atoms (d). (d). AlthoughAlthough thethe compositionthecomposition composition of of the ofthe studiedthe studied studied alloy alloyalloy is close is closeclose to the toto stoichiometric the the stoichiometric stoichiometric composition composition composition of Heusler of of Heusleralloys,Heusler we alloys, refer alloys, we to ourreferwe refer alloy to ourto asour alloy the alloy Heusler-type as as the the Heusler-type Heusler-type alloy. In alloy.alloy. accordance In In accordance accordance with with the with the more themore recentmore recent recent phase phasediagramsphase diagrams (see diagrams below), (see (see webelow), below), assume we we thatassume assume the A2 thatthat and thth B2ee latticesA2 andand coherently B2B2 lattices lattices coexistcoherently coherently at the coexist nanoscalecoexist at the at level the nanoscale level (cuboids with the size of dozens of nm embedded in a matrix) at high temperatures. nanoscale(cuboids with level the (cuboids size of with dozens the of size nm of embedded dozens of in nm a matrix)embedded at high in a temperatures.matrix) at high temperatures. 2. Materials and Methods 2. Materials Materials and and Methods Methods The alloy was prepared using vacuum induction melting and casting under argon into a The alloy was prepared using vacuum induction melting and casting under argon into a cylindrical Thecylindrical alloy castwas of prepared diameter 20using mm andvacuum exploitable induction length melting of 200 mm. and The casting nominal under composition argon ofinto a cylindricalcast ofthe diameter alloy, cast 25 of at. 20 diameter % mm Al and and 2025 exploitable at.mm % Co,and is exploitable marked length ofin parts 200lengthmm. of isothermal of The200 nominalmm. sections The composition nominalof the ternary compositionof Fe–Al– the alloy, of the25at. alloy,Co % diagram Al 25 and at. % 25 in Al at.Figure and % Co 252 ,[20]. isat. marked %The Co, grains is in marked parts are large of in isothermal andparts have of isothermalan sections elongated of sections the shape ternary with of the Fe–Al–Coa mean ternary aspect diagram Fe–Al– Coin Figurediagramratio2 of[ 4:1;20in ].Figure the The mean grains2 [20].width areThe is largeapproximately grains and are have large 250 an μandm, elongated seehave Figure an shape elongated3. with ashape mean with aspect a mean ratio aspect of 4:1; ratiothe mean of 4:1; width the mean is approximately width is approximately 250 µm, see 250 Figure μm,3. see Figure 3. 40 60 40 60 a) 873 K b) 923 K 40 60 40 60 50 50 50 50 a) 873 K B2 b) 923 K B2 50 50 5060 50 40 60 40 a a e t e t. F F B2 D0 % B2 % 3 % C % C . t 70 40 30 o t 70 60 4030 o a 60 a a a 0 3 e t e D t. F + F % D0 A2 % 3 % 2 C % C . 80 B 20 80. 20 t 70 2+ 30 o t 70 30 o a A 2 a A2+B2 +B0 3 A2D 2+ 90 A 10 90 10 B2 80 2+ 20 80 20 A2A 2 B2+B2* B2 A2 A2+B2 B2 +B B2+B2* B2* 2 100 A A2+B2* 0 100 0 90 10 90 10 0 102030405060 0 102030405060 A2 at. %B2+B2* Al B2 A2 at. % Al B2 B2+B2* B2* 100 A2+B2* 0 100 0 Figure 2. Details of the isothermal sections of the Fe–Al–Co system at (a) 873 K, calculated, and (b) 9230 K, experimental, 102030405060 according to Kozakai et al. [20]. 0 102030405060 at. % Al at. % Al Figure 2. Details ofof thethe isothermal isothermal sections sections of of the the Fe–Al–Co Fe–Al–Co system system at (a at) 873 (a) K,873 calculated, K, calculated, and ( band) 923 (b K,) 923experimental, K, experimental, according according to Kozakai to Kozakai et al. [20 et]. al. [20]. Crystals 2020, 10, 52 3 of 9 Crystals 2020, 10, x FOR PEER REVIEW 3 of 9 FigureFigure 3. BackscatteredBackscattered electron electron micrograph micrograph of meta metallographicllographic section of the uncrept alloy. Creep tests were performed in uniaxial compressioncompression on samples with a gauge length of 10 mm 2 and a cross-section of 5 × 5 mm2.. The The compressive compressive testing testing allows allows creep creep deformation deformation not not affected affected by × the onset of fracture processes to bebe studied.studied. On On the the other hand, because of of the need for a specificspecific compressive cage (reversible grips), the test arra arrangementngement does not allow rapid cooling of the test specimen, and not even cooling under the eeffectffect of load. Effective Effective observation of the microstructure after creep is, therefore,therefore, limited.limited. The The samples were prepared by travelling wire electro-discharge machining and fine fine grinding of the surfaces. The tests were performed under a constant load on a dead-weight creepcreep machine machine that that was was constructed constructed in-house in-house in a protectivein a protective atmosphere atmosphere of dry purified.
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