crystals Review A Review on the Properties of Iron Aluminide Intermetallics Mohammad Zamanzade 1,*, Afrooz Barnoush 2 and Christian Motz 1 Received: 31 August 2015; Accepted: 4 January 2016; Published: 14 January 2016 Academic Editor: Duc Nguyen-Manh 1 Department of Material Science and Methods (MWW), Saarland University, Saarbrücken D-66041, Germany; [email protected] 2 Department of Engineering Design and Materials, Norwegian University of Science and Technology, Trondheim No-7491, Norway; [email protected] * Correspondence: [email protected]; Tel.: +49-681-302-5163; Fax: +49-681-302-5015 Abstract: Iron aluminides have been among the most studied intermetallics since the 1930s, when their excellent oxidation resistance was first noticed. Their low cost of production, low density, high strength-to-weight ratios, good wear resistance, ease of fabrication and resistance to high temperature oxidation and sulfurization make them very attractive as a substitute for routine stainless steel in industrial applications. Furthermore, iron aluminides allow for the conservation of less accessible and expensive elements such as nickel and molybdenum. These advantages have led to the consideration of many applications, such as brake disks for windmills and trucks, filtration systems in refineries and fossil power plants, transfer rolls for hot-rolled steel strips, and ethylene crackers and air deflectors for burning high-sulfur coal. A wide application for iron aluminides in industry strictly depends on the fundamental understanding of the influence of (i) alloy composition; (ii) microstructure; and (iii) number (type) of defects on the thermo-mechanical properties. Additionally, environmental degradation of the alloys, consisting of hydrogen embrittlement, anodic or cathodic dissolution, localized corrosion and oxidation resistance, in different environments should be well known. Recently, some progress in the development of new micro- and nano-mechanical testing methods in addition to the fabrication techniques of micro- and nano-scaled samples has enabled scientists to resolve more clearly the effects of alloying elements, environmental items and crystal structure on the deformation behavior of alloys. In this paper, we will review the extensive work which has been done during the last decades to address each of the points mentioned above. Keywords: aluminide intermetallics; alloy design; brittleness and ductility; corrosion; oxidation; hydrogen embrittlement 1. Introduction Transition metal (TM)—aluminide intermetallics including TiAl, NiAl, FeAl and Fe3Al have unique properties, e.g., high melting points, enhanced oxidation resistance, relatively low density, and can be used as soft magnetic materials [1–7]. Early TM-aluminides have fcc-based crystal structure in contrast to the bcc-based crystal structure of late transition metal alloys. Due to the strongly attractive chemical bonding between the bi-metallic species, they are ordered and have stoichiometry. However, the energy of interatomic bonds differs from the early TM (TiAl, VAl) to the late TM alloys (CoAl, NiAl and FeAl). The calculated heats of formation for aluminides with equiatomic composition are plotted in Figure1[ 8]. The heat of formation is remarkable for the case of Ni aluminides in comparison to the Ti aluminides [9]. However, in both cases Al atoms act as an electron donor. The middle TM aluminides are shown to have much less charge transfer and the lowest degrees of ordering. In the case of late TM alloys, charge transfer from Al to the TM and hybridization between Al sp and transition Crystals 2016, 6, 10; doi:10.3390/cryst6010010 www.mdpi.com/journal/crystals Crystals 20162016,, 66,, 1010 22 of of 2928 degrees of ordering. In the case of late TM alloys, charge transfer from Al to the TM and hybridization metalbetween d-states Al sp areand the transition driving metal force d-states in bonding are the [8]. driving Therefore, force the in Fe-Albonding neighbors [8]. Therefore, are energetically the Fe–Al favoredneighbors and are Al-Al energetically neighbors favored are avoided. and Al–Al neighbors are avoided. Figure 1. 1. HeatsHeats of formation of formation for transition-metal for transition-metal aluminides aluminides (with equiatomic (with equiatomic composition). composition). Square Squareand circle and symbols circle symbols are experimental are experimental and theore and theoreticaltical values, values, respective respectively.ly. Re-plotted Re-plotted from from the thereference reference [8]. [8]. Aluminides were first first aimed to be usedused atat highhigh temperaturestemperatures insteadinstead of Ni-basedNi-based superalloys. Hence, some of the thermo-mechanicalthermo-mechanical properties of the aluminides have been reviewed and compared with the CMSX-4 Ni-basedNi-based superalloyssuperalloys inin FigureFigure2 2[ [10–13].10–13]. The six major advantages of aluminides for high temperaturetemperature structural applications over the Ni-basedNi-based superalloyssuperalloys include: (i) higher meltingmelting temperatures where the meltingmelting temperatures definedefine the upper limit of thethe serviceservice temperature and are indicators of of the the temperature temperature range range at at which which diff diffusion-controlledusion-controlled processes processes start start to dominate; to dominate; (ii) (ii)lower lower densities densities where where the lower the lower densities densities (especially (especially in the case in the of caseγ-TiAl of intermetallics)γ-TiAl intermetallics) result in resultlower inoperating lower operating stresses that stresses make that it possible make it possibleto fabricat toe fabricatesmaller and smaller lighter and comp lighteronents components which, in which, turn, inresult turn, in resultbetter inengine better accelerations engine accelerations due to the due lower to the mass lower of the mass rotating of the parts; rotating (iii) parts;better (iii)oxidation better oxidationresistance resistancebecause of becausethe high ofaluminum the high aluminumcontent; (iv) content; lower ductile (iv) lower to brittle ductile transition to brittle temperature transition temperature(DBTT); (v) similar (DBTT); thermal (v) similar expansion thermal coefficients expansion as coefficients the bcc steels as the and bcc (vi) steels low andcosts (vi) of lowproduction, costs of production,since they do since not they generally do not generallyincorporate incorporate rare and rarestrategic and strategicelements elements [10–13]. [10In– 13contrast,]. In contrast, the three the threenegative negative features features of the ofintermetal the intermetallicslics at elevated at elevated temperatures temperatures are (i) arelow (i) strength; low strength; (ii) limited (ii) limited creep creepresistance; resistance; and (iii) and high (iii) highthermal thermal conductivity. conductivity. At Atlow low to tomoderate moderate temperatures, temperatures, most most of of the intermetallics additionally suffer suffer from from poor ductility (Figure (Figure 22)) andand lowlow fracturefracture toughnesstoughness [[14,15].14,15]. These problems significantlysignificantly impedeimpede thethe widewide usage of intermetallics because the machining of alloys (at(at lowlow temperature)temperature) becomesbecomes veryvery difficult.difficult. Liu etet al.al.[ 16[16]] presented presented the the ductile ductile behavior behavior of aluminidesof aluminides in dry in oxygen,dry oxygen, where where the fracture the fracture strain isstrain about is 17.6%about for17.6% Fe-36.5 for at.Fe-36.5 % Al. at. In % general, Al. In general, increasing increasing the Al concentration the Al concentration decreases thedecreases density the of materialsdensity of and materials enhances and the enhances protective the oxide protective layer atox highide layer temperatures at high temperatures [17,18]. However, [17,18]. the However, existence ofthe high existence aluminum of high concentration aluminum hasconcentration negative side has effects negative [19,20 side]. The effects reaction [19,20 of]. Al The atoms reaction with waterof Al resultsatoms with in the water production results of in hydrogen the production atoms, whichof hydrogen are responsible atoms, which for the are low responsible ductility of Fe-Alfor the based low intermetallicductility of Fe-Al alloys based in moisture-containing intermetallic alloys atmospheres in moisture-containing [21–24] (Figure atmospheres3). [21–24] (Figure 3). Crystals 2016, 6, 10 3 of 29 Crystals 2016, 6, 10 3 of 28 Crystals 2016, 6, 10 3 of 28 Figure 2.FigureMechanical 2. Mechanical properties properties of transition-metalof transition-metal aluminides aluminides [10–13]. [10–13 (a].) Equiatomic (a) Equiatomic composition composition (e.g., 1 Fe:1 Al) and (b) three TM: 1 Al compositions. Melting temperature of CMSX-4 Ni-based (e.g., 1 Fe:1 Al) and (b) three TM: 1 Al compositions. Melting temperature of CMSX-4 Ni-based super-alloy ≈ 1450 °C, Density ≈ 8.70 gr/cm3, Ductility ≈ 3%. ˝ 3 super-alloyFigure« 1450 2. MechanicalC, Density properties« 8.70 of gr/cmtransition-metal, Ductility aluminides« 3%. [10–13]. (a) Equiatomic composition The(e.g., rate 1 Fe:1of a Al) charge-transfer and (b) three TM: process 1 Al compositions. such as that Melting of hydrogen temperature adsorption of CMSX-4 depends Ni-based on the Thepotential ratesuper-alloy of across a charge-transfer the ≈ 1450 sample-solution °C, Density process≈ 8.70 (environment) gr/cm such3, Ductility asinte ≈ that rface,3%. ofwhich hydrogen can