Iron aluminide

Iron aluminides are intermetallic compounds of iron and aluminium - they typically contain ~18% Al or more.

For the ~50% Al ferroalloy used in ferrous metallurgy, see Ferroaluminum.

Good oxide and sulfur resistance, with strength comparable to steel alloys, and low cost of materials have made these compounds of metallurgical interest - however low ductility and issues with hydrogen embrittlement are barriers to their processing and use in structural applications.

Overview

High corrosion resistance of Iron alloys containing more than 18% aluminium was first noted in the 1930s.[1] Their tensile strength compares favorably with steels, whilst utilizing only common elements; however they have low ductility at room temperature, and strength drops of substantially over 600 °C.[1] The alloys also have good sulfide and oxidation resistance, good wear resistance, and lower density than steels.[2] Peak strength and hardness is reached at the Fe₃Al stoichiometric region.[1] Although Al gives corrosion resistance via an oxide film surface, reaction (with water) may also give rise to embrittlement via hydrogen produced in the reaction between Al and H₂O.[1]

Chromium (2-6%) improves room temperature ductility, though the mechanism is (As of 1990) not fully understood - it may reduce hydrogen embrittlement possibly via its ability to stabilise the FeAl phase.[1] Chromium may also facilitate slipping via crystal dislocations; and its contribution to surface passivation may also aid ductility by preventing embrittling water reactions.[3] A disordered alloy (designated FAPY) containing ~16% Al, ~5.4% Cr plus ~0.1% Zr, C, and Y, with ~1% Mo showed much improved ductility, only dropping substantially under ~200C (cf 650C for Fe₃Al) - this alloy is also cold workable.[2]

Phases

Below ~18-20% (atomic) Al the aluminium exists as a solid solution in iron. Above this concentration there are FeAl (B3 phase) and Fe₃Al (DO₃ phase) existing in the form of caesium chloride (CsCl) and α-bismuth trifluoride (BiF₃) crystal structures.[1] Above ~550 °C the Fe₃Al phase is transformed in FeAl (and Fe).[3]

Above ~50% Al (atomic) Fe₅Al₈, FeAl₂, Fe₂Al₅, and Fe₄Al₁₃ are also known - the Al rich phases show high brittleness.[3]

Preparation

The reaction between Al and Fe to generate iron aluminide is exothermic. Production from direct melting of Al and Fe is economical, but any water in the charge produces issues with the generation of hydrogen which shows solubility in the iron aluminide, leading to gas voids. Blowing with argon or vacuum melting alleviates this.[2]

Large grain size is greatly deleterious to ductility, especially with Fe₃Al, and is encountered in cast iron aluminides.[2]

Coatings of iron aluminide can be prepared by chemical vapor deposition onto iron.[4]

Uses

Potential uses for iron alumides include : electrical heating elements, piping and other work for high temperature process including piping for coal gasification and for superheater and re-heater tubes.[1] It has also been suggested as a structural material for lunar use.[5]

References

1. McKamey, C. G. (1996), "Iron Aluminides", Physical Metallurgy and processing of Intermetallic Compounds, pp. 351–391
2. Sikka, Vinod K. (1994), "Processing and Applications of Iron Aluminides", TMS Annual Meeting Proceedings Publications
3. Zamanzade, Mohammad; Barnoush, Afrooz; Motz, Christian (2016), "A Review on the Properties of Iron Aluminide Intermetallics", Crystals, 6 (10): 10, doi:10.3390/cryst6010010
4. John, J.T.; Sundararaman, M.; Dubey, V.; Srinivasa, R.S. (2013), "Structural characterisation of iron aluminide coatings prepared by chemical vapour deposition", Materials Science and Technology, 29 (3): 357–363, doi:10.1179/1743284712Y.0000000105
5. Landis, Geoffrey A. (2006), Materials Refining for Solar Array Production on the Moon

External links

• Raebe, D., Iron-Aluminides