Iron aluminides are intermetallic compounds of iron and aluminium - they typically contain ~18% Al or more. 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 alloyscontaining more than 18% aluminium was first noted in the 1930s. 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. The alloys also have good sulfide and oxidation resistance, good wear resistance, and lower density than steels. Peak strength and hardness is reached at the Fe3Al stoichiometric region. Although Al gives corrosion resistance via an oxide film surface, reaction may also give rise to embrittlement via hydrogen produced in the reaction between Al and H2O. Chromium improves room temperature ductility, though the mechanism is not fully understood - it may reduce hydrogen embrittlement possibly via its ability to stabilise the FeAl phase. Chromium may also facilitate slipping via crystal dislocations; and its contribution to surface passivation may also aid ductility by preventing embrittling water reactions. A disordered alloy 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 - this alloy is also cold workable.
Phases
Below ~18-20% Al the aluminium exists as a solid solution in iron. Above this concentration there are FeAl and Fe3Al existing in the form of caesium chloride and α-bismuth trifluoride crystal structures. Above ~550 °C the Fe3Al phase is transformed in FeAl. Above ~50% Al Fe5Al8, FeAl2, Fe2Al5, and Fe4Al13 are also known - the Al rich phases show high brittleness.
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. Large grain size is greatly deleterious to ductility, especially with Fe3Al, and is encountered in cast iron aluminides. Coatings of iron aluminide can be prepared by chemical vapor deposition onto iron.
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. It has also been suggested as a structural material for lunar use.
