Microstructural evolution in iron aluminide Fe-28Al-2C after high-temperature hydrogen treatment

High-temperature hydrogen attack (HA) in a carbon-alloyed iron aluminide of composition Fe-28.1Al-2.1C (in at. pct) has been studied. Well-polished samples were exposed to hydrogen gas (1 atm) at 700 °C and 900 °C for different times. The characteristics of HA were evaluated by two-dimensional microstructural analysis, performed on the rolling plane of the as-received and hydrogen-treated samples. The carbon-alloyed iron aluminide contained carbides in two different morphologies: blocky and needle shaped. The compositions of these carbides have been determined. The blocky carbides were preferentially attacked along the {111} and {110} crystallographic planes. The interfaces between the blocky carbides and grains were mainly affected by the hydrogen treatment. The needle-shaped carbides dissolved in the matrix after relatively short times. The degree of attack generally increased with increases in treatment temperature and time of exposure, as long as the surface was not completely covered with a protective oxide layer.