A FeAl40 Grade 3 intermetallic is plasma-nitrided at 600 °C for 15 min in a 95% N2/5% H2 gas mixture.
A thin outer sublayer (2 μm) is composed of the γ′-Fe4N and exhibits a moderate hardness (550–850 Hv).
A thick and hard inner sublayer (36 μm, 1400 Hv) consists in a lamellar morphology of hex. AlN and α-Fe lamellae.
A mechanism of “discontinuous” precipitation occurs during nitriding of Al-rich Fe-Al alloy (40 at.%).