Multiscale analysis of an ODS FeAl40 intermetallic after plasma-assisted nitriding

• 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.%).

The binary B2 (ordered bcc-type structure) FeAl40 Grade 3 aluminide containing 40 at.% Al was nitrided at 600 °C for 15 min with a pulsed DC plasma in a 95% N2/5% H2 gas mixture. The nitrided layer was described at the micro-, nano- and atomic scales. The nitrided layer consists of a thin outer sublayer (∼1 μm in thickness) containing γ′-Fe4N and having a moderate hardness (870 Hv) below which a thicker (∼36 μm in thickness) and harder (1400 Hv) inner sublayer is formed. While the outer sublayer is completely depleted in Al, the inner sublayer is made of a mixture of hex. AlN and α-Fe phases regularly structured at different levels: i) At the micrometre scale, observations showed a network of α-Fe “wave-like” veins (∼2 μm in length and ∼100 nm in width) located at the grain boundaries and preferentially aligned parallel to the nitrided surface. ii) At the nanometre scale, observations evidenced that such veins are embedded in a “lamellar-like” morphology matrix alternately composed of α-Fe and α-Fe/hex. AlN lamellae (∼300 nm in length and ∼10 nm in width). iii) At the atomic scale, analysis pointed out that each α-Fe/hex. AlN lamella presents a finer structural arrangement in which hex. AlN-rich clusters (∼4 nm in diameter) are trapped between thin α-Fe walls (∼2 nm in thickness). The diversity of the morphologies evidenced in the present study highlights the complexity of the nitriding mechanisms of iron-based aluminides. The observations support however a mechanism of discontinuous precipitation in the inner nitrided sublayer which seems to be strongly promoted by the high amount of Al in the FeAl40 grade 3 alloy (40 at.%).