Multiscale analysis of an ODS FeAl40 intermetallic after plasma-assisted nitriding Article - Octobre 2016

J. Martin, A. Martinavicius, S. Bruyère, H.P. van Landeghem, C. Gendarme, F. Danoix, R. Danoix, A. Redjaïmia, T. Grosdidier, T. Czerwiec

J. Martin, A. Martinavicius, S. Bruyère, H.P. van Landeghem, C. Gendarme, F. Danoix, R. Danoix, A. Redjaïmia, T. Grosdidier, T. Czerwiec, « Multiscale analysis of an ODS FeAl40 intermetallic after plasma-assisted nitriding  », Journal of Alloys and Compounds, octobre 2016, pp. 418-426. ISSN 0925-8388

Abstract

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.%).

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