An investigation on nitrogen uptake and microstructure of equimolar quaternary FeCoNiCr high entropy alloy after active-screen plasma nitriding

Xiao Tao*, Yepeng Yang, Jiahui Qi, Biao Cai, W. Mark Rainforth, Xiaoying Li, Hanshan Dong

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

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Abstract

Under nitrogen diffusion treatments, N-expanded austenite (γN) can form at the surface of self-passivating Fe-Cr, Ni-Cr, and Co-Cr alloys at low temperatures, which provides beneficial hardening and enhancements in wear resistance without reducing corrosion resistance. Given the wide research interests in multicomponent equimolar alloys, an equimolar quaternary FeCoNiCr high entropy alloy (HEA) was investigated after active-screen plasma nitriding at 430–480 °C in this study. Firstly, the formation of γN-FeCoNiCr case at 430 °C was demonstrated with the bright case appearance after metallographic etching, the lattice expansion under XRD, the FCC electron diffraction patterns and the shear bands under TEM. Secondly, the thick treatment cases at ∼9–16 μm first indicated that N interstitial diffusion was not sluggish in the FeCoNiCr surface. Thirdly, analogous to stainless steels, the onset of dark regions in the etched γN-FeCoNiCr case was owing to the formation of a cellular mixture of CrN + γ-(Fe, Co, Ni) nano-lamellae at elevated treatment temperatures. The residual bright regions in γN-FeCoNiCr at 480 °C showed ∼1–3 nm CrN nanoprecipitates with no substantial Cr segregation. Additionally, a significant nanocrystalline layer was seen at the topmost surface at 480 °C, which is most likely associated with the high substrate Cr content.
Original languageEnglish
Article number113593
JournalMaterials Characterization
Volume208
Early online date22 Dec 2023
DOIs
Publication statusPublished - 1 Feb 2024

Bibliographical note

Acknowledgement
The authors would first like to thank the financial support from Smart Factory Hub (ESIF 06R1702266). The authors acknowledge Prof. Bin Liu at Central South University for providing the materials. The authors also wish to acknowledge the EPSRC ‘Friction: The Tribology Enigma’ Programme Grant (grant number EP/001766/1) and The Henry Royce Institute for Advanced Materials, for access to the JEOL JEM-F200 (funded through EPSRC grant EP/R00661X/1).

Keywords

  • Surface alloying
  • Plasma nitriding
  • Expanded austenite
  • High entropy alloys
  • Microstructure

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