Effect of Nitriding Potential KN on the Formation and Growth of a \u201cWhite Layer\u201d on Iron Aluminide Alloy

Ngoc Minh Le,Horst Biermann,Anke Dalke,Christian Schimpf

doi:10.1007/s11663-020-02029-x

Abstract

This paper investigates the effect of nitriding potential under well-defined gas nitriding conditions on the formation and growth of a compound layer called “white layer” on a FeAl40 (with the composition of 40 at. pct Al) iron aluminide alloy. The nitriding potential was systematically varied in the range of 0.1 to 1.75 bar−1/2 at 590 °C for 5 hour nitriding time with an ammonia-hydrogen-nitrogen atmosphere. Characterization of the microstructure and phases formed within the white layer was performed using optical and scanning electron microscopy, X-ray diffraction (XRD), electron backscatter diffraction (EBSD), and glow discharge optical emission spectroscopy (GDOES). Experimental results indicated that the nitriding potential strongly influences morphology and crystal structure of the white layer. The nitride compound layer consists of the phases γ′-Fe4N, ε-Fe2-3N, and AlN. A mechanism is proposed for the formation and growth of the white layer, depending on the effect of the nitriding potential.

Highlights

ALUMINIDE intermetallic alloys, based on phases of TiAl, FeAl, and Fe3Al, have been of great scientific as well as industrial interest due to their beneficial properties at elevated temperatures such as excellent oxidation and creep resistance, and low density compared to Ni-based superalloys
Spies et al.[9] carried out gas nitriding on an iron aluminide FeAl40 alloy at the above-mentioned range of temperature in ammonia atmosphere controlled nitriding potential KN. They reported two disadvantages regarding the diffusion of nitrogen atoms at nitriding temperatures above 650 °C: (i) the nitrogen potential KN was strongly reduced related to the thermal decomposition of NH3 since the growth of the nitride layer was reduced, and (ii) an ‘‘external nitriding’’ process took place more strongly
The results demonstrated that the white layer formation was due to the development of iron nitride phases by an external nitriding mechanism

Summary

INTRODUCTION

ALUMINIDE intermetallic alloys, based on phases of TiAl, FeAl, and Fe3Al, have been of great scientific as well as industrial interest due to their beneficial properties at elevated temperatures such as excellent oxidation and creep resistance, and low density compared to Ni-based superalloys. Spies et al.[9] carried out gas nitriding on an iron aluminide FeAl40 alloy at the above-mentioned range of temperature in ammonia atmosphere controlled nitriding potential KN. They reported two disadvantages regarding the diffusion of nitrogen atoms at nitriding temperatures above 650 °C: (i) the nitrogen potential KN was strongly reduced related to the thermal decomposition of NH3 since the growth of the nitride layer was reduced, and (ii) an ‘‘external nitriding’’ process took place more strongly. Based on the obtained results, the effect of nitriding potential on the formed phases is thoroughly discussed, and an optimal KN value is given for this alloy

EXPERIMENTAL DETAILS

Surface Roughness and Topography

Elemental Depth Distribution

Phase Composition and Structure of the White Layer

Mechanism of the Development of White Layers on B2-FeAl

CONCLUSIONS