Abstract

A novel technique has been developed to produce Fe–Al intermetallic coatings on steel. This technique applies mechanical vibration to a retort, which is loaded with Al powder, alumina filler, ammonium chloride activator and FeCrAl alloy balls. The operation temperature was from 440 to 600 °C. This technique produced coatings with thickness of 17 μm for 15 min and 90 μm for 120 min treatment at 560 °C. The coatings appear to be homogeneous, with a high density and free of porosity, and have excellent adherence to the substrate. The coatings consisted mainly of η-Fe₂Al₅ with small amounts of θ-FeAl₃ and β-FeAl, and exhibited a nano-structure. Microstructure studies suggested that the formation of the intermetallic phases at a low temperature has a complex mechanism, including the formation of a thin Al layer on the substrate by ball milling; Al-rich phases nucleation, growth and formation of an initial alloy layer; severe plastic deformation which increases the local temperature and produces a nano-structure; and fast outward diffusion of Fe and formation of Fe–Al intermetallics. This technique reduced the treatment temperature and duration significantly compared with the conventional Al pack cementation processes, providing a new approach to industrial diffusion coatings with great energy and time savings.

Keywords: A. Iron aluminides (based on Fe₂Al₅); A. Nano-structured materials; C. Coatings, intermetallic and otherwise; C. Mechanical alloying and milling; F. Electron microscopy, scanning

Article Outline

1. Introduction

2. Experimental

2.1. Apparatus and experimental processes

2.2. Specimens preparation

2.3. Microstructure characterization

3. Results

3.1. Effects of temperature and duration on coating thickness

3.2. Cross-section of the coatings

3.3. Surface micrographs of the coatings

3.4. XRD results

4. Discussion

5. Conclusions

Acknowledgements

References