Unusual precipitation of amorphous silicon nitride upon nitriding Fe-2at.%Si alloy

Abstract

Silicon-nitride precipitation in ferritic Fe-2at.%Si alloy was investigated upon nitriding in NH 3/H 2 gas mixtures, using light microscopy, hardness measurements, scanning and transmission electron microscopy, X-ray diffraction and electron-probe microanalysis for microstructural characterisation. Surprisingly, ideally weak nitriding behaviour occurred upon nitriding thick (1mm) recrystallised Fe-2at.%Si alloy specimens. This phenomenon can be attributed to the onset of silicon-nitride precipitation only after a certain degree of nitrogen supersaturation has been established at all depths in the specimen. Silicon-nitride precipitates formed inside the ferrite grains and also along the ferrite grain boundaries. The precipitates were amorphous and had a stoichiometry of Si3 N4. The amorphous nature of the tiny precipitates has a thermodynamic origin. Nitride precipitation occurred very slowly due to the very large volume misfit of the nitride with the matrix. An anomalous non-monotonous hardness change occurred with increasing nitriding time, which was ascribed to the initially fully elastic accommodation of precipitate/matrix misfit. The nitrogen uptake rate increased upon continued nitriding as a result of self-catalysis. The possible, favourable application of amorphous silicon-nitride precipitates as grain-growth inhibitors in the production of grain-oriented electrical steel is discussed. © 2012 Copyright Taylor and Francis Group, LLC.