Unusual Precipitation of Amorphous Silicon Nitride upon Nitriding Fe-2at.%Si Alloy

Silicon-nitride precipitation in ferritic Fe–2at.%Si alloy was investigated upon nitriding in NH3/H2 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 (1 mm) 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 Si3N4. 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 occur...