Unexpected low-temperature crystallization of amorphous silicon nitride into α-Si3N4 in a ferritic Fe–Si matrix

Annealing conditions leading to the crystallization of amorphous silicon nitride occurring upon nitriding of Fe–Si alloys have been found. A brief thermodynamic explanation for the occurrence of this unexpected amorphous-to-crystal transition is given. The crystalline precipitates were found to be α-Si3N4 and follow the orientation relationship given by (11¯0)α-Fe//(101¯0)α-Si3N4 with [111]α-Fe//[0001]α-Si3N4[111]α-Fe//[0001]α-Si3N4. This transition happens in precipitates with a constant Si3N4 composition. This structural change also leads to a morphological change from the initial cube-like shape to a hexagonal prism.