Competitive precipitation of amorphous and crystalline silicon nitride in ferrite: Interaction between structure, morphology, and stress relaxation

We present a detailed analysis based on both experimental and modeling approaches of the unique silicon nitride precipitation sequence recently observed in ferritic Fe–Si alloys upon nitriding. At 570 °C, silicon nitride forms as an amorphous phase of size-dependent cuboidal morphology which results from the symmetry of matrix crystal structure. These amorphous precipitates are stable over remarkably long treatment durations. However, we demonstrate here that it is possible to trigger a transition to the precipitation of a crystalline modification of Si3N4 by switching to a denitriding medium at the same temperature. This change in structure is associated with a change from the cube-like morphology to a hexagonal prism shape. This unique phenomenon, driven by a classical nucleation and growth process, can be explained by the shift of stress-relief mechanism related to the change of atmosphere. The plentiful availability of nitrogen atoms during nitriding allows a strong relaxation of the precipitation-induced stress, which is energetically more favorable to the amorphous phase. Upon annealing in a low nitrogen activity medium, nitrogen in solid solution diffuses outward and no longer relieves the precipitation-induced stress. In that configuration, the crystalline modification of Si3N4 becomes more stable owing to its lower associated stress. It precipitates in a hexagonal prism shape, which is the equilibrium shape dictated by the crystal symmetries of both the matrix and the precipitates.