Atomic Resolution Microscopy of Nitrides in Steel

MN and CrMN type nitride precipitates in 12%Cr steels have been investigated using atomic resolution microscopy. The MN type nitrides were observed to transform into CrMN both by composition and crystallography as Cr diffuses from the matrix into the MN precipitates. Thus a change from one precipitate type to another does not necessarily involve nucleation of the new precipitate type followed by dissolution of the old precipitates.

By studying the interface between these nitrides and the matrix, it could be observed that the MN and CrMN type precipitates had a few nanometers thick amorphous layer between the crystalline nitride and ferrite matrix. Usually precipitates are described as having (semi) coherent or incoherent interfaces, but in this case it is more energetically favourable to create an amorphous layer instead of the incoherent interface.