Nitriding of ternary iron-based alloys: metastable mixed nitrides, excess nitrogen and stress-depth profile

摘要

For the first time it could be shown that, upon nitriding Fe-Cr-Al alloys, the thermodynamical stable cubic CrN and hexagonal A1N nitrides do not develop. Instead, a mixed Cr_xAl_(1-x)N nitride develops of rock-salt crystal-structure type exhibiting a Bain orientation relationship with the ferritic matrix: the Al atoms are "dragged" into the developing cubic CrN precipitates. Similar observations were made upon nitriding Fe-Cr-Ti alloys. The formation of these mixed Cr_xAl_(1-x)N precipitates is associated with the incorporation of large amounts of (mobile and immobile) "excess nitrogen" and the development of large residual stresses. Creep, associated with residual stress relaxation, occurs during nitriding, giving rise to complex residual stress-depth profiles.