Nitride precipitation in compositionally heterogeneous alloys : nucleation growth and coarsening during nitriding

A theoretical approach is proposed to take into the account nucleation, growth and coarsening of nitrides in alloys featuring heterogeneous nitrogen content. It is based on physical considerations and accounts for both bulk nitrogen diffusion, which results from the nitriding process and nitrides precipitation kinetics. It predicts local information such as average particle density, radius and volume fraction of nitrides as a function of depth. The work presented in this paper leads to two important conclusions. First, the precipitation rate is not so high that precipitation can be considered as infinitely faster than the diffusion of nitrogen into the bulk. Second, the precipitation state at a given depth depends on the local interaction between nucleation, growth and coarsening phenomena, themselves depending on the local nitrogen content. Finally, the precipitation in alloys with heterogeneous nitrogen content induced by nitriding is radically different from classical precipitation in a single phase where the driving force for precipitation is consumed