ABSTRACT

To study the formation and evolution of the Cr(V,Nb)N (modified Z-phase) in 12%Cr steels, a model alloy has been designed with a very low carbon content and no additions of W and Mo. In such a system, the formation of nitrides and particularly of the Z-phase is highly promoted without cluttering from carbides and Laves phases. The Z-phase evolution has been investigated during long time ageing at temperatures in the range 600°C-700°C by transmission electron microscopy and x-ray diffraction on precipitates extracted by selective electrolytic dissolution of the matrix. It has been demonstrated that the formation of the Z-phase proceeds at the expenses of pre-existing MN nitrides, having composition (V,Nb)N, and does not involve nucleation processes. The diffusion of Cr into MN particles induces a change in their cubic crystal lattice which, after long ageing times, transforms into the tetragonal structure typical of Cr(V,Nb)N. Detailed transmission electron investigations have also revealed the presence of hybrid particles, composed by regions with a MN-type chemical composition and cubic crystal structure and regions with a chemical composition and a tetragonal lattice typical of Cr(V,Nb)N. It has been demonstrated that Z-phase formation is faster along grain boundaries and, in the early stages of formation, their presence induces a selective depletion of (V,Nb)N in the surrounding matrix.