Alloying effects on spinodal decomposition induced nano-sized X-N clustering during low-temperature nitriding of austenitic Fe-35Ni-X alloys

The addition of Cr is vital for the formation of the expanded austenite (γ_N) supersaturated by nitrogen (N) during low-temperature nitriding of austenitic steels. Cr-N clustering in γ_N promoted by Cr-N attractive interaction was recently reported for low-temperature nitrided Fe-35Ni-Cr alloys. However, the effects of other elements having different X-N interactions remain unclear. In the present work, the effect of nitride-forming element X (X = Al, Mo, Mn, Cr, V) addition on the low-temperature nitriding behavior and nanostructure evolution of γ_N is systematically investigated for austenitic Fe-35Ni-10X (at%) alloys. The addition of X promotes surface N absorption, lattice expansion and thus hardening of γ_N in the order of Mn<Al<Mo<Cr<V. Transmission electron microscopy (TEM) and three-dimensional atom probe (3DAP) analyses have revealed the formation of nano-scale non-equilibrium X-N clusters in γ_N as spinodally modulated structures and γ′-Fe₄N type long-range ordering (LRO) of N near the surface for the Mo, Cr, and V-added alloys. However, the γ_N formed in Al or Mn-added alloys shows no X-N clustering or LRO of N. The chemical driving force, strain energy and modulation wavelength for coherent spinodal decomposition were calculated thermodynamically under multi-compositional concentration fluctuations. The driving force for spinodal decomposition increases in the order of X-N attractive interaction, namely Mn<Mo<Cr<V. The calculations agree with the nanostructures observed where the alloy with stronger X-N attractive interaction showed a smaller modulation wavelength.