Effect of alloying element content on ion-nitriding behavior of Fe-Ti alloys

Abstract

The ion-nitriding behavior of iron alloys with a titanium content of between 1.07 and 2.58 wt.% was investigated in the α-phase region. The behavior was found to be analogous to the internal oxidation behavior of iron alloys: An internal-nitriding layer, where small TiN precipitates are dispersed, as well as a very thin surface layer of γ′-Fe₄N were formed. A parabolic rate law holds for growth of the internal-nitriding layer. The kinetics of growth of the internalnitriding layer is discussed according to the rate equation of internal-oxidation, giving the diffusion coefficient of nitrogen, D ^app_N , in the layer. The measured D ^app_N decreases as the volume fraction of TiN, f, increases, indicating that the diffusion of nitrogen is apparently inhibited by the existence of TiN precipitates. Furthermore, the diffusion coefficient of nitrogen in α-iron was evaluated by extrapolating D ^app_N to f=0, being in good agreement with that reported previously. The f-dependence of D ^app_N is discussed in terms of the effective area for diffusion of nitrogen in α-iron.