Thermochemistry and nature of nitrogen absorption in nitrogenated FeTi alloys

Abstract Four different chemical species of nitrogen have been identified in nitrogenated FeTi alloys: the most tightly-bound nitrogen atoms contained in the mononitride (TiN), nitrogen atoms adsorbed at the ferrite-TiN interface, nitrogen dissolved in the ferrite matrix, and that in the iron-nitride phase produced when FeTi alloys are nitrogenated above a critical nitrogen activity. During the initial nitrogenation treatments of all alloys at 400 or 500°C, the dispersed molecules or platelets adsorb one additional interface N atom per TiN unit to produce TiN 2 . The nitrogen solubility in ferrite containing titanium nitride particles is significantly enhanced relative to that in pure iron, and, from the observed solution thermodynamics, this nitrogen appears to be uniformly distributed throughout a uniformly dilated ferrite matrix. The thermodynamic activity of nitrogen needed to convert the ferrite phase to Fe 4 N(γ′) in the nitrogenated FeTi alloys was found to increase with an increase in the concentration of the dispersed titanium nitride that causes an increase in yield strength. This finding is compatible with the prediction that the growth of γ′ in ferrite involves a martensitic-type transformation requiring plastic deformation of the ferrite lattice.