Thermodynamic and structural studies on nitrided Fe–1.62%Mn and Fe–0.56%V alloys

Precipitation in the diffusion zone of Fe–1.62wt.%Mn and Fe–0.56wt.%V alloys nitrided at 843 K was studied by TEM and XRD analyses. Recent thermodynamic data have been used to interpret the different results in both systems. In the manganese containing alloy, the thermodynamically stable θ-Mn6N5 phase precipitates in the diffusion zone. Around the maximum hardness, in a narrow zone between 10 and 150 μm below the γ′/α interface, needle shaped precipitates are also present oriented along three mutually orthogonal directions. This phase, designated θ′, belongs to one of the three space groupsP 4m 2c 2c, P4n 2n 2c or P 4n 2b 2m.X-ray diffraction depth profiles show that the presence of theses precipitates is closely related to the macrostress in the diffusion layer. In the case of the vanadium containing alloy, nitriding simultaneously produces VN precipitates and a tweed structure which annealing experiments reveal to be metastable. EPMA studies show that excess nitrogen is present in the Fe–V alloy. Sidebands are observed only on the (200)α peaks and are attributed to the microstrain gradient at a given nitrogen concentration, due to the periodicity of the tweed structure. The tweed contrast is due to a tetragonal distortion of the matrix, and not to a spinodal microstructure.