Transmission electron microscopy study on the cross-sectional microstructure of an ion-nitriding layer

The cross-sectional microstructure of an ion-nitrided layer on an A(3) steel (C = 0.15, Si = 0.2, Mn = 0.5, and balance Fe, in wt pct) was studied by transmission electron microscopy (TEM), and its electron diffraction patterns were analyzed. It has been shown that the compound layer consists of columnar epsilon-Fe2-3N and gamma'-Fe4N. The former precipitates thin gamma'-Fe4N phases related to each other by a 180 deg turning twin, and the orientation relationship between epsilon-Fe2-3N and gamma'-Fe4N is {111}(gamma')//(0001)(epsilon), [1 (1) over bar 0](gamma')//[11 (2) over bar 0](epsilon) The columnar gamma'-Fe4N has a stacking fault substructure and accompanying lattice distortion. There are islandlike ferrite crystals between the columnar crystals. Near the compound layer is mostly gamma'-Fe4N with a coexisting small amount of alpha-Fe. The diffuse layer is composed of Guinier-Preston (GP) zones, alpha ''-Fe16N2, gamma'-Fe4N, and alpha-Fe. The gamma'-Fe4N with long period structure considered as the ordering of the stacking fault was found. The characters and transformation mechanism of the case were discussed.