Microstructural investigation on the precipitation of α" nitrides and α"→γ′ nitride transformation in ion-nitrided pure iron

The precipitated characteristics of alpha " -Fe16N2 nitrides in the diffusion layer of ion-nitrided pure iron were investigated with transmission electron microscopy (TEM) and high-resolution electron microscopy (HREM). Three sets of alpha " nitrides, whose habit planes are (100)(alpha), (010)(alpha), and (001)(alpha), respectively, do not precipitate simultaneously from the diffusion layer, which is different from the normal homogeneous precipitation in Fe-N alloys. Unlike the typical disc-shaped morphology reported widely, the alpha " nitrides in the diffusion layer appear as ribbonlike slices. They grow on {001}(alpha) matrix planes with a parallel orientation relationship, and the direction of their length is parallel to the < 110 > (alpha) direction. The interface between the alpha " nitride and alpha matrix and a 7 deg [111]/(112) low-angle-tilt grain boundary in the alpha " nitride were examined with HREM. The distributions of dislocations at the interface and the grain boundary were investigated. During microstructural examination, it was observed that a gamma'-Fe4N nitride could grow on an alpha " nitride directly. The orientation relationship during the alpha " --> gamma' nitride transformation was determined as to be (001)(gamma')//(1 (1) over bar0)(alpha "),[110](gamma')//[111](alpha ").