| As an important technique of surface modification, nitriding of materials has been used widely in industry. The iron nitride films are also regarded as good candidates for magnetic recording materials because of their good magnetic properties. The microstructural examination of nitrided layer and the study of nitride in it, are essential and necessary for the improvement of their mechanical and magnetic properties. As we know, high resolution electron microscopy can give structural information at atomic scale. However, little results about nitrided layer have been reported in the previous literatures.In the present paper, the microstructure, orientation relationship, crystal defect, interface, long range order, super structure as well as imaging of nitrogen atom in the nitrided layer, have been studied by electron diffraction and high resolution electron microscopy. The results were summarized as follows:1. The nitride layer consists of compound layer and diffusion layer from the surface to the interior. In the compound layer, there are two sublayers, i.e. column layer and transition layer. The column layer has lamellar structure, resulting from the alternative array of e lamella and y' lamella. The interface between e and y' lamella is coherent, except that the growing steps exist at the interface. No defects were observed in each e or y' lamella. In transition layer, a lots of stacking faults and twins were observed in y'. The reaction between {lll}y and {112}y. twins can form regular twinning steps in y' grains.2. There are two kinds of y' precipitate in diffusion layer. One is full of parallel stacking faults (type I), while the other does not have such parallel striations (type II). It was observed that sliding took place among the segments of type I y'. The type II y' was found to have a different substructure consisting of dislocations, stacking faults, low angle grainboundaries as well as mis-orientated grains. The long range order of nitrogen atoms along <012}r direction, was observed in type II y' for the first time. It is believed that e nitride can only be found in compound layer. However, s phase, which appears as a subgrain in type II y', was observed in diffusion layer.3. The precipitation of a" in the diffusion layer is heterogeneous, which is different from the reported homogeneous precipitation in the bulk samples. Unlike the typical disc-shaped plate, a" has ribbon-like slice morphology. It precipitates from {100}a plane, and grows along (011)a direction. This unusual precipitated characteristic comes from the gradient of nitrogen content in the diffusion layer, which was not reported previously. The a"-y' nitride transformation was directly observed in the diffusion layer, which gives definite evidence for the in situ nucleation of y' on a" nitride. The diffracted characteristic of Jack-1 and Jack-2 structures of a", was analyzed theoretically for the first time. It was proved that the Jack-2 structure, which was thought had fee-related diffraction patterns in the previous literature, also has bcc-related diffraction patterns as Jack-1 structure.4. A new method, which is used to distinguish e, e' and e" supper structures, was proposed and used in the experiment. A series of diffraction patterns from e" super structure, were obtained with large scale tilting technique. The existence of e" was verified for the first time after its discovery according to X-ray diffraction.5. In order to distinguish s'-Fe2N and s'-Fe3N nitrides, the idea of "Imaging of Light Atom" was first used to analyze these two super structures. The nitrogen atoms in e'-Fe2N nitride were imaged with high resolution electron microscope. Different configurations of nitrogen atoms as well as iron atoms, were displayed.
|
PDF Full Text Request