The precipitated characteristics of ␣ Љ-Fe 16 N 2 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 ␣ Љ nitrides, whose habit planes are (100) ␣ , (010) ␣ , and (001) ␣ , 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 ␣ Љ nitrides in the diffusion layer appear as ribbonlike slices. They grow on {001} ␣ matrix planes with a parallel orientation relationship, and the direction of their length is parallel to the ͗110͘ ␣ direction. The interface between the ␣ Љ nitride and ␣ matrix and a 7 deg [111]/(112) low-angle-tilt grain boundary in the ␣ Љ 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 ␥ Ј-Fe 4 N nitride could grow on an ␣ Љ nitride directly. The orientation relationship during the ␣ Љ → ␥ Ј nitride transformation was determined as to be (001) ␥ Ј //(110) ␣ Љ , [110] ␥ Ј //[111] ␣ Љ .
Small additions of Hf to directionally solidified NiAl–Cr(Mo) eutectic resulted in precipitation of a high density of Heusler phase Ni2AlHf along with fine G-phase Ni16Hf6Si7. The Heusler phase was mainly located on the grain boundary region. The fine G-phase formed in the presence of Si, which was a contamination resulting from contact with ceramic shell molds during directional solidification of the alloy. These fine G-phases were cuboidal in shape and coherent with the NiAl matrix. After hot isostatic pressing and aging treatment, the fine G-phases completely disappeared. The density of the Heusler phase was partially reduced, and the Heusler particles precipitated preferentially on the NiAl/Cr(Mo) interfaces and grain boundaries of the NiAl matrix. Some Heusler particles precipitated locally within the NiAl matrix, and small amounts of them precipitated within the Cr(Mo) phase. The structures of the NiAl/Ni2AlHf and NiAl/Ni16Hf6Si7 interfaces were investigated by high-resolution electron microscopy. The habit plane of the fine G-phase was {001}NiAl. This result was in good agreement with calculation based on the linear elastic theory. The misfit dislocation network on the NiAl/Ni2AlHf (110) interface was calculated from the O-lattice model and compared with the observation, which showed good agreement.
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