Identification of fine precipitate particles in unidirectionally solidified NiAlMo eutectic alloys by means of edx and sad analyses

An attempt is made to identify the fine precipitate particles formed in the α-Mo fibers in the unidirectionally solidified NiAlMo eutectic alloys. The energy dispersive X-ray (EDX) microanalysis and the selected area diffraction (SAD) analysis are used for this identification. Since all particles in the fibers are smaller than the electron probe size, the method proposed by Cliff et al. [Developments in Electron Microscopy and Analysis, p. 63. Inst. Phys., London (1983)] is adopted in the EDX analysis in order to exclude effects from the surrounding α-Mo phase. In the SAD analysis, the diffraction patterns are taken at the particles in the transverse (001)_α and the longitudinal (110)_α sections. These analyses show that the fine particles in the α-Mo fibers are the γ' phase having the fixed composition identical to the γ' matrix around the α-Mo fibers, the crystal structure of L1₂ type and the NW orientation relationship with the α-Mo phase. With reference to the recent phase diagram of Miracle et al. [Metall. Trans. 15A, 481 (1984)], this γ' formation in the α-Mo fibers is understood as the stable phase formation and hence precludes any possibility of carbides, metastable phases or the detrimental δ-NiMo phase. Discussion is extended for observing well-defined diffraction patterns in examining the orientation relationship between the f.c.c. γ' phase and the b.c.c. a phase.