Three distinct second phases were observed to form heterogeneously at grain and deformation twin boundaries when Hastelloy C-276 was aged in the temperature range of 923 to 1173 K. The most abundant was the faulted, molybdenum rich μ phase. The next most abundant phase was molybdenum rich M6C carbides, and the third phase, which was observed very infrequently, was tentatively identified as theP phase. The compositions of these phases were remarkably similar, and the need to employ several electron microscopy techniques is emphasized. The origin of the μ phase is discussed in the light of its chemistry.

Electron Microscopic Analysis of Heterogeneous Precipitates in Hastelloy C-276

The initial stages of ordering in Ni3Mo and Ni4Mo have been investigated by transmission electron microscopy and diffraction. The development of long-range order in Ni3Mo is associated with decomposition into the two metastable phases Ni2Mo and Ni4Mo, which are eventually replaced by the equilibrium Ni3Mo at a later stage of ordering. Evidence for the presence of metastable Ni2Mo phase was also found during the early stages of ordering of Ni4Mo. The presence of the metastable phase Ni2Mo at Ni4Mo composition and that of Ni2Mo and Ni4Mo at Ni3Mo composition can be explained in terms of the recent thermodynamic calculations of the ground states of ordered binary alloys by Cahn and his co-workers.

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Yasumasa Mizutani

Papers

Structural Changes during Isothermal Annealing of the Quenched Ni 3 Mo Alloy (I)