Bimodal size distributions of γ′ precipitates in Ni-Al-Mo-II. Transmission electron microscopy

A double step aging procedure was used to produce bimodal size distributions of precipitates in two Ni-Al-Mo alloys. The evolution of the microstructure during the decomposition was studied using transmission electron microscopy. Shape and changes in the spatial correlations of the precipitate ensemble as well as precipitate size distributions were determined for a very wide time scale. Particle shape transitions during aging were monitored by the determination of aspect-ratio distributions. The results are compared with small-angle neutron scattering analysis in the preceding paper and good agreement is found between the size distributions determined by the two techniques. In the alloy with high coherency strains peculiar phenomena were observed during aging, e.g. coalescence among the small precipitates, preferential growth of small precipitates in the presence of larger ones, and a dramatic narrowing of the size distribution of large precipitates indicating a uniformization in size towards late stages of aging.