BIMODAL SIZE DISTRIBUTIONS OF GAMMA' PRECIPITATES IN NI-AL-MO .2. TRANSMISSION ELECTRON-MICROSCOPY

A double step aging procedure was used to produce bimodal size distrib utions of precipitates in two Ni-Al-Mo alloys. The evolution of the mi crostructure during the decomposition was studied using transmission e lectron microscopy. Shape and changes in the spatial correlations of t he precipitate ensemble as well as precipitate size distributions were determined for a very wide time scale. Particle shape transitions dur ing aging were monitored by the determination of aspect-ratio distribu tions. The results are compared with small-angle neutron scattering an alysis 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 nar rowing of the size distribution of large precipitates indicating a uni formization in size towards late stages of aging.