A coherent precipitate known as gamma prime develops in iron-nickel base alloys when alloying additions of aluminum and titanium are included. The gamma prime phase is an ordered fcc structure of Ll2 symmetry having an Ni3(A1, Ti) composition. Gamma prime is generally found as a fine spherical or cubic precipitate, uniformly distributed throughout the material. Imaging is usually difficult because lattice mismatches are small and only superlattice reflections of weak intensity are available for dark field imaging. This paper presents evidence that the gamma prime phase can be formed by discontinuous precipitation, developing a typical cellular structure.

An experimental alloy, Fe-14.9Cr-34.8Ni-l.5Al-l.98Ti-.004C (weight percent), has been given a solutionizing treatment of 1145°C for 1 hour followed by an air quench and an aging treatment of 815°C for 1 hour. Examination of the microstructure by TEM indicated that a cellular precipitate had developed extensively at grain boundaries.