appear as fine, spherical precipitate (Weatherly, 1973).

This spherical morphology will change as the lattice

mismatch between the gamma prime and gamma matrix

changes. The spherical precipitates occur for mismatches

of -0.3% above which cube morphologies predominate,

independent of size or volume fraction of gamma prime

(Merrick, 1978). When gamma prime precipitates as cubes,

the cube habit is (100) gamma//(100) gamma prime.

 When the gamma prime precipitate size is small

(100-300 Angstroms), the coherency of the precipitate is

not lost and can be maintained by a tetragonal distortion

at the matrix/precipitate interface (Merrick, 1978). When

coherency is lost, the lattice mismatch between the two

phases can be accommodated by a dislocation network. This

network has been characterized for Ni based superalloys by

Lasalmonie and Strudel (1975). Since the morphology of

the gamma prime is a sensitive function of lattice

mismatch, it follows that this mismatch can be varied by

making alloy additions to the binary alloy which will

partition preferentially to one or the other of the two

predominant phases. In pure binary alloys, Phillips

reports this lattice mismatch at 0.53% (Phillips, 1966).

In ternary and higher order alloys, the mismatch is widely

variable due to elemental partitioning differences between

the two phases.

 Gamma prime is a unique intermetallic phase. Its

major contributions to strength are the result of both