antiphase boundary formation and modulus strengthening

(Sims and Hagel, 1972). The strength of the gamma prime

increases with temperature, an anomaly not yet fully

explained. The phase also remains fully ordered to very

high temperatures (Pope and Garin, 1977).

 In the early stages of gamma prime precipitation in

Ni-Al alloys, side band satellites in x-ray powder

diffraction patterns appear. These satellites were first

thought to correspond to periodic modulations in structure

(Kelly and Nicholson, 1963). These presumed modulations

lead to the speculation that the mechanism of Ni-Al

decomposition was spinodal. Cahn (1961) originally

suggested this possibility. The data of Corey et al.

(1973) and Gentry and Fine (1972) suggest that this

mechanism is possible at high supersaturations. Faulkner

and Ralph (1972) studied the early stages of precipitation

in a more dilute Ni-Al 6.5 wt.% alloy using FIM and

conclude that the spinodal ordering mechanism is

unlikely. They suggest that the sidebands are due to

particle morphology changes during the early stages of

decomposition.

 The nucleation of the gamma prime could not likely

explain the "macro" order in the microstructure, the large

uniformly sized and distributed gamma prime precipitates

that are present in the alloys studied here. Ardell et

al. (1966) explain this ordered microstructure with a