Ni-17% Mo. Nesbit and Laughlin (1978) studied off

stoichiometric Ni-16.7% Mo. Two mechanisms of ordering

are suggested from their study:

 1. the ordered phase (Ni4Mo) may form

 heterogeneously from the disordered

 supersaturated solid solution by heterogeneous

 nucleation, or

 2. the ordered Ni4Mo phase may form homogeneously

 throughout the gamma matrix by one of the

 following mechanisms:

 a. spinodal clustering followed by ordering

 within the solute region,

 b. spinodal ordering, and

 c. continuous ordering in which the final

 equilibrium structure evolves continuously

 from a low amplitude quasi-homogeneous

 concentration wave.

 Their results show that ordering at 750 C takes place

by the homogeneous nucleation of Ni4Mo. At 700 C,

reasonable evidence exists for the mechanism to be

spinodal ordering. Continuous ordering is not plausible

since the SRO maxima do not correspond to maxima in the

long range ordered state. They did not age at a high

enough temperature to draw any conclusion about the

possibility of heterogeneous nucleation of Ni4Mo.