On the roles of clusters during intragranular nucleation in the absence of static defects

In some alloy systems, e.g., Cu-Co, prior formation of clusters does not affect the nucleation of precipitates, whereas in others, e.g., Al-Cu-X, where X can be Mg, Ag, Cd, In, Sri, Si, or Ge (singly or in combination), clusters can not only accelerate the nucleation kinetics of the first phase to form but can also change the identity of this phase. When solute atoms in a cluster are strongly bound to one another but none has a high-binding energy to vacancies, clusters will tend to dissolve and to cease forming as a result of the loss of quenched-in vacancies before the incubation time for nucleation has elapsed unless the ambient vacancy concentration is high enough to sustain the clustering process. When solutes are also strongly bound to vacancies, however, clusters can survive through and beyond the incubation time. Two mechanisms are examined through which such clusters can assist nucleation. (a) One or more of the atom species constituting the clusters adsorbs at the interfaces of the nuclei and thereby reduces their interfacial energy. (b) At plate- (or lath-) shaped nuclei formed with an appreciable shear-strain energy, segregation of large atoms to the regions under tension within the local strain field and of small atoms and/or vacancies to local regions under compression can markedly reduce this strain energy. Nucleation taking place in the absence of static defects, such as aggregates, dislocations, internal boundaries, or particles, is clearly homogeneous when clusters do not assist the nucleation process. However, atom probe results have demonstrated the existence of clusters surviving long enough to assist nucleation and in so doing suggest that the definition of "heterogeneous nucleation" be extended to include even these continuously fluctuating heterogeneities.