It was noted that, in some alloy systems (e.g., Cu-Co), the prior formation of clusters did not affect the nucleation of precipitates, whereas in others (e.g., Al-Cu-X; where X was Mg, Ag, Cd, In, Sn, Si, or Ge - singly or in combination), clusters could not only accelerate the nucleation kinetics of the first phase to form but could also change the identity of this phase. When solute atoms in a cluster were strongly bound to one another  but none had a high-binding energy to vacancies, clusters tended to dissolve and to cease forming as a result of a loss of quenched-in vacancies before the incubation time for nucleation had elapsed; unless the ambient vacancy concentration was high enough to maintain the clustering process. When the solutes were also strongly bound to vacancies, the clusters could survive beyond the incubation time. Two mechanisms were examined via which such clusters could assist nucleation. That is: one or more of the atom species constituting the clusters was adsorbed at the interfaces of the nuclei and thereby reduced their interfacial energy. Alternatively, in the case of plate- (or lath-) shaped nuclei formed with an appreciable shear-strain energy, the 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 could markedly reduce the strain energy. Nucleation which

took place in the absence of aggregates, dislocations, internal boundaries or particles, was clearly homogeneous when clusters did not assist nucleation. Atom probe results had revealed the existence of clusters which survived long enough to assist nucleation. It was therefore concluded that the definition of heterogeneous nucleation be extended so as to include such continuously fluctuating heterogeneities.

On the Roles of Clusters during Intragranular Nucleation in the Absence of Static Defects. J.F.Nie, B.C.Muddle, H.I.Aaronson, S.P.Ringer, J.P.Hirth: Metallurgical and Materials Transactions A, 2002, 33[6], 1649-58