On the roles of clusters during intragranular nucleation in the absence of static defects
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INTRODUCTION A. Intragranular Heterogeneous Nucleation THAT intragranular nucleation tends strongly to occur at defects has long been appreciated.[1] With optical microscopy, some examples of nucleation at precipitates of another phase or even of the same phase[2] can be verified. With the advent of transmission electron microscopy (TEM), dislocations could also be validated as nucleation sites.[3] Nucleation at small vacancy-based defects has long been postulated[4] but will require advanced application of the still evolving three-dimensional atom probe[5] (3DAP) technique for verification and especially for detailed characterization of these sites. All of the defects listed in this paragraph can be regarded as static, particularly at the scale of the incubation times usually required to achieve steady-state nucleation kinetics. With the advent of the 3DAP, experimental investigation has established, as long suspected from less direct evidence,[6] that clusters of solute atoms can expedite significantly the nucleation of (usually) transition phases.[7] Unlike J.F. NIE, Senior Lecturer, and B.C. MUDDLE, Professor and Head, are with the School of Physics and Materials Engineering, Monash University, Victoria 3800, Australia. H.I. AARONSON, R.F. Mehl University Professor Emeritus, Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, is Visiting Professor, School of Physics and Materials Engineering, Monash University, Contact e-mail: aaronson @andrew.cmu.edu S.P. RINGER, formerly Senior Lecturer, Department of Materials Engineering, Monash University, is Associate Professor and Director, Australian Key Centre for Microscopy & Microanalysis, The University of Sydney, 2006, NSW, Australia. J.P. HIRTH, Professor Emeritus, Washington State University, Pullman, WA 99164, and Ohio State University, Columbus, OH 43210, is at 114 E. Ramsey Canyon Rd., Hereford, AZ 85615. Manuscript submitted March 29, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
the intragranular nucleation sites noted previously, clusters serve as dynamic nucleation sites. Clusters are continuously forming and disappearing and do so on timescales probably somewhat shorter than those involved in the formation of individual critical nuclei. Of especial importance is the view emphasized by Cohen[6] that homogeneous nucleation does not occur during phase transformations in which clusters play an active role. Instead, a seamless sequence connects clusters to the first formed precipitates. The view developed in some detail in the present article is quite different, namely that clusters represent another form of nucleation site and that their assistance to the formation of the first precipitates is appropriately described as heterogeneous nucleation. B. Cluster-Aided Intragranular Nucleation Clusters are yet another type of intragranular defect that may aid nucleation.[6] This assistance may either increase the nucleation kinetics of a given (usually transition) phase or cause replacement of this phase by another.[
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