When 10:30 AM - 11:30 AM Jan 13, 2017
Where 1670 Beyster Building
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Nucleation Reactions in Metallic Glass Alloys


John H. Perepezko
Materials Science and Engineering, University of Wisconsin-Madison

A growing number of metallic alloys have been discovered that can be synthesized as amorphous phases either during rapid melt quenching or by slow cooling of bulk volumes. In these systems crystallization reactions are effective in yielding nanoscale microstructures. For example, with amorphous Al alloys crystal densities can reach levels of 1022 -1023 m-3  The crystallization kinetics determinations support a heterogeneous nucleation, but it is also evident that the kinetics are affected by transient effects. The initial nucleation appears to derive from quenched-in atomic arrangements to yield a high nucleation number density. The decaying growth rate at long time can be related to the impingement of diffusion fields from neighboring nanocrystals, but other factors may operate at short times.  Along with the advances in basic understanding of alloying to promote vitrification, there is a recognition that the structural models of amorphous alloys require refinements to account for local interactions and nanoscale heterogeneities that represent short range order as well as the development of medium range order. These are important issues that impact the analysis of the crystallization behavior during devitrification and the amorphous phase thermal stability. Beyond crystallization reactions, nucleation is critical for promoting useful ductility for structural applications of bulk metallic glasses. In this case, plastic deformation is concentrated in a narrow shear band that propagates rapidly. By promoting copious shear band nucleation throughout the volume of a bulk metallic glass, a more homogeneous deformation can be realized along with a useful ductility. The study and analysis of nucleation reactions for these different situations requires a consideration of the stochastic nature of nucleation and the influence of heterogeneous sites. These are new developments that offer exciting possibilities for control of nanoscale microstructures and deformation behavior as well as challenges for the fundamental understanding of the reaction mechanisms. 

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