Frontiers for Stable Nanocrystalline Materials
Nanocrystalline grain sizes give rise to many interesting and desirable properties in bulk materials, including strength, wear resistance, corrosion resistance, and enhanced functional properties. However, nanoscale grain structures are “hard to make and hard to keep”: their structural instability is easily lost through coarsening, and as a result, typically extreme out-of-equilibrium processes are needed to produce them. Stabilizing such nanostructures is not only necessary to avoid their gradual coarsening, but also to open the range of viable processing routes to include some that scale well to production. This talk will review the strategy that has become a dominant one for the stabilization of nanocrystalline materials: selective alloying of grain boundaries. The concept of alloy design for the stabilizing effects of grain boundary segregation will be reviewed, including views on how modeling and computation is needed to elaborate our understanding of alloy structure and thermodynamics at grain boundaries. Whereas binary alloy design has led to a number of scientific and commercial successes, many more significant challenges await in the ternary and multinary alloy systems most likely to be used in practice. Finally, the extension of these concepts to scalable processing methods will generally involve additional alloying constraints and the development of new processing science. Recent successes in powder processing and 3D printing of nanocrystalline materials will be highlighted.