Retaining the Nano in Nanocrystalline Alloys

When nanocrystalline metals first became available, their remarkably high strength seemed to open up many interesting design applications, but several adverse properties were soon noted. A particularly vexing problem was a microstructural instability that decreases strength over time. The small crystal grain size creates a large grain boundary area; the associated high interfacial energy drives coarsening (increase in grain size) that leads to softening. For many advanced alloys, the coarsening problem is accelerated by the prolonged high temperatures associated with processing. Attempts to prevent grain growth in a nanocrystalline metal by alloying have been hit and miss, and in general they have only retarded coarsening. On page [951][1] of this issue, Chookajorn et al. ( 1 ) present a solution to this microstructural instability problem. They performed thermodynamic calculations on a series of candidate binary alloys to determine which are stabilized by grain boundary segregation, not only against coarsening but also against phase separation. A rational method is made available to design nanocrystalline alloys that meet operational requirements, even at elevated temperatures. [1]: /lookup/doi/10.1126/science.1224737