Nanocrystalline Metals and Alloys

Metals get stronger as their grain size is reduced. This behavior, described by the Hall-Petch relation, is exemplified by nanocrystalline metals. Their exceptionally small grains give them extraordinary strength, but there are drawbacks. Among these are the difficulty in achieving a nanoscale grain size and maintaining it during processing. The smaller the grain size, the more favorable it is for the grains to coalesce and grow. The grains can be “stabilized” at a small size by adding a second element or second phase. The secondary addition will either reduce the driving force for grain growth (thermodynamic stabilization) or will physically hinder the grain growth (kinetic stabilization). There are specific parameters required of the solute(s) added to the metal. Often these are elements or compounds that are significantly different in size or stable at high temperature. Two examples are zirconium or tungsten added to copper. Zirconium atoms are much larger than copper atoms and they segregate to grain boundaries and reduce the driving force for growth. Tungsten, on the other hand, exists in the form of discrete particles which pin the grain boundaries from moving. Images of these materials are below (click on images for full resolution).

Related publications are listed below (for full list, see Publications).

 

Publications on Nanocrystalline Metals and Alloys: