Design of a stable nanocrystalline alloy

Abstract

Nanocrystalline materials are inherently unstable due to their high material fraction of grain boundaries, preventing their improved properties from being used in application. To stabilize the nanoscale grain size against rampant growth, past literature has proposed lowering the grain boundary energy through solute segregation to the boundaries. This approach has seen varied experimental success, using a metric of segregation strength to select an alloying element. In those alloys were some measure of stabilization is gained through alloying, precipitation of a second phase disrupts the necessary segregation state and triggers grain growth. This work considers the total stability of a nanocrystalline alloy -- both stability against grain growth and stability against second phase precipitation -- by examining the changes in free energy associated with segregation and nanostructuring. It is discovered that the relationship of segregation and mixing enthalpies for a particular system dictates the nanocrystalline stability of the alloy. Nanocrystalline stability maps are constructed for several conditions that can be used to predict an alloy's ability to support a nanostructure. Also revealed by the generation of stability maps are new types of stable nanocrystalline phases - in addition to the expected nano-phase stabilized by solute decoration of the grain boundary, a solute-rich nanocrystalline phase is expected to be stable under some conditions, as is an amorphous phase. To connect material systems to the results predicted by this work, a new model for the enthalpy of segregation was derived.