Interplay between thermodynamic and kinetic stabilization mechanisms in nanocrystalline Fe-Mg alloys

Mechanisms governing thermal stability of nanocrystalline metal alloys have typically been classified as either “kinetic” or “thermodynamic”, although both should generally be expected to manifest at the same time and either act constructively, or compete. The aim of this work is to present a quantitative study of thermal stability in ball-milled Fe-Mg alloys with two concurrent stabilization mechanisms at play—grain boundary segregation and Zener pinning by oxide nanoparticles—to assess their relative potency and to explore the interplay between them. To that end, we studied the alloys' configuration and attendant thermal stability by varying their composition, annealing time, temperature and atmosphere. At the mesoscale, both grain growth and oxide evolution were tracked by in-situ annealing during x-ray diffraction, as well as by electron microscopy and atom probe tomography. At the nanoscale, the local grain boundary chemistry was also probed. We discuss our results, which lie between the expectations from each separate mechanism, in terms of two primary interactions them. These results also suggest a method to control both grain and oxide precipitate size within a unified framework, and thus helps elaborate the design space for nanocrystalline alloys.