Manipulation of solute partitioning mechanisms for nanocrystalline stability

We describe an approach that enables the activation of two distinct nanocrystalline stability mechanisms in the same Ni(P) alloy by controlling the annealing temperature and chemical composition. Using a two-step heating process, it is shown that a low temperature first stage anneal (350 °C/1 h) drives the solute in a Ni-1at%P alloy to decorate the grain boundaries (GBs) in achieving thermodynamic stabilization at higher temperatures (550 °C). Atom probe tomography reveals a very low contamination level of species (< 0.2 at.%) that concretely demonstrates this stability mechanism. If the annealing avoids this initial, low temperature step and directly reaches the higher temperatures (550 °C), the solute precipitates a stable Ni ₃P phase pinning the GBs. Until this secondary phase forms, the grains coarsen significantly. On changing the alloy chemistry to Ni-4at.%P, in either the two-step (350 °C/1 h then 550 °C) or single-step (directly to 550 °C) anneal, the solute precipitates the Ni ₃ P phase and engages the Zener pinning mechanism. Comparing the two stabilization mechanisms, thermodynamic versus Zener pinning, the thermodynamic mechanism yields a more refined, stable grain size with annealing temperature even though the pinning strength from a uniform distribution of solute is found to be lower than that of the Zener mechanism. These results are discussed using complementary atomistic simulations.