Manipulation of solute partitioning mechanisms for nanocrystalline stability

XY Zhou and A Gupta and GJ Tucker and GB Thompson, ACTA MATERIALIA, 208, 116662 (2021).

DOI: 10.1016/j.actamat.2021.116662

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 degrees 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 degrees 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 degrees C), the solute precipitates a stable Ni3P 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 degrees C/1 h then 550 degrees C) or single-step (directly to 550 degrees C) anneal, the solute precipitates the Ni3P 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. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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