On the extrinsic Hall-Petch to inverse Hall-Petch transition in nanocrystalline Ni-Co electrodeposits

The Hall-Petch to inverse Hall-Petch (HP-IHP) transition of an electrodeposited nanocrystalline Ni-32at%Co alloy was examined by annealing the as-plated alloy to obtain grain sizes ranging between 18 - 239 nm. We describe this transition as an extrinsic HP-IHP transition (extrinsic to the fabrication process) as opposed to the intrinsic HP-IHP transition observed in materials in their as-plated state (intrinsic to the fabrication process). The extrinsic transition of the Ni-32at%Co alloy took place in the mid-stage of abnormal grain growth, at an average grain size of ~30 nm. Grain boundary relaxation governs the IHP region and maximum hardness was achieved at initial development of bimodal grain size; meanwhile late-stage abnormal grain growth and further normal grain growth led to the reduction in hardness in the HP region. Segregation of sulfur impurities to grain boundaries, crystallographic orientation and annealed twin development did not have significant contributions to the observed change in hardness.

Automated summary

In this study, the Hall-Petch to inverse Hall-Petch transition of electrodeposited nanocrystalline Ni-32at%Co alloy was investigated. The transition was found to occur in the mid-stage of abnormal grain growth, with grain boundary relaxation governing the inverse Hall-Petch region. Late-stage abnormal grain growth and further normal grain growth led to a reduction in hardness in the Hall-Petch region. The segregation of sulfur impurities, crystallographic orientation, and annealed twin development did not significantly contribute to the observed change in hardness.