The Age-Hardening Mechanism of Nanocrystalline Ni-P Alloys Synthesized by Electrodeposition

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In this study, we aim to clarify the strengthening mechanism of age-hardenable Ni-P alloy where very fine Ni3P precipitates have been believed to harden the nanocrystalline structures by the so-called precipitation hardening. Specimens were synthesized by electrodeposition followed by aging treatment at temperatures ranging from 200 to 600°C. As-deposited structures were amorphous and became crystallized into nanocrystals by the subsequent aging treatment accompanying Ni3P precipitation. Microstructures, and distributions of grain size of Ni matrix and Ni3P particles size were examined in detail by field-emission type transmission electron microscopy. After the aging treatment under 300°C, average grain size of Ni matrix was smaller than 20nm with finer Ni3P precipitates inside the Ni grains. On the other hand, after aging at 350°C and higher temperatures, both the grain size of Ni matrix and Ni3P particle size were grown to be comparable. Maximum hardness was obtained after the aging treatment at 350°C, and it was harder than that predicted by rule-of-mixture between pure Ni and Ni3P. Therefore, the additional strengthening may operate, which is not associated with the precipitate hardening. Less hardening by the precipitate in nanocrystalline structures is associated with a different deformation mechanism such as grain boundary sliding, which may operate in nanocrystalline regime.