抄録
Strengthening mechanism of age-hardenable electrodeposited Ni-P alloy was investigated focusing on the role of Ni3P precipitates in nanocrystalline structures. 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, in particular 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 or bellow, average grain size of Ni matrix was approximately 10 nm with finer Ni3P precipitating intra- or intergranulary. On the other hand, after aging at 350°C 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 the rule-of-mixture between pure Ni and Ni3P. In both the cases, the additional strengthening may operate, which is not associated with the classical 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 domain.
