Age-hardening mechanism for nanocrystalline Ni-P alloys synthesized by electrodeposition

Strengthening mechanism of age-hardenable electrodeposited Ni-P alloy was investigated focusing on the role of Ni3P precipitates in nanocrystalline structures. Specimens of P content ranging from 3.0 to 12.5 wt.% were synthesized by electrodeposition followed by aging treatment at a temperature between 473 and 773 K. The specimens became harder with higher P content after the electrodeposition and aging treatment. As-electrodeposited structures comprised only Ni phase, and Ni3P precipitated by the subsequent aging treatment. Vickers hardness of electrodeposits showed the maximum after aging at 673 K, and was higher than that predicted by the rule of mixture of Ni and Ni3P phases. Distribution of parent Ni grain size and Ni3P precipitate size was examined in detail by field-emission type transmission electron microscopy. The maximum hardness was obtained when the grain size of Ni matrix and Ni3P particle size were comparable. The hard interphase boundary to the dislocation slip is included as a possible mechanism which is responsible for the hardening by the aging treatment rather than the classical precipitation hardening.