Grain size dependent phase stability of pulse electrodeposited nano-grained Co-Ni films

Grain size dependence of phase transformation and phase stability for nano-grained alloys remains controversial, due to the lack of high quality nano-grain alloys and coupling with the growth of grains upon heating. Less impurity, nano-grained Co-30.8 wt% Ni films with a mean grain size of 13 nm were prepared by pulse electrodeposition. The as-received nano-grained Co-Ni films consist of both the low-temperature-phase HCP phase and the high-temperature-phase FCC phase. Phase transformation from HCP to FCC in the nano-grained Co-Ni upon heating was not observed until the growth of grain took place at about 320 degrees C which was about 100 degree higher than the reverse martensitic transformation (HCP to FCC) temperature of coarse-grained Co-Ni alloy with the same composition. On the other hand, the forward martensitic transformation from FCC to HCP, which was greatly suppressed in the annealed samples, had not been observed even the samples cooled to the liquid nitrogen temperature. The experiment result shows that both the MCP low-temperature-phase and the FCC high-temperature-phase in the nano-grained Co-30.8 wt% Ni films have an abnormal stability. (C) 2012 Elsevier B.V. All rights reserved.