Influence of Annealing on the Microstructure and Mechanical Properties of Electrodeposited Nanocrystalline Nickel

Abstract

The evolution of the microstructure and mechanical properties of electrodeposited nanocrystalline Ni with different annealing procedures was studied systematically. For the annealed specimens hardness decreases with increasing average grain size but the dependence changes at different grain size ranges. The specimens annealed at a low temperature show higher hardness compared to the as-deposited nanocrystalline Ni, despite an increased measured average grain size. In association with this hardening an increase in elastic modulus and a decrease in microstrain was observed after annealing. With increasing annealing temperature both the tensile strength and the fracture strain were observed to decrease, this is companied with a transition from ductile to brittle in the fracture surfaces. These results indicated that the mechanical behaviour of nanocrystalline Ni depends not only on the average grain size but also on the grain boundary structure. A change in the grain boundary state arising from annealing may be responsible for the observed increase in hardness and elastic modulus as well as the deterioration of tensile properties.

Electrodeposition and Characterization of Nanocrystalline Ni–B with Low Boron Content for MEMS Applications

“…Figure 5 shows the grain size of pure NC Ni and NC Ni-B electrodeposits as a function of heat treatment temperature. The grain size of NC Ni-B electrodeposits remained below 100 nm at 573 K regardless of the boron content, whereas abnormal grain growth occurred in pure NC Ni at around 473 K. (13,24) These results show that, in terms of micromorphology, alloying nickel with boron significantly enhances the thermal stability of NC Ni.…”

“…8(b), the elastic modulus of the as-plated NC Ni-B electrodeposits is around 200 GPa, which is similar to those of pure NC Ni and conventional polycrystalline Ni found in the literature. (24,26) However, as the heat treatment temperature increases, the elastic modulus of Ni-B electrodeposits is found to significantly increase regardless of the boron content, while the elastic modulus of pure NC Ni remains at a similar value. The maximum value of the elastic modulus is 291.4 GPa for Ni-0.19B electrodeposits heat treated at 673 K. Although it is speculated that Ni x B precipitators increase the elastic modulus, because they are migrated into the grain boundary region and hinder the grain boundary movement, further research should be conducted to identify the exact mechanism of this increase.…”