Effect of low-temperature annealing on tensile behavior of electrodeposited bulk nanocrystalline Ni-W alloys

Relatively low-temperature thermal treatment of nanocrystalline metals results in grain boundary relaxation and the absence of clear grain growth. However, the manner in which the relaxation affects the tensile properties of bulk nanocrystalline metals is unclear. In this study, the effect of low-temperature annealing on the tensile behavior of nanocrystalline metals was studied using electrodeposited Ni-W alloys as a model system. Tensile tests were conducted using the electrodeposited bulk nanocrystalline Ni-W alloys in the as-deposited state and after annealing at 200-300 °C for 24 h. After annealing, the yield strength and tensile strength increased by 0.25-0.34 and 0.13-0.18 GPa, respectively, while uniform elongation decreased from 4.8% to 2.5%. Good local elongation of 6.0-7.0%, which was comparable with the value in the as-deposited state, was observed after annealing at 200 and 250 °C. However, annealing at 300 °C induced a degradation of local elongation to 2.6%. The results of this study indicate that reduction of excess boundary defects affects dislocation emission from grain boundaries, leading to increasing yield strength and decreasing uniform elongation, whereas the relaxation has no effect on the local elongation. In addition, discussion of the microstructures of Ni-W alloys also suggest the possibility that the failure mode changes at annealing temperatures near 250-300 °C by changing the shape of grain boundaries more linearly, in addition to a reduction of excess boundary defects.