Specimen size effect on superplastic flow behavior of electrodeposited nanocrystalline nickel


Nanocrystalline nickel foils with a grain size of ~12 nm are produced by pulsed current electrodeposition using a modified Watts electrolytic bath. Saccharin, a sulfur-based organic compound, used as a grain refiner and stress reliever to produce free-standing Ni foils. Nano-Ni tensile specimens displayed superplastic elongations (>400 %) at high strain rate of 3×10-1 s-1 and relatively low temperature of 777 K. Tensile specimens with different gauge lengths and thicknesses exhibited significant specimen size effect on maximum stress, elongation to failure and flow behavior under superplastic conditions. An increased total elongation to failure with increasing the specimen thickness and decreasing the gauge length was observed in the electrodeposited Ni foils. The stress-strain behavior of Ni changed from strain hardening to strain softening with increasing the specimen thickness. Strain rate jump tests indicated a very high strain rate sensitivity index values (m~0.5—0.8) for nano-Ni specimens at 777 K with a slight increased value of m with increasing the specimen thickness. The as-deposited nano-Ni samples showed substantial grain growth during heating and deformation as well at the testing temperature of 777 K. Microstructural investigation revealed that the total number of grains across specimen thickness and the formation of oxide layer during high temperature deformation can influence the superplasticity characteristics of nanocrystalline Ni.

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