Microhardness and tensile strength of electrochemically synthesized nickel-cobalt binary alloy sheets exfoliated from a dumbbell-shaped titanium cathode

Abstract

Nanocrystalline nickel–cobalt (Ni–Co) binary alloy sheets were fabricated through electroforming in an acidic aqueous bath using exfoliation from a metallic titanium cathode. Cobalt content in Ni–Co alloy sheets ranged from 28.8 at% to 72.0 at% depending on experimental parameters, such as cathodic overpotential and bath composition. The surface roughness (R a) of the electroformed alloy sheets significantly decreased down to 1.5 μm as saccharin sodium dihydrate was added as an additive to the acidic aqueous solution bath. X-ray diffraction profiles and transmission electron microscopy images indicated that the electroformed Ni–Co alloy sheets have a nanocrystalline structure (grain size ≈ 30 nm). The lattice constant of the electroformed Ni–Co alloy sheets increased with an increase in cobalt content (i.e. solute atom concentration). The mechanical properties were significantly improved because of the synergistic effects of crystal grain refinement and solid solution strengthening. The microhardness and tensile strength of the electroformed Ni–Co alloy sheets reached 609 kgf mm−2 and 1757 MPa (X Co = 49.9 at%), respectively. The tensile strength of the electroformed Ni–Co alloy sheets in this study significantly exceeded that of solidified Ni–Co alloys (approximately 370 MPa). Therefore, this study offers a technique to enhance the mechanical properties of electroformed Ni–Co alloy sheets.