Tailoring compositional gradient in electrodeposited Ni-Co alloys to achieve strength-ductility synergy and microstructural stability at nanoscale

The gradient nanocrystalline (NC) materials are expected to provide unique mechanical properties that are difficult to achieve in homogenous NC metals and alloys. The performance of the electrodeposited NC Ni-Co system is of interest as an alternative to hard chrome deposits. The present work focuses on the experimental investigation of compositionally gradient Ni-Co alloy with a gradual increase in Co content from zero to 25 wt% (NC Gr-NiCo). A free-standing, compositionally gradient alloy foils with a thickness of ∼200 μm are synthesized using pulsed electrodeposition, leveraging anomalous co-deposition. The homogeneous foils possessing grain sizes of ∼30–35 nm are also developed by adjusting the composition of the anode. The aim is to evaluate the effect of gradient in Co content on microstructural evolution, and hardness and tensile properties of NC NiCo alloy in as-deposited and heat-treated conditions, using electron microscopes, nanoindentation and tensile test, and differential scanning calorimetry (DSC), respectively. The gradient alloy (NC Gr-NiCo) shows better strength and ductility than the counterpart homogeneous Ni and NiCo alloy. The improvement in strength and ductility has been attributed to the synergistic effect of multiple strengthening mechanisms, such as strain gradient plasticity and heterogeneous induced deformation due to varying composition. Upon heat treatment, the degree of gradient in terms of grain size is enhanced in NC Gr-NiCo alloy. A transition from abnormal grain growth to normal grain growth has been observed in NC Ni containing Co, possibly, because of the reduced boundary migration owing to the increased twin fraction with Co content.