Dislocation-mediated stress relaxation in megasonic assisted nickel electrodeposition

The inherent internal stress generated during nickel electrodeposition induces warpage of electroformed mold inserts and degrades the precision of optical components. Herein, we propose a cathode-localized megasonic-assisted electroforming strategy with an integrated vibrating cathode to achieve directional, efficient interfacial energy input. The results show that localized megasonic vibration simultaneously reduces internal stress and improves hardness of nickel electrodeposits. The internal stress decreases from 203.0 MPa to 165.4 MPa at 2 A dm^-2, while the hardness increases from 252.8 HV to 298.6 HV at 5 A dm^-2. Combined SEM, XRD, TKD and electrochemical measurements indicate that megasonic streaming accelerates interfacial mass transfer, refines nickel grains and elevates dislocation density. The abundant grain boundaries act as dislocation storage sites to relieve internal stress, while grain refinement strengthens nickel deposits via the Hall-Petch rule. Hydrogen evolution analysis shows suppressed hydrogen evolution is not the primary cause of stress reduction. Finally, the low-stress and high-hardness nickel molds are applied for fabrication of polymer Fresnel lenses via grayscale lithography, electroforming and hot embossing processes, verifying the scalability of this process chain for micro-optical device manufacturing.