Crystal Structure and Mechanical Properties of Ni-W Alloy Deposits

초록

The fabrication of amorphous Ni-W alloys by electrodeposition, and the change of crystal structure and mechanical properties with tungsten (W) content and heat-treatment were studied. Electrodeposition was conducted with various W mole fractions in a nickel sulfate solution using citric acid as a complexing agent. Current efficiency was sharply decreased when W mole fraction of higher than 0.74 due to severe H2 evolution with increasing cathode overvoltage. The surface glossiness was increased and roughness was decreased with increasing W content in the Ni-W alloy deposit. X-ray diffraction peak was broadened with increasing W content and an amorphous pattern was appeared clearly at 49wt.%W. It was confirmed that Ni-W alloy deposit was transformed from crystalline to amorphous structure between 41 and 49wt.%W. These results were verified through TEM analysis. Hardness of Ni-W alloy deposits were increased with increasing W content because of the solid solution hardening and the grain size refining effects. When the heat-treatment temperature was increased, the hardness was increased to reach the maximum of 1300VHN at 600℃ with Ni-58wt.%W deposit. The precipitation of intermetallic compounds such as Ni4W and NiW seemed to contribute to the increase of hardness. However, the hardness was decreased at over 600℃ because of the precipitation of pure W which was supersaturated in Ni matrix.