Enhancement in mechanical properties of bulk nanocrystalline Fe–Ni alloys electrodeposited using propionic acid

A method for enhancement of the mechanical properties of electrodeposited bulk nanocrystalline Fe–Ni alloys was explored. Bulk nanocrystalline Fe–Ni alloys were prepared using electrolytes that primarily consisted of iron sulfate and nickel sulfamate with propionic acid. Varying the concentration of propionic acid in the deposition bath produced no significant change in the C, S, or Ni content or grain sizes of the electrodeposited alloys. In contrast, the hardness of bulk nanocrystalline Fe–Ni alloys increased from 4.0 to 5.0 GPa with the increase of propionic acid concentration from 0 to 3.0 g/L. In addition, the tensile strength also increased from 1.6 to 1.7 GPa while maintaining a good tensile ductility of ~10%. These increases for electrodeposited alloys can be explained by grain boundary relaxation strengthening. After annealing at relatively mild temperatures, the alloys electrodeposited without propionic acid exhibited higher strength than at the as-deposited state, while the strength of the alloys electrodeposited using 3.0 g/L propionic acid remained unchanged. The results of this study indicate that the addition of propionic acid to the deposition bath could remove the excess grain boundary defects of Fe–Ni alloys in as-deposited states, requiring no additional thermal treatments and enhancing the mechanical strength to high levels.