Mechanical and fracture behaviour of the three-scale hierarchy structure in As-deposited and annealed nanocrystalline electrodeposited Ni-Fe alloys

The growing interest in nanocrystalline (nc) materials is driven by their outstanding combination of mechanical and functional properties. Electrodeposited nc Ni-Fe alloys have received considerable attention, thanks to their unique strength and to their thermal and magnetic properties. However, the research on the relationship between microstructure, mechanical properties and fracture behaviour, under both As-deposited and annealed conditions, is still rather limited. In this paper, 48wt% Fe electrodeposited nc Ni-Fe alloy foils were tested, by means of tensile and nanoindentation tests, under both As-deposited and annealed conditions (300-800 degrees C). High-resolution FESEM images revealed that the As-deposited microstructure consisted of an unforeseen, evident anisotropic nested three-scale hierarchy nc structure, namely from the nanocrystalline (similar to 10nm) to the sub-micron (up to 250nm) scale, across a mesostructure (155-165nm), here denoted as the characteristic grain structure. Such a nested nc grain structure of the electrodeposited nc Ni-Fe alloy, which resembles the one of a corncob (corncob-like structure), explained the unique anisotropic mechanical properties of the foils after nanoindentation, tensile and oblique-bending fracture tests. The mechanical properties (i.e. tensile strength, elongation, yield strength and hardness) of the foils annealed up to 320 degrees C were found to be improved, in comparison with the As-deposited counterparts, whereas those annealed above 320 degrees C were deteriorated.