The mechanical behavior of a layered nanostructured Ni with an alternating growth of ultrafine grains and nano-sized grains fabricated by electrodeposition

A layered nanostructured (NS) Ni with an alternating growth from the ultrafine-grains (UFG) layer with a thickness of 5 µm and the nano-sized grains (NG) layer with a thickness of 10 µm along the across-section is fabricated by periodically controlling the current density during electrodeposition. It is revealed by transmission electron microscope (TEM) and X-ray diffractometer (XRD) that the alternating growth of well-defined layers consists of the ultrafine-grains with the mean grain size of about 500 nm along a (200) preferential growth and the nano-grains with the mean size of 20 nm with a random growth. Tensile tests reveal that in comparison with the monolithic UFG and NG Ni, the layered NS Ni exhibits a higher tensile ductility of near 13% elongation to failure while maintaining a moderate ultimate tensile strength of near 1300 MPa. The improvement in the ductility for the layered NS Ni can be mainly attributed to two effects including the increased strain hardening ability sourced from the action of these specific microstructure involving the periodical distribution of ultrafine-grains, the existence of interface and the alternation of crystal orientation between the layers on the dislocation activities and alleviating the local stress concentration caused by the periodic coordination deformation of the UFG layer in the NG substrate. A larger homogeneous surface deformation trace with the uplift and collapse feature is observed for the layered NG Ni in comparison with the monolithic UFG and NG Ni, which further indicates a large uniform plastic deformation ability that the layered NG Ni holds. Additionally, the mutual restraint and coordination deformation between the UFG and NG layer also leads to the formation of a fracture feature with a smaller and uniform dimple structure.