Multilayer Structure Control for Improving Tensile Strength and Ductility in Electroformed Bulk Nanocrystalline Iron-Nickel Alloys

Multilayer structures for improving both the strength and ductility of electroformed multilayer nanocrystalline iron-nickel alloy composed of alternately stacked nickel-rich (fcc) alloy and iron-rich (bcc) alloy layers were explored. Multilayer structures were produced from single electrolyte mainly composed of nickel sulfamate and iron chloride using a sequence-controlled DC power supply. Nanocrystalline Fe-Ni alloy specimens with multilayer structures ranging from 3 to 75 layers were produced. The strength and elongation increase gradually in specimens with layers ranging from 25 to 45. When the number of layers was in the range of 45 to 51, both the tensile strength and elongation increased significantly. The specimen with 51 layers showed the ultimate tensile strength of 2.12 GPa and elongation of 12.5%. As the number of layers further increased, the tensile strength and elongation decreased, and the 75-layered specimen fractured with almost no plastic deformation. Interphase boundaries contribute to the strengthening and deformation of multilayer alloys; however, as the interfacial spacing decreases, the bcc alloy layer with a low thickness hardens and loses its ability to deform, resulting in brittle fracture. It was shown that the optimum number of layers for enhancing both strength and ductility is governed by the interfacial spacing, namely, the thickness of the bcc alloy layer in the multilayer material.