Tensile strength and ductility of ultra-fine-grained nickel processed by severe plastic deformation

The structure and mechanical properties of Ni subjected to severe plastic deformation by means of equal channel angular pressing (ECAP) followed by cold rolling or high-pressure torsion were investigated. A material with uniform ultra-fine-grained (UFG) microstructure and mean grain size in the range from 100 to 600 nm, depending on the processing procedure, was obtained. Tensile tests at room temperature revealed ultimate tensile strength values in the range from 840 to 1270 MPa and an elongation to fracture in the range from 7 to 12% depending on the processing method. Annealing at 200 °C of the sample deformed by ECAP and cold rolling led to the best combination of strength and plasticity. The strength of the material was well above the level expected from the Hall–Petch rule. We explained this high strength by taking into account dislocations trapped at grain boundaries (TLDs). In ultra-fine-grained (UFG) materials the specific surface of grain boundaries is high and therefore the TLDs density is high as well. The fracture and deformation relief indicate an important role of grain-boundary sliding and grain rotation as deformation mechanisms. The UFG high-strength Ni with relatively good ductility has a potential for applications in particular in microsystems, where the cross section of the material is in the micrometer range but still must cross a sufficient number of grains to look structurally uniform.