Improving ductility in ultrafine grained nickel with porosity and segregation via deformation

When bulk nanostructured (NS) materials are prepared via consolidation of individual particles, agglomerates or clusters, extraneous defects, such as porosity, insufficient bonding, and impurities are sometimes introduced leading to the degradation of ductility. In this study we propose to examine the hypothesis that deformation can be used to ameliorate the negative effects of these artifacts on the ductility of NS alloys. The approach involved cryomilling and forging to synthesize bulk NS nickel with porosity (95.5% theoretical density) and nitrogen grain boundary (GB) segregation. The results demonstrate that cold rolling resulted in an increase in tensile ductility from 2 to ∼4%, with a slight decrease in yield strength from 1150 to 1050 MPa. Microstructural analyses suggest that the elimination of nano-porosity together with the physical breakdown of a continuous nitrogen layer at GBs during cold rolling is possibly responsible for the observed ductility enhancement, and in the case of the latter phenomena, a corresponding decrease in strength also.