In nanostructured metals and alloys, the negative grain-size effect on yield strength, called “negative Hall-Patch relationship,” was known to be caused by grain boundary sliding. Furthermore, another important factor affecting the short and long-term deformation is the type and amount of interstitials present at grain boundaries. The interstitials, such as boron, sulfur, carbon, etc. have significant influence on deformation and fracture behavior at room as well as elevated temperatures in nanostructured nickel. To understand the effect of interstitials such as sulfur and boron in grain boundaries on deformation behavior, various mechanical tests including uniaxial and cyclic tensile tests, creep tests and DMA tests were performed along with X-ray diffraction analysis and TEM observation. Although each of these techniques reveals only partial information on the deformation mechanisms, the combined experiments have brought us closer to a comprehensive understanding on the deformation mechanisms. The nano-nickel sheets used in current experiments have grain sizes of 15 and 30 nm, respectively, and contain sulfur ranging from 0.03 to 0.12 at % and boron of 0.085 at %
The results show that the presence of such interstitials enhances yield strength and creep resistance, but reduces anelastic deformation and elongation. All the evidences point to the fact that the grain boundary deformation is the dominant mechanism in nano nickel with grain sizes less than 30 nm, while the role of lattice dislocations in deformation appears to be minimal at room temperature. The internal friction of nanostructured nickel has been investigated using the dynamic mechanical analyzer, in which the storage modulus, loss modulus and tangent delta were determined as functions of frequency (0.01–200 Hz) and temperature (300 K–473 K). The results show that the internal friction increases with decreasing grain size in nanostructured nickel. The energy loss is far greater in nanostructured nickel than polycrystalline nickel for the temperatures and frequencies investigated. The interstitials such as sulfur and boron as dopants to grain boundaries have a strong influence on the short- and long-term strength, and viscous deformation of nanocrystalline nickel alloys including anelastic relaxation
