Nanocrystalline metals are polycrystalline metals with grain sizes in the nanometer range. They have attracted significant interest in recent years due to their unique mechanical and electrical properties. The main objective of this thesis is to develop continuum-scale descriptions of nanoscale deformation and failure mechanisms in nanocrystalline metals. The research has focused on three speci...

A series of tensile testing was carried out on the ultrafine grained 5083 Al alloy, which was fabricated by equal channel angular pressing (ECAP) with different ECAP strains, at low temperature superplastic (LTS) temperature of 548K. This investigation was aimed at examining the effect of the ECAP strain inducing different microstructure in the alloy on the deformation mechanisms at LTS regime....

The plastic behavior of crystalline materials is mainly controlled by the nucleation and motion of lattice dislocations. We report in situ dynamic transmission electron microscope observations of nanocrystalline nickel films with an average grain size of about 10 nanometers, which show that grain boundary-mediated processes have become a prominent deformation mode. Additionally, trapped lattice...

The utility of the notion of generalized disclinations in materials science is discussed within the physical context of modeling interfacial and bulk line defects like defected grain and phase boundaries, dislocations and disclinations. The Burgers vector of a disclination dipole in linear elasticity is derived, clearly demonstrating the equivalence of its stress field to that of an edge disloc...

Grain-boundary (GB) diffusion creep (Coble creep) is the dominant deformation mechanism for the fine-grained materials under low stress and at elevated temperature [1]. From the classical Coble creep formula [1], the strain rate is proportional to the external stress, and inversely proportional to the cube of the grain size: 3 ~ − d σ ε& . During creep deformation the grains become elongated in...

Microstructure evolution during “abc” deformation of commercial-purity titanium was investigated. Continuous dynamic recrystallization (CDRX) played a dominant role in the microstructural changes, although mechanical twinning also contributed to some extent. It found that CDRX developed preferentially at original grain boundaries, thus resulting necklace-type microstructure. This effect attribu...

A special mechanism/mode of plastic deformation occurring through stress-driven rotations of low-angle grain boundaries (GBs) near crack tips in nanocrystalline and ultrafinegrained (UFG) materials is theoretically described. It is demonstrated that such rotations of GBs represent energetically favorable processes in wide ranges of parameters characterizing precracked specimens with nanocrystal...

An alloy can be created without melting, by violently deforming mixtures of different powders. As first discovered by Benjamin, inert oxides can also be introduced into the microstructure using this technique. The resulting dispersion-strengthened alloyed powders are then consolidated using hot-isostatic pressing and extrusion, to produce a solid with a very fine grain structure on a sub-microm...

The properties of materials change, sometimes catastrophically, as alloying elements and impurities accumulate preferentially at grain boundaries. Studies of bicrystals show that regular atomic patterns often arise as a result of this solute segregation at high-symmetry boundaries, but it is not known whether superstructures exist at general grain boundaries in polycrystals. In bismuth-doped po...