Year

2015

Degree Name

Doctor of Philosophy

Department

School of Mechanical, Materials and Mechatronics Engineering

Abstract

Molecular dynamics simulations are performed to investigate the deformation mechanisms of nanotwinned materials. The simulations oftextured polycrystalline Cu under tensile loading parallel to the twin boundary (TB) reveal that the transmissions of dislocations dominate the plastic deformation. The majority of the TBs retain their initial coherency even after a considerable deformation. The tensile strength monotonically increases as the twin spacing decreases. The main strengthening effect in nanotwinned Cu results from TB restricting the dislocation transmission across TB. Dislocation processes involved in the slip-twin interactions are identified at atomic level, including the direct and indirect transmissions. The direct transmission involves either the successive transmission of the leading and trailing partials by the Fleischer cross-slip model or the absorption and desorption of the extended dislocation by the Friedel-Escaig cross-slip mechanism. In contrast, the indirect transmission involves the formation of special superjogs. The persistent slip transfer leaves zigzag slip traces on the cross-sectional view. The plastic anisotropy of individual grains leads to an inhomogeneous deformation, which results in the formation of intersected slip bands on the plane view.

The simulations of nanotwinned Cu with different orientations reveal a dynamic transition in deformation mechanisms as the TB orientation changes. The dislocation activities in nanotwinned Cu obey the Schmid’s law. Three distinct dislocation processes are identified. When the TB is parallel or inclined by an angle less than 30° to the loading direction, the plastic deformation is dominated by the transmission of extended screw dislocations, which involves the successive transmissions of the leading and trailing dislocations and leaves the TB intact. As the angle of inclination increases, TB migration takes place in addition to slip transfer. When the TB is inclined by an angle between 30° and 60° with respect to the loading axis, TB migration governs the deformation. The nanotwinned samples can be fully twinned or detwinned to form single crystals depending on the TB orientations, which eliminates the TBs. When the TB is inclined by an angle between 60° and 90° to the loading axis, dislocation-TB interactions dominate the deformation, where deformation twins and shear bands are formed.

Deformation twinning occurs under specific crystallographic orientations. The formation of deformation twins involves the motion of Shockley partial dislocations on adjacent {111} slip planes. Two twinning mechanisms are identified based on the arrangement of these Shockley partials. The first type of twins involves the overlap of Shockley partials of different types, which occurs when the TB is inclined by an angle between 15° and 30° to the loading direction. Thin twin plates embedded in the original grains are formed, which are not sable and can be easily eliminated under further deformation. The second type of twins results from the passage of Shockley dislocations of the same type, which takes place when the TB is inclined by an angle between 75° and 90° to the loading axis. Deformation twins are well developed along one primary twinning system in samples with inclination angles close to 75°, while multiple twinning systems are activated in samples with inclination angles close to 90°. The twinning dependence on the crystallographic orientation agrees with available experimental results to some extent.

The simulations of nanotwinned Fe reveal a dynamic transition in the deformation mechanism as the TB orientation changes. The slip activities in nanotwinned Fe cannot be explained by the Schmid’s law alone. The TB orientation determines three distinct dislocation processes. When the TBs are parallel or inclined less than 30° to the loading direction, the samples eventually fracture in an almost brittle manner. The initial yielding in samples with slanted TBs is due to TB migration. When the TBs are inclined with angles between 45° and 60° to the loading direction, TB migration dominates the plastic deformation. The nanotwinned samples can be fully detwinned or twinned to form single crystals depending on the TB orientations, which eliminates the original TBs. When the TBs are inclined by more than 74° to the loading axis, dislocation-TB interactions are the main deformation mechanism, which forms complicated dislocation networks. Under vertical orientations, extensive slip transmission takes place in samples with small twin spacing, while twinning occurs in addition to conventional dislocation slip in samples with large twin spacing.

Share

COinS