Dislocation

Various modes of transmission electron microscopy including aberration corrected imaging were used in order to unravel the fundamental mechanisms controlling the formation of growth twins and the evolution of twin boundaries under mechanical and hydrogen loading modes in nanocrystalline (nc) palladium thin films. The latter were produced by electron-beam evaporation and sputter deposition and subjected to uniaxial tensile deformation as well as hydriding/dehydriding cycles. The results show that the twins form by dissociation of grain boundaries. The coherency of S3{111} coherent twin boundaries considerably decreases with deformation due to dislocation/twin boundary interactions while S3{112} inco-herent twin boundaries dissociate under hydrogen cycling into two-phase boundaries bounding a new and unstable 9R phase. The effect of these elementary mechanisms on the macroscopic behavior of the palladium films is discussed and compared to recent experimental and simulation works in the literature. The results provide insightful information to guide the production of well-controlled population of growth twins in high stacking fault energy nc metallic thin films. The results also indicate directions for further enhancement of the mechanical properties of palladium films as needed for instance in palladium-based membranes in hydrogen applications.

Classical theories on deformation twinning in face centered cubic (fcc) metals and alloys suggest that mechanical twinning is initiated in high stress conditions, in stage III hardening, at low temperatures or in high strain rate deformation conditions. The twin orientation is achieved by a homogeneous shearing of the matrix, which requires the highly coordinated glide of a/6 < 1 1 2 >-type partial dislocations with the same Burgers vector, with exactly one dislocation gliding on each successive {111}-type twinning planes. Twinning is initiated after slip on the primary slip system and the conjugate system has occurred and a high stress is available to activate the twin nuclei formed by dislocation interactions.
The twin formation modifies the prior dislocation structure, with some initially glissile dislocations of the parent crystal being transformed to sessile dislocations in the twin. The Burgers vector of some dislocations may change when they are transformed in the twin matrix. This process contributes to the strain hardening of the deformation twins. In addition, the dynamic Hall–Petch effect also contributes to strain hardening in polycrystals undergoing deformation twinning as twin boundaries are strong obstacles to glide and the formation of micro-twins gradually reduces the dislocation mean free path. Mechanical twinning in fcc metals does not twin single crystals or grains entirely, and the deformed microstructure always contains layers with the parent orientation alternating with twinned ones. Several deformation twin formation mechanisms have been proposed for polycrystalline metals and alloys, such as the emission of dislocations from periodic grain boundaries ledges, which assumes that twins can be generated directly, without the need to form suitable dislocation junctions. Using molecular dynamics and discrete dislocation dynamics simulation, Weinberger et al. argued that the emission of surface dislocations, nucleated at atomically sharp corners, compensates for dislocation
starvation during the deformation of micro-pillars in which dislocation starvation occurs. Zhu et al. also argued in favor of twinning by partial dislocation nucleation at surfaces, but they did not propose an actual mechanism for the generation of partial dislocations from surfaces. There are fundamental problems with the assumption that surfaces or grain boundaries can be a source of twinning dislocations, such as the fact that the emission of a partial dislocation loop requires a high critical shear stress, and that twin formation requires a precise partial dislocation sequence, i.e. one single partial dislocation on every {111} type slip plane. Partial dislocation emission from a surface has not conclusively been shown to be compatible with these requirements.
When small scale compression testing (in a nano-indentation setup) is used to analyze the deformation of micro-pillars, a characteristic size effect is observed [10,11]: the shear stress to initiate plastic flow increases with decreasing pillar diameter. Greer proposed a dislocation starvation model to explain the size effect. Dislocation source-hardening, which occurs in small samples due to the physical limitation of the dislocation source size, was proposed to account for the considerable strengthening in micron-sized single crystals. The size dependence of the strength of single-ended dislocation sources was verified by three-dimensional discrete dislocation dynamics simulation. Yu reported that the twinning stress in micro-pillars was also size-dependent. Seo et al. suggested that the strength of Au nano-wires, in which twins were formed, was inversely
related to their diameter. Size effects were also reported for TWIP steel micro-pillars deformed in compression. In the present study, the twinning-induced plasticity mechanism of TWIP steel was studied by means of compression tests
on bulk single crystals and micro-pillars, oriented either for perfect dislocation glide or, respectively, for deformation twinning. Transmission electron microscopy (TEM) was used to investigate dislocation interactions and the evolution of the deformation microstructure in the micro-pillars. The dislocation interactions were characterized at the initial stage of the deformation in which
no microstructural evolution had occurred in order to verify whether the yielding mechanism of the micro-pillars was associated with twin formation.
In summary we here study bulk and micro-pillar single crystals regarding the twinning-induced plasticity mechanism in austenitic Fe-22 wt%Mn-0.6 wt%C TWIP steel. Compression of micro-pillars oriented either for deformation-induced twinning or for perfect dislocation glide was carried out for pillars with diameters
in the range of 600 nm to 4 um. The same size dependence of the critical resolved shear stress was observed for both orientations. The critical micro-pillar diameter for size-independent plasticity was approximately 7.6 um. Partial dislocation-mediated formation of twins and e-martensite was observed in micro-pillars oriented for twinning by transmission electron microscopy. The elastic–plastic transition in micro-pillars oriented for deformation twinning did not involve twinning, and dislocation–dislocation interactions were a necessary precondition for twin formation.