Using transmission electron microscopy, we have analyzed dislocations in AlN nucleation layers and GaN films grown by metallorganic chemical vapor deposition (MOCVD) on the (0001) surface of epitaxially-grown 4H-SiC mesas with and without steps. For 4H-SiC substrates free of SiC surface steps, half-loop nucleation and glide parallel to the AlN/SiC interfacial plane play the dominant role in strain relief, with no mechanism for generating threading dislocations. In contrast, 4H-SiC mesa surfaces with steps give rise to regions of high stress at the heteroepitaxial interface, thereby providing an environment conducive to the nucleation and growth of threading dislocations, which act to accommodate misfit strain by the tilting of threading edge dislocations.

We investigated plastic deformation of fully dense electrodeposited nanocrystalline (NC) Ni with an average grain size of 30-40nm. We studied the microstructural evolution during cold rolling of NC Ni to a reduction in thickness up to 76% (true strain equivalent ∼1.42). We determined changes in texture, grain morphology, grain boundary character and grain sizes as a function of cold rolling strain, using X-ray diffraction and transmission electron microscopy TEM. We compared our results of the NC Ni with our own results for cold rolled coarse grained (CG) Ni. Differences and similarities in deformation behavior are discussed with respect to well documented findings in the literature.

We compute the strain fields and the interactions between dislocations at the junctions of classical small-angle grain boundaries. It is shown that, in contrast with the results for infinite small-angle boundaries, there are always forces acting on the dislocations in the arrays that define the grain boundaries, and that there is also an excess elastically stored energy associated with the triple junction (TJ). The forces on the dislocations and the excess stored energy of the TJ are shown to vary with the dihedral angles formed by the grain boundaries, and that the “equilibrium” dihedral angle based upon the Herring equation and the energies of the individual grain boundaries does not correspond to any kind of force or energy minimum. This relates to an unwarranted assumption in Herring's original derivation, that no interactions occur between the grain boundaries that make up a TJ.

Twin boundary motion is the mechanism that drives the plastic deformation in magnetic shape memory alloys (MSMAs), and is largely dependent on the twin microstructure of the MSMA. The twin microstructure is established during the martensitic transformation, and can be influenced through thermo-magneto-mechanical training. For self-accommodated and ineffectively trained martensite, twin thickness and magnetic-field-induced strain (MFIS) are very small. For effectively trained crystals, a single crystallographic domain may comprise the entire sample and MFIS reaches the theoretical limit. In this paper, a numerical simulation is presented describing the twin microstructures and twin boundary motion of self-accommodated martensite using disclinations and disconnections (twinning dislocations). Disclinations are line defects such as dislocations, however with a rotational displacement field. A quadrupole solution was chosen to approximate the defect structure where two quadrupoles represent an elementary twin double layer unit. In the simulation, the twin boundary was inclined to the twinning plane which required the introduction of twinning disconnections, which are line defects with a stress field similar to dislocations. The shear stress - shear strain properties of self-accommodated martensite were analyzed numerically for different initial configurations of the twin boundary (i.e. for different initial positions of the disconnections). The shear stress - shear strain curve was found to be sensitive to the initial configuration of disconnections. If the disconnections are very close to boundaries of hierarchically higher twins – such as is the case for self-accommodated martensite, there is a threshold stress for twin-boundary motion. If the disconnections are spread out along the twin boundary, twinning occurs at much lower stress.