The Role of Geometrically Necessary Dislocations in Cantilever Beam Bending Experiments of Single Crystals

Mapping pure plastic strains against locally applied stress: Revealing toughening plasticity

The deformation of all materials can be separated into elastic and plastic parts. Measuring the purely plastic component is complex but crucial to fully characterize, understand, and engineer structural materials to "bend, not break." Our approach has mapped this to answer the long-standing riddle in materials mechanics: The low toughness of body-centered cubic metals, where we advance an experimentally led mitigative theory. At a micromechanically loaded crack, we measured in situ the stress state applied locally on slip systems, and the dislocation content, and then correlatively compared with the occurrence-or not-of toughness-inducing local plasticity. We highlight limitations and potential misinterpretations of commonly used postmortem transmission electron imaging. This should enable better-informed design for beneficial plasticity and strength in crystalline and amorphous solids alike.

Gradient Crystal Plasticity: A Grain Boundary Model for Slip Transmission

Interaction between dislocations and grain boundaries (GBs) in the forms of dislocation absorption, emission, and slip transmission at GBs significantly affects size-dependent plasticity in fine-grained polycrystals. Thus, it is vital to consider those GB mechanisms in continuum plasticity theories. In the present paper, a new GB model is proposed by considering slip transmission at GBs within the framework of gradient polycrystal plasticity. The GB model consists of the GB kinematic relations and governing equations for slip transmission, by which the influence of geometric factors including the misorientation between the incoming and outgoing slip systems and GB orientation, GB defects, and stress state at GBs are captured. The model is numerically implemented to study a benchmark problem of a bicrystal thin film under plane constrained shear. It is found that GB parameters, grain size, grain misorientation, and GB orientation significantly affect slip transmission and plastic behaviors in fine-grained polycrystals. Model prediction qualitatively agrees with experimental observations and results of discrete dislocation dynamics simulations.

Microbending Experiments on Pure Magnesium with Nonbasal Slip Orientation

In the present study, in situ microbending experiments on magnesium single crystalline microcantilevers are presented. Microcantilevers with pentagonal cross-section were fabricated by focus ion beam. Two basic crystallographic orientations of the microcantilevers were investigated: {0001} and {10-10}, i.e., the c-axis perpendicular to and parallel with the cantilever top surface, respectively. After bending, the longitudinal sections of the microcantilevers were analyzed using electron backscatter diffraction to investigate the crystal lattice rotations and accumulated deformations. The stress levels in the loaded cantilevers are strongly dependent on the crystal orientation. Extension twins were found in the {10-10} cantilevers.