Direct imaging of short-range order and its impact on deformation in Ti-6Al.

Chemical short-range order (SRO) within a nominally single-phase solid solution is known to affect the mechanical properties of alloys. While SRO has been indirectly related to deformation, direct observation of the SRO domain structure, and its effects on deformation mechanisms at the nanoscale, has remained elusive. Here, we report the direct observation of SRO in relation to deformation using energy-filtered imaging in a transmission electron microscope (TEM). The diffraction contrast is enhanced by reducing the inelastically scattered electrons, revealing subnanometer SRO-enhanced domains. The destruction of these domains by dislocation planar slip is observed after ex situ and in situ TEM mechanical testing. These results confirm the impact of SRO in Ti-Al alloys on the scale of angstroms. The direct confirmation of SRO in relationship to dislocation plasticity in metals can provide insight into how the mechanical behavior of concentrated solid solutions by the material's thermal history

Accurate, rapid identification of dislocation lines in coherent diffractive imaging via a min-max optimization formulation

Defects such as dislocations impact materials properties and their response during external stimuli. Defect engineering has emerged as a possible route to improving the performance of materials over a wide range of applications, including batteries, solar cells, and semiconductors. Imaging these defects in their native operating conditions to establish the structure-function relationship and, ultimately, to improve performance has remained a considerable challenge for both electron-based and x-ray-based imaging techniques. However, the advent of Bragg coherent x-ray diffractive imaging (BCDI) has made possible the 3D imaging of multiple dislocations in nanoparticles ranging in size from 100 nm to1000 nm. While the imaging process succeeds in many cases, nuances in identifying the dislocations has left manual identification as the preferred method. Derivative-based methods are also used, but they can be inaccurate and are computationally inefficient. Here we demonstrate a derivative-free method that is both more accurate and more computationally efficient than either derivative- or human-based methods for identifying 3D dislocation lines in nanocrystal images produced by BCDI. We formulate the problem as a min-max optimization problem and show exceptional accuracy for experimental images. We demonstrate a 260x speedup for a typical experimental dataset with higher accuracy over current methods. We discuss the possibility of using this algorithm as part of a sparsity-based phase retrieval process. We also provide the MATLAB code for use by other researchers

Traumatic Spondylopelvic Dissociation: A Report of Two Cases of Spondylolisthesis at L5-S1 and Review of Literature.

Study Design Retrospective chart review and review of literature. Objective Few case reports of traumatic L5-S1 displacement have been presented in the literature. Here we present two cases of traumatic spondylolisthesis showing both anterior and posterior displacement, the treatment algorithm, and a review of the literature. Methods The authors conducted a retrospective review of representative patients and a literature review of traumatic spondylolisthesis at the L5-S1 junction. Two representative patients were identified with traumatic spondylolisthesis: one with an anterior dissociation, and the other with a posterior dissociation. Results Radiographic, computed tomography, and magnetic resonance imaging illustrated the bony and soft tissue injury found in each patient, as well as the final stabilization and outcomes. Operative stabilization was necessary, and both patients were treated with open reduction internal fixation. The patient with posterior dissociation had complete recovery without neurologic sequelae. The patient with anterior dissociation had persistent bilateral L5-S1 radiculopathy with intact rectal tone, due to neurologic compression. Conclusions Few cases of traumatic spondylopelvic dissociation that are isolated to the L5-S1 disk space are described in the literature. We examined both an anterior and a posterior dissociation and treated both with L5-S1 posterior spinal fusion. The patient with anterior dissociation had persistent L5-S1 root injury; however, the patient with posterior dissociation had no neurologic deficits. This is the opposite of what is expected based on anatomy. These cases offer insight into the management of anterior and posterior L5-S1 spondylopelvic dissociation

Polycrystalline graphene and other two-dimensional materials

Graphene, a single atomic layer of graphitic carbon, has attracted intense attention due to its extraordinary properties that make it a suitable material for a wide range of technological applications. Large-area graphene films, which are necessary for industrial applications, are typically polycrystalline, that is, composed of single-crystalline grains of varying orientation joined by grain boundaries. Here, we present a review of the large body of research reported in the past few years on polycrystalline graphene. We discuss its growth and formation, the microscopic structure of grain boundaries and their relations to other types of topological defects such as dislocations. The review further covers electronic transport, optical and mechanical properties pertaining to the characterizations of grain boundaries, and applications of polycrystalline graphene. We also discuss research, still in its infancy, performed on other 2D materials such as transition metal dichalcogenides, and offer perspectives for future directions of research.Comment: review article; part of focus issue "Graphene applications