Grain boundary velocity and curvature are not correlated in Ni polycrystals.

Grain boundary velocity has been believed to be correlated to curvature, and this is an important relationship for modeling how polycrystalline materials coarsen during annealing. We determined the velocities and curvatures of approximately 52,000 grain boundaries in a nickel polycrystal using three-dimensional orientation maps measured by high-energy diffraction microscopy before and after annealing at 800°C. Unexpectedly, the grain boundary velocities and curvatures were uncorrelated. Instead, we found strong correlations between the boundary velocity and the five macroscopic parameters that specify grain boundary crystallography. The sensitivity of the velocity to grain boundary crystallography might be the result of defect-mediated grain boundary migration or the anisotropy of the grain boundary energy. The absence of a correlation between velocity and curvature likely results from the constraints imposed by the grain boundary network and implies the need for a new model for grain boundary migration.

Reconstruction of Grains in Polycrystalline Materials From Incomplete Data Using Laguerre Tessellations.

Far-field three-dimensional X-ray diffraction microscopy allows for quick measurement of the centers of mass and volumes of a large number of grains in a polycrystalline material, along with their crystal lattice orientations and internal stresses. However, the grain boundaries-and, therefore, individual grain shapes-are not observed directly. The present paper aims to overcome this shortcoming by reconstructing grain shapes based only on the incomplete morphological data described above. To this end, cross-entropy (CE) optimization is employed to find a Laguerre tessellation that minimizes the discrepancy between its centers of mass and cell sizes and those of the measured grain data. The proposed algorithm is highly parallel and is thus capable of handling many grains (>8,000). The validity and stability of the CE approach are verified on simulated and experimental datasets.

Abnormal grain growth mediated by fractal boundary migration at the nanoscale.

Modern engineered materials are composed of space-filling grains or domains separated by a network of interfaces or boundaries. Such polycrystalline microstructures have the capacity to coarsen through boundary migration. Grain growth theories account for the topology of grains and the connectivity of the boundary network in terms of the familiar Euclidian dimension and Euler's polyhedral formula, both of which are based on integer numbers. However, we recently discovered an unusual growth mode in a nanocrystalline Pd-Au alloy, in which grains develop complex, highly convoluted surface morphologies that are best described by a fractional dimension of ∼1.2 (extracted from the perimeters of grain cross sections). This fractal value is characteristic of a variety of domain growth scenarios-including explosive percolation, watersheds of random landscapes, and the migration of domain walls in a random field of pinning centers-which suggests that fractal grain boundary migration could be a manifestation of the same universal behavior.

Direct observation of grain rotations during coarsening of a semisolid Al-Cu alloy.

Sintering is a key technology for processing ceramic and metallic powders into solid objects of complex geometry, particularly in the burgeoning field of energy storage materials. The modeling of sintering processes, however, has not kept pace with applications. Conventional models, which assume ideal arrangements of constituent powders while ignoring their underlying crystallinity, achieve at best a qualitative description of the rearrangement, densification, and coarsening of powder compacts during thermal processing. Treating a semisolid Al-Cu alloy as a model system for late-stage sintering-during which densification plays a subordinate role to coarsening-we have used 3D X-ray diffraction microscopy to track the changes in sample microstructure induced by annealing. The results establish the occurrence of significant particle rotations, driven in part by the dependence of boundary energy on crystallographic misorientation. Evidently, a comprehensive model for sintering must incorporate crystallographic parameters into the thermodynamic driving forces governing microstructural evolution.

Atom-by-atom observation of grain boundary migration in graphene.

Grain boundary (GB) migration in polycrystalline solids is a materials science manifestation of survival of the fittest, with adjacent grains competing to add atoms to their outer surfaces at each other's expense. This process is thermodynamically favored when it lowers the total GB area in the sample, thereby reducing the excess free energy contributed by the boundaries. In this picture, a curved boundary is expected to migrate toward its center of curvature with a velocity proportional to the local radius of boundary curvature (R). Investigating the underlying mechanism of boundary migration in a 3D material, however, has been reserved for computer simulation or analytical theory, as capturing the dynamics of individual atoms in the core region of a GB is well beyond the spatial and temporal resolution limits of current characterization techniques. Here, we similarly overcome the conventional experimental limits by investigating a 2D material, polycrystalline graphene, in an aberration-corrected transmission electron microscope, exploiting the energy of the imaging electrons to stimulate individual bond rotations in the GB core region. The resulting morphological changes are followed in situ, atom-by-atom, revealing configurational fluctuations that take on a time-averaged preferential direction only in the presence of significant boundary curvature, as confirmed by Monte Carlo simulations. Remarkably, in the extreme case of a small graphene grain enclosed within a larger one, we follow its shrinkage to the point of complete disappearance.

Transformations of carbon adsorbates on graphene substrates under extreme heat.

We describe new phenomena of structural reorganization of carbon adsorbates as revealed by in situ atomic-resolution transmission electron microscopy (TEM) performed on specimens at extreme temperatures. In our investigations, a graphene sheet serves as both a quasi-transparent substrate for TEM and as an in situ heater. The melting of gold nanoislands deposited on the substrate surface is used to evaluate the local temperature profile. At annealing temperatures around 1000 K, we observe the transformation of physisorbed hydrocarbon adsorbates into amorphous carbon monolayers and the initiation of crystallization. At temperatures exceeding 2000 K the transformation terminates in the formation of a completely polycrystalline graphene state. The resulting layers are bounded by free edges primarily in the armchair configuration.

Enhancing the magnetoviscosity of ferrofluids by the addition of biological nanotubes.

Applying a magnetic field to many ferrofluids leads to a significant increase in viscosity, but the phenomenon has yet to find technological exploitation because of the thinning caused by even weak shear flows. We have discovered that the addition of plant-virus-derived nanotubes to a commercial ferrofluid can give rise to a dramatic enhancement in magnetoviscosity and a suppression of shear thinning. The dependence of this effect on nanotube aspect ratio and surface charge, both of which were varied biotechnologically, is consistent with a "scaffolding" of magnetic particles into quasi-linear arrays. Direct support for this explanation is derived from transmission electron micrographs, which reveal a marked tendency for the magnetic nanoparticles to decorate the outside surface of the virus nanotubes.

Management of splenic trauma in the pediatric hemophiliac patient: Case series and review of the literature.

In July and August 1998, 3 patients who attend the Hemophilia Treatment Center required emergency admission to the authors' hospital for management of hemorrhagic shock caused by splenic injury. Computed tomography was used to diagnose and grade the splenic injuries, which ranged from II to IV on the organ injury scale. Two patients had Christmas disease (Factor IX deficiency) and were treated with splenorrhaphy and factor IX replacement. One patient who has severe von Willebrand disease (Type 3) had grade II splenic injury that required splenectomy to secure hemostasis. The coagulopathic deficiency was aggressively treated in each patient. All patients required operative intervention with attempted splenorrhaphy. All patients survived their operative experience, and none suffered a rebleeding episode. With correction of the coagulopathy throughout the perioperative period and local hemostatic control by operative techniques, salvage procedures for splenic injury were successful for 2 of these 3 patients.