http://www.ae.utexas.edu/courses/ase324_huang/ncmetals.pdf

Deformation in nanocrystalline metals

weave, architecture, yarn crimp, and the various mechanisms of energy absorption of the fabric, discussed later. To briefly compare various commercial brands of fabrics, Zylon woven fabrics absorb nearly twice the energy per unit areal density specific energy absorbed SEA than both Kevlar and Spectra, when gripped on all four edges, and almost 12 times that of aluminum fuselage skin. The ballistic impact performance of PBO systems is substantially superior to Kevlar 29 systems and marginally better than Kevlar KM2 systems 4. It is reported in Ref. 5 that Spectra fibers are ten times stronger than steel. Previous review work includes a database of early ballistic fabric research by Cunniff 6, and the mechanisms influencing ballistic performance of woven fabrics by Cheeseman and Bogetti 7.

Ballistic Impact of Dry Woven Fabric Composites: A Review

To fully understand the plastic deformation of metallic polycrystalline materials, the physical mechanisms by which a dislocation interacts with a grain boundary must be identified. Recent atomistic simulations have focused on the discrete atomic scale motions that lead to either dislocation obstruction, dislocation absorption into the grain boundary with subsequent emission at a different site along the grain boundary, or direct dislocation transmission through the grain boundary into the opposing lattice. These atomistic simulations, coupled with foundational experiments performed to study dislocation pile-ups and slip transfer through a grain boundary, have facilitated the development and refinement of a set of criteria for predicting if dislocation transmission will occur and which slip systems will be activated in the adjacent grain by the stress concentration resulting from the dislocation pile-up. This article provides a concise review of both experimental and atomistic simulation efforts focused on the details of slip transmission at grain boundaries in metallic materials and provides a discussion of outstanding challenges for atomistic simulations to advance this field.

Insights on slip transmission at grain boundaries from atomistic simulations

Sequential slip transfer across grain boundaries (GB) has an important role in size-dependent propagation of plastic deformation in polycrystalline metals. For example, the Hall–Petch effect, which states that a smaller average grain size results in a higher yield stress, can be rationalised in terms of dislocation pile-ups against GBs. In spite of extensive studies in modelling individual phases and grains using atomistic simulations, well-accepted criteria of slip transfer across GBs are still lacking, as well as models of predicting irreversible GB structure evolution. Slip transfer is inherently multiscale since both the atomic structure of the boundary and the long-range fields of the dislocation pile-up come into play. In this work, concurrent atomistic-continuum simulations are performed to study sequential slip transfer of a series of curved dislocations from a given pile-up on Σ3 coherent twin boundary (CTB) in Cu and Al, with dominant leading screw character at the site of interaction. A Frank-Read source is employed to nucleate dislocations continuously. It is found that subject to a shear stress of 1.2 GPa, screw dislocations transfer into the twinned grain in Cu, but glide on the twin boundary plane in Al. Moreover, four dislocation/CTB interaction modes are identified in Al, which are affected by (1) applied shear stress, (2) dislocation line length, and (3) dislocation line curvature. Our results elucidate the discrepancies between atomistic simulations and experimental observations of dislocation-GB reactions and highlight the importance of directly modeling sequential dislocation slip transfer reactions using fully 3D models. Three-dimensional modelling has uncovered important mechanistic clues to strengthening polycrystalline metals through plastic deformation. A team led by David McDowell at the Georgia Institute of Technology in the US used concurrent atomistic-continuum (CAC) simulations to investigate the challenging problem of how curved dislocations pile-up and interact with special ‘twin’ grain boundaries in copper and aluminum when the metals are subjected to mechanical strain. These line defects move until they meet barriers such as grain boundaries separating crystalline regions, where they ‘pile-up’ behind the leading defect and may inhibit further defects from forming, a process known as work hardening. The multiscale CAC technique coarse grains the lattice using 3-D rhombohedra, and then applies an integral form finite element method to describe dislocation motion between elements, critical for understanding work hardening.

Sequential slip transfer of mixed-character dislocations across Σ3 coherent twin boundary in FCC metals: a concurrent atomistic-continuum study

A review on atomistic simulation of grain boundary behaviors in face-centered cubic metals

The interactions between edge dislocations and grain boundaries---dislocation pileup, dislocation absorption, and dislocation transmission---are studied by performing quasicontinuum simulations. The $⟨112⟩$ asymmetrical tilt grain boundaries with different misorientation angles are used. The atomic configurations and stress fields of equilibrium and nonequilibrium asymmetrical grain boundaries are investigated in detail by comparison with analytical models. The influence of the grain boundary structure on the stress concentration due to dislocation pileup and the accommodation of extrinsic dislocations in the grain boundaries are also examined by using low- and high-angle grain boundaries. The critical forces on the dislocation in small-angle tilt grain boundaries for it to eject from the boundaries are evaluated by atomic simulations, and the results are compared with dislocation theory. It is also found that the rearrangement of the grain boundary dislocations with local grain boundary sliding in the local region, where the extrinsic dislocation is absorbed, is the characteristic accommodation mechanism of low-angle asymmetrical grain boundaries. The effects of the interaction between dislocations and grain boundaries on the mechanical properties of coarse-grained metals with dislocation sources in their grain and on those of nanocrystalline metals without sources in their grain are also discussed.

Interaction mechanism between edge dislocations and asymmetrical tilt grain boundaries investigated via quasicontinuum simulations

The static and dynamic drape behavior of polyester Shingosen fabrics is investigated using the new mechanical parameter of dynamic drapability, that is, the dynamic drape coefficient with swinging motion Dd. The Dd of the Peach Face fabric is small and that of the New Worsted fabric is large. On the other hand, there are almost no differences between each group of Shingosen fabrics in node numbers and conventional static drape coefficients. In classifying production characteristics, yam-processing fabrics show larger values of Dd than other fiber-production and fabric-finishing samples. In classifying fiber characteristics contractile fiber and ultra-fine fiber Shingosen fabrics show smaller values of Dd than irregular fiber fabrics.

Polyester "Shingosen" Fabrics Characterized by Dynamic Drape Coefficient with Swinging Motion:

Static and dynamic drape behaviors of polyester woven fabrics are s died through representative finishing stages using two sets of fabrics with different conditions of relaxation and weight reducti...

Changes in the Static and Dynamic Drape Coefficients of Polyester Fabrics Through the Finishing Stages

The effect of extrinsic grain boundary dislocations (EGBDs) in nonequilibrium grain boundaries on the mechanical properties of ultrafine-grained metals is investigated by molecular dynamics simulations. Aluminum bicrystal models containing cracks and EGBDs impinged from the crack tips are prepared. First, the dependence of the local grain boundary structure on the accommodation mechanism of EGBDs, and on its stress field is studied. Then, the shielding effect of EGBDs on the emissions of dislocations from crack tips is investigated, and the effect of nonequilibrium grain boundaries on the intragranular deformation is discussed. Finally, to investigate the relationship between EGBDs and intergranular deformations, shear loading is applied to the bicrystal models. It is found that extrinsic grain boundaries function as the intergranular deformation source, and the Burgers vector components of the EGBDs lead to anisotropic grain boundary sliding.

Effect of Extrinsic Grain Boundary Dislocations on Mechanical Properties of Ultrafine-Grained Metals by Molecular Dynamics Simulations

The influence of defects in nanograins, e.g. stacking faults and twinnings, on mechanical properties of nanocrystalline materials is studied by molecular dynamics simulations. Two types of many-body interatomic potential based on aluminium are adopted to investigate the influence of stacking fault energy on the deformation mechanism of nanocrystalline metals: one accurately reproduces the energy value of stacking faults for aluminium; the other underestimates the energy value for aluminium. Three different deformation processes are performed to nanocrystalline models with high or low stacking fault energy, and crystal slips occur in both models. In the case of the high stacking fault energy, crystal deformation occurs by perfect dislocations and no defects exist in the grains. Therefore, the strength almost recovers after relaxation. On the other hand, for low stacking fault energy, stacking faults remain through the grains after the crystal slips by partial dislocations. Consequently, these defects cause anisotropy in the mechanical properties of the simulated nanocrystalline metals.

Sukenori Shintaku

Papers

Interaction mechanism between edge dislocations and asymmetrical tilt grain boundaries investigated via quasicontinuum simulations

Polyester "Shingosen" Fabrics Characterized by Dynamic Drape Coefficient with Swinging Motion:

Changes in the Static and Dynamic Drape Coefficients of Polyester Fabrics Through the Finishing Stages

Effect of Extrinsic Grain Boundary Dislocations on Mechanical Properties of Ultrafine-Grained Metals by Molecular Dynamics Simulations

Defect-induced anisotropy in mechanical properties of nanocrystalline metals by molecular dynamics simulations