A Predictive Framework for Dislocation-Density Pile-Ups in Crystalline Systems With Coincident Site Lattice and Random Grain Boundaries

2017 ◽  
Vol 139 (2) ◽  
Author(s):  
David M. Bond ◽  
Mohammed A. Zikry
Keyword(s):  
Dislocation Density ◽  
Grain Boundaries ◽  
Coincident Site Lattice ◽  
Local Stress ◽  
Face Centered Cubic ◽  
Slip Systems ◽  
Triple Junctions ◽  
High Angle ◽  
Site Lattice ◽  
Wide Range

Evolving dislocation-density pile-ups at grain-boundaries (GBs) spanning a wide range of coincident site lattice (CSL) and random GB misorientations in face-centered cubic (fcc) bicrystals and polycrystalline aggregates has been investigated. A dislocation-density GB interaction scheme coupled to a dislocation-density-based crystalline plasticity formulation was used in a nonlinear finite element (FE) framework to understand how different GB orientations and GB-dislocation-density interactions affect local and overall behavior. An effective Burger's vector of residual dislocations was obtained for fcc bicrystals and compared with molecular dynamics (MDs) predictions of static GB energy, as well as dislocation-density transmission at GB interfaces. Dislocation-density pile-ups and accumulations of residual dislocations at GBs and triple junctions (TJs) were analyzed for a polycrystalline copper aggregate with Σ1, Σ3, Σ7, Σ13, and Σ21 CSLs and random high-angle GBs to understand and predict the effects of GB misorientation on pile-up formation and evolution. The predictions indicate that dislocation-density pile-ups occur at GBs with significantly misoriented slip systems and large residual Burger's vectors, such as Σ7, Σ13, and Σ21 CSLs and random high-angle GBs, and this resulted in heterogeneous inelastic deformations across the GB and local stress accumulations. GBs with low misorientations of slip systems had high transmission, no dislocation-density pile-ups, and lower stresses than the high-angle GBs. This investigation provides a fundamental understanding of how different representative GB orientations affect GB behavior, slip transmission, and dislocation-density pile-ups at a relevant microstructural scale.

Fracture Strengths of Individual Grain Boundaries in Ni3Ai Using a Miniaturized Disk Bend Test

1991 ◽  
Vol 238 ◽  
Author(s):  
Douglas E. Meyers ◽  
Alan J. Ardell
Keyword(s):  
Plastic Deformation ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Grain Growth ◽  
Coincident Site Lattice ◽  
Bend Test ◽  
High Angle ◽  
Load Drop ◽  
Site Lattice ◽  
Special Boundaries

ABSTRACTThe results of our initial efforts at measuring the fracture strengths of grain boundaries In Ni3Al using a miniaturized disk-bend test are presented. The samples tested were 3 mm in diameter and between 150 and 300 μm thick. An Ingot of directlonally-solidlfled, boron-free Ni3Al containing 24% Al was annealed between 1300 and 1350 °C to induce grain growth, producing many grain boundaries In excess of 1.5 mm in length. Specimens were cut from these In such a way that one long grain boundary was located near a diameter of the specimen. The relative orientations of the grains on either side of the boundary were determined from electron channeling patterns. Low-angle boundaries are so strong they do not fracture; Instead the samples deform In a completely ductile manner. High-angle boundaries always fracture, but only after considerable plastic deformation of the two grains flanking them. Fracture is Indicated by a load drop in the load vs. displacement curves. A method involving extrapolation of the elastic portion of these curves to the displacement at fracture is used to estimate the fracture stresses. This procedure yields consistent values of the fracture strengths of high-angle boundaries. The measured stresses are large (∼2 to 3 GPa), but considerably smaller than those required for the fracture of special boundaries, as predicted by computer simulations. No correlation was found between the fracture stresses or loads and the geometry of the high-angle boundaries, many of which are close to, but deviate from, coincident site lattice orientations.

The evolution of grain-boundary cracking evaluated through in situ tensile-creep testing of Udimet alloy 188

2008 ◽  
Vol 23 (2) ◽  
pp. 500-506 ◽  
Author(s):  
C.J. Boehlert ◽  
S.C. Longanbach ◽  
M. Nowell ◽  
S. Wright
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Creep Deformation ◽  
Surface Deformation ◽  
Coincident Site Lattice ◽  
Tensile Creep ◽  
Face Centered Cubic ◽  
High Angle ◽  
Electron Backscattered Diffraction

In situ scanning electron microscopy was performed during elevated-temperature (⩽760 °C) tensile-creep deformation of a face-centered-cubic cobalt-based Udimet 188 alloy to characterize the deformation evolution and, in particular, the grain boundary-cracking evolution. In situ electron backscatter diffraction observations combined with in situ secondary electron imaging indicated that general high-angle grain boundaries were more susceptible to cracking than low-angle grain boundaries and coincident site-lattice boundaries. The extent of general high-angle grain-boundary cracking increased with increasing creep time. Grain-boundary cracking was also observed throughout subsurface locations as observed for postdeformed samples. The electron backscattered diffraction orientation mapping performed during in situ tensile-creep deformation proved to be a powerful means for characterizing the surface deformation evolution and in particular for quantifying the types of grain boundaries that preferentially cracked.

On Predicting Nucleation of Microcracks Due to Slip-Twin Interactions at Grain Boundaries in Duplex Near γ-TiAl

2008 ◽  
Vol 130 (2) ◽  
Author(s):  
D. Kumar ◽  
T. R. Bieler ◽  
P. Eisenlohr ◽  
D. E. Mason ◽  
M. A. Crimp ◽  
...  
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Crack Nucleation ◽  
Coincident Site Lattice ◽  
Local Stress ◽  
Fracture Initiation ◽  
Critical Parameters ◽  
Geometrical Parameters ◽  
Slip Systems ◽  
Weak Boundaries

Simkin et al. (2003, “A Factor to Predict Microcrack Nucleation at Gamma-Gamma Grain Boundaries in TiAl,” Scr. Mater., 49(2), 149–154) proposed a relationship for predicting crack initiation in γ-TiAl in a scenario where a mechanical twin interacts with a grain boundary. This correlation (quantified using a fracture initiation parameter or fip) was based only on the geometry of the Burgers vectors as they are related to slip transfer across the grain boundary and the Mode I type opening force experienced by the grain boundary. Generally, a fip is a mathematical combination of factors that allow weak boundaries to be probabilistically identified in the context of a state of stress. This paper further develops this approach by considering the inclusion of the mismatch between the slip planes in the grain boundary and a parameter that accounts for the different elastic properties in adjoining grains. Also, the significance of primary twin (slip) systems versus secondary slip systems is assessed. When compared to fips that can be constructed through a variety of other combinations of nine geometrical parameters that could affect grain boundary damage nucleation, the fip obtained by multiplying Simkin’s original parameter by Emin∕Emax, the ratio of Young’s modulus in the stress direction in the two grains, is best able to distinguish between cracked and intact grain boundary populations. Cracked and intact boundaries are also characterized to assess tilt and twist character and whether they are low Σ (or coincident site lattice) boundaries (using a cubic criterion). It is also shown that fips based on Σ values or the tilt and twist character of the boundary lead to an unacceptably high probability of incorrectly distinguishing between cracked and intact grain boundaries, implying that these are not critical parameters affecting crack nucleation at the grain boundary in duplex near-γ TiAl. The paper closes with a discussion on how combined microscopic and crystal plasticity finite element analyses provide insights on local stress-strain relationships that can be used to evaluate a fip in the context of heterogeneous deformation in multigrain ensembles.

Observations of dislocation interactions with recrystallized grain boundaries

1988 ◽  
Vol 46 ◽  
pp. 628-629
Author(s):  
C. W. Price
Keyword(s):  
Dislocation Density ◽  
Grain Boundary ◽  
Strain Rate ◽  
Grain Boundaries ◽  
Dislocation Structure ◽  
Hot Deformation ◽  
Static Recrystallization ◽  
High Dislocation Density ◽  
High Angle ◽  
Dislocation Interactions

Little evidence exists on the interaction of individual dislocations with recrystallized grain boundaries, primarily because of the severely overlapping contrast of the high dislocation density usually present during recrystallization. Interesting evidence of such interaction, Fig. 1, was discovered during examination of some old work on the hot deformation of Al-4.64 Cu. The specimen was deformed in a programmable thermomechanical instrument at 527 C and a strain rate of 25 cm/cm/s to a strain of 0.7. Static recrystallization occurred during a post anneal of 23 s also at 527 C. The figure shows evidence of dissociation of a subboundary at an intersection with a recrystallized high-angle grain boundary. At least one set of dislocations appears to be out of contrast in Fig. 1, and a grainboundary precipitate also is visible. Unfortunately, only subgrain sizes were of interest at the time the micrograph was recorded, and no attempt was made to analyze the dislocation structure.

Evaluation of Plastic Work Density, Strain Energy and Slip Multiplication Intensity at Some Typical Grain Boundary Triple Junctions

2010 ◽  
Vol 654-656 ◽  
pp. 1283-1286 ◽  
Author(s):  
Tetsuya Ohashi ◽  
Michihiro Sato ◽  
Yuhki Shimazu
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Crystal Plasticity ◽  
Strain Energy ◽  
Plastic Work ◽  
Slip Systems ◽  
Triple Junctions ◽  
Plastic Slip ◽  
Boundary Triple

Plastic slip deformations of tricrystals with simplified geometries are numerically analyzed by a FEA-based crystal plasticity code. Accumulation of geometrically necessary (GN) dislocations, distributions of the total slip, plastic work density and GN dislocations on slip systems, as well as some indices for the intensity of slip multiplication are evaluated. Results show that coexistence of GN dislocations on different slip systems is prominent at triple junctions of grain boundaries.

Tougher ultrafine grain Cu via high-angle grain boundaries and low dislocation density

2008 ◽  
Vol 92 (8) ◽  
pp. 081903 ◽  
Author(s):  
Y. H. Zhao ◽  
J. F. Bingert ◽  
Y. T. Zhu ◽  
X. Z. Liao ◽  
R. Z. Valiev ◽  
...  
Keyword(s):  
Dislocation Density ◽  
Grain Boundaries ◽  
Ultrafine Grain ◽  
High Angle ◽  
Low Dislocation Density

The (111)/(100) interface in cubic materials

1994 ◽  
Vol 52 ◽  
pp. 522-523
Author(s):  
Sundar Ramamurthy ◽  
C. Barry Carter
Keyword(s):  
Grain Boundaries ◽  
Coincident Site Lattice ◽  
Interface Plane ◽  
Polycrystalline Thin Films ◽  
Cubic Materials ◽  
Wide Range ◽  
Low Energies ◽  
Cubic System ◽  
The Stability ◽  
Microscopy Techniques

Descriptions of crystalline interfaces have concentrated on grain boundaries using the framework of the coincident-site-lattice (CSL) model. Within this framework, an interface-plane scheme can be formulated to describe all types of crystalline interfaces, homophase or heterophase. Low Σ values, the inverse density of CSL sites, are associated with boundaries with low energies. However, stable interfaces that cannot be described by such models are often found experimentally in thin-film, semi-bulk and bulk forms. The stability of such interfaces thus depends on factors other than the geometry of the interface.Crystallography of the (111)/(100) interface in the cubic system has been studied extensively in a wide range of materials using different electron-microscopy techniques. Recently, this interface has spurred interest in the growth of tri-crystal microstructures to study junctions in polycrystalline thin films where three identical grain boundaries meet along a line. In this paper we review the (111)/(100) interface showing different examples from our recent work.

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