Mechanisms of Intergranular Fracture

1998 ◽  
Vol 539 ◽  
Author(s):  
Diana Farkas
Keyword(s):  
Grain Boundary ◽  
Crack Tip ◽  
Boundary Region ◽  
Failure Behavior ◽  
Cleavage Crack ◽  
Quasi Equilibrium ◽  
Dislocation Emission ◽  
Embedded Atom ◽  
Crack Tips ◽  
Ductile Behavior

AbstractWe present a study of the atomistic mechanisms of crack propagation along grain boundaries in metals and alloys. The failure behavior showing cleavage crack growth and/or crack-tip dislocation emission is demonstrated using atomistic simulations for an embedded-atom model. The simulations follow the quasi-equilibrium growth of a crack as the stress intensity applied increases. Dislocations emitted from crack tips normally blunt the crack and inhibit cleavage, inducing ductile behavior. When the emitted dislocations stay near the crack tip (sessile dislocations), they do blunt the crack but brittle cleavage can occur after the emission of a sufficient number of dislocations. The fracture process occurs as a combination of dislocation emission/micro-cleavage portions that are controlled by the local atomistic structure of the grain boundary. The grain boundary is shown to be a region where dislocation emission is easier, a mechanism that competes with the lower cohesive strength of the boundary region.

Kinetics Of Dislocation Emission From Crack Tips And The Brittle To Ductile Transition Of Cleavage Fracture.

1995 ◽  
Vol 409 ◽  
Author(s):  
A.S. Argon ◽  
G. Xu ◽  
M. Ortiz
Keyword(s):  
Crack Tip ◽  
Boundary Integral ◽  
Boundary Integral Method ◽  
Cleavage Crack ◽  
Dislocation Emission ◽  
Brittle To Ductile Transition ◽  
Central Elements ◽  
Crack Tips ◽  
Inclined Planes ◽  
Kinetics Of

AbstractSeveral activation configurations of dislocation embryos emanating from cleavage crack tips at the verge of propagating have been analyzed in detail by the variational boundary integral method, as central elements of the rate controlling process of nucleation governed fracture transitions from brittle cleavage to tough forms, as in the case for BCC transition metals. The configurations include those on inclined planes, oblique planes and crack tip cleavage ledges. Surface ledge production resistance is found to have a very strong embrittling effect. Only nucleation on oblique planes near a free surface and at crack tip cleavage ledges are found to be energetically feasible to explain brittle-to-ductile transition temperatures in the experimentally observed ranges.

Atomistic Study of Cleavage and Dislocation Emission in NiAl

1994 ◽  
Vol 364 ◽  
Author(s):  
M. Ludwig ◽  
P. Gumbsch
Keyword(s):  
Finite Element ◽  
Mixed Mode ◽  
Crack Tip ◽  
Mode I ◽  
Crack Plane ◽  
Dislocation Emission ◽  
Dislocation Generation ◽  
Embedded Atom ◽  
Atomistic Study ◽  
Eam Potential

AbstractThe atomistic processes during fracture of NiAl are studied using a new embedded atom (EAM) potential to describe the region near the crack tip. To provide the atomistically modeled crack tip region with realistic boundary conditions, a coupled finite element - atomistic (FEAt) technique [1] is employed. In agreement with experimental observations, perfectly brittle cleavage is observed for the (110) crack plane. In contrast, cracks on the (100) plane either follow a zig-zag path on (110) planes, or emit dislocations. Dislocation generation is studied in more detail under mixed mode I/II loading conditions.

Some aspects of forces and fields in atomic models of crack tips

1991 ◽  
Vol 6 (12) ◽  
pp. 2565-2577 ◽  
Author(s):  
R.G. Hoagland ◽  
M.S. Daw ◽  
J.P. Hirth
Keyword(s):  
Crack Tip ◽  
Lattice Parameter ◽  
Crack Model ◽  
Dislocation Emission ◽  
Linear Elastic ◽  
Partial Dislocations ◽  
Atomistic Models ◽  
Crack Tips ◽  
Eam Potential ◽  
Crack Faces

This paper examines the stresses and displacement gradients in atomistic models of cracks based on an EAM potential devised for aluminum. Methods for computing these quantities are described. Results are presented for two models differing in terms of the orientations of the crack relative to the crystal, a [100] (010) orientation that behaves in a brittle fashion and a [111] (110) orientation that emits partial dislocations prior to extending. Both models display lattice trapping. The stresses in the brittle crack model are compared with the linear elastic prediction and found to be in remarkably good agreement to within distances of about one lattice parameter of the crack tip and at the free surface where contributions from sources other than strain energy (e.g., surface tension) influence the results. Similar results are observed for the ductile model until dislocation emission occurs. The largest stresses that develop just prior to crack extension or dislocation emission are used to estimate the ratio of theoretical tensile strength to shear strength in this material. Eshelby's conservation integrals, F and M, are also computed. F is found to be essentially contour independent and in agreement with the linear elastic prediction in both models until dislocation emission occurs, at which point a large screening contribution arises from the emitted partials. The contour size dependence of M reveals some interesting features of the crack tip including a slight wobble of the crack tip inside its potential well with changing applied K and the existence of forces acting to move the crack faces apart as blunting occurs.

Coupled effect of grain boundary sliding and dislocation emission on fracture toughness of nanocrystalline materials

2016 ◽  
Vol 01 (02) ◽  
pp. 1650008 ◽  
Author(s):  
Q. H. Fang ◽  
L. C. Zhang
Keyword(s):  
Grain Size ◽  
Grain Boundary ◽  
Crack Growth ◽  
Nanocrystalline Materials ◽  
Crack Tip ◽  
Grain Boundary Sliding ◽  
Dislocation Emission ◽  
Disclination Dipole ◽  
Coupled Effect ◽  
Wedge Disclination

This paper establishes a theoretical model to explore the coupled effect of grain boundary (GB) sliding deformation and crack tip dislocation emission on the critical stress intensity factor (SIF) for crack growth in ultrafine-grained and nanocrystalline materials (NCMs). The model postulates that the stress concentration near a crack tip initiates GB sliding. It is found that GB sliding leads to the formation of wedge disclination dipole at the triple junctions of grain boundaries. Under the external load and stress fields produced by wedge disclinations, dislocations are emitted from crack tips but will stop at the opposite GBs. The influence of the wedge disclination dipole and the dislocation emitted from crack tip on the critical SIF for crack growth is investigated. The model prediction shows that the critical SIF varies with the decrement of grain size, and that there is a critical grain size corresponding to a minimum value of SIF. Compared with the pure brittle fracture in NCMs at the grain sizes of tens of nanometers, the combined deformation mechanisms can bring an increase of the critical SIF for crack growth.

Modeling of crack tip dislocation emission in B2 intermetallic alloys

1992 ◽  
Vol 7 (4) ◽  
pp. 919-925 ◽  
Author(s):  
Michael F. Bartholomeusz ◽  
John A. Wert
Keyword(s):  
Slip System ◽  
Fracture Mode ◽  
Crack Tip ◽  
Intermetallic Alloys ◽  
Dislocation Emission ◽  
Thermally Activated ◽  
Ordered Intermetallic ◽  
Macroscopic Fracture ◽  
Crack Tips ◽  
Active Slip

A model has been previously proposed to describe the energy associated with emission of dissociated superlattice dislocations from crack tips in ordered intermetallic alloys. In the present paper, the model is applied to several B2 intermetallic alloys. The results of the analysis reveal a correlation between the range of slip system orientations for which emission of a dislocation from a crack tip is energetically favorable and the macroscopic fracture mode of the alloy. Additionally, the effects of changing the active slip system, increasing the thermal energy available for thermally activated dislocation emission, and changing the {111} APB energy on the fracture mode of NiAl and FeAl are discussed.

An atomic model of crack tip deformation in aluminum using an embedded atom potential

1990 ◽  
Vol 5 (2) ◽  
pp. 313-324 ◽  
Author(s):  
R. G. Hoagland ◽  
M. S. Daw ◽  
S. M. Foiles ◽  
M. I. Baskes
Keyword(s):  
Crack Tip ◽  
Atomic Displacement ◽  
Crack Plane ◽  
Dislocation Emission ◽  
Displacement Fields ◽  
Linear Elastic ◽  
Embedded Atom ◽  
Equilibrium Configurations ◽  
Field Dislocation ◽  
Atom Potential

The atomic configuration at the tip of a mode 1 crack in aluminum is modeled by means of molecular dynamics calculations using an embedded atom potential. This potential intrinsically incorporates many-body contributions. This paper is concerned with the characteristics of the atomic displacement fields in comparison to the linear elastic predictions and dislocation emission phenomena. Three crack/crystal orientations are examined in which the crack plane–crack propagation directions are (010)-[100], (10)-[110], and (10)-[111]. The first two models behaved in a brittle fashion as dislocation emission did not occur for reasons associated with the use of periodic boundary conditions parallel to the crack front. For the models which remained atomically sharp, the positions of the atoms near the crack tip in equilibrium configurations are different from the linear elastic predictions but, to first order, retain an r1/2 dependence, with smaller K, and with the origin displaced behind the physical crack tip. This near tip region is also observed to be elastically softer than in the far field. Dislocation emission readily proceeds in the (10)-[111] model by the sequential emission of partials with attendant nonzero uz displacements. The blunting is characterized by the creation of two corner defects that separate as emission occurs and relaxation of the strains in the region initially confronted by the crack tip. Additional features of the results are discussed.

Atomistic Simulation of Dislocation Interactions with a σ = 5 (210) Grain Boundary during Nanoindentation of Ni

2004 ◽  
Vol 821 ◽  
Author(s):  
Ho Jang ◽  
Diana Farkas
Keyword(s):  
Grain Boundary ◽  
Boundary Region ◽  
Dislocation Motion ◽  
Dislocation Loops ◽  
Ni Substrate ◽  
Embedded Atom ◽  
Dislocation Interactions ◽  
Slip Planes ◽  
Dynamics Simulations ◽  
Atom Configuration

AbstractMolecular dynamics simulations of nanoindentation were performed using embedded atom potentials. The indentation was simulated on a Ni substrate using a diamond-like spherical indenter. In this study, we focused on the interaction of a σ=5 (210) grain boundary with the dislocations that were nucleated and expanded as loops under the indenter during nano- indentation. The results showed that dislocation loops were nucleated beneath the surface and propagated on multiple {111} slip planes. These dislocations impinged into the grain boundary in the course of nanoindentation. The lattice dislocations changed the atom configuration at the boundary region as they merged into the grain boundary and at later stages they transmitted across the grain boundary. The results also showed that the presence of a grain boundary affected the indenting speed and dislocation motion during nanoindentation, with the grain boundary retarding the indentation process.

Ab Initio Electronic Structure Calculations of the Σ 5 (210) [001] Tilt Grain Boundary in Ni3Al

1997 ◽  
Vol 472 ◽  
Author(s):  
Gang Lu ◽  
Nicholas Kioussis
Keyword(s):  
Electronic Structure ◽  
Grain Boundary ◽  
Boundary Energy ◽  
Boundary Region ◽  
Embedded Atom Method ◽  
Full Potential ◽  
Plane Wave Method ◽  
Embedded Atom ◽  
Tilt Grain Boundary ◽  
Linearized Augmented Plane Wave

ABSTRACTThe atomic and the electronic structure of the Σ 5 (210) [001] tilt grain boundary in Ni3Al have been calculated using the full potential linearized-augmented plane-wave method. The strain field normal to the boundary plane and the excess grain boundary volume are calculated and compared with the results obtained using the embedded-atom method (EAM). The interlayer strain normal to the grain boundary oscillates with increasing distance from the grain boundary. The bonding charge distributions suggest that bonding in the boundary region is significantly different from that in the bulk. The grain boundary energy and the Griffith cohesive energy are calculated and compared with the EAM results.

Sign in / Sign up

Export Citation Format

Share Document