Modeling of crack tip dislocation emission in B2 intermetallic alloys

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.

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

Journal of Materials Research ◽

10.1557/jmr.1991.2565 ◽

1991 ◽

Vol 6 (12) ◽

pp. 2565-2577 ◽

Cited By ~ 56

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.

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

MRS Proceedings ◽

10.1557/proc-409-29 ◽

1995 ◽

Vol 409 ◽

Cited By ~ 6

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.

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Mechanisms of Intergranular Fracture

MRS Proceedings ◽

10.1557/proc-539-291 ◽

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.

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Bonding and stability in the ordered alloys FeV and CoTi. Comparison with FeTi

Collection of Czechoslovak Chemical Communications ◽

10.1135/cccc19852093 ◽

1985 ◽

Vol 50 (10) ◽

pp. 2093-2100

Author(s):

Štěpán Pick ◽

Mojmír Tomášek ◽

Mojmír Šob

Keyword(s):

Phase Diagrams ◽

Partial Wave ◽

Partial Wave Analysis ◽

Intermetallic Alloys ◽

Wave Analysis ◽

Ordered Alloys ◽

Ordered Intermetallic ◽

Cscl Structure ◽

Ionic Effects

Partial wave analysis together with the qualitative examination of hybridization has been performed for two ordered intermetallic alloys with CsCl structure, FeV, and CoTi. The results resemble those obtained previously for FeTi, although important deviations are present as well. The stabilization of the ordered phase is again due to ionic effects. Qualitative arguments are suggested to explain the small stability of the CsCl phase of FeV and some differences in the FeTi and CoTi phase diagrams.

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Atomistic Study of Cleavage and Dislocation Emission in NiAl

MRS Proceedings ◽

10.1557/proc-364-389 ◽

1994 ◽

Vol 364 ◽

Cited By ~ 4

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.

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