Brittle and ductile crack-tip behavior in magnesium

Wu, Zhaoxuan; Curtin, W.A.

doi:10.1016/j.actamat.2015.01.023

Cited by 82 publications

(51 citation statements)

References 34 publications

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“…It is well-known that continuum fracture mechanics is unable to explain many important fracture phenomena, including lattice trapping [7][8][9], crack tip instabilities [10][11][12][13], and crack velocities in steady-state [14], all of which depend intimately on the details of bonding between atoms [15]. Atomistic simulations have therefore become increasingly popular for studying crack tip deformation mechanisms and their implications for ductility [16], both in quasi-static [17][18][19][20] and dynamic [21][22][23][24] conditions. In the latter case, model interatomic potentials have found great utility in molecular dynamics simulations [12,13,[25][26][27][28][29].…”

Section: Introductionmentioning

confidence: 99%

Crack tip blunting and cleavage under dynamic conditions

Rajan

2016

Journal of the Mechanics and Physics of Solids

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In structural materials with both brittle and ductile phases, cracks often initiate within the brittle phase and propagate dynamically towards the ductile phase. The macroscale, quasistatic toughness of the material thus depends on the outcome of this microscale, dynamic process. Indeed, dynamics has been hypothesized to suppress dislocation emission, which may explain the occurrence of brittle transgranular fracture in mild steels at low temperatures [1]. Here, crack tip blunting and cleavage under dynamic conditions is explored using continuum mechanics and molecular dynamics simulations. The focus is on two questions: 1) whether dynamics can affect the energy barriers for dislocation emission and cleavage, and 2) what happens in the dynamic "overloaded" situation, in which both processes are energetically possible. In either case, dynamics may shift the balance between brittle cleavage and ductile blunting, thereby affecting the intrinsic ductility of the material. To explore these effects in simulation, a novel interatomic potential is used for which the intrinsic ductility is tunable, and a novel simulation technique is employed, termed a "dynamic cleavage test," in which cracks can be run dynamically at a prescribed energy release rate into a material. Both theory and simulation reveal, however, that the intrinsic ductility of a material is unaffected by dynamics. The energy barrier to dislocation emission appears to be identical in quasi-static and dynamic conditions, and, in the overloaded situation, dislocation emission is kinetically preferred. Thus, dynamics cannot embrittle a ductile material, and the origin of brittle failure in certain alloys (e.g., mild steels) appears to be unrelated to dynamic effects at the crack tip.

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Section: Introductionmentioning

confidence: 99%

Crack tip blunting and cleavage under dynamic conditions

Rajan

2016

Journal of the Mechanics and Physics of Solids

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“…In this case, the critical K Ie is K Ie = β(θ, φ, C, ψ)γ usf (2) where θ, φ, and γ usf are the angle between the slip and crack planes, the complementary angle of the crack front and the slip direction, and the unstable stacking fault, respectively. Details for calculating β within anisotropic elasticity are also given in [22]. The same formulation can be used for the case of the symmetric tilt GBs 70 in cubic metals, which is the case studied here.…”

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Atomistic study of hydrogen embrittlement of grain boundaries in nickel: I. Fracture

Tehranchi

Curtin

2017

Journal of the Mechanics and Physics of Solids

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Hydrogen ingress into a metal is a persistent source of embrittlement. Fracture surfaces are often intergranular, suggesting favorable cleave crack growth along grain boundaries (GBs) as one driver for embrittlement. Here, atomistic simulations are used to investigate the effects of segregated hydrogen on the behavior of cracks along various symmetric tilt grain boundaries in fcc Nickel. An atomistic potential for Ni-H is first recalibrated against new quantum level computations of the energy of H in specific sites within the NiΣ5(120)⟨100⟩ GB. The binding energy of H atoms to various atomic sites in the NiΣ3(111) (twin), NiΣ5(120)⟨100⟩, NiΣ99(557)⟨110⟩, and NiΣ9(221)⟨110⟩ GBs, and to various surfaces created by separating these GBs into two possible fracture surfaces, are computed and used to determine equilibrium H concentrations at bulk H concentrations typical of embrittlement in Ni. Mode I fracture behavior is then studied, examining the influence of H in altering the competition between dislocation emission (crack blunting; "ductile" behavior) and cleavage fracture ("brittle" behavior) for intergranular cracks. Simulation results are compared with theoretical predictions (Griffith theory for cleavage; Rice theory for emission) using the computed surface energies. The deformation behavior at the GBs is, however, generally complex and not as simple as cleavage or emission at a sharp crack tip, which is not unexpected due to the complexity of the GB structures. In cases predicted to emit dislocations from the crack tip, the presence of H atoms reduces the critical load for emission of the dislocations and no cleavage is found. In the cases predicted to cleave, the presence of H atoms reduces the cleavage stress intensity and makes cleavage easier, including NiΣ9(221)⟨110⟩ which emits dislocations in the absence of H. Aside from the one unusual NiΣ9(221)⟨110⟩ case, no tendency is found for H to cause a ductile-to-brittle transformation for cracks along GBs in Ni, either according to theory or simulation for initial equilibrium H segregation and with no, or limited, H diffusion near the newly-created fracture surfaces. The NiΣ3(111) twin boundary does not absorb H at all, suggesting that embrittlement is more difficult in materials with higher fraction of such twin boundaries, as found experimentally. Experimental observations of cleavage-like failure are thus presumably caused by mechanisms involving H diffusion or dynamic crack growth.

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“…Therefore, the procedures to obtain accurate SF information in pure metals are readily available. Accurate γ sf and γ usf thus obtained can be used to better predict dislocation dissociation [4] and crack-tip behavior in hcp metals [53]. In SF calculations of metals with solid solution alloying, the NEB method can be applied to obtain a γ-line, but the result depends on the choice of the supercell size N 1 and N 2 , in addition to the usual N 3 -dependence, and will depend on how the solutes are distributed.…”

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Comprehensive first-principles study of stable stacking faults in hcp metals

2017

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The plastic deformation in hcp metals is complex, with the associated dislocation core structures and properties not well understood on many slip planes in most hcp metals. A first step in establishing the dislocation properties is to examine the stable stacking fault energy and its structure on relevant slip planes. However, this has been perplexing in the hcp structure due to additional in-plane displacements on both side of the slip plane. Here, density functional theory guided by crystal symmetry analysis is used to study all relevant stable stacking faults in 6 hcp metals (Mg, Ti, Zr, Re, Zn, Cd). Specially, the stable stacking fault energy, position, and structure on the Basal, Prism I and II, Pyramidal I and II planes are determined using all-periodic supercells with full atomic relaxation. All metals show similar stacking fault position and structure as dictated by crystal symmetry, but the associated stacking fault energy, being governed by the atomic bonding, differs significantly among them. Stacking faults on all the slip planes except the Basal plane show substantial out-of-plane displacements while stacking faults on the Prism II, Pyramidal I and II planes show additional in-plane displacements, all extending to multiple atom layers. The in-plane displacements are not captured in the standard computational approach for stacking faults, and significant differences are shown in the energies of such stacking faults between the standard approach and fully-relaxed case. The existence of well-defined stable stacking fault on the Pyramidal planes suggests zonal dislocations are unlikely. Calculations on the equilibrium partial separation further suggests c + a dissociation into three * Corresponding author Email address: zhaoxuan.wu@epfl.ch (Zhaoxuan Wu) Preprint submitted to Acta Materialia October 9, 2016This is a pre-print of the following article: Yin, Binglun; Wu, Zhaoxuan; Curtin, W.A. Acta Materialia 2017, 123, 223--234.. The formal publication is available at http://dx.doi.org/10.1016/j.actamat.2016.10.042 partials on the Pyramidal I plane is unlikely and c dissociation on Prism planes is unlikely to be stable against climb-dissociation onto the Basal planes in these metals.

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Brittle and ductile crack-tip behavior in magnesium

Cited by 82 publications

References 34 publications

Crack tip blunting and cleavage under dynamic conditions

Crack tip blunting and cleavage under dynamic conditions

Atomistic study of hydrogen embrittlement of grain boundaries in nickel: I. Fracture

Comprehensive first-principles study of stable stacking faults in hcp metals

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