Accurate atomistic simulation of (<i>a</i>/2) 〈111〉 screw dislocations and other defects in bcc tantalum

Yang, Liusai; Söderlind, Per; Moriarty, John A.

doi:10.1080/01418610108214446

Cited by 122 publications

(112 citation statements)

References 46 publications

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“…With the Ta4 potentials, the calculated relaxed vacancy formation energy at ambient pressure is then in better agreement with both experiment and DFT calculations [36], and the calculated melt curve [21,37] is significantly lowered, as discussed below in Sec. IIB1.…”

Section: A Mgpt Interatomic Potentialssupporting

confidence: 66%

“…As in other recent Ta4 MGPT applications [2,4,6,21,28,31,36,37], all of the melt methods discussed below apply the Ta6.8x potentials using the advanced matrix representation of MGPT [6], as implemented in the parallel molecular dynamics code ddcMD [39]. In ddcMD, the angular functions L, P, and M in Eqs.…”

Section: B Methods Of Melt Calculationmentioning

confidence: 99%

“…In the case of Ta, we have refined this basic scheme over a number of generations of MGPT potentials, with the closely related Ta3 [2] and Ta4 [2,36] potentials being the first published versions.…”

Section: A Mgpt Interatomic Potentialsmentioning

confidence: 99%

“…A few Z-method MGPT-MD melt results were previously reported in Ref. 22 based on version-4 MGPT potentials (denoted here as Ta4), which have also been used in other recent Ta applications [2,4,6,21,28,31,36,37]. In Sec.…”

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confidence: 99%

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Polymorphism and melt in high-pressure tantalum

2012

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Recent small-cell (< 150-atom) quantum molecular dynamics (QMD) simulations for Ta based on density functional theory (DFT) have predicted a hexagonal omega (hex-ω) phase more stable than the normal bcc phase at high temperature (T) and pressure (P) above 70 GPa [Burakovsky et al., Phys. Rev. Lett. 104, 255702 (2010)]. Here we examine possible high-T , P polymorphism in Ta with complementary DFT-based model generalized pseudopotential theory (MGPT) multi-ion interatomic potentials, which allow accurate treatment of much larger system sizes (up to ~ 80000 atoms). We focus on candidate bcc, A15, fcc, hcp, and hex-ω phases for the high-T , P phase diagram to 420 GPa, studying the mechanical and relative thermodynamic stability of these phases for both small and large computational cells. Our MGPT potentials fully capture the T = 0 DFT energetics of these phases, while MGPT-MD simulations demonstrate that the higher-energy fcc, hcp and hex-ω structures are only mechanically stabilized at high temperature by large, size-dependent, anharmonic vibrational effects, with the stability of the hex-ω phase also being found to be a sensitive function of its c / a ratio. Both twophase and Z-method melting techniques have been used in MGPT-MD simulations to determine relative phase stability and its size dependence. In the large-cell limit, the twophase method yields accurate equilibrium melt curves for all five phases, with bcc producing the highest melt temperatures at all pressures and hence being the most stable phase of those considered. The two-phase bcc melt curve is also in good agreement with dynamic experimental data as well as with the MGPT melt curve calculated from bcc and 2 liquid free energies. In contrast, we find that the Z method produces only an upper bound to the equilibrium melt curve in the large-cell limit. For the bcc and hex-ω structures, however, this is a close upper bound within 5% of the two-phase results, although for the A15, fcc, and hcp structures, the Z-melt curves are 25-35% higher in temperature than the two-phase results. Nonetheless, the Z method has allowed us to study melt size effects in detail. We find these effects to be either small or modest for the cubic bcc, A15, and fcc structures, but to have a large impact on the hexagonal hcp and hex-ω melt curves, which are dramatically pushed above that of bcc for simulation cells less than 150 atoms. The melt size effects are driven by and closely correlated with similar size effects on the mechanical stability and the vibrational anharmonicity. We further show that for the same simulation cell sizes and choice of c / a ratio, the MGPT-MD bcc and hex-ω melt curves are in good agreement with the QMD results, so the QMD prediction is confirmed in the small-cell limit. But in the large-cell limit, the MGPT-MD hex-ω melt curve is always lowered below that of bcc for any choice of c / a , so bcc is the most stable phase.We conclude that for the non-bcc Ta phases studied, one requires simulation cells of at least 250-500 atoms to be free ...

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Section: A Mgpt Interatomic Potentialssupporting

confidence: 66%

Section: B Methods Of Melt Calculationmentioning

confidence: 99%

Section: A Mgpt Interatomic Potentialsmentioning

confidence: 99%

mentioning

confidence: 99%

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Polymorphism and melt in high-pressure tantalum

2012

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“…Hence, the atomic scale geometry of the defect core is coupled to the long-range strain field in the surrounding medium. Sinclair et al introduced flexible boundary conditions for studying defects in bulk materials[1] such as cracks [2,3], dislocations [4][5][6][7], vacancies with classical potentials and isolated screw or edge dislocations with density-functional theory [8][9][10][11]. Flexible boundary conditions use the LGF corresponding to the specific geometry of the problem.…”

Section: Introductionmentioning

confidence: 99%

Lattice Green’s function for crystals containing a planar interface

Ghazisaeidi

Trinkle

2010

Phys. Rev. B

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Flexible boundary condition methods couple an isolated defect to a harmonically responding medium through the bulk lattice Green's function; in the case of an interface, interfacial lattice Green's functions. We present a method to compute the lattice Green's function for a planar interface with arbitrary atomic interactions suited for the study of line defect/interface interactions.The interface is coupled to two different semi-infinite bulk regions, and the Green's function for interface-interface, bulk-interface and bulk-bulk interactions are computed individually. The elastic bicrystal Green's function and the bulk lattice Green's function give the interaction between bulk regions. We make use of partial Fourier transforms to treat in-plane periodicity. Direct inversion of the force constant matrix in the partial Fourier space provides the interface terms. The general method makes no assumptions about the atomic interactions or crystal orientations. We simulate a screw dislocation interacting with a (1012) twin boundary in Ti using flexible boundary conditions and compare with traditional fixed boundary conditions results. Flexible boundary conditions give the correct core structure with significantly less atoms required to relax by energy minimization. This highlights the applicability of flexible boundary conditions methods to modeling defect/interface interactions by ab initio methods.

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Anomalous Slip - A Feature of High-Purity Body-Centred Cubic Metals

Seeger¹,

Wasserbäch²

2002

phys. stat. sol. (a)

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The hitherto poorly understood phenomenon of anomalous slip in body-centred cubic (bcc) metal crystals, constituting a striking violation of Schmid's law of resolved shear stresses in limited ranges of temperature and crystallographic orientation of the stress axis, is shown to be a natural consequence of the fact that the slip planes of a 0 h111i=2 screw dislocations change from f110g at low temperatures to f112g at higher temperatures. Above the temperature T T of this change, which is responsible for the so-called lower bend of the flow-stress-temperature relationship, in the orientation range of anomalous slip the two slip systems with the largest resolved shear stress have different slip directions. The screw dislocations of these systems may avoid jog formation and generation of atomic defects by cross-slipping to the anomalous f110g plane. This mechanism accounts for the dependence of anomalous slip on temperature, crystallographic orientation, and purity. The variation from metal to metal is shown to be correlated with the elastic anisotropy. Mechanisms that might be responsible for this variation, in particular for the absence of anomalous slip in bcc iron, are discussed.

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Accurate atomistic simulation of (a/2) 〈111〉 screw dislocations and other defects in bcc tantalum

Cited by 122 publications

References 46 publications

Polymorphism and melt in high-pressure tantalum

Polymorphism and melt in high-pressure tantalum

Lattice Green’s function for crystals containing a planar interface

Anomalous Slip - A Feature of High-Purity Body-Centred Cubic Metals

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