Lubrication of dislocation glide in MgO by hydrous defects

Skelton, Richard; Walker, Andrew

doi:10.1007/s00269-018-0957-y

Cited by 5 publications

(9 citation statements)

References 70 publications

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“…Bulatov and Kaxiras 1997) distribution of dislocation density along the glide plane. Here we use an implementation of the discrete approach previously used for modeling lubrication of dislocation glide in MgO (Skelton and Walker 2018). The total energy of such a finite distribution of dislocations at a distance R from the dislocation line is where E LONG-RANGED is the elastic strain energy of the dislocation contained within radius R and E INTERNAL is the internal energy of the dislocation distribution.…”

Section: Methodsmentioning

confidence: 99%

“…The inelastic component of the dislocation energy is parametrized here using atomistic calculations of generalized stacking fault (GSF; Christian and Vítek 1970) energies on olivine slip planes, using empirical interatomic potentials to model interactions between ions. PN modeling has been used to investigate glide lubrication by interstitial hydrogen atoms (Lu et al 2001) and vacant lattice sites (Lu and Kaxiras 2002) in fcc Al, and {2H Mg } X defects in MgO (Skelton and Walker 2018). We consider only cation vacancies on the M1 and M2 sub-lattices, represented using Kröger-Vink notation (Kröger and Vink 1956) at fO 2 or through incorporation of water under hydrous conditions.…”

Section: Introductionmentioning

confidence: 99%

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Lubrication of dislocation glide in forsterite by Mg vacancies: insights from Peierls-Nabarro modeling

Skelton¹,

Walker²

2018

Preprint

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Dislocation glide is an important contributor to the rheology of olivine under conditions of high stress and low to moderate temperature, such as occur in mantle wedges. Interactions between point defects and dislocation core may alter the Peierls stress,  p , and has been suggested that vacancy-related defects may selectively enhance glide on certain slip systems, changing the olivine deformation fabric.In this study, the Peierls-Nabarro model, parameterized by generalized stacking fault (GSF) energies calculated atomistically using empirical interatomic potentials, is used to determine the effect of bare concentration at the glide plane of 0.125 defects/site is sufficient to reduce cross-over to 0.7 GPa. This may account for the existence of the B-type olivine deformation fabric in the corners of mantle wedges.

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Section: Methodsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Lubrication of dislocation glide in forsterite by Mg vacancies: insights from Peierls-Nabarro modeling

Skelton¹,

Walker²

2018

Preprint

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“…Generalized stacking fault energies, which can be used to parameterize a Peierls-Nabarro model for dislocation glide, are reduced in MgO by the presence of protonated vacancies at the slip plane (Skelton and Walker, 2018). However, this reduction depends on the orientation of the O-H bond, and glide is lubricated only by {2H Mg } X defects whose O-H bond vectors are perpendicular to the glide plane normal (i.e.…”

Section: {2h Mg } X Defectsmentioning

confidence: 99%

“…The presence of vacancy-related defects near dislocations may influence strain rates, especially under conditions of low temperature or high stress, where deformation is controlled by dislocation glide. High-stress experiments have found that the incorporation of protonated defects (often referred to as "water") in (Mg,Fe) 2 SiO 4 olivine may reduce the Peierls stress by up to a factor of two (Katayama and Karato, 2008), and Peierls-Nabarro modeling suggests that protonated vacancies may have a similar effect on the Peierls stress of dislocations in MgO (Skelton and Walker, 2018). Recent measurements of seismic attentuation in Fo90 olivine under oxidizing conditions demonstrated that bare M-site + vacancies produced to charge balance the oxidation of Fe 2+ to Fe 3+ can have similar effects on the mechanical properties of mantle minerals as protonated vacancies (Cline et al, 2018).…”

Section: Introductionmentioning

confidence: 99%

Segregation of bare and protonated Mg vacancies to dislocation cores in MgO

Skelton¹,

Walker²

2018

Preprint

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Water can be incorporated into the lattice of mantle minerals in the form of protons charge-balanced by the creation of cation vacancies. These protonated vacancies, when they interact with dislocations, increase strain rates by enhancing dislocation climb and, potentially, by reducing the Peierls barrier to glide. We use atomic scale simulations to investigate segregation of Mg vacancies to atomic sites within the core regions of dislocations in MgO. Energies are computed for bare and protonated Mg vacancies occupying atomic sites close to 1/2<110> screw dislocations, and 1/2<110>{100} and 1/2<110>{110} edge dislocations. These are compared with energies for equivalent defects in the bulk lattice to determine segregation energies for each defect. Mg vacancies preferentially bind to 1/2<110>{100} edge dislocations, with calculated minimum segregation energies of -3.54 eV for {VMg}′′ and -4.56 eV for {2HMg}X. The magnitudes of the minimum segregation energies calculated for defects binding to 1/2<110>{110} edge or 1/2<110> screw dislocations are considerably lower. Interactions with the dislocation strain field lift the 3-fold energy degeneracy of the {2HMg}X defect in MgO. For edge dislocations, defect configurations in which the O-H bond vector is perpendicular to the glide plane normal are preferred, which may have implications for the ability of protonated vacancies to influence dislocation glide.

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“…The presence of vacancy-related defects near dislocations may influence strain rates, especially under conditions of low temperature or high stress, where deformation is controlled by dislocation glide. High-stress experiments have found that the incorporation of protonated defects (often referred to as "water") in (Mg,Fe) SiO 4 olivine may reduce the Peierls stress by up to a factor of two (Katayama and Karato, 2008), and Peierls-Nabarro modeling suggests that protonated vacancies may have a similar effect on the Peierls stress of dislocations in MgO (Skelton and Walker, 2018). Recent measurements of seismic attentuation in Fo90 olivine under oxidizing conditions demonstrated that bare cation site vacancies produced to charge balance the oxidation of Fe 2+ to Fe 3+ can have similar effects on the mechanical properties of mantle minerals as protonated vacancies (Cline et al, 2018).…”

Section: Introductionmentioning

confidence: 99%

Interactions between bare and protonated Mg vacancies and dislocation cores in MgO

Skelton

Walker

2019

Phys Chem Minerals

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Water can be incorporated into the lattice of mantle minerals in the form of protons charge-balanced by the creation of cation vacancies. These protonated vacancies, when they interact with dislocations, influence strain rates by affecting dislocation climb, pinning the dislocation, and, potentially, by altering the Peierls barrier to glide. We use atomic scale simulations to investigate segregation of Mg vacancies to atomic sites within the core regions of dislocations in MgO. Energies are computed for bare and protonated Mg vacancies occupying atomic sites close to ½<110> screw dislocations, and ½<110>{100} and ½<110>{110} edge dislocations. These are compared with energies for equivalent defects in the bulk lattice to determine segregation energies for each defect. Mg vacancies preferentially bind to ½<110>{100} edge dislocations, with calculated minimum segregation energies of-3.54 eV for V Mg ″ and-4.56 eV for 2 H Mg x. The magnitudes of the minimum segregation energies calculated for defects binding to ½<110>{110} edge or ½<110> screw dislocations are considerably lower. Interactions with the dislocation strain field lift the 3-fold energy degeneracy of 1 the 2 H Mg x defect in MgO. These calculations show that Mg vacancies interact strongly with dislocations in MgO, and may be present in sufficiently high concentrations to affect dislocation mobility in both the glide-and climb-controlled creep regimes.

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Lubrication of dislocation glide in MgO by hydrous defects

Cited by 5 publications

References 70 publications

Lubrication of dislocation glide in forsterite by Mg vacancies: insights from Peierls-Nabarro modeling

Lubrication of dislocation glide in forsterite by Mg vacancies: insights from Peierls-Nabarro modeling

Segregation of bare and protonated Mg vacancies to dislocation cores in MgO

Interactions between bare and protonated Mg vacancies and dislocation cores in MgO

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