Thermodynamics of Cottrell atmospheres tested by atomistic simulations

Cahn, John W.

doi:10.1016/j.actamat.2016.07.013

Cited by 34 publications

(19 citation statements)

References 26 publications

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“…() documented a possible geological occurrence. Mishin and Cahn () showed how atomistic computer simulations of Al segregation on a dislocation in Ni are in agreement with LC theory and similar studies could be made of minerals. Local equilibrium (4) shows that under stress a correlation between crystal orientation and chemistry could be expected, again if mobile components equilibrated and were then frozen in: this is a testable idea.…”

Section: Discussionmentioning

confidence: 59%

The effects of stress on reactions in the Earth: Sometimes rather mean, usually normal, always important

Wheeler

2018

Journal Metamorphic Geology

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Stress affects chemical processes on all scales in the Earth but the magnitude of its effect is debated. Here, I give a new synthesis of the theory that describes the effects of stress on chemistry, elaborating upon work in Materials Science which is built from fundamental thermodynamic laws, and show its significance in Earth Science.There are separate but compatible relationships describing what happens (1) at interfaces and (2) within grains. (1) The main chemical effects of stress in the Earth are due to variations in normal stress along grain interfaces and between interfaces with different orientations. For reactions involving diffusion these variations give effects on mineral stability broadly equivalent to pressure changes of (molar volume)/(molar volume change during reaction) 9 (stress variation). The volume ratio is generally large and so the effects of normal stress variations are always important since all stressed rocks have interfaces supporting different normal stresses. There is no global chemical equilibrium in a stressed system, so reaction kinetics contribute to ongoing evolution until stresses relax: this evolution can include deformation by diffusion creep and pressure solution, possibly with new mineral growth. These effects are relevant for predicting the conditions for reactions involving fluids, such as serpentinite formation and breakdown (relevant for the Earth's volatile cycles) and for other reactions such as ringwoodite breakdown (relevant for understanding the 660 km mantle discontinuity). (2) Within stressed solid solution grains it is not possible to define chemical potentials of all chemical components since one has to be specified as "immobile." The chemical potential of a "mobile" component such as an exchange vector can be defined. It depends on the "partial molar strain," a second rank tensor defining the variation in unit cell geometry with composition. In cubic crystals the partial molar strain is isotropic and the chemical potential of a mobile component depends on mean stress. In other crystal systems the partial molar strain is anisotropic and the chemical potential depends on a "weighted" mean stress; orientation as well as magnitude of stress has an influence. I propose "chemical palaeopiezometry"-the possibility of measuring past stress levels via chemistry. Examples show that stress variations in hundreds of MPa to GPa are required to produce 2% variations in composition but high stresses and/or precise chemical analyses will allow this proposal to be tested. High stresses around inclusions and dislocations could be targeted. So, the weighted mean stress inside grains has an | INTRODUCTIONIt is undeniable that reactions in the Earth, with or without the involvement of fluids in pores, often occur under stress. I use the word reaction here in the broadest sense to include growth of new phases from old (either adjacent or separated), chemical change in existing phases and growth and dissolution of existing phases during diffusion creep and pressure solution....

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Section: Discussionmentioning

confidence: 59%

The effects of stress on reactions in the Earth: Sometimes rather mean, usually normal, always important

Wheeler

2018

Journal Metamorphic Geology

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“…The approach is embraced by many modern studies of phase transformations by theory and numerical modeling ( 3 – 9 ), and its predictions for the impact of stress on the chemical potential and, thereby, on the driving force for diffusion during lithium insertion are a current topic in the active field of battery research ( 10 – 12 ). The Larché–Cahn theory is also at the heart of an ongoing controversy about the extent of solute enrichment in Cottrell atmospheres and its consequences for stress shielding in dislocation plasticity ( 13 – 18 ). It is, therefore, surprising that central predictions of the theory have not seen direct verification by experiment.…”

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confidence: 99%

Verifying Larché–Cahn elasticity, a milestone of 20th-century thermodynamics

Shi

Markmann

Weißmüller

2018

Proc. Natl. Acad. Sci. U.S.A.

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SignificanceBy concatenating the fields of continuum mechanics and alloy thermodynamics in a unified framework, the theory of open system elasticity by Francis Larché and John W. Cahn achieved one of the key advances in thermodynamics of the 20th century. The theory’s implications for the coupling between stress and composition are generally well acknowledged, yet selected issues are under debate. It is, therefore, dissatisfactory that the predictions for the open system elastic parameters—as the theory’s central materials descriptors—await experimental verification. Here, we report that the variation of the open system Young’s modulus of palladium hydride with composition is in excellent agreement with the prediction. This confirms a cornerstone of materials’ theory.

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“…In particular, the atom radius and the associated molar volume of Ag could promote the formation of Cottrell atmospheres [37]. On the other hand, there was also a chemical effect, which is the driving force for segregation.…”

Section: Discussion On the Effect Of Alloy Elements (Ag) On Plc Effectmentioning

confidence: 99%

Portevin-Le Chatelier effect triggered by complex loading paths in an Al–Cu aluminium alloy

Ren

Morgeneyer

Mazière

et al. 2018

Philosophical Magazine

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Serrated flow has been observed during non monotonic tensile tests of an Al-Cu aluminium alloy in the naturally aged state. The associated propagative localization bands were observed by digital image correlation (DIC). In particular, the Portevin-Le Chatelier (PLC) effect and also Lüders bands were observed in interrupted tests during which the specimen was held for a length of time and also in tests with partial unloading followed by a holding time. Increasing strain rate jumps also triggered the PLC effect. These observations indicate the existence of the PLC effect in this material which was formerly considered insensitive to it at room temperature under monotonic loading conditions. There is no evidence of PLC serrations during constant strain rate tests. A strain ageing finite element model is used that captures the experimentally found PLC triggering effects.

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Thermodynamics of Cottrell atmospheres tested by atomistic simulations

Cited by 34 publications

References 26 publications

The effects of stress on reactions in the Earth: Sometimes rather mean, usually normal, always important

The effects of stress on reactions in the Earth: Sometimes rather mean, usually normal, always important

Verifying Larché–Cahn elasticity, a milestone of 20th-century thermodynamics

Portevin-Le Chatelier effect triggered by complex loading paths in an Al–Cu aluminium alloy

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