Stable atomic structure of NiTi austenite

Zarkevich, Nikolai A.; Johnson, Duane D.

doi:10.1103/physrevb.90.060102

Cited by 29 publications

(59 citation statements)

References 25 publications

(58 reference statements)

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“…27 Recent attempts using small systems to include finite temperatures effects into B2 stability analyses have given contradictory results. 28,29 On the other hand, B19' at the experimental monoclinic angle γ of 98…”

Section: Introductionmentioning

confidence: 99%

Ab initio simulations of phase stability and martensitic transitions in NiTi

2016

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For NiTi based alloys, the shape memory effect is governed by a transition from a low-temperature martensite phase to a high-temperature austenite phase. Despite considerable experimental and computational work, basic questions regarding the stability of the phases and the martensitic phase transition remain unclear even for the simple case of binary, equiatomic NiTi. We perform ab initio molecular dynamics simulations to describe the temperature-dependent behavior of NiTi and resolve several of these outstanding issues. Structural correlation functions and finite temperature phonon spectra are evaluated to determine phase stability. We show that finite temperature, entropic effects stabilize the experimentally observed martensite (B19') and austenite (B2) phases while destabilizing the theoretically predicted (B33) phase. Free energy computations based on ab initio thermodynamic integration confirm these results and permit estimates of the transition temperature between the phases. In addition to the martensitic phase transition, we predict a new transition between the B33 and B19' phases. The role of defects in suppressing phase transformation temperatures is discussed.

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Section: Introductionmentioning

confidence: 99%

Ab initio simulations of phase stability and martensitic transitions in NiTi

2016

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“…4. Energy (meV/atom) relative to austenite (A) for the martensitic transformation in NiTi from the low-T BCO 8 to the high-T austenite, 7 represented by a Ni 54 Ti 54 unit cell with 324 degrees of freedom. C2-NEB finds the TS at 1 meV/atom.…”

Section: Applicationsmentioning

confidence: 99%

“…We apply C2-NEB to study (A) a deformation of the martensitic structure in the NiTi shape-memory alloy; [7][8][9][10][11] (B) its more complicated transformation to the austenite; and FIG. 4.…”

Section: Applicationsmentioning

confidence: 99%

Nudged-elastic band method with two climbing images: Finding transition states in complex energy landscapes

Zarkevich

Johnson

2015

The Journal of Chemical Physics

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The nudged-elastic band (NEB) method is modified with concomitant two climbing images (C2-NEB) to find a transition state (TS) in complex energy landscapes, such as those with a serpentine minimal energy path (MEP). If a single climbing image (C1-NEB) successfully finds the TS, then C2-NEB finds it too. However, improved stability of C2-NEB makes it suitable for more complex cases, where C1-NEB misses the TS because the MEP and NEB directions near the saddle point are different. Generally, C2-NEB not only finds the TS, but also guarantees, by construction, that the climbing images approach it from the opposite sides along the MEP. In addition, C2-NEB provides an accuracy estimate from the three images: the highestenergy one and its climbing neighbors. C2-NEB is suitable for fixed-cell NEB and the generalized solid-state NEB. KeywordsMaterials Science and Engineering, Density functional theory, Nickel, Shape memory effect, Optimization, Band structure DisciplinesEngineering Physics | Metallurgy The nudged-elastic band (NEB) method is modified with concomitant two climbing images (C2-NEB) to find a transition state (TS) in complex energy landscapes, such as those with a serpentine minimal energy path (MEP). If a single climbing image (C1-NEB) successfully finds the TS, then C2-NEB finds it too. However, improved stability of C2-NEB makes it suitable for more complex cases, where C1-NEB misses the TS because the MEP and NEB directions near the saddle point are different. Generally, C2-NEB not only finds the TS, but also guarantees, by construction, that the climbing images approach it from the opposite sides along the MEP. In addition, C2-NEB provides an accuracy estimate from the three images: the highest-energy one and its climbing neighbors. C2-NEB is suitable for fixed-cell NEB and the generalized solid-state NEB. C 2015 AIP Publishing LLC.[http://dx

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“…While there have been many attempts to reveal the competing structures and key barriers, they relied on intuition but have not identified the actual path or structures. First and foremost, rather than a simple (but unstable) B2 structure always used heretofore, the stable high-temperature austenite phase was recently discovered to be a more complex structure [3], with configurations displaying prominent, correlated static displacements but remaining B2-like on average, acting similar to a phonon glass [4].…”

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confidence: 99%

Shape-Memory Transformations of NiTi: Minimum-Energy Pathways between Austenite, Martensites, and Kinetically Limited Intermediate States

Zarkevich

Johnson

2014

Phys. Rev. Lett.

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NiTi is the most used shape-memory alloy, nonetheless, a lack of understanding remains regarding the associated structures and transitions, including their barriers. Using a generalized solid-state nudge elastic band (GSSNEB) method implemented via density-functional theory, we detail the structural transformations in NiTi relevant to shape memory: those between body-centered orthorhombic (BCO) groundstate and a newly identified stable austenite ("glassy" B2-like) structure, including energy barriers (hysteresis) and intermediate structures (observed as a kinetically limited R-phase), and between martensite variants (BCO orientations). All results are in good agreement with available experiment. We contrast the austenite results to those from the often-assumed, but unstable B2. These high-and low-temperature structures and structural transformations provide much needed atomic-scale detail for transitions responsible for NiTi shape-memory effects.

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Stable atomic structure of NiTi austenite

Cited by 29 publications

References 25 publications

Ab initio simulations of phase stability and martensitic transitions in NiTi

Ab initio simulations of phase stability and martensitic transitions in NiTi

Nudged-elastic band method with two climbing images: Finding transition states in complex energy landscapes

Shape-Memory Transformations of NiTi: Minimum-Energy Pathways between Austenite, Martensites, and Kinetically Limited Intermediate States

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