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Phase Field Theory Of Surface- And Size-Induced Microstructures, Valery I. Levitas, D.W. Lee, D. L. Preston

#### Phase Field Theory Of Surface- And Size-Induced Microstructures, Valery I. Levitas, D.W. Lee, D. L. Preston

*Valery I. Levitas*

New surface- and size-induced microstructures are found as analytic solutions to a phase field theory of first-order phase transformations. A recently developed exact stability criterion, based on most destabilizing fluctuations, is used to analyze the stability and physical interpretation of each microstructure. Conditions for barrierless surface nucleation, i.e. relationship between surface energy, driving force for the transformation and sample size, are found. If they are met, some of these microstructures are destroyed resulting in the barrierless transformation to alternative phases.

Ginzburg-Landau Theory Of Microstructures: Stability, Transient Dynamics, And Functionally Graded Nanophases, Valery I. Levitas, D. L. Preston, Dong Wook Lee

#### Ginzburg-Landau Theory Of Microstructures: Stability, Transient Dynamics, And Functionally Graded Nanophases, Valery I. Levitas, D. L. Preston, Dong Wook Lee

*Valery I. Levitas*

The stability, transient dynamics, and physical interpretation of microstructures obtained from a Ginzburg-Landau theory of first-order phase transformations are studied. The Jacobi condition for stability fails numerically, thus an alternative exact stability criterion, based on critical (most destabilizing) fluctuations, is developed. The degree-of-stability parameter is introduced to quantify the physical stability of long-lived unstable microstructures. For nanofilms, the existence of functionally graded nanophases is demonstrated. Numerical simulations indicate that graded nanophases can be produced by dissolving material from both surfaces of a nanofilm. Stability under finite fluctuations and post-bifurcation microstructure evolution are investigated numerically.

Kinetics Of Strain-Induced Structural Changes Under High Pressure, Valery I. Levitas, Oleg M. Zarechnyy

#### Kinetics Of Strain-Induced Structural Changes Under High Pressure, Valery I. Levitas, Oleg M. Zarechnyy

*Valery I. Levitas*

A mechanism-based microscale kinetic theory for strain-induced structural changes (SCs) (that includes phase transformations (PTs) and chemical reactions (CRs)) is developed. Time is not an independent parameter in this theory; instead, plastic strain is a time-like parameter. Kinetics depends essentially on the ratio of the yield strengths of phases. Stationary and nonstationary solutions of the kinetic equations are analyzed for various cases, including SCs between two phases in an inert matrix and between three phases in silicon and germanium. A number of experimental phenomena are explained, and material parameters controlling the kinetics of strain-induced SCs are determined. This includes the ...

Strain-Induced Disorder, Phase Transformations, And Transformation-Induced Plasticity In Hexagonal Boron Nitride Under Compression And Shear In A Rotational Diamond Anvil Cell: In Situ X-Ray Diffraction Study And Modeling, Valery I. Levitas, Yanzhang Ma, Javad Hashemi, Mark Holtz, Necip Guven

#### Strain-Induced Disorder, Phase Transformations, And Transformation-Induced Plasticity In Hexagonal Boron Nitride Under Compression And Shear In A Rotational Diamond Anvil Cell: In Situ X-Ray Diffraction Study And Modeling, Valery I. Levitas, Yanzhang Ma, Javad Hashemi, Mark Holtz, Necip Guven

*Valery I. Levitas*

Plastic shear significantly reduces the phase transformation (PT) pressure when compared to hydrostatic conditions. Here, a paradoxical result was obtained: PT of graphitelike hexagonal boron nitride (hBN) to superhard wurtzitic boron nitride under pressure and shear started at about the same pressure(∼10GPa) as under hydrostatic conditions. In situ x-ray diffraction measurement and modeling of the turbostratic stacking fault concentration (degree of disorder) and PT in hBN were performed. Under hydrostaticpressure, changes in the disorder were negligible. Under a complex compression and shear loading program, a strain-induced disorder was observed and quantitatively characterized. It is found that the strain-induced disorder ...

Interfacial And Volumetric Kinetics Of The Β→Δ Phase Transition In The Energetic Nitramine Octahydro-1,3,5,7-Tetranitro-1,3,5,7-Tetrazocine Based On The Virtual Melting Mechanism, Valery I. Levitas, Laura B. Smilowitz, Bryan F. Henson, Blaine W. Asay

#### Interfacial And Volumetric Kinetics Of The Β→Δ Phase Transition In The Energetic Nitramine Octahydro-1,3,5,7-Tetranitro-1,3,5,7-Tetrazocine Based On The Virtual Melting Mechanism, Valery I. Levitas, Laura B. Smilowitz, Bryan F. Henson, Blaine W. Asay

*Valery I. Levitas*

In the recent papers, 1,2 we presented a thermodynamic and kinetic model of the β→δ phase transformation (PT) in the organic energetic crystal octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX). It was based on the hypothesis that the kinetics of the β→δ PT is governed by the thermodynamics of melting of the δ phase. In particular, the activation energy for growth was found to be equal to the heat of fusion Δhδ→m . Nucleation was modeled empirically by the reversible first-order kinetics. The proposed kinetics described the experimental data quite well, however, a number of questions ...

Solid-Solid Phase Transformation Via Internal Stress-Induced Virtual Melting, Significantly Below The Melting Temperature. Application To Hmx Energetic Crystal, Valery I. Levitas, Bryan F. Henson, Laura B. Smilowitz, Blaine W. Asay

#### Solid-Solid Phase Transformation Via Internal Stress-Induced Virtual Melting, Significantly Below The Melting Temperature. Application To Hmx Energetic Crystal, Valery I. Levitas, Bryan F. Henson, Laura B. Smilowitz, Blaine W. Asay

*Valery I. Levitas*

We theoretically predict a new phenomenon, namely, that a solid−solid phase transformation (PT) with a large transformation strain can occur via internal stress-induced virtual melting along the interface at temperatures significantly (more than 100 K) below the melting temperature. We show that the energy of elastic stresses, induced by transformation strain, increases the driving force for melting and reduces the melting temperature. Immediately after melting, stresses relax and the unstable melt solidifies. Fast solidification in a thin layer leads to nanoscale cracking which does not affect the thermodynamics or kinetics of the solid−solid transformation. Thus, virtual melting represents ...

Crystal-Amorphous And Crystal-Crystal Phase Transformations Via Virtual Melting, Valery I. Levitas

#### Crystal-Amorphous And Crystal-Crystal Phase Transformations Via Virtual Melting, Valery I. Levitas

*Valery I. Levitas*

A new mechanism of crystal (c)–amorphous (a) and c-c phase transformations (PTs) and internal stress relaxation via virtual melting (VM) induced by internal stresses was justified thermodynamically and kinetically. VM removes interface friction, reduces kinetic barrier, increases atomic mobility, and can reduce thermodynamic melting temperature. We combine VM and nonequilibrium PT diagrams to develop new scenarios of c-a and c-c PTs. Results are applied for a new interpretation of c-c and c-a PT mechanisms in ice Ih and are also applicable for other materials.

Solid-Solid Phase Transformation Via Internal Stress-Induced Virtual Melting: Additional Confirmations, Valery I. Levitas, Laura B. Smilowitz, Bryan F. Henson, Blaine W. Asay

#### Solid-Solid Phase Transformation Via Internal Stress-Induced Virtual Melting: Additional Confirmations, Valery I. Levitas, Laura B. Smilowitz, Bryan F. Henson, Blaine W. Asay

*Valery I. Levitas*

Recently, we predicted a mechanism of solid-solid phase transformation (PT) via virtual melting at 121K below the melting temperature. We report additional experimental and theoretical results for PTs among three polymorphs of the energetic material HMX, α, β, and δ that support this mechanism. In particular: (a) the predicted velocity of interface propagation for β→δ PT and overall kinetics of δ→β PT are in agreement with experiment; (b) the energy of internal stresses is sufficient to reduce the melting temperature from 520to400K for δ→β PT; (c) the nanocracking that appears during solidification does not change the PT ...

Transformation-Induced Plasticity And Cascading Structural Changes In Hexagonal Boron Nitride Under High Pressure And Shear, Valery I. Levitas, Yanzhang Ma, Javad Hashemi

#### Transformation-Induced Plasticity And Cascading Structural Changes In Hexagonal Boron Nitride Under High Pressure And Shear, Valery I. Levitas, Yanzhang Ma, Javad Hashemi

*Valery I. Levitas*

In situx-ray diffraction study and modeling of the degree of disorder, s, and phase transformation (PT) in hexagonal hBN were performed. It was proven that changes in s are strain-induced and that s can be used to quantify plastic strain. During the strain-induced hBN→wurtzitic wBN PT, the transformation-induced plasticity (TRIP) was exposed and quantified. TRIP exceeds conventional plasticity by a factor of 20. Cascading structural changes were revealed. Strain-induced disorder explains why PT under hydrostatic and nonhydrostatic conditions started at the same pressure∼10GPa. For the same disorder, plastic shear reduces PT pressure by a factor of 3–4.

Strain-Induced Disorder And Phase Transformation In Hexagonal Boron Nitride Under Quasi-Homogeneous Pressure: In Situ X-Ray Study In A Rotational Diamond Anvil Cell, Valery I. Levitas, J. Hashemi, Y. Z. Ma

#### Strain-Induced Disorder And Phase Transformation In Hexagonal Boron Nitride Under Quasi-Homogeneous Pressure: In Situ X-Ray Study In A Rotational Diamond Anvil Cell, Valery I. Levitas, J. Hashemi, Y. Z. Ma

*Valery I. Levitas*

One of the challenges in characterization of strain-induced transformations is to create uniform pressure. In this letter, conditions for nearly homogeneous pressure distribution are predicted and achieved experimentally. Compared to hydrostatic loading, plastic shear generally reduces the transformation pressure significantly. We observed, however, an unexpected phenomenon: the transformation of hexagonal to superhard wurtzitic BN under pressure and shear initiated at a pressure comparable to that in hydrostatic compression ($\sim 10$ \ensuremath{{\rm ~GPa}}). In situ X-ray diffraction revealed that plastic shear increases the disorder, while hydrostatic compression does not. This increase neutralizes the transition pressure reduction caused by shear. For ...

Microscale Simulation Of Martensitic Microstructure Evolution, Valery I. Levitas, Alexander V. Idesman, Dean L. Preston

#### Microscale Simulation Of Martensitic Microstructure Evolution, Valery I. Levitas, Alexander V. Idesman, Dean L. Preston

*Valery I. Levitas*

A new model for the evolution of multivariant martensitic microstructure in single crystals and polycrystals is developed. In contrast with Landau-Ginzburg models, which are limited in practice to nanoscale specimens, this new scale-free model is valid for length scales greater than 100 nm and without an upper bound. It is based on a thermodynamic potential in the volume fractions of the martensitic variants that exhibits an instability resulting in microstructure formation. Simulated microstructures in elastic single crystals and polycrystals under uniaxial loading are in qualitative agreement with those observed experimentally.

Solid-Solid Phase Transformation Via Virtual Melting Significantly Below The Melting Temperature, Valery I. Levitas, Bryan F. Henson, Laura B. Smilowitz, Blaine W. Asay

#### Solid-Solid Phase Transformation Via Virtual Melting Significantly Below The Melting Temperature, Valery I. Levitas, Bryan F. Henson, Laura B. Smilowitz, Blaine W. Asay

*Valery I. Levitas*

A new phenomenon is theoretically predicted, namely, that solid-solid transformation with a relatively large transformation strain can occur through virtual melting along the interface at temperatures significantly (more than 100 K) below the melting temperature. The energy of elastic stresses, induced by transformation strain, increases the driving force for melting and reduces the melting temperature. Immediately after melting, the stresses relax and the unstable melt solidifies. Fast solidification in a thin layer leads to nanoscale cracking, which does not affect the thermodynamics and kinetics of solid-solid transformation. Seven theoretical predictions are in quantitative agreement with experiments conducted on the β ...

A Microscale Model For Strain-Induced Phase Transformations And Chemical Reactions Under High Pressure, Valery I. Levitas

#### A Microscale Model For Strain-Induced Phase Transformations And Chemical Reactions Under High Pressure, Valery I. Levitas

*Valery I. Levitas*

A simple strain-controlled kinetic equation for strain-induced phase transformations and chemical reactions is thermodynamically derived. This model is applied to explain various mechanochemical phenomena observed under compression and shear of materials in diamond or Bridgman anvils. In particular, it explains zero-pressure hysteresis and the appearance of new phases, especially strong phases, which were not obtained without shear. Also an explanation was obtained as to why a nonreacting matrix with a yield stress higher (lower) than that for reagents significantly accelerates (slows down) the reactions. Some methods to characterize and control strain-induced transformations and reactions are suggested.

High-Pressure Mechanochemistry: Conceptual Multiscale Theory And Interpretation Of Experiments, Valery I. Levitas

#### High-Pressure Mechanochemistry: Conceptual Multiscale Theory And Interpretation Of Experiments, Valery I. Levitas

*Valery I. Levitas*

Fifteen mechanochemical phenomena observed under compression and plastic shear of materials in a rotational diamond anvil cell (RDAC) are systematized. They are related to strain-induced structural changes (SCs) under high pressure, including phase transformations (PTs) and chemical reactions. A simple, three-scale continuum thermodynamic theory and closed-form solutions are developed which explain these phenomena. At the nanoscale, a model for strain-induced nucleation at the tip of a dislocation pile-up is suggested and studied. At the microscale, a simple strain-controlled kinetic equation for the strain-induced SCs is thermodynamically derived. A macroscale model for plastic flow and strain-induced SCs in RDAC is developed ...

Three-Dimensional Landau Theory For Multivariant Stress-Induced Martensitic Phase Transformations. Iii. Alternative Potentials, Critical Nuclei, Kink Solutions, And Dislocation Theory, Valery I. Levitas, Dean L. Preston, Dong Wook Lee

#### Three-Dimensional Landau Theory For Multivariant Stress-Induced Martensitic Phase Transformations. Iii. Alternative Potentials, Critical Nuclei, Kink Solutions, And Dislocation Theory, Valery I. Levitas, Dean L. Preston, Dong Wook Lee

*Valery I. Levitas*

In part III of this paper, alternative Landau potentials for the description of stress-and temperature-induced martensitic phase transformations under arbitrary three-dimensional loading are obtained. These alternative potentials include a sixth-degree (2-4-6) polynomial in Cartesian order parameters and a potential in hyperspherical order parameters. Each satisfies all conditions for the correct description of experiments. The unique features of the potentials are pointed out and a detailed comparison of the potentials is made for NiAl alloy. Analytic solutions of the one-dimensional time-independent Ginzburg-Landau equations for the 2-3-4 and 2-4-6 potentials for a constant-stress tensor and invariant-plane strain are obtained and compared. Solutions ...