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Full-Text Articles in Mechanical Engineering

An Exact Formulation For Exponential-Logarithmic Transformation Stretches In A Multiphase Phase Field Approach To Martensitic Transformations, Anup Basak, Valery I. Levitas Jan 2020

An Exact Formulation For Exponential-Logarithmic Transformation Stretches In A Multiphase Phase Field Approach To Martensitic Transformations, Anup Basak, Valery I. Levitas

Aerospace Engineering Publications

A general theoretical and computational procedure for dealing with an exponential-logarithmic kinematic model for transformation stretch tensor in a multiphase phase field approach to stress- and temperature- induced martensitic transformations with N martensitic variants is developed for transformations between all possible crystal lattices. This kinematic model, where the natural logarithm of transformation stretch tensor is a linear combination of natural logarithm of the Bain tensors, yields isochoric variant-variant transformations for the entire transformation path. Such a condition is plausible and cannot be satisfied by the widely used kinematic model where the transformation stretch tensor is linear in Bain tensors. Earlier ...


Tensorial Stress−Strain Fields And Large Elastoplasticity As Well As Friction In Diamond Anvil Cell Up To 400 Gpa, Valery I. Levitas, Mehdi Kamrani, Biao Feng Oct 2019

Tensorial Stress−Strain Fields And Large Elastoplasticity As Well As Friction In Diamond Anvil Cell Up To 400 Gpa, Valery I. Levitas, Mehdi Kamrani, Biao Feng

Aerospace Engineering Publications

Various phenomena (fracture, phase transformations, and chemical reactions) studied under extreme pressures in diamond anvil cell are strongly affected by fields of all components of stress and plastic strain tensors. However, they could not be measured. Here, we suggest a coupled experimental−theoretical−computational approach that allowed us (using published experimental data) to refine, calibrate, and verify models for elastoplastic behavior and contact friction for tungsten (W) and diamond up to 400 GPa and reconstruct fields of all components of stress and large plastic strain tensors in W and diamond. Despite the generally accepted strain-induced anisotropy, strain hardening, and path-dependent ...


Nde In-Process For Metal Parts Fabricated Using Powder Based Additive Manufacturing, Leonard J. Bond, Lucas W. Koester, Hossein Taheri Mar 2019

Nde In-Process For Metal Parts Fabricated Using Powder Based Additive Manufacturing, Leonard J. Bond, Lucas W. Koester, Hossein Taheri

Aerospace Engineering Publications

Ensuring adequate quality for additive manufactured (AM) materials presents unique metrology challenges to the on-line process measurement and nondestructive evaluation (NDE) communities. AM parts now have complex forms that are not possible using subtractive manufacturing and there are moves for their use in safety criticality components. This paper briefly reviews the status, challenges and metrology opportunities throughout the AM process from powder to finished parts. The primary focus is on new acoustic signatures that have been demonstrated to correlate process parameters with on-line measurement for monitoring and characterization during the build. In-process, quantitative characterization and monitoring of material state is ...


Slip Of Shuffle Screw Dislocations Through Tilt Grain Boundaries In Silicon, Hao Chen, Valery Levitas, Liming Xiong Feb 2019

Slip Of Shuffle Screw Dislocations Through Tilt Grain Boundaries In Silicon, Hao Chen, Valery Levitas, Liming Xiong

Aerospace Engineering Publications

In this paper, molecular dynamics (MD) simulations of the interaction between tilt grain boundaries (GBs) and a shuffle screw dislocation in silicon are performed. Results show that dislocations transmit into the neighboring grain for all GBs in silicon. For Σ3, Σ9 and Σ19 GBs, when a dislocation interacts with a heptagon site, it transmits the GB directly. In contrast, when interacting with a pentagon site, it first cross slips to a plane on the heptagon site and then transmits the GB. The energy barrier is also quantified using the climbing image nudged elastic band (CINEB) method. Results show that Σ3 ...


Imaging Stress And Magnetism At High Pressures Using A Nanoscale Quantum Sensor, S. Hsieh, P. Bhattacharyya, C. Zu, T. Mittiga, T. J. Smart, F. Machado, B. Kobrin, T. O. Hohn, N. Z. Rui, Mehdi Kamrani, S. Chatterjee, S. Choi, M. Zaletel, V. V. Struzhkin, J. E. Moore, Valery I. Levitas, R. Jeanloz, N. Y. Yao Dec 2018

Imaging Stress And Magnetism At High Pressures Using A Nanoscale Quantum Sensor, S. Hsieh, P. Bhattacharyya, C. Zu, T. Mittiga, T. J. Smart, F. Machado, B. Kobrin, T. O. Hohn, N. Z. Rui, Mehdi Kamrani, S. Chatterjee, S. Choi, M. Zaletel, V. V. Struzhkin, J. E. Moore, Valery I. Levitas, R. Jeanloz, N. Y. Yao

Aerospace Engineering Publications

Pressure alters the physical, chemical and electronic properties of matter. The development of the diamond anvil cell (DAC) enables tabletop experiments to investigate a diverse landscape of high-pressure phenomena ranging from the properties of planetary interiors to transitions between quantum mechanical phases. In this work, we introduce and utilize a novel nanoscale sensing platform, which integrates nitrogen-vacancy (NV) color centers directly into the culet (tip) of diamond anvils. We demonstrate the versatility of this platform by performing diffraction-limited imaging (~600 nm) of both stress fields and magnetism, up to pressures ~30 GPa and for temperatures ranging from 25-340 K. For ...


Morphology And Stress Evolution During The Initial Stages Of Intergranular Corrosion Of X70 Steel, Denizhan Yavas, Abdullah Alshehri, Pratyush Mishra, Pranav Shrotriya, Ashraf F. Bastawros, Kurt R. Hebert Sep 2018

Morphology And Stress Evolution During The Initial Stages Of Intergranular Corrosion Of X70 Steel, Denizhan Yavas, Abdullah Alshehri, Pratyush Mishra, Pranav Shrotriya, Ashraf F. Bastawros, Kurt R. Hebert

Aerospace Engineering Publications

Pipeline steels are vulnerable to stress corrosion cracking (SCC) during intergranular corrosion (IGC) at potentials of active dissolution in moderately alkaline carbonate-bicarbonate solutions. Morphology evolution accompanying IGC has not been fully described, despite the relevance of the corrosion geometry to crack initiation. The present article reports a characterization of concurrent morphology and mechanical stress development during the initial stages of IGC of X70 steel in sodium bicarbonate solution, in the potential range of high SCC susceptibility. Morphology was revealed by scanning electron microscope examination of cross sections through the IGC layer, and stress evolution was monitored by curvature interferometry. At ...


Resonance Analysis Of A High Temperature Piezoelectric Disc For Sensitivity Characterization, Prathamesh N. Bilgunde, Leonard J. Bond Jul 2018

Resonance Analysis Of A High Temperature Piezoelectric Disc For Sensitivity Characterization, Prathamesh N. Bilgunde, Leonard J. Bond

Aerospace Engineering Publications

Ultrasonic transducers for high temperature (200 °C+) applications are a key enabling technology for advanced nuclear power systems and in a range of chemical and petro-chemical industries. Design, fabrication and optimization of such transducers using piezoelectric materials remains a challenge. In this work, experimental data-based analysis is performed to investigate the fundamental causal factors for the resonance characteristics of a piezoelectric disc at elevated temperatures. The effect of all ten temperature-dependent piezoelectric constants (ε33, ε11, d33, d31, d15, s11, s12, s13, s33, s44) is studied numerically on both the radial and thickness mode resonances of a piezoelectric disc. A sensitivity ...


Nanoscale Multiphase Phase Field Approach For Stress- And Temperature-Induced Martensitic Phase Transformations With Interfacial Stresses At Finite Strains, Anup Basak, Valery I. Levitas Apr 2018

Nanoscale Multiphase Phase Field Approach For Stress- And Temperature-Induced Martensitic Phase Transformations With Interfacial Stresses At Finite Strains, Anup Basak, Valery I. Levitas

Aerospace Engineering Publications

A thermodynamically consistent, novel multiphase phase field approach for stress- and temperature-induced martensitic phase transformations at finite strains and with interfacial stresses has been developed. The model considers a single order parameter to describe the austenite↔martensitic transformations, and another N order parameters describing N variants and constrained to a plane in an N-dimensional order parameter space. In the free energy model coexistence of three or more phases at a single material point (multiphase junction), and deviation of each variant-variant transformation path from a straight line have been penalized. Some shortcomings of the existing models are resolved. Three different kinematic ...


Wear Evolution And Stress Distribution Of Single Cbn Superabrasive Grain In High-Speed Grinding, Jingwei Wang, Tianyu Yu, Wenfeng Ding, Yucan Fu, Ashraf F. Bastawros Jan 2018

Wear Evolution And Stress Distribution Of Single Cbn Superabrasive Grain In High-Speed Grinding, Jingwei Wang, Tianyu Yu, Wenfeng Ding, Yucan Fu, Ashraf F. Bastawros

Aerospace Engineering Publications

In this study, both finite element analysis (FEA) and experimental observations were used to investigate the single CBN grain wear in high-speed grinding of Inconel 718 superalloy. The wear characteristics for each grinding pass were numerically assessed utilizing the tensile and compressive strength limits of the cutting grain. Additionally, stress distribution within the grain, chip formation and grinding force evolution during multiple passes were investigated. The combined experimental and numerical results show that the CBN grain wear has two major modes: the macro fracture on the grain top surface propagating from the rake surface, and the micro fracture near the ...


Thermodynamically Consistent And Scale-Dependent Phase Field Approach For Crack Propagation Allowing For Surface Stresses, Valery I. Levitas, Hossein Jafarzadeh, Gholam Hossein Farrahi, Mahdi Javanbakht Jan 2018

Thermodynamically Consistent And Scale-Dependent Phase Field Approach For Crack Propagation Allowing For Surface Stresses, Valery I. Levitas, Hossein Jafarzadeh, Gholam Hossein Farrahi, Mahdi Javanbakht

Aerospace Engineering Publications

A thermodynamically-consistent phase field approach for crack propagation which includes the following novel features is presented. (1) Scale dependency was included by relating the length scale to the number of cohesive interatomic planes at the crack tip. Because of this, the developed theory is applicable from the atomistic to the macroscopic scales. (2) The surface stresses (tension) are introduced by employing some geometrical nonlinearities even in small strain theory. They produce multiple contributions to the Ginzburg-Landau equation for crack propagation. (3) Crack propagation in the region with compressive closing stresses is eliminated by employing a stress-state-dependent kinetic coefficient in the ...


Phase-Field Approach For Stress- And Temperature-Induced Phase Transformations That Satisfies Lattice Instability Conditions. Part 2. Simulations Of Phase Transformations Si I↔↔ Si Ii, Hamed Babaei, Valery I. Levitas Jan 2018

Phase-Field Approach For Stress- And Temperature-Induced Phase Transformations That Satisfies Lattice Instability Conditions. Part 2. Simulations Of Phase Transformations Si I↔↔ Si Ii, Hamed Babaei, Valery I. Levitas

Aerospace Engineering Publications

A complete system of equations of the advanced phase-field theory for martensitic phase transformations (PTs) under a general stress tensor is presented. Theory includes a fully geometrically nonlinear formulation for the general case of finite elastic and transformational strains as well as anisotropic and different elastic properties of phases. Material parameters are calibrated, in particular, based on the crystal lattice instability conditions from atomistic simulations for martensitic PTs between cubic Si I and tetragonal Si II phases under complex triaxial compression-tension loading. A finite element algorithm and numerical procedure is developed and implemented in the code deal.II. Various 3D ...


Comparison Of Acoustoelastic Lamb Wave Propagation In Stressed Plates For Different Measurement Orientations, Ning Pei, Leonard J. Bond Oct 2017

Comparison Of Acoustoelastic Lamb Wave Propagation In Stressed Plates For Different Measurement Orientations, Ning Pei, Leonard J. Bond

Aerospace Engineering Publications

High order Lamb waves are investigated for the effects of stress on both symmetrical and anti-symmetrical modes in an aluminum plate for wave propagation and load parallel. Data are compared with those for the case of load and measurement axis perpendicular. It is the S1 mode which exhibits significantly higher sensitivity to stress than other Lamb modes. For aluminum the use of the S1 mode for stress measurement is found to be about six times more sensitive, than bulk waves, for the load-measurement axes parallel case and this compares with about ten times for the case of load-measurement axes perpendicular.


Performance And Modeling Of Paired Polishing Process, Tianyu Yu, David Thomas Asplund, Ashraf F. Bastawros, Abhijit Chandra Jan 2016

Performance And Modeling Of Paired Polishing Process, Tianyu Yu, David Thomas Asplund, Ashraf F. Bastawros, Abhijit Chandra

Aerospace Engineering Publications

Paired polishing process (PPP) is a variant of the chemical mechanical polishing process which facilitates defect mitigation via minimization of maximum force as well as effective planarization via profile driven determination of force gradient. The present embodiment of PPP machine employs two polishing wheels, radially spanning the wafer surface on a counter-gimbaled base. The PPP machine is deployed to experimentally investigate the role of the process parameters on the surface roughness evolution, and the effective material removal rate. Two sets of copper and aluminum blanket layers were polished under a range of applied down force, polishing wheel speed and transverse ...


Multiple Pure Tone Noise Prediction, Fei Han, Anupam Sharma, Umesh Paliath, Chingwei Shieh Dec 2014

Multiple Pure Tone Noise Prediction, Fei Han, Anupam Sharma, Umesh Paliath, Chingwei Shieh

Aerospace Engineering Publications

This paper presents a fully numerical method for predicting multiple pure tones, also known as “Buzzsaw” noise. It consists of three steps that account for noise source generation, nonlinear acoustic propagation with hard as well as lined walls inside the nacelle, and linear acoustic propagation outside the engine. Noise generation is modeled by steady, part-annulus computational fluid dynamics (CFD) simulations. A linear superposition algorithm is used to construct full-annulus shock/pressure pattern just upstream of the fan from part-annulus CFD results. Nonlinear wave propagation is carried out inside the duct using a pseudo-two-dimensional solution of Burgers׳ equation. Scattering from nacelle ...


Strain-Induced Phase Transformation Under Compression In A Diamond Anvil Cell: Simulations Of A Sample And Gasket, Biao Feng, Valery I. Levitas, Yanzhang Ma Jan 2014

Strain-Induced Phase Transformation Under Compression In A Diamond Anvil Cell: Simulations Of A Sample And Gasket, Biao Feng, Valery I. Levitas, Yanzhang Ma

Aerospace Engineering Publications

Combined high pressure phase transformations (PTs) and plastic flow in a sample within a gasket compressed in diamond anvil cell (DAC) are studied for the first time using finite element method. The key point is that phase transformations are modelled as strain-induced, which involves a completely different kinetic description than for traditional pressure-induced PTs. The model takes into account, contact sliding with Coulomb and plastic friction at the boundaries between the sample, gasket, and anvil. A comprehensive computational study of the effects of the kinetic parameter, ratio of the yield strengths of high and low-pressure phases and the gasket, sample ...


Unambiguous Gibbs Dividing Surface For Nonequilibrium Finite-Width Interface: Static Equivalence Approach, Valery I. Levitas Jan 2014

Unambiguous Gibbs Dividing Surface For Nonequilibrium Finite-Width Interface: Static Equivalence Approach, Valery I. Levitas

Aerospace Engineering Publications

The definition of all properties of the nonequilibrium interface depends on the choice of the position of the dividing surface. However, the definition of its position has been an unsolved problem for more than a century. A missing principle to unambiguously determine the position of the Gibbs’ dividing surface is found: the principle of static equivalence. A sharp interface (dividing surface) is statically equivalent to a nonequilibrium finite-width interface with distributed tensile stresses if it possesses (a) the same resultant force, equal to the interface energy, and (b) the same moment, which is zero about the interface position. Each of ...


Multiple Twinning And Variant-Variant Transformations In Martensite: Phase-Field Approach, Valery I. Levitas, Arunabha M. Roy, Dean L. Preston Aug 2013

Multiple Twinning And Variant-Variant Transformations In Martensite: Phase-Field Approach, Valery I. Levitas, Arunabha M. Roy, Dean L. Preston

Aerospace Engineering Publications

A phase-field theory of transformations between martensitic variants and multiple twinning within martensitic variants is developed for large strains and lattice rotations. It resolves numerous existing problems. The model, which involves just one order parameter for the description of each variant-variant transformation and multiple twinnings within each martensitic variant, allows one to prescribe the twin interface energy and width, and to introduce interface stresses consistent with the sharp interface limit. A finite-element approach is developed and applied to the solution of a number of examples of twinning and combined austenite-martensite and martensite-martensite phase transformations (PTs) and nanostructure evolution. A similar ...


Prediction Of Aerodynamic Tonal Noise From Open Rotors, Anupam Sharma, Hsuan-Nien Chen Aug 2013

Prediction Of Aerodynamic Tonal Noise From Open Rotors, Anupam Sharma, Hsuan-Nien Chen

Aerospace Engineering Publications

A numerical approach for predicting tonal aerodynamic noise from ‘‘open rotors’’ is presented. ‘‘Open rotor’’ refers to an engine architecture with a pair of counter-rotating propellers. Typical noise spectra from an open rotor consist of dominant tones, which arise due to both the steady loading/thickness and the aerodynamic interaction between the two bladerows. The proposed prediction approach utilizes Reynolds Averaged Navier–Stokes (RANS) Computational Fluid Dynamics (CFD) simulations to obtain near- field description of the noise sources. The near-to-far-field propagation is then carried out by solving the Ffowcs Williams–Hawkings equation. Since the interest of this paper is limited ...


Plastic Flows And Phase Transformations In Materials Under Compression In Diamond Anvil Cell: Effect Of Contact Sliding, Biao Feng, Valery I. Levitas, Oleg M. Zarechnyy Jul 2013

Plastic Flows And Phase Transformations In Materials Under Compression In Diamond Anvil Cell: Effect Of Contact Sliding, Biao Feng, Valery I. Levitas, Oleg M. Zarechnyy

Aerospace Engineering Publications

Modeling of coupled plastic flows and strain-induced phase transformations (PTs) under high pressure in a diamond anvil cell is performed with the focus on the effect of the contact sliding between sample and anvils. Finite element software ABAQUS is utilized and a combination of Coulomb friction and plastic friction is considered. Results are obtained for PTs to weaker, equal-strength, and stronger high pressure phases, using different scaling parameters in a strain-controlled kinetic equation, and with various friction coefficients. Compared to the model with cohesion, artificial shear banding near the constant surface is eliminated. Sliding and the reduction in friction coefficient ...


Phase Field Approach To Interaction Of Phase Transformation And Dislocation Evolution, Valery I. Levitas, Mahdi Javanbakht Jun 2013

Phase Field Approach To Interaction Of Phase Transformation And Dislocation Evolution, Valery I. Levitas, Mahdi Javanbakht

Aerospace Engineering Publications

Phase field approach to coupled evolution of martensitic phase transformations (PTs) anddislocation is developed. A fully geometrically nonlinear formulation is utilized. The finite element method procedure is developed and applied to study the hysteretic behavior and propagation of an austenite (A)–martensite (M) interface with incoherency dislocations, the growth and arrest of martensitic plate for temperature-induced PT, and the evolution of phase and dislocation structures for stress-induced PT. A similar approach can be developed for the interaction of dislocations with twins and diffusive PTs described by Cahn-Hilliard theory.


Strain-Induced Phase Transformations Under Compression, Unloading, And Reloading In A Diamond Anvil Cell, Biao Feng, Oleg M. Zarechnyy, Valery I. Levitas May 2013

Strain-Induced Phase Transformations Under Compression, Unloading, And Reloading In A Diamond Anvil Cell, Biao Feng, Oleg M. Zarechnyy, Valery I. Levitas

Aerospace Engineering Publications

Strain-induced phase transformations (PTs) in a sample under compression, unloading, and reloading in a diamond anvil cell are investigated in detail, by applying finite element method. In contrast to previous studies, the kinetic equation includes the pressure range in which both direct and reverse PTs occur simultaneously. Results are compared to the case when “no transformation” region in the pressure range exists instead, for various values of the kinetic parameters and ratios of the yield strengths of low and high pressure phases. Under unloading (which has never been studied before), surprising plastic flow and reverse PT are found, which were ...


Anisotropic Compositional Expansion And Chemical Potential For Amorphous Lithiated Silicon Under Stress Tensor, Valery I. Levitas, Hamed Attariani Apr 2013

Anisotropic Compositional Expansion And Chemical Potential For Amorphous Lithiated Silicon Under Stress Tensor, Valery I. Levitas, Hamed Attariani

Aerospace Engineering Publications

Si is a promising anode material for Li-ion batteries, since it absorbs large amounts of Li. However, insertion of Li leads to 334% of volumetric expansion, huge stresses, and fracture; it can be suppressed by utilizing nanoscale anode structures. Continuum approaches to stress relaxation in LixSi, based on plasticity theory, are unrealistic, because the yield strength of LixSi is much higher than the generated stresses. Here, we suggest that stress relaxation is due to anisotropic (tensorial) compositional straining that occurs during insertion-extraction at any deviatoric stresses. Developed theory describes known experimental and atomistic simulation data. A method to reduce stresses ...


Interface Stress For Nonequilibrium Microstructures In The Phase Field Approach: Exact Analytical Results, Valery I. Levitas Feb 2013

Interface Stress For Nonequilibrium Microstructures In The Phase Field Approach: Exact Analytical Results, Valery I. Levitas

Aerospace Engineering Publications

An exact expression for the temperature-dependent interface stress tensor (tension) and energy is derived within a phase field approach. The key problem, of which part of the thermal energy should contribute to the surface tension, is resolved with the help of an analytical solution for a nonequilibrium interface. Thus, for a propagating interface at any temperature, the interface stress tensor represents biaxial tension with magnitude equal to the temperature-dependent interface energy. Explicit expressions for the distributions of interface stresses are obtained for a nonequilibrium interface and a critical nucleus. The results obtained are applicable for various phase transformations (solid-solid, melting-solidification ...


Coupled Phase Transformations And Plastic Flows Under Torsion At High Pressure In Rotational Diamond Anvil Cell: Effect Of Contact Sliding, Biao Feng, Valery I. Levitas Jan 2013

Coupled Phase Transformations And Plastic Flows Under Torsion At High Pressure In Rotational Diamond Anvil Cell: Effect Of Contact Sliding, Biao Feng, Valery I. Levitas

Aerospace Engineering Publications

A three-dimensional large-sliding contact model coupled with strain-induced phase transformations (PTs) and plastic flow in a disk-like sample under torsion at high pressure in rotational diamond anvil cell (RDAC) is formulated and studied. Coulomb and plastic friction are combined and take into account variable parameters due to PT. Results are obtained for weaker, equal-strength, and stronger high pressure phases, and for three values of the kinetic coefficient in a strain-controlled kinetic equation and friction coefficient. All drawbacks typical of problem with cohesion are overcome, including eliminating mesh-dependent shear band and artificial plastic zones. Contact sliding intensifies radial plastic flow, which ...


Advanced Phase-Field Approach To Dislocation Evolution, Valery I. Levitas, Mahdi Javanbakht Oct 2012

Advanced Phase-Field Approach To Dislocation Evolution, Valery I. Levitas, Mahdi Javanbakht

Aerospace Engineering Publications

The phase-field approach to dislocations is conceptually advanced. Large strain formulation is developed. A local thermodynamic potential eliminates stress dependence of the Burgers vector and reproduces the desired local stress-strain curve, as well as the desired, mesh-independent, dislocation height for any dislocation orientation. A gradient energy contains an additional term, which excludes localization of dislocation within a height smaller than the prescribed height but does not produce artificial interface energy and dislocation widening. Problems for nucleation and evolution of multiple dislocations along the single and multiple slip systems, and the interaction of dislocations with an austenite (A)-martensite (M) interface ...


Crystal-Crystal Phase Transformation Via Surface-Induced Virtual Premelting, Valery I. Levitas, Zhaohui Ren, Yuewu Zeng, Gaorong Han Jun 2012

Crystal-Crystal Phase Transformation Via Surface-Induced Virtual Premelting, Valery I. Levitas, Zhaohui Ren, Yuewu Zeng, Gaorong Han

Aerospace Engineering Publications

A mechanism for crystal-crystal phase transformations (PTs) via surface-induced virtual premelting is justified thermodynamically and confirmed experimentally for the PTs in PbTiO3 nanofibers. When the thickness of the surface melt (which appears much below the melting temperature, especially for nano-objects) exceeds the size of the critical product nucleus, nucleation and growth of the product crystal occur. For nanowires, premelting starts near the smallest size, and hydrodynamic flow driven by reduction in the external surface leads to a large change in shape and further promotion of crystal-crystal PT. During the product crystal-growth stage, virtual melting is observed experimentally within the ...


Reply To “Comment On 'Mechanochemical Continuum Modeling Of Nanovoid Nucleation And Growth In Reacting Nanoparticles'″, Valery I. Levitas, Hamed Attariani Jun 2012

Reply To “Comment On 'Mechanochemical Continuum Modeling Of Nanovoid Nucleation And Growth In Reacting Nanoparticles'″, Valery I. Levitas, Hamed Attariani

Aerospace Engineering Publications

The problem of the mechanisms of void nucleation in nanoparticles and related problem of the stresses and their relaxation in nanoparticles are of significant importance and far from being resolved. It is not surprising that there are several approaches with different and, in the given case, opposite assumptions. Thus, in ref 1, surface tension is neglected, and volumetric strain due to oxidation is introduced in the oxide shell. It is stated that for linear lattice mismatch α ≥ 0.3 large tensile stresses in a metal core are sufficient for void nucleation.


High-Density Amorphous Phase Of Silicon Carbide Obtained Under Large Plastic Shear And High Pressure, Valery I. Levitas, Yanzhang Ma, Emre Selvi, Jianzhe Wu, John A. Patten Feb 2012

High-Density Amorphous Phase Of Silicon Carbide Obtained Under Large Plastic Shear And High Pressure, Valery I. Levitas, Yanzhang Ma, Emre Selvi, Jianzhe Wu, John A. Patten

Aerospace Engineering Publications

In situ x-ray diffraction study of the hexagonal 6H SiC under pressure and shear in rotational diamond anvil cell is performed that reveals phase transformation to the new high-density amorphous (hda) phase SiC. In contrast to known low-density amorphous SiC, hda-SiC is promoted by pressure and unstable under pressure release. The critical combination of pressure ∼30 GPa and rotation of an anvil of 2160° that causes disordering is determined.


Coupled Plastic Flow And Phase Transformation Under Compression Of Materials In A Diamond Anvil Cell: Effects Of Transformation Kinetics And Yield Strength, Oleg M. Zarechnyy, Valery I. Levitas, Yanzhang Ma Jan 2012

Coupled Plastic Flow And Phase Transformation Under Compression Of Materials In A Diamond Anvil Cell: Effects Of Transformation Kinetics And Yield Strength, Oleg M. Zarechnyy, Valery I. Levitas, Yanzhang Ma

Aerospace Engineering Publications

The large-strain problem on phase transformations (PTs) under compression in a diamond anvil cell is studied in detail using the finite-element method. The combined effect of transformation kinetics and ratios of the yield strengths of low- and high-pressure phases is examined. Some experimental phenomena (e.g., plateaus in pressure distribution and plasticflow to the center of a sample) are reproduced. Results are applied to interpretation of experimental data and characterization of strain-induced PTs.


Mechanochemical Continuum Modeling Of Nanovoid Nucleation And Growth In Reacting Nanoparticles, Valery I. Levitas, Hamed Attariani Jan 2012

Mechanochemical Continuum Modeling Of Nanovoid Nucleation And Growth In Reacting Nanoparticles, Valery I. Levitas, Hamed Attariani

Aerospace Engineering Publications

Hollow nanoparticles (NPs) are produced by void nucleation and growth during chemical reactions. However, there is no proper understanding of nucleation and growth mechanisms and their predictive modeling. Models based on the Kirkendall effect predict the process time, which is larger by orders of magnitude than in experiment. This is why some works propose that a large tensile pressure in the core causes void nucleation. Here, a continuum-mechanics approach for nucleation and growth of a nanovoid in reacting NPs based on the Kirkendall effect is developed. In contrast to previous approaches, void nucleation and the effects of stresses are treated ...