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Full-Text Articles in Mechanical Engineering
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 the …
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→δ …
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. …