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Full-Text Articles in Mechanical Engineering

Coherent Solid/Liquid Interface With Stress Relaxation In A Phase-Field Approach To The Melting/Solidification Transition, Valery I. Levitas, Kamran Samani Oct 2011

Coherent Solid/Liquid Interface With Stress Relaxation In A Phase-Field Approach To The Melting/Solidification Transition, Valery I. Levitas, Kamran Samani

Aerospace Engineering Publications

An advanced Ginzburg-Landau (GL) approach to melting and solidification coupled with mechanics is developed. It is based on the concept of a coherent solid-liquid interface with a transformation strain tensor, the deviatoric part of which is described by a thermodynamically consistent kinetic equation. Due to the relaxation of the elastic energy, a promoting contribution to the driving force for phase transformation in the GL equation appears, both for melting and solidification. Good agreement with known experiments is obtained for Al nanoparticles for the size-dependent melting temperature and temperature-dependent thickness of the surface molten layer. All types of interface stress distributions ...


Surface-Induced Phase Transformations: Multiple Scale And Mechanics Effects And Morphological Transitions, Valery I. Levitas, Mahdi Javanbakht Oct 2011

Surface-Induced Phase Transformations: Multiple Scale And Mechanics Effects And Morphological Transitions, Valery I. Levitas, Mahdi Javanbakht

Aerospace Engineering Publications

Strong, surprising, and multifaceted effects of the width of the external surface layer Δξ and internal stresses on surface-induced pretransformation and phase transformations (PTs) are revealed. Using our further developed phase-field approach, we found that above some critical Δξ*, a morphological transition from fully transformed layer to lack of surface pretransformation occurs for any transformation strain εt. It corresponds to a sharp transition to the universal (independent of εt), strongly increasing the master relationship of the critical thermodynamic driving force for PT Xc on Δξ. For large εt, with increasing Δξ, X ...


Phase-Field Modeling Of Fracture In Liquid, Valery I. Levitas, Alexander V. Idesman, Ameeth K. Palakala Aug 2011

Phase-Field Modeling Of Fracture In Liquid, Valery I. Levitas, Alexander V. Idesman, Ameeth K. Palakala

Aerospace Engineering Publications

Phase-field theory for the description of the overdriven fracture in liquid (cavitation) in tensile pressure wave is developed. Various results from solid mechanics are transferred into mechanics of fluids. Thermodynamic potential is formulated that describes the desired tensile pressure–volumetric strain curve and for which the infinitesimal damage produces infinitesimal change in the equilibrium bulk modulus. It is shown that the gradient of the order parameter should not be included in the energy, in contrast to all known phase-field approaches for any material instability. Analytical analysis of the equations is performed. Problems relevant to the melt-dispersion mechanism of the reaction ...


Phase Transition And Structure Of Silver Azide At High Pressure, Dongbin Hou, Fuxiang Zhang, Cheng Ji, Trevor Hannon, Hongyang Zhu, Jianzhe Wu, Valery I. Levitas, Yanzhang Ma Jul 2011

Phase Transition And Structure Of Silver Azide At High Pressure, Dongbin Hou, Fuxiang Zhang, Cheng Ji, Trevor Hannon, Hongyang Zhu, Jianzhe Wu, Valery I. Levitas, Yanzhang Ma

Aerospace Engineering Publications

ilver azide (AgN 3) was compressed up to 51.3 GPa. The results reveal a reversible second-order orthorhombic-to-tetragonal phase transformation starting from ambient pressure and completing at 2.7 GPa. The phase transition is accompanied by a proximity of cell parameters aand b, a 3° rotation of azide anions, and a change of coordination number from 4-4 (four short, four long) to eight fold. The crystal structure of the high pressure phase is determined to be inI4/mcm space group, with Ag at 4a, N1 at 4d, and N2 at 8h Wyckoff positions. Both of ...


Size And Mechanics Effects In Surface-Induced Melting Of Nanoparticles, Valery I. Levitas, Kamran Samani Jan 2011

Size And Mechanics Effects In Surface-Induced Melting Of Nanoparticles, Valery I. Levitas, Kamran Samani

Aerospace Engineering Publications

Various melting-related phenomena (like surface melting, size dependence of melting temperature, melting of few nm-size particles and overheating at a very fast heating rate) are of great fundamental and applied interest, although the corresponding theory is still lacking. Here we develop an advanced phase-field theory of melting coupled to mechanics, which resolves numerous existing contradictions and allowed us to reveal exciting features of melting problems. The necessity of introducing an unexpected concept, namely, coherent solid-melt interface with uniaxial transformation strain, is demonstrated. A crossover in temperature dependence of interface energy for radii below 20 nm is found. Surface-induced premelting and ...