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    Effect of surface stresses on elastic behavior of a screw dislocation inside the wall of a nanotube

    , Article Physica Status Solidi (B) Basic Research ; Volume 248, Issue 6 , 2011 , Pages 1437-1441 ; 03701972 (ISSN) Shodja, H. M ; Gutkin, M. Y ; Moeini Ardakani, S. S ; Sharif University of Technology
    Behavior of a screw dislocation inside a nanotube (NT) is considered in the context of surface elasticity. The elastic fields as well as the image force acting over the dislocation are derived and analyzed in detail. In contrast with the result of classical elasticity, the screw dislocation is shown to be repelled by free surfaces and occupy two stable equilibrium positions near them. The image force strongly depends on the NT's inner and outer radii as well as surface elastic characteristics  

    Effect of source strength on dislocation pileups in the presence of stress gradients

    , Article Philosophical Magazine ; Volume 95, Issue 20 , 2015 , Pages 2175-2197 ; 14786435 (ISSN) Zamani, Z ; Shishvan, S. S ; Assempour, A ; Sharif University of Technology
    The behaviour of a dislocation pileup with a finite-strength source is investigated in the presence of various stress gradients within a continuum model where a free-dislocation region exists around the source. Expressions for dislocation density and stress field within the pileup are derived for the situation where there are first and second spatial gradients in applied stress. For a pileup configuration under an applied stress, yielding occurs when the force acting on the leading dislocations at the pileup tips reaches the obstacle strength, and at the same time, it is required that the source be at the threshold stress for dislocation production. A numerical methodology is presented to... 

    Analysis of displacement and strain fields of a screw dislocation in a nanowire using gradient elasticity theory

    , Article Scripta Materialia ; Volume 59, Issue 3 , August , 2008 , Pages 368-371 ; 13596462 (ISSN) Shodja, H. M ; Davoudi, K. M ; Gutkin, M. Y ; Sharif University of Technology
    Displacement and strain fields of a screw dislocation in a nanowire are considered within the theory of gradient elasticity. The gradient solution of the corresponding boundary value problem is derived and discussed in detail. It is shown that the dislocation fields do not contain classical jumps and singularities at the dislocation line. The maximum values of the dislocation displacement and elastic strain strongly depend on both the dislocation position and nanowire radius, thus demonstrating a nonclassical size effect. © 2008 Acta Materialia Inc  

    Microstructural characterization of HIP consolidated NiTi–nano Al2O3 composites [electronic resource]

    , Article Journal of Alloys and Compounds ; 5 September 2014, Volume 606, Pages 21–26 Farvizi, M ; Ebadzadeh, T ; Vaezi, M. R ; Yoon, E. Y ; Kim, Y. J ; Kim, H. S ; Simchi, A. (Abdolreza) ; Sharif University of Technology
    The microstructure and phase transformational behavior of NiTi-based composites reinforced with 6 wt.% of α-alumina nanoparticles have been investigated. Two kinds of starting materials, elemental Ni–Ti and prealloyed austenitic NiTi, were used to prepare the composites. The samples were consolidated using a hot isostatic pressing method. The X-ray diffraction results showed that while unreinforced NiTi mainly contained B2 phase at room temperature, martensitic B19′ phase appeared in the microstructure after addition of the α-alumina nanoparticles. The differential scanning calorimetry measurements indicated that the martensitic transformation temperatures were elevated in the composite... 

    Modeling of the mutual effect of dynamic precipitation and dislocation density in age hardenable aluminum alloys

    , Article Journal of Alloys and Compounds ; Volume 683 , 2016 , Pages 527-532 ; 09258388 (ISSN) Khani Moghanaki, S ; Kazeminezhad, M ; Sharif University of Technology
    Elsevier Ltd  2016
    A model has been proposed to capture the complex strain rate effect on dynamic precipitation of GP zones in an age-hardenable aluminum alloy. The contributions of vacancies and dislocations to dynamically formed GP zones have been specified in the model. It has been demonstrated that the proposed model is capable for predicting the contribution of each dynamic precipitation mechanisms, accurately, which are acting during deformation. Furthermore, the vacancy and dislocation evolutions during deformation have been considered in this modeling. The effect of strain rate by considering different mechanisms of dynamic precipitation of GP zones has been studied and confirmed by experimental data... 

    A new microstructural model based on dislocation generation and consumption mechanisms through severe plastic deformation

    , Article Computational Materials Science ; Volume 50, Issue 3 , January , 2011 , Pages 1123-1135 ; 09270256 (ISSN) Hosseini, E ; Kazeminezhad, M ; Sharif University of Technology
    A new model on the evolution of dislocation structure of cell forming metals and alloys through severe plastic deformation is presented. Following previous approaches, the model considers a cellular dislocation structure consisted of two phases: cell interiors and cell walls. The model distinguishes edge and screw dislocations in terms of three categories: mobile dislocations, immobile dislocations in cell interiors and immobile dislocations in cell walls. Then considering physical and geometrical assumptions for each dislocation category, an evolutional law is derived, based on some dislocation interaction mechanisms such as dislocation generation, annihilation, locking and migration. The... 

    Surface/interface effects on the formation of misfit dislocation in a core-shell nanowire

    , Article Philosophical Magazine ; Volume 94, Issue 5 , 11 February , 2014 , Pages 492-519 ; ISSN: 14786435 Enzevaee, C ; Gutkin, M. Y ; Shodja, H. M ; Sharif University of Technology
    The misfit strain within the core of a two-phase free-standing core-shell nanowire resulting in the generation of an edge misfit dislocation or an edge misfit dislocation dipole at the core-shell interface is considered theoretically within both the classical and surface/interface elasticity approaches. The critical conditions for the misfit dislocation generation are studied and discussed in detail with special attention to the non-classical surface/interface effect. It is shown that this effect is significant for fine cores of radius smaller than roughly 20 interatomic distances. The positive and negative surface/interface Lamé constants mostly make the generation of the misfit dislocation... 

    A criterion for slip transfer at grain boundaries in Al

    , Article Scripta Materialia ; Volume 178 , 2020 , Pages 408-412 Alizadeh, R ; Peña Ortega, M ; Bieler, T. R ; LLorca, J ; Sharif University of Technology
    Acta Materialia Inc  2020
    The slip transfer phenomenon was studied at the grain boundaries of pure Aluminum by means of slip trace analysis. Either slip transfer or blocked slip was analyzed in more than 250 grain boundaries and the likelihood of slip transfer between two slip systems across the boundary was assessed. The experimental results indicate that slip transfer was very likely to occur if the residual Burgers vector, ∆b, was below 0.35b and the Luster–Morris parameter was higher than 0.9, and that the ratio of the Luster–Morris parameter and the residual Burgers vector has a threshold above which slip transfer is probable. © 2019 Acta Materialia Inc  

    On the modeling of the static recrystallization considering the initial grain size effects

    , Article Materials Science and Engineering A ; Volume 486, Issue 1-2 , 2008 , Pages 202-207 ; 09215093 (ISSN) Kazeminezhad, M ; Sharif University of Technology
    Modeling of the static recrystallization in deformed copper specimens with different initial grain sizes is carried out based on a previous dislocation-grain size interaction model and a Monte Carlo simulation. From the dislocation-grain size interaction model, the stored energy of the deformed copper is calculated considering the interaction of the dislocations due to the different initial grain sizes. Then, utilizing the stored energy and Monte Carlo simulation the kinetic of recrystallization and recrystallized grain sizes are obtained. The JMAK plots of the modeling results show that, in conditions of 2D modeling and site-saturated nucleation, the Avrami exponent is 2 ± 0.1. The time for... 

    Mechanical properties and γ/γ' interfacial misfit network evolution: A study towards the creep behavior of Ni-based single crystal superalloys

    , Article Mechanics of Materials ; Volume 171 , 2022 ; 01676636 (ISSN) Khoei, A. R ; Youzi, M ; Tolooei Eshlaghi, G ; Sharif University of Technology
    Elsevier B.V  2022
    The aim of this study is to investigate the role of the temperature, stress, and rhenium (Re) on the γ/γ' interfacial misfit dislocation network and mechanical response of Ni-based single crystal superalloys. After aging at elevated temperatures, mismatch between the two phases results in an interfacial dislocation network to relieve the coherency stress. Molecular dynamics (MD) simulations have been performed to study the properties of the (100), (110), and (111) phase interface crystallographic directions. Increasing temperature disperses the atomic potential energy at the interface diminishing the strength and stability of the networks. In the case of loading, when a constant strain rate... 

    An Investigation of Alloying Elements Effect on Creep Behavior of Ni-Based Super Alloys Using Molecular Dynamics

    , M.Sc. Thesis Sharif University of Technology Youzi, Mehrdad (Author) ; Khoei, Amir Reza (Supervisor)
    The aim of this study is to investigate the role of the temperature, stress, strain rate, and rhenium (Re) on the γ\/γ^' interfacial misfit dislocation network and mechanical response of Ni-based single crystal superalloys. The interfacial network plays an important role in the superior creep behavior of Ni-based superalloys. Therefore, a comprehensive understanding of this topic would help us to go through the impact of various factors on the mechanical and creep properties at high temperatures to achieve an optimal design. Due to the mismatch between the two phases, a dislocation network forms after aging at high temperatures to alleviate the stress field. The (100), (110), and (111)... 

    On phase transformations in a Ni-based superalloy

    , Article Philosophical Magazine ; Volume 93, Issue 10-12 , Mar , 2013 , Pages 1351-1370 ; 14786435 (ISSN) Asgari, S ; Sharghi Moshtaghin, R ; Sadeghahmadi, M ; Pirouz, P ; Sharif University of Technology
    In this study, a particular nickel-based superalloy has been studied by transmission electron microscopy and the results are discussed in terms of phase transformations that may have taken place. The alloy has been found to consist of three major phases; a FCC γ phase containing a random distribution of nearly spherical γ′ - Ni3Al(Ti) - precipitates and elongated thin Ni3Ti(Ta) platelets with a hexagonal structure. The orientation relationships between the different phases have been determined; loss of coherency and possible occurrence of misfit-relieving interfacial dislocations are discussed and compared with experimental observations, and possible mechanisms of phase transformations are... 

    Interface effect on the formation of a dipole of screw misfit dislocations in an embedded nanowire with uniform shear eigenstrain field

    , Article European Journal of Mechanics, A/Solids ; Volume 51 , May–June , 2015 , Pages 154-159 ; 09977538 (ISSN) Shodja, H. M ; Enzevaee, C ; Gutkin, M. Y ; Sharif University of Technology
    Elsevier Ltd  2015
    The critical condition for the generation of a screw misfit dislocation dipole (MDD) at the interface between a nanowire (NW) with uniform shear misfit strain and its surrounding unbounded matrix within surface/interface elasticity theory is of particular interest. The analysis is carried out using the complex potential variable method. It is shown that the critical radius of the NW corresponding to the onset of the MDD generation decreases with the increase in the uniform shear eigenstrain inside the NW as well as when the stiffness of the NW increases with respect to the matrix. The critical radius strongly depends on the non-classical interface parameter. Comparison is made with classical... 

    Nanoindentation study of cementite size and temperature effects in nanocomposite pearlite: A molecular dynamics simulation

    , Article Current Applied Physics ; Volume 16, Issue 9 , 2016 , Pages 1015-1025 ; 15671739 (ISSN) Ghaffarian, H ; Karimi Taheri, A ; Ryu, S ; Kang, K ; Sharif University of Technology
    Elsevier B.V 
    We carry out molecular dynamics simulations of nanoindentation to investigate the effect of cementite size and temperature on the deformation behavior of nanocomposite pearlite composed of alternating ferrite and cementite layers. We find that, instead of the coherent transmission, dislocation propagates by forming a widespread plastic deformation in cementite layer. We also show that increasing temperature enhances the distribution of plastic strain in the ferrite layer, which reduces the stress acting on the cementite layer. Hence, thickening cementite layer or increasing temperature reduces the likelihood of dislocation propagation through the cementite layer. Our finding sheds a light on... 

    Dislocation structure and strength evolution of heavily deformed tantalum

    , Article International Journal of Refractory Metals and Hard Materials ; Volume 27, Issue 3 , 2009 , Pages 605-610 ; 02634368 (ISSN) Hosseini, E ; Kazeminezhad, M ; Sharif University of Technology
    A unified model composed of the flow function model, dislocation model and Taylor theory is used to investigate the evolution of dislocation density, cell size, and strength of tantalum during ECAP process. From the flow function model, strain and strain rate distributions are achieved and then using a modified version of three-dimensional ETMB model, the dislocation density, cell size, and strength are predicted. The predicted dislocation density, cell size, and strength are compared with the experimental data and a remarkable agreement is obtained. In addition, the effect of dynamic recovery on the strength of the processed tantalum is modeled and compared with other materials. © 2008... 

    Analysis of stress field of a screw dislocation inside an embedded nanowire using strain gradient elasticity

    , Article Scripta Materialia ; Volume 61, Issue 4 , 2009 , Pages 355-358 ; 13596462 (ISSN) Davoudi, K. M ; Gutkin, M. Yu ; Shodja, H. M ; Sharif University of Technology
    The stress field of a screw dislocation inside an embedded nanowire is considered within the theory of strain-gradient elasticity. It is shown that the stress singularity is removed and all stress components are continuous and smooth across the interface, in contrast with the results obtained within the classical theory of elasticity. The maximum magnitude of dislocation stress depends greatly on the dislocation position, the nanowire size, and the ratios of shear moduli and gradient coefficients of the matrix and nanowire materials. © 2009 Acta Materialia Inc  

    A study on the mobility of [formula presented] edge dislocation in low-carbon α-Fe and its interactions with damage cascade: on picosecond time scale using molecular dynamics simulations

    , Article Journal of Nuclear Materials ; Volume 527 , 2019 ; 00223115 (ISSN) Zamzamian, M ; Feghhi, A. H ; Samadfam, M ; Sharif University of Technology
    Elsevier B.V  2019
    Molecular dynamics simulations were used to propose a closed-form expression for the mobility of the [Formula presented] edge dislocation in low-carbon α-Fe (up to 0.1 at.% C) at temperatures of 300, 400 and 500 K and applied shear stresses of 10–100 MPa. Considering this parameter helps us to understand the effect of damage cascade on the dislocation mobility. The results confirmed that the point defect clusters at the thermal spike stage of the cascade (that they can be considered as an unstable precipitation-like phase), the distance of damage cascade relative to the center of the dislocation core and forming carbon-vacancy (C–V) complexes are some rather stronger obstacles for movement... 

    Modeling of dislocation density and strength on rheoforged A356 alloy during multi-directional forging

    , Article Computational Materials Science ; Vol. 81 , 2014 , pp. 284-289 Kavosi, J ; Saei, M ; Kazeminezhad, M ; Dodangeh, A ; Sharif University of Technology
    In this study, a hybrid model is presented to predict the dislocation density and strength evolution of the rheoforged non-dendritic A356 alloy during multi-directional forging. Regarding the characteristics of non-dendritic A356 alloy, combination of Shear Lag and Nes models is used for the eutectic structure, and Nes model is used for the α-Al globular phase. The aspect ratio variations of Si particles in eutectic structure during 3 passes of multi-directional forging do not change the model predictions, significantly. Model predictions on shear stress are in good agreement with experimental results of shear punch test  

    Modelling the temperature rise effect through high-pressure torsion

    , Article Materials Science and Technology (United Kingdom) ; Volume 32, Issue 12 , 2016 , Pages 1218-1222 ; 02670836 (ISSN) Parvin, H ; Kazeminezhad, M ; Sharif University of Technology
    Taylor and Francis Ltd 
    An approach composed of the thermodynamics-based dislocation model and the Taylor theory is used to investigate the evolution of microstructure and flow stress during high-pressure torsion (HPT). The incremental temperature rise is considered through the modelling of HPT. The temperature can affect the annihilation of dislocations and thus the dislocation density. The model predicts the dislocation density, sub-grain size and flow stress during HPT. The modelling results are compared with the experimental data and the modelling results without considering the incremental temperature rise. A remarkable agreement is observed between the modelling results with considering the temperature rise... 

    Flow stress evolution in further straining of severely deformed Al

    , Article Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science ; Volume 50, Issue 5 , 2019 , Pages 2371-2380 ; 10735623 (ISSN) Charkhesht, V ; Kazeminezhad, M ; Sharif University of Technology
    Springer Boston  2019
    To investigate the flow stress evolution in further straining of severely deformed Al sheets, a comprehensive model which considers both mechanical and metallurgical alterations is needed. In this study, constrained groove pressing (CGP) as a severe plastic deformation method, and a flat rolling process for further straining are utilized. Using basic mechanical models, strain and strain rate were calculated for this process. Dislocation density and flow stress evolutions were predicted by utilizing initial mechanical data, considering the ETMB (Y. Estrin, L. S. Toth, A. Molinari, and Y. Brechet) dislocation density model. Based on these model predictions, the combination of the CGP process...