Sharif Digital Repository

Achieving a high speed, unidirectional water flow through carbon nanotubes (CNTs) is a key factor in designing novel nanofluidic devices. In this study, utilizing molecular dynamics (MD) simulations, we propose a novel nanoscale water pump for directed water transportation using charged rotating CNTs. Two basic conditions for stable water flow, including thermodynamic nonequilibrium and spatial asymmetry, are provided by introducing partial charges on carbon atoms of the channel with asymmetric patterns and its rotation, respectively. We demonstrate that the performance of the water pump is proportional to the gradient of a linear charge distribution and angular velocity of the rotation. Our... 

There are many engineering applications in which composite materials are required to satisfy two or more criteria regarding physical and mechanical properties. In this article, Al-matrix nanocomposites reinforced with different volume fractions of SiC nanoparticles (~ 50 nm; up to 6%) were processed by powder metallurgy (P/M) routes through mechanical milling and hot consolidation techniques. Microstructural studies showed that nano-metric Al2O3 particles with a size of ~ 20 nm and volume fraction of ~ 2% were formed and distributed in the metal matrix, owing to the surface oxides breaking. Microstructural analysis also revealed that the size of cellular structure and the density of... 

In this paper, the effects of rocking and fixed base prestressed columns and conventional reinforced concrete columns on the response of frames are investigated. Also, the influence of some selected rocking base prestressed columns on the response of the concrete frame was studied. Three types of a concrete frame with conventional Reinforced Concrete columns, rocking, and fixed base prestressed columns were modeled using the finite element method in Opensees software. The details of the simulation of the rocking and fixed base columns in Opensees are described precisely. The results obtained from the models were compared with those of the literature to evaluate the validity of the results.... 

In order to investigate the evolution of microstructure and flow stress during non-isothermal annealing, aluminum samples were subjected to strain magnitudes of 1, 2 and 3 by performing 2, 4 and 6 passes of multi-directional forging. Then, the samples were non-isothermally annealed up to 150, 200, 250, 300 and 350 °C. The evolution of dislocation density and flow stress was studied via modeling of deformation and annealing stages. It was found that 2, 4 and 6 passes multi-directionally forged samples show thermal stability up to temperatures of 250, 250 and 300 °C, respectively. Modeling results and experimental data were compared and a reasonable agreement was observed. It was noticed that... 

The effects of thermal history on the microstructure and mechanical properties of a friction-stir-processed Al–Mg–TiO2 (3 vol.-%, 20 nm) nanocomposite were studied. It is shown that, with increases in peak temperature, a more uniform distribution of nanoparticles in the metal matrix, and a refined grain structure, are attained. Transmission electron microscopy indicated that the mechanism of grain refinement is influenced by the hard inclusions, changing from discontinuous to continuous dynamic recrystallisation. A fine-grained nanocomposite (average grain size of 3 µm) with a uniform distribution of nanoparticles is obtained after four fully-overlapping passes at 1200 rev min−1 and 100 mm... 

Reactive friction stir processing (RFSP) has been employed to modify the surface properties of AA5052 Al-Mg alloy through grain refinement and the distribution of ultrafine hard nanoparticles (2 to 6% volume fractions of 50 nm titanium dioxide). The heat generated induced solid state reactions between the metal matrix and the reinforcement to form Al3Ti/MgO inclusions. The role of grain refinement and distributed hard nanoparticles on the tribological behavior of the alloy under dry sliding wear condition was evaluated. The wear rates and friction coefficient as well as macro- and micro-features of the worn surfaces indicate that the wear mechanism (at 3–7 kgf and 0.5 m/s) is abrasive. The... 

Aluminum matrix nanocomposites were fabricated via friction stir processing of an Al–Mg alloy with pre-inserted TiO2 nanoparticles at different volume fractions of 3%, 5% and 6%. The nanocomposites were annealed at 300–500 °C for 1–5 h in air to study the effect of annealing on the microstructural changes and mechanical properties. Microstructural studies by scanning and transmission electron microscopy showed that new phases were formed during friction stir processing due to chemical reactions at the interface of TiO2 with the aluminum matrix alloy. Reactive annealing completed the solid-state reactions, which led to a significant improvement in the ductility of the nanocomposites (more... 

Friction stir processing (FSP) is a solid state route with a capacity of preparing fine grained nanocomposites from metal sheets. In this work, we employed this process to finely distribute TiO2 nanoparticles throughout an Al–Mg alloy, aiming to enhance mechanical properties. Titanium dioxide particles (30 nm) were preplaced into grooves machined in the middle of the aluminium alloy sheet and multipass FSP was afforded. This process refined the grain structure of the aluminium alloy, distributed the hard nanoparticles in the matrix and promoted solid state chemical reactions at the interfaces of the metal/ceramic particles. Detailed optical and electron microscopic studies showed that the... 

Submerged friction-stir processing under cryogenic conditions was employed to fabricate ultrafine-grained nanocomposites with enhanced mechanical characteristics. Al-Mg alloy sheet with 3vol% TiO2 nanoparticles were processed under air (ambient temperature), a water-dry ice medium (~-25°C), and liquid nitrogen. It is shown that a homogenous distribution of reinforcement particles throughout the metal matrix is attained at a rotational speed of 1400rpm and a traverse velocity of 50mm/min after 4 passes. In situ formation of Al3Ti and MgO nanophases during multi-pass processing is shown by transmission electron microscopy. Under the cryogenic cooling condition, ultrafine grains and cellular... 

A fine-grained aluminum-matrix hybrid nanocomposite reinforced with TiO2, MgO and Al3Ti nanoparticles was prepared via reactive friction stir processing (FSP) of an Al-Mg sheet with pre-placed TiO2 particles (50nm; 3.1vol%). The microstructure of the hybrid nanocomposite comprises high-angle grain boundaries (~90%) with an average size of 2μm and hard inclusions with sizes in the range of 30-50nm. Evaluation of the hot deformation behavior of the nanocomposite by uniaxial tensile testing at different temperatures (300-450°C) and strain rates (0.001-0.1s-1) shows that the deformation apparent activation energy of the nanocomposite is 137kJmol-1 at ≤300°C. The values of the activation energy... 

A fine-grained Al-Mg/Al3Ti nanocomposite was fabricated by friction stir processing (FSP) of an aluminum-magnesium (AA5052) alloy with pre-placed titanium powder in the stirred zone. Microstructural evolutions and formation of intermetallic phases were analyzed by optical and electron microscopic techniques across the thickness section of the processed sheets. The microstructure of the nanocomposite consisted of a fine-grained aluminum matrix (1.5 μm), un-reacted titanium particles (<40 μm) and reinforcement particles of Al3Ti (<100 nm) and Mg2Si (<100 nm). Detailed microstructural analysis indicated solid-state interfacial reactions between the aluminum... 

Friction stir welding (FSW) of dissimilar joints has recently attracted great interest for the fabrication of bimetallic and layered composite structures. In this paper, dissimilar joining of an aluminum-matrix hybrid nanocomposite to commercial pure aluminum is reported for the first time. An aluminum hybrid nanocomposite reinforced with Al2O3 (2 vol%; 15 nm) and SiC (2 vol%; 50 nm) nanoparticles was prepared by powder metallurgy routes including mechanical milling and hot powder consolidation techniques. Different joint designs at various ranges of rotational (w) and traverse velocities (v) were evaluated to determine process window for the dissimilar solid-state welding. Macro- and... 

Highly cold worked (HCW) low carbon steel sheets with cellular structure in the range of 200 to 300 nm are fabricated via constrained groove pressing process. Joining of the sheets is carried out by resistance spot welding process at different welding currents and times. Thereafter, failure behavior of these welded samples during tensile-shear test is investigated. Considered concepts include; failure load, fusion zone size, failure mode, ultimate shear stress, failure absorbed energy, and fracture surface. The results show that HCW low carbon steel spot welds have higher failure peak load with respect to the as-received one at different welding currents and times. Also, current limits for... 

Constrained groove pressing as a severe plastic deformation method is utilized to produce ultra-fine grained low carbon steel sheets. The ultra-fine grained sheets are joined via resistance spot welding process and the characteristics of spot welds are investigated. Resistance spot welding process is optimized for welding of the sheets with different severe deformations and their results are compared with those of as-received samples. The effects of failure mode and expulsion on the performance of ultra-fine grained sheet spot welds have been investigated in the present paper and the welding current and time of resistance spot welding process according to these subjects are optimized.... 

The welding of nanostructured low carbon steel sheets produced by severe plastic deformation (SPD) has been considered in the present paper. Constrained groove pressing (CGP) method is used for imposing the severe plastic deformation to the steel sheets as a large pre-strain. The SPDed sheets are joined using resistance spot welding (RSW) process. The results show that severe plastic deformation can effectively increase the electrical resistivity of steel sheets; therefore it can affect the microstructure and mechanical properties of spot welds. Microstructure and mechanical properties of fusion zone, heat affected zone (HAZ), recrystallized zone and base metal of SPDed sheets are... 

The need for simplified physical models representing frequency dependent soil impedances has been the motivation behind many researches throughout history. Generally, such models are generated to capture impedance functions in a wide range of excitation frequencies, which leads to relatively complex models. That is while there is just a limited range of frequencies that really influence the response of the structure. Here, a new methodology based on the response-matching concept is proposed, which can lead to the development of simpler discrete models. The idea is then used to upgrade an existing simple model of surface foundations to the case of embedded foundations. The applicability of... 

In this study a severe plastic deformation method called constrained groove pressing (CGP) is used for imposing a high magnitude of strain into the low carbon steel sheets. Microstructural changes during process are examined by X-ray diffraction and optical observations. The grain size evolution during severe plastic deformation is studied using Williamson-Hall analysis on XRD pattern of the deformed samples. In effective strain of 4.64, ferrite grains with a submicron size of 200-300nm are achieved. The results show that constrained groove pressing can effectively refine the coarse-grained structure to an ultra fine grain range. Mechanical properties changes due to microstructure evolution... 

We studied the microstructure and mechanical characteristics of spot welded specimens, fabricated from low carbon steel sheets with different microstructures. Both ultra-fine grained (UFG) steel sheet and coarse grained (CG) steel sheet were used. The refined microstructure of the UFG steel has been produced by severe plastic deformation (SPD) using the constrained groove pressing (CGP) method. The grain size of the base metals was approximately 260. nm and 30. μm in diameter, respectively, in the UFG and CG steels. Examining the microstructure of a cross section cut through the spot weld reveals a similar grain size and phase distribution in the nugget on both the sides of the initial... 

A new modification of constrained groove pressing (CGP) process named as constrained groove pressing-cross route (CGP-CR) was suggested for severe plastic deformation (SPD) of sheet form metals with great potential for fabricating high strength nanostructured sheets. This process is based on the conventional CGP process including some modifications. One pass of this process includes eight stages (four corrugation and four flattening) and involves 90° cross-rotation between each two stages. As a result of each CGP-CR pass, a strain magnitude of ∼2.32 is imparted to the sample. To simulate the process, finite element modeling (FEM) was carried out using three-dimensional finite element... 

A modified method of severe plastic defomation (SPD) entitled constrained groove pressing-cross route (CGP-CR) was introduced for imposing a high magnitude of equivalent strain of about 2.32 per pass on the sheet form samples. The major benefit of this improved route compared to previous common route was the more homogeneity of strain in the rolling (RD) and transverse (TD) directions of sheets. In this study, low carbon steel samples were used for examination of evolutions in microstructure and mechanical properties during SPD via CGP-CR process. Mechanical properties improvement were measured by tensile and macro hardness tests. The results indicate that CGP-CR process can effectively...