Sharif Digital Repository

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... 

Microstructure and mechanical properties of AA2024 after severe plastic deformation (SPD) and non-isothermal annealing were investigated. The non-isothermal treatment was carried out on the severely deformed AA2024, and the interaction between restoration and precipitation phenomena was investigated. Differential scanning calorimetry, hardness and shear punch tests illustrate that static recovery and dissolution of GPB zones/Cu–Mg co-clusters occur concurrently through non-isothermal annealing. Scanning electron microscope and electron backscatter diffraction illustrate that non-isothermal annealing of deformed AA2024 up to 250 °C promotes the particle-free regions and also particle... 

The recrystallization behavior of an Al-Cu-Mg alloy is investigated in multi-directionally forged over-aged and solution treated alloys, during non-isothermal annealing. Deformation and non-isothermal annealing are performed with a Gleeble 3800 thermo-mechanical simulator. The hardness measurements show that there is a thermal stability in mechanical properties during non-isothermal annealing up to 250 °C with heating rate of 10 K·min− 1. Differential scanning calorimetry curves of deformed over-aged and solution treated alloys describe the related precipitation phenomena. EBSD maps demonstrate that partially recrystallized and fully recrystallized microstructures appear in deformed... 

Concurrent precipitation in an age hardenable aluminum alloy determines recrystallization behavior especially grains structure and recrystallization texture during continuous heating. In this study, to investigate the effect of concurrent precipitation on the recrystallization texture and grains structure, a solution treated Al-Cu-Mg alloy has been multi-directionally forged at room temperature and continuously heated with different heating rates (800, 1600 and 2500 K·min− 1) up to 450 °C using Gleeble 3800 thermo-mechanical simulator. At lower heating rates (800 K·min− 1), Goss texture component {110} 〈001〉 (Φ1 = 90°/0°, Φ = 90°/45°, Φ2 = 45°/90°) is developed during continuous heating and... 

Particle stimulated nucleation (PSN) is one of the important mechanisms of recrystallization which is investigated in this research during non-isothermal annealing of multi-directionally forged solution treated aluminum alloy 2024. Non-isothermal annealing is performed up to temperatures of 380 °C and 450 °C at heating rates of 10, 100 and 200 K·min− 1 using a Gleeble 3800 thermo-mechanical simulator. In order to investigate the microstructure, field emission gun scanning electron microscope (FEG-SEM) equipped with electron backscatter diffractometer (EBSD) and differential scanning calorimetry (DSC) are utilized. The results show that the heating rate has a significant effect on the PSN and... 

The effects of initial heat treatments, i.e. solution treating and over ageing, on deformation behavior of an age hardenable Al-Cu-Mg alloy, including mechanical properties and texture evolution, during multi-directional forging (MDF) are investigated. Hardness measurements, electron back scatter diffraction (EBSD) maps, and differential scanning calorimetry (DSC) are carried out. Mechanical behaviors of the alloy in terms of compressive stress and hardness test after MDF are investigated. In both solution treated and over-aged alloys, the compressive stress during MDF shows an increase up to the second pass, and by further straining the flow stress is decreased. However, shear and... 

Post-uniform deformation energy of materials is defined as absorbed energy per unit area after necking. The energy is constant for a material type and its experienced specific processing history and also depends on its mechanical parameters as workhardening exponent, strain rate sensitivity, post-necking extension and inhomogeneity factor. Different methods such as single and multiple tensile testing had been proposed in the literature to calculate tearing energy, but the effect of post-necking extension had not been expressed explicitly. A new model by implementing uniform and failure elongations with the combination of Unwin theory is introduced. Based on the model, it was shown that... 

The non-isothermal aging behaviour of a newly developed Al–Zn–Mg–Cu alloy containing 17 wt-% Zn was investigated. Hardness and shear punch tests demonstrated that during non-isothermal aging, the mechanical properties of the alloy first increased and then decreased. The best properties were obtained in a sample which was non-isothermally aged upto 250°C with heating rate of 20°C min−1, due to the presence of η′/η (MgZn2) phases. This was confirmed by differential scanning calorimetery. After homogenisation, residual eutectic phases remained at triple junctions or in a spherical form. During aging, these phases transformed into rodlike S (Al2CuMg)-phase at 400°C, with sizes ranging from 50 to... 

The non-isothermal aging behaviour of a newly developed Al–Zn–Mg–Cu alloy containing 17 wt-% Zn was investigated. Hardness and shear punch tests demonstrated that during non-isothermal aging, the mechanical properties of the alloy first increased and then decreased. The best properties were obtained in a sample which was non-isothermally aged upto 250°C with heating rate of 20°C min−1, due to the presence of η′/η (MgZn2) phases. This was confirmed by differential scanning calorimetery. After homogenisation, residual eutectic phases remained at triple junctions or in a spherical form. During aging, these phases transformed into rodlike S (Al2CuMg)-phase at 400°C, with sizes ranging from 50 to... 

In this study, a new magnetic nanocomposite was synthesized via radical polymerization of methyl acrylate onto modified magnetic nanoparticles followed by the functionalization of the methyl ester groups with ethylenediamine and sodium chloroacetate. The generated magnetic nanocomposite was characterized by FT-IR, TEM, SEM, TGA, VSM, XRD and elemental analysis. Its key role as an adsorbent for the removal of typical cationic dyes, methyl violet and malachite green was investigated in terms of pH, contact time and initial dye concentration. The resulted adsorbent displays excellent adsorption capacities for cationic dyes which are more effective than most of the adsorbents reported so far.... 

This paper studies the scheduling problem in transit networks in order to decrease transfer waiting time. Transfer waiting time is calculated based on headway and departure time of intersecting routes and is divided into two parts. The first part can be reduced by changing departure times and was studied by the authors previously. The focus of the present research, however, is to minimize the second part of the transfer waiting time, dependent on the headways. The proposed optimization model in this paper includes both parts and is a nonlinear mathematical programming model. The model is decomposed to the departure time setting model (DSM) and the headway setting model (HSM). A solution... 

Representing nonlinear systems by linear models along with structured or unstructured uncertainties and applying robust control strategies could reduce the computational complexity in comparison with implementing the nonlinear model predictive controllers. In this paper design of robust model predictive controllers which are based on special classes of nonlinear systems representations called Wiener and Hammerstein are presented. The proposed algorithms approximate the nonlinear systems by uncertain linear models and reduce online the computational demands in the control implementation. The advantages of the proposed approaches are illustrated by two examples  

Abstract This paper proposes a bumpless transfer method to overcome the problem of switching jumps in a scheduled robust model predictive control approach. A scheduled robust model predictive controller implements a set of local robust model predictive controllers based on an on-line switching strategy. This method could enlarge the domain of attraction efficiently but the transient response might be hampered by spikes appearing at the moment of switching between adjacent local controllers. The proposed algorithm could enhance the transient response by implementing some intermediate controllers augmented to the main control scheme to solve the problem without needing more computation. The... 

This paper proposes a tracking control strategy for nonlinear systems without needing a prior knowledge of the reference trajectory. The proposed method consists of a set of local controllers with appropriate overlaps in their stability regions and an on-line switching strategy which implements these controllers and uses some augmented intermediate controllers to ensure steering the system states to the desired set points without needing to redesign the controller for each value of set point changes. The proposed approach provides smooth transient responses despite switching among the local controllers. It should be mentioned that the stability regions of the proposed controllers could be... 

Sectionalizing switches (SSs) are installed in distribution networks to provide ring and maneuver points, thereby increasing service reliability. These switches are either manual switches (MSs) or remote controlled switches (RCSs). The switches are usually assumed to be fully reliable. However, in practice, their performance is not always ideal. Actually, the switches may sometimes experience malfunction, which reduces their ability to enhance system reliability. The present study proposes a model to consider the malfunction possibility of the switches in their optimal placement problem. The model helps to minimize the total costs of SSs as well as the interruption costs incurred by...