Sharif Digital Repository

Laser-assisted friction stir processing was used to modify the microstructure of an as-cast gamma prime strengthened nickel base superalloy. The grain structure of top region of the stir zone, where full dissolution of gamma prime precipitates occurred during heating, is controlled by discontinuous dynamic recrystallization mechanism and the pinning effect of refined carbides and cooling gamma prime precipitates. In the bottom of the stir zone, the partial dissolved gamma prime precipitates play important role in grain structure development by their pinning effect in retarding grain growth during cooling as well as by a continuous recrystallization mechanism  

In this study, a mathematical model is presented for predicting temperature and velocity fields together with dynamic and static microstructural changes during hot rod rolling utilizing a finite element model. For doing so, heat transfer and plasticity equations are coupled with the additivity rule and Avrami rate-equation to consider the dynamic and static microstructural changes as well as to evaluate the effects of temperature and strain rate variations on flow stress of rolling metal. To verify the results of the employed model, a comparison is made between the measured surface temperature during rod rolling and the predicted data that confirms the validity of the model. © 2003 Elsevier... 

The role of silicon on austenite recrystallization was investigated using hot compression experiments on a low carbon and a high silicon steel samples. The models were proposed to describe the hot deformation behavior of metals regarding dynamic recovery. The rate of static recrystallization was found lower in the silicon steel in comparison with low carbon steel. Silicon was found to produce a lower rate of dynamic recovery due to its effect on the austenite staking fault energy  

A mathematical model is proposed for evaluating flow behaviour under hot deformation conditions. The effects of dynamic recovery and recrystallisation as well as temperature and strain rate variations are considered in the model by means of Bergstrom's approach and the additivity rule for strain. To verify the model, hot compression tests for three grades of steel together with upsetting experiments are carried out. Comparison between experimental and theoretical results confirms the reliability of the model. © 2003 IoM Communications Ltd. Published by Maney for the Institute of Materials, Minerals and Mining  

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... 

The hot deformation behavior of A-286 superalloy has been characterized using hot compression experiments in the temperatures between 1000 and 1100 °C and strain rates varying between 0.001 and 0.1 s-1. In addition, hot workability of this alloy has been analyzed by employing flow-localization parameter. The results show that both kinds of softening mechanism, dynamic recovery and dynamic recrystallization, occur during hot working, where at 1000 °C the main mechanism is dynamic recovery and at higher temperatures and strain rate of 0.001-0.01 s-1 dynamic recrystallization takes place. Calculations demonstrates that this alloy mainly have a good workability for the utilized deformation... 

The recrystallization behavior of a commercial nickel-cobalt base superalloy, AEREX 350, is investigated by means of hardness test, X-ray diffraction, and microscopy. It is found that the alloy resists recrystallization up to a high temperature of 1025 °C. Recrystallized grains are readily formed at grain boundaries below this temperature; however, the growth of these new grains is inhibited by Widmanstätten η particles having coherent facets with the nickel matrix (γ). The passage of the recrystallization front results in coherency loss and consequently dissolution of the η platelets. Recrystallization proceeds with a discontinuous precipitation of the η phase behind the moving boundary  

The texture, microstructure, and mechanical properties of severely deformed aluminum subjected to friction stir processing (FSP) with SiC nanoparticles under ambient and cryogenic temperatures are comprehensively investigated. Even one pass of FSP in the ambient temperature results in the formation of quite large grain structure with random texture in the heat affected zone (HAZ). It suggests the occurrence of static recrystallization and subsequent grain growth because of the appreciable strain stored within the base metal. One FSP pass under condition of cryogenic temperature causes to the development of bimodal grain size distribution in HAZ, including the elongated grains with interior... 

An examination of the influence of rolling path change on the static recrystallization behavior of commercial purity aluminum was performed in the present work. Aluminum strips were cold rolled to a reduction of 50 % under various rolling sequences, i.e. single-pass, double-pass from one direction and with reverse directions, and were then annealed in 290 °C for different durations, while mechanical evaluations such as hardness and tensile tests were used to study the mechanical response of cold deformed and annealed samples. It was indicated that a variation in the recrystallization kinetics of the cold rolled aluminum strips takes place when the rolling path is altered from single to... 

In this article, the effects of hot rolling parameters on static recrystallization, mechanical properties, and final microstructures of a low carbon steel have been investigated. A numerical analysis was first employed to determine distributions of temperature, strain, and strain rate during hot rolling and then the predicted results were combined with the additivity rule together with Avrami equation to evaluate the progress of non-isothermal static recrystallization after hot rolling. In the next stage, hot rolling experiments have been performed under different rolling conditions to assess the effects of rolling layout on microstructures and mechanical properties of the rolled steel. The... 

An Artificial Neural Network (ANN) model has been designed for predicting the effects of alloying elements (Fe, Si, Al, Mn) on the recrystallization behavior and microstructural changes of 70/30 brass. The model introduced here considers the content of alloying elements, temperature, and time of recrystallization as inputs while percent of recrystallization is presented as output. It is shown that the designed model is able to predict the effect of alloying elements well. It is also shown that all alloying elements strongly affect the recrytallization kinetics, and all slow down the recrystallization process. The effect of alloying elements on the activation energy for recrystallization has... 

A model has been developed for evaluation of recrystallization kinetics during hot rolling of a low carbon steel. For doing so, the finite difference and the finite element methods have been coupled to consider the interconnections among temperature, strain rate, strain and the kinetics of recrystallization within the steel under non-isothermal hot rolling conditions. To verify the model predictions, hot rolling experiments are carried out and the roll forces and microstructure of the rolled metal are compared with the predicted results. The comparison between the two sets of data shows a good agreement. © 2006 Elsevier B.V. All rights reserved  

An ultrafine-grained Al6063/Al2O3 (0.8 vol.%, 25 nm) nanocomposite was prepared via powder metallurgy route through reactive mechanical alloying and hot powder extrusion. Scanning electron microcopy, transmission electron microscopy, and back scattered electron diffraction analysis showed that the grain structure of the nanocomposite is trimodal and composed of nano-size grains (< 0.1 μm), ultrafine grains (0.1–1 μm), and micron-size grains (> 1 μm) with random orientations. Evaluation of the mechanical properties of the nanocomposite based on the strengthening-mechanism models revealed that the yield strength of the ultrafine-grained nanocomposite is mainly controlled by the high-angle... 

Ultrafine-grained (UFG) Al6063/Al2O3 (0.8 vol%, 25 nm) nanocomposite was prepared via powdr metallurgy route. The grain structure of the nanocomposite composed of nano-size grains (< 0.1 μm), ultrafine grains (0.1-1 μm) and micron-size grains (>1 μm) with random orientations. It was found that the yield strength of the UFG nanocomposite is mainly controlled by the Orowan mechanism rather than the grain boundaries. The deformation activation energy at temperature ranges of T <300 ˚C and 300 ˚C ≤T < 450 ˚C was determined to be 74 and 264 kJ mol-1, respectively. At the higher temperatures, significant deformation softening was observed due to dynamic recrystallization of non-equilibrium grain... 

Al6063 powder was subjected to severe plastic deformation via high-energy mechanical milling to prepare ultrafine-grained (UFG) aluminium alloy. Uniaxial compression test at various temperatures between 300 and 450 °C and strain rates between 0.01 and 1 s-1 was carried out to evaluate hot workability of the material. Microstructural studies were performed by EBSD and TEM. The average activation energy and strain rate sensitivity of the hot deformation process were determined to be 280 kJ mol-1 and 0.05, respectively. The deformation temperature and applied strain rate significantly affected the grain structure of UFG Al alloy. A finer grain structure was obtained at lower temperatures and... 

An ultrafine-grained Al6063/Al2 O3 (0.8vol.%, 25nm) nanocomposite was prepared via powder metallurgy route through reactive mechanical alloying and hot powder extrusion. Scanning electron microcopy, transmission electron microscopy, and back scattered electron diffraction analysis showed that the grain structure of the nanocomposite is trimodal and composed of nano-size grains (<0.1μm), ultrafine grains (0.1-1μm), and micron-size grains (>1μm) with random orientations. Evaluation of the mechanical properties of the nanocomposite based on the strengthening-mechanism models revealed that the yield strength of the ultrafine-grained nanocomposite is mainly controlled by the high-angle grain... 

In this work, non-isothermal annealing of cold-rolled 304L austenitic stainless steel was studied at temperatures ranging between 400 and 800 °C. In the first place, a dual-phase structure containing work-hardened austenite and strain-induced martensite was produced by means of multi-pass plate rolling, and then, the deformed steel was subjected to annealing heat treatments. To determine the distribution of stored strain energy in the rolling operations, an elastic–plastic finite-element modeling was conducted. Afterwards, an artificial neural network technique was coupled with the thermal-cellular automata model for prediction of martensite reversion, temperature history, and... 

A Monte Carlo model on the basis of hardness input is developed to predict the annealing microstructure of deformed specimens in tensile, compression, and tensile + compression tests. From experimental value of hardness, the stored energy of the deformed specimens is calculated and entered into the Monte Carlo model. The consistency between the simulation results and experimental data shows that the developed model based on hardness input can be more practical since the effect of different deformation states is considered for estimating of stored energy. © 2008 Elsevier B.V. All rights reserved