Sharif Digital Repository

The effect of temperature on the short-range order (SRO) structures, deformation mechanisms and failure modes of metallic glasses (MGs) is of fundamental importance for their practical applications. However, due to lack of direct structural information at the atomistic level from experiments and the absence of previous molecular dynamics (MD) simulations to reproduce experimental observations over a wide range of temperature, this issue has not been well understood. Here, by carefully constructing the atomistic models of Cu64Zr36 and Fe80W20 MGs, we are able to reproduce the major deformation modes observed experimentally, i.e. single shear banding (SB) at low temperatures, multiple... 

In this work, neural networks are used for estimation of flow stress of AA5083 with regard to dynamic strain ageing that occurs in certain deformation conditions and varies flow stress behavior of the metal being deformed. The input variables are selected to be strain rate, temperature and strain and the output value is the flow stress. In the first stage, the appearance and terminal of dynamic strain aging are determined with the aid of tensile testing at various temperatures and strain rates and subsequently for the serrated flow and the smooth yielding domains different neural networks are constructed based on the achieved results. While a feed-forward backpropagation algorithm is... 

A thermo-mechanical model is developed to predict metal behavior during hot working operations. At first, a neural network model is trained to calculate flow stress of deforming metal as a function of temperature, strain and strain rate and then by coupling the neural network model and a thermo-viscoplastic finite element model, temperature and velocity fields during hot open die forging process are predicted. To examine the model, hot nonisothermal upsetting on a low carbon steel is performed while force-displacement behavior and temperature history during hot working are recorded. A good agreement is observed between the predicted data and the measured results. © 2007 Elsevier B.V. All... 

This paper describes the application of three-dimensional finite element analysis in hot slab rolling of aluminum alloys. To assess the rolling behavior of materials, finite element program Abaqus/Explicit has been employed and the hot rolling process was modeled in three dimensions. The temperature distribution, variation of flow stress, and flow pattern were determined during hot rolling of 6063 and 3003 aluminum alloys. Also, with the aid of the predicted results the geometrical changes and the rolling forces were determined. To check the validity of the employed model and equations, hot rolling experiments were conducted and roll force and temperature of the samples have been measured... 

A hybrid algorithm based on the finite element method, Monte Carlo model, and Hall-Petch relationship is utilized to predict the mechanical properties of the rods after cold forging at different degrees of deformations and heat treatments at different temperatures and times. The results show that the flow stress and hardness of the rods after forging and those of the forged rods after the heat treatments are decreased from their center to surface. However, with increasing the temperature and time of the heat treatment the flow stress and hardness are decreased, their effects are not considerable. In addition, the distribution of the mechanical properties of the forged rods after the heat... 

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... 

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... 

Two-dimensional finite element analysis together with stream function and neural network models are employed to determine thermo-mechanical behavior during hot strip rolling of AA5083. An appropriate velocity field and stream function is first determined using the rule of volume constancy and upper bound theorem and then temperature field within the metal is predicted by means of a two-dimensional conduction-convection model. In order to consider the effect of flow stress and its dependence on temperature, strain and strain rate, a neural network model is also employed in the analysis. Based on the performed tensile tests, two different neural network models are constructed one for smooth... 

AISI 4340 steel was heated to 910 °C for 1 h then directly transferred to 750 °C and intercritically annealed for different times to obtain different ferrite volume fractions and then isothermally held at 350 °C for 40 min followed by air cooling to room temperature. Samples of these steels with dual phase ferrite-bainite structure were tensile tested at room temperature. The tensile flow stress data for this steel with different ferrite volume fraction was analyzed in term of Hollomon equation. It is seen that the two Hollomon equations can describe the flow behavior adequately and found that the work hardening takes place in two stages which each equation belong to the one of work... 

A mathematical model is proposed to predict the flow stress behaviour of aluminium alloys under hot rolling conditions. To do so, a dislocation model for evaluating flow stress during deformation is coupled with a finite element analysis to access metal behaviour under non-isothermal and variable strain rate conditions. Then, with the aid of the proposed model, a hot strip rolling process was simulated. In order to verify modelling results, flow stress behaviour of an aluminium alloy is studied employing hot compression tests in various temperatures and strain rates and the model was examined on this material. Non-isothermal hot rolling experiments were carried out and good agreement was... 

In this study, the prediction of flow stress in 304 stainless steel using artificial neural networks (ANN) has been investigated. Experimental data earlier deduced-by [S. Venugopal et al., Optimization of cold and warm workability in 304 stainless steel using instability maps, Metall. Trans. A 27A (1996) 126-199]-were collected to obtain training and test data. Temperature, strain-rate and strain were used as input layer, while the output was flow stress. The back propagation learning algorithm with three different variants and logistic sigmoid transfer function were used in the network. The results of this investigation shows that the R2 values for the test and training data set are about... 

A mathematical model has been proposed to predict flow behavior of steel under hot deformation conditions with regard to the effects of dynamic recovery and dynamic recrystallization. For this purpose, Kocks-Mecking dislocation model together with a first order rate equation have been utilized. To associate temperature and strain rate variations on flow behavior of deforming metal, a thermo-viscoplastic finite element model has been coupled with the proposed model. To verify the modeling results, hot rolling experiments were performed and roll forces at various temperatures and rolling speeds were recorded. A good agreement was found between the predicted and the experimental data. © 2005... 

Commercially pure aluminum sheets, AA 1050, are processed by accumulative roll bonding (ARB) up to eight cycles to achieve ultrafine-grained (UFG) aluminum as primary material for mechanical testing. Optical microscopy and electron backscattering diffraction analysis are used for microstructural analysis of the processed sheets. Strain rate sensitivity (m-value) of the specimens is measured over a wide range of strain rates by stress relaxation test under plane strain compression. It is shown that the flow stress activation volume is reduced by decrease of the grain size. This reduction which follows a linear relation for UFG specimens, is thought to enhance the required effective (or... 

In this study, artificial neural network (ANN) was used to model the hot deformation behavior of 7075 aluminum alloy during compression test, in the strain rate range of 0.0003-1 s-1 and temperature range of 200-450 C. The inputs of the model were temperature, strain rate, and strain, while the output of the model was the flow stress. The feed-forward back-propagation network with two hidden layers was built and successfully trained at different deformation domains by Levenberg-Marquardt training algorithm. Comparative analysis of the results obtained from the hyperbolic sine, the power law constitutive equations, and the ANN shows that the newly developed ANN model has a better performance... 

Two-internal variable thermodynamics model is presented to investigate the evolution of microstructure and flow stress during severe plastic deformation. Previous studies have shown that due to heterogeneous distribution of dislocations during severe plastic deformation, the use of multivariable models is needed. In this regard, a two-internal variable model is presented. In the present paper, the dislocation densities in the subgrain boundaries and interiors are considered as internal variables. The model uses general laws of thermodynamics and describes the evolution of the dislocation densities on the basis of parameters such as the self-diffusion activation energy and the stacking fault... 

In this work, an optimum multilayer perceptron neural network is developed to model the correlation between hot working parameters (temperature, strain rate and strain) and flow stress of IN625 alloy. Three variations of standard back propagation algorithm (Broyden, Fletcher, Goldfarb and Shanno quasi-Newton, Levenberg-Marquardt and Bayesian) are applied to train the model. The results show that, in this case, the best performance, minimum error and shortest converging time are achieved by the Levenberg-Marquardt training algorithm. Comparing the predicted values and the experimental values reveals that a well trained network is capable of accurately calculating the flow stress of the alloy... 

The relation between porosity and permeability parameters in carbonated rocks is complicated and indistinct. Flow units are defined with aim of better understanding reservoir unit flow behavior and relation between porosity and permeability. Flow units reflect a group of rocks with same geological and physical properties which affect fluid flow, but they do not necessarily coincide with boundary of facies. In each flow unit homogeneity of data is preserved and this homogeneity fades in the boundaries. Here, in this study, three methods are used for identification of flow units and estimation of average porosity and permeability in three wells of Tabnaak gas field located in south of Iran.... 

A mathematical model has been developed to predict stress-strain curve up to the peak stress at hot deformation. This model is based on the linear estimation of work hardening rate-stress curve up to the peak stress. This equation is expressed in terms of peak stress, peak strain. In addition, in order to find the value of peak strain, Zenner-Hollomon parameter is modified. The predicted results are found to be in accord with the experimental flow stress curves which can be used to predict the required deformation forces in hot deformation processes. © 2008 Elsevier Ltd. All rights reserved  

Using a mechanical model and dislocation density based model, the evolutions of dislocation density and flow stress of pure copper during constrained groove pressing (CGP) process are investigated. In this regard, the strain and strain rate are achieved from the mechanical model and then input into the dislocation model. To verify the predicted flow stress, the process of constrained groove pressing is performed on the sheets of pure copper from one to three passes. The predicted flow stresses are compared with the experimental data and a good agreement is observed. Also, it is found that during the straining of the copper sheet in CGP process, the dislocation density and strength dropping... 

In this paper, a method for modeling of elastic-plastic hardening materials under large deformations is proposed. In this model the generalized strain rate tensor is used. Such a tensor is obtained on the basis of the method which was introduced by the authors. Based on the generalized strain rate tensor, a flow rule, a Prager-type kinematic hardening equation and a kinematic decomposition is proposed and the governing equations for such materials are obtained. As an application, the governing equations for the simple shear problem are solved and some results are compared with those in the literature. Copyright © 2008 by ASME