Sharif Digital Repository

Most devices based on shape memory alloys experience both finite deformations and non-proportional loading conditions in engineering applications. This motivates the development of constitutive models considering finite strain as well as martensite variant reorientation. To this end, in the present article, based on the principles of continuum thermodynamics with internal variables, a three-dimensional finite strain phenomenological constitutive model is proposed taking its basis from the recent model in the small strain regime proposed by Panico and Brinson (J Mech Phys Solids 55:2491-2511, 2007). In the finite strain constitutive model derivation, a multiplicative decomposition of the... 

The effect of aluminium content on the formation mechanism, volume fraction, morphology, and particle size distribution of graphite has been investigated. Addition of aluminium to ductile iron causes some fundamental changes in iron-carbon phase diagrams and, as a result, improves graphite formation during eutectic transformation. Results reveal that aluminium compounds have been formed in the core of graphite nodules; thus aluminium plays an important role in the formation of graphite nodules. Furthermore, it is indicated that an increase in the aluminium content also leads to an increase in the number of graphite nodules and a decrease in the nodule size. By using electron probe... 

A thermo-elastic analysis coupled with a phase change model is employed to evaluate thermal stresses as well as progress of austenite decomposition after hot strip rolling operations. The thermo-mechanical finite element analysis is utilized to assess the distributions of temperature and thermal stresses during cooling while at the same time, a model based on the additivity rule is used to determine the rate of austenite to ferrite decomposition. The proposed model is applicable to multi-pass rolling schedules while the effects of different process parameters including the rolling speed and cooling configuration of the run-out table can be taken into account. In order to verify the... 

This paper investigates the metallurgical and mechanical response of 2304 duplex stainless steel-as an interesting candidate for automotive body-in-white applications-to resistance spot welding. The results showed the high cooling rate associated with the resistance spot welding process suppressed the postsolidification ferrite-austenite transformation leading to improper ferrite-austenite phase balance with a reduced volume fraction of austenite and consequent precipitation of chromium-rich nitrides. The effects of welding current, as the key parameter determining the weld heat input, on the austenite volume fraction and precipitation are discussed. The minimum fusion zone hardness... 

The technological and industrial needs for development of fully dense nanocomposites have led to significant advances in spark plasma sintering (SPS) technique and its enhanced forms. This technique has opened up a new prospect over carbon nanotube (CNT)-metal matrix nanocomposites (MMNCs) with superior physical or mechanical characteristics. To date, a large number of authentic papers have been published over this ongoing field, but have not been comprehensively reviewed. The pertinent research works cover some significant aspects of CNT-MMNCs requiring a concise review on (i) the potential phase transformations of pure CNTs and microstructure evolution; (ii) the novel approaches for... 

The inclusion of carbon nanotubes (CNTs) into metallic systems has been the main focus of recent literature. The aim behind this approach has been the development of a new property or improvement of an inferior one in CNT-dispersed metal matrix nanocomposites. Although it has opened up new possibilities for promising engineering applications, some practical challenges have restricted the full exploitation of CNTs’ unique characteristics. Non-uniform dispersion of CNTs in the metallic matrix, poor interfacial adhesion at the CNT/metal interface, the unfavorable chemical reaction of CNTs with the matrix, and low compactability are the most significant challenges, requiring more examination.... 

Recent uses of intelligent composites in biomedical appliances aggrandize the necessity of bonding-strength improvement in NiTi/silicone matrix interface. SEM micrographs and pull-out tests are employed to determine the strength of the NiTi/silicone bonds in a flexible composite piece. Greater adhesion strengths are obtained due to the presence of thin oxide layer, surface roughness and frictional forces between the embedded-wires and the contacting phase. Effect of curing treatment on phase transformation temperatures of the wires is determined by electrical resistivity (ER) measurements. Results show that the curing treatment shifts the transition points of the wires towards higher... 

Highly stable, water-based barium titanate (BaTiO3) sols were developed by a low cost and straightforward sol-gel process. Nanocrystalline barium titanate thin films and powders with various Ba:Ti atomic ratios were produced from the aqueous sols. The prepared sols had a narrow particle size distribution in the range 21-23 nm and they were stable over 5 months. X-ray diffraction pattern revealed that powders contained mixture of hexagonal- or perovskite-BaTiO3 as well as a trace of Ba2Ti 13O22 and Ba4Ti2O27 phases, depending on annealing temperature and Ba:Ti atomic ratio. Highly pure barium titanate with cubic perovskite structure achieved with Ba:Ti = 50:50 atomic ratio at the high... 

Nickel titanium shape memory alloys (NiTi-SMAs) were successfully produced from elemental Ni/Ti powders by powder metallurgical method and then subjected to age treatment. Microstructure was examined by SEM and XRD and phase transformation temperatures were measured by dilatometric method. The phase transformation temperatures increased with both duration and temperature of the age treatment. The porous product exhibited desirable shape memory effect. © 2009 Elsevier B.V. All rights reserved  

Biphase TiO2 nanoparticles have been prepared by sol-gel method. Water/titanium molar ratio (r) has been used to control the hydrolysis and condensation of titanium isopropoxide in solution producing titanium oxide with two different polymorphs. The influence of crystallite size and morphology of prepared TiO2 on the phase transformation of the resultant materials has been investigated. Synthesized powders were characterized by X-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Different trends can be observed in the phase transformation and particle growth of the prepared titanium oxide nanomaterial. It was concluded that, the rate of particle... 

Graphitization in steels is a subject which has been studied temporarily in past decades. The importance of this phenomenon is due to the fact that graphite formation in steels can ease machining treatment and increase cold-work capability of steels. Weight reduction of the parts and the dedicated bioenvironmental properties of these steels are considered as a valuable replacement for lead containing steels for researchers. Anyway there has been always the assumption that this operation is long and not economically cost effective. For this reason due to the different conditions of applied heat treatments, the effects of the temperature and the time period of heat treatment on the... 

A mathematical model has been proposed for evaluation of velocity and temperature fields in hot forging operations for axisymmetric parts. The model can be applied to determine forging load, strain rate, strain and temperature distributions. In addition, the effects of temperature and strain rate on metal flow have been considered through simultaneous modelling of dynamic phase transformation within the deforming metal. Hot forging experiments have been carried out under different working conditions and the results have been compared with the predictions. A good agreement between the simulated and experimental results was found. © 2004 Elsevier Ltd. All rights reserved  

In recent years, there have been some endeavors in order to characterize and model Shape Memory Alloy (SMA) behavior both in tension and compression. However, the one-dimensional behavior of SMA has been mostly studied for the case of wire elements subjected to tension. The objective of this paper is to analyze the behavior of Ni-55.7% wt Ti SMA in tension, compression, and at various temperatures. The effects of cycling, annealing, and friction on the mechanical behavior of this material in compression are discussed as well and, where applicable, compared to those of tension. The compressed SMA rings are finally used as clamping-force actuators in loosed bolted joints. In the second part of... 

The mathematical modeling of a bolted joint with a Shape Memory Alloy (SMA) actuator ring is presented. In this way, phase transformation, constitutive and displacement compatibility equations are used. The obtained model is utilized to show how the application of the SMA actuator ring in the bolted joint can help the joint to recover a significant amount of its lost clamping force. The model is run using the experimental data obtained for the Ni-55.7%wt Ti SMA in the companion paper (Part 1). The mechanical properties of SMA are calculated using the mentioned equations and compared to the experimental findings. It is observed that the final clamping force obtained theoretically is close to... 

A study has been made to determine the influence of the carbon content on the kinetics of dynamic recrystallization and recovery as well as flow stress during hot deformation of carbon steels. For this purpose, single hit hot compression experiments at various strain rates and temperatures on two grades of carbon steel together with the Avrami-type kinetics equation and Bergstrom approach have been utilized. The results show that the apparent hot deformation activation energy decreases with increasing carbon content and this phenomenon results in a faster dynamic recrystallization and recovery and a lower flow stress at high temperatures and/or low strain rates in high carbon steels, while... 

In this paper the velocity and temperature fields during hot strip rolling are determined using a rigid-viscoplastic finite element method together with a microstructural model for handling dynamic phase transformation during hot deformation. Based on these fields, the strain distributions within the rolled metal at different positions in the deformation zone are estimated. The analysis is capable of considering the effects of metallurgical phase transformations as well as various process parameters such as initial strip temperature, rolling speed, lubrication on the strain inhomogeneity produced during the hot rolling process. To assess the reliability of the theoretical analysis, a... 

This paper presents a mathematical model for predicting the temperature distribution and austenite microstructural changes during hot rolling of steel bars and rod. The model is based on the finite element method for evaluating the temperature distribution and Wusatowski approach to determine the strain field within the deformation zone. Also, by employing double and single hit hot compression experiments, the kinetics of dynamic and static recrystallization of a low carbon steel are determined to predict the flow stress as well as the phase changes in austenite range during and after the hot rolling process. To deal with the correlation between the metal flow and thermal behaviour and...