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    Laser welding of NiTi shape memory alloy: Comparison of the similar and dissimilar joints to AISI 304 stainless steel [electronic resource]

    , Article Optics and Laser Technology ; Volume 54, 2013, Pages 151–158 Mirshekari, G. R. (Golamreza) ; Saatchi, A ; Kermanpur, A ; Sadrnezhaad, K ; Sharif Univercity of Technology
    Abstract
    The unique properties of NiTi alloy, such as its shape memory effect, super-elasticity and biocompatibility, make it ideal material for various applications such as aerospace, micro-electronics and medical device. In order to meet the requirement of increasing applications, great attention has been given to joining of this material to itself and to other materials during past few years. Laser welding has been known as a suitable joining technique for NiTi shape memory alloy. Hence, in this work, a comparative study on laser welding of NiTi wire to itself and to AISI 304 austenitic stainless steel wire has been made. Microstructures, mechanical properties and fracture morphologies of the... 

    Microstructural characterization of HIP consolidated NiTi–nano Al2O3 composites [electronic resource]

    , Article Journal of Alloys and Compounds ; 5 September 2014, Volume 606, Pages 21–26 Farvizi, M ; Ebadzadeh, T ; Vaezi, M. R ; Yoon, E. Y ; Kim, Y. J ; Kim, H. S ; Simchi, A. (Abdolreza) ; Sharif University of Technology
    Abstract
    The microstructure and phase transformational behavior of NiTi-based composites reinforced with 6 wt.% of α-alumina nanoparticles have been investigated. Two kinds of starting materials, elemental Ni–Ti and prealloyed austenitic NiTi, were used to prepare the composites. The samples were consolidated using a hot isostatic pressing method. The X-ray diffraction results showed that while unreinforced NiTi mainly contained B2 phase at room temperature, martensitic B19′ phase appeared in the microstructure after addition of the α-alumina nanoparticles. The differential scanning calorimetry measurements indicated that the martensitic transformation temperatures were elevated in the composite... 

    Mechanical- activated phase formation of NiTi in the presence of nanoparticles [electronic resource]

    , Article Journal of Nano ; October 2013, Volume 08, Issue 05 Farvizi, M ; Ebadzadeh, T ; Vaezi, M. R ; Simchi, A. (Abdolreza) ; Kim, H. S ; Sharif University of Technology
    Abstract
    Effect of Al2O3 nanoparticles (80 nm) on the grain structure and phase formation in Ni-50Ti system during high-energy mechanical alloying (MA) was studied. While the formation of NiTi B2 phase occurs progressively during MA, it is shown that the hard inclusions cause abrupt phase formation at short milling times, particularly at higher nano-Al2O3 contents. High-resolution transmission electron microscopy showed significant grain refinement in the presence of alumina nanoparticles to sizes less than 10 nm, which precedes the formation of semicrystalline structure and reduces the diffusion length and thus accelerates the phase formation. The composite powder reached steady-state MA condition... 

    What is the copper thin film thickness effect on thermal properties of NiTi/Cu bi-layer?

    , Article Materials Research Express ; Volume 4, Issue 2 , 2017 ; 20531591 (ISSN) Fazeli, S ; Vahedpour, M ; Sadrnezhaad, S. K ; Sharif University of Technology
    Institute of Physics Publishing  2017
    Abstract
    Molecular dynamics (MD) simulation was used to study of thermal properties of NiTi/Cu. Embedded atom method (EAM) potentials for describing of inter-atomic interaction and Nose-Hoover thermostat and barostat are employed. The melting of the bi-layers was considered by studying the temperature dependence of the cohesive energy and mean square displacement. To highlight the differences between bi-layers with various copper layer thickness, the effect of copper film thickness on thermal properties containing the cohesive energy, melting point, isobaric heat capacity and latent heat of fusion was estimated. The results show that thermal properties of bi-layer systems are higher than that of... 

    The effect of chemical composition on enthalpy and entropy changes of martensitic transformations in binary NiTi shape memory alloys

    , Article Journal of Alloys and Compounds ; Volume 487, Issue 1-2 , 2009 , Pages 363-366 ; 09258388 (ISSN) Khalil Allafi, J ; Amin Ahmadi, B ; Sharif University of Technology
    2009
    Abstract
    In the present research work the binary NiTi alloys with various compositions in the range of 50.3-51 at.% Ni were used. Samples have been annealed at 850 °C for 15 min and then quenched in water. In order to characterize transformation temperatures and enthalpy changes of the forward and the reverse martensitic transformation, Differential Scanning Calorimetric (DSC) experiments were performed. The enthalpy and entropy changes as a function of Ni atomic content have been thermodynamically investigated. Results show that enthalpy and entropy changes of martensitic transformation decrease when Ni atomic content increases. The variation of enthalpy and entropy of martensitic transformation... 

    Phase transformation behavior of porous NiTi alloy fabricated by powder metallurgical method

    , Article Materials Science and Engineering C ; Volume 29, Issue 7 , 2009 , Pages 2203-2207 ; 09284931 (ISSN) Hosseini, S. A ; Sadrnezhaad, S. K ; Ekrami, A ; Sharif University of Technology
    2009
    Abstract
    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  

    The effect of deformation heating on restoration and constitutive equation of a wrought equiatomic NiTi alloy

    , Article Journal of Materials Engineering and Performance ; Volume 21, Issue 4 , April , 2012 , Pages 516-523 ; 10599495 (ISSN) Ahadi, A ; Karimi Taheri, A ; Karimi Taheri, K ; Sarraf, I. S ; Abbasi, S. M ; Sharif University of Technology
    Abstract
    In this study, a set of constitutive equation corrected for deformation heating is proposed for a near equiatomic NiTi shape memory alloy using isothermal hot compression tests in temperature range of 700 to 1000 °C and strain rate of 0.001 to 1 s -1. In order to determine the temperature rise due to deformation heating, Abaqus simulation was employed and varied thermal properties were considered in the simulation. The results of hot compression tests showed that at low pre-set temperatures and high strain rates the flow curves exhibit a softening, while after correction of deformation heating the softening is vanished. Using the corrected flow curves, the power-law constitutive equation of... 

    Characterization of sputtered NiTi shape memory alloy thin films

    , Article Scientia Iranica ; Volume 16, Issue 3 B , 2009 , Pages 248-252 ; 10263098 (ISSN) Sanjabi, S ; Naderi, M ; Zare Bidaki, H ; Sadrnezhaad, Kh ; Sharif University of Technology
    2009
    Abstract
    During recent years, many investigations have been carried out to determine how to select different materials for the making of Micro Electro Mechanical Systems (MEMS) and bio-MEMS. The NiTi shape memory alloy thin film has been much regarded as a promising candidate for MEMS due. to its unique shape memory effect and high energy output. In this research, NiTi thin film was fabricated using a sputtering technique from separate elemental Ni and Ti targets. The characterizations of the deposited films were, investigated using different analysis techniques, such as Field Emission SEM, DSC, XR.D, electrical resistivity measurement and nanoindentation. © Sharif University of Technology, June 2009... 

    The effect of Cu addition on the hot deformation behavior of NiTi shape memory alloys

    , Article Journal of Alloys and Compounds ; Volume 499, Issue 1 , June , 2010 , Pages 57-62 ; 09258388 (ISSN) Morakabati, M ; Kheirandish, Sh ; Aboutalebi, M ; Karimi Taheri, A ; Abbasi, S. M ; Sharif University of Technology
    2010
    Abstract
    The influence of Cu addition on the hot deformation behavior of NiTi shape memory alloys was investigated using hot compression test. A series of alloys with different Cu contents of Ti50.4Ni49.6-xCux (x = 0, 3, 5, 7 at.%) were deformed under compression to a true strain of 0.7 at the temperature range of 700-1000 °C with 100 °C intervals and constant strain rate of 0.1 s-1. The stress-strain curves showed that the addition of Cu to NiTi alloy made the flow curves shift upward. This was confirmed by the calculated critical stress, σc, obtained from inflections in θ-σ plots, which is attributed to the formation of high strength Cu containing precipitates and solid solution hardening caused by... 

    Powder metallurgical fabrication and characterization of nanostructured porous NiTi shape-memory alloy

    , Article Materials and Manufacturing Processes ; Volume 21, Issue 8 , 2006 , Pages 727-735 ; 10426914 (ISSN) Sadrnezhaad, Kh ; Arami, H ; Keivan, H ; Khalifezadeh, R ; Sharif University of Technology
    2006
    Abstract
    Production of NiTi alloy from elemental powders was conducted by mechanical alloying (MA) and sintering of the raw materials. Effects of milling time and milling speed (RPM) on crystallite size, lattice strain, and XRD peak intensities were investigated by X-ray analysis of the alloy. Powder compaction and sintering time and temperature effects on pore percentage of the as-mixed and the mechanically alloyed samples were empirically evaluated. The crystallite size of the mechanically alloyed Ni50Ti50 samples decreased with MA duration and with the milling speed. Depending on the crystal structure of the raw materials, the lattice strain increases with the milling duration. Metallographic... 

    Mechanical-activated phase formation of niti in the presence of nanoparticles

    , Article Nano ; Volume 8, Issue 5 , 2013 ; 17932920 (ISSN) Farvizi, M ; Ebadzadeh, T ; Vaezi, M. R ; Simchi, A ; Kim, H. S ; Sharif University of Technology
    2013
    Abstract
    Effect of Al2O3 nanoparticles (80 nm) on the grain structure and phase formation in Ni-50Ti system during high-energy mechanical alloying (MA) was studied. While the formation of NiTi B2 phase occurs progressively during MA, it is shown that the hard inclusions cause abrupt phase formation at short milling times, particularly at higher nano-Al 2O3 contents. High-resolution transmission electron microscopy showed significant grain refinement in the presence of alumina nanoparticles to sizes less than 10 nm, which precedes the formation of semicrystalline structure and reduces the diffusion length and thus accelerates the phase formation. The composite powder reached steady-state MA condition... 

    Laser welding of NiTi shape memory alloy: Comparison of the similar and dissimilar joints to AISI 304 stainless steel

    , Article Optics and Laser Technology ; Volume 54 , 2013 , Pages 151-158 ; 00303992 (ISSN) Mirshekari, G. R ; Saatchi, A ; Kermanpur, A ; Sadrnezhaad, S. K ; Sharif University of Technology
    2013
    Abstract
    The unique properties of NiTi alloy, such as its shape memory effect, super-elasticity and biocompatibility, make it ideal material for various applications such as aerospace, micro-electronics and medical device. In order to meet the requirement of increasing applications, great attention has been given to joining of this material to itself and to other materials during past few years. Laser welding has been known as a suitable joining technique for NiTi shape memory alloy. Hence, in this work, a comparative study on laser welding of NiTi wire to itself and to AISI 304 austenitic stainless steel wire has been made. Microstructures, mechanical properties and fracture morphologies of the... 

    Effect of hot rolling on microstructure and transformation cycling behaviour of equiatomic NiTi shape memory alloy

    , Article Materials Science and Technology (United Kingdom) ; Volume 28, Issue 6 , 2012 , Pages 727-732 ; 02670836 (ISSN) Ahadi, A ; Rezaei, E ; Karimi Taheri, A ; Sharif University of Technology
    2012
    Abstract
    In this study, a near equiatomic NiTi shape memory alloy was hot rolled at 800°C using thethickness reductions of 30 and 50%. Optical and transmission electron microscopy, together withX-ray diffraction were used to demonstrate the microstructural changes associated with the hotrolling at different thickness reductions. Repeated transformation cycling was employed toinvestigate the evolution of R phase during cycling. Microstructural observations revealed thepresence of deformation twins embedded in an elongated grain matrix in the hot rolled material.Moreover, it was found that with increasing degree of thickness reduction, the size and number ofdeformation twins increased throughout the... 

    Property change during nanosecond pulse laser annealing of amorphous NiTi thin film

    , Article Bulletin of Materials Science ; Volume 35, Issue 3 , 2012 , Pages 357-364 ; 02504707 (ISSN) Sadrnezhaad, S. K ; Yasavol, N ; Ganjali, M ; Sanjabi, S ; Sharif University of Technology
    2012
    Abstract
    Nanosecond lasers of different intensities were pulsed into sputter-deposited amorphous thin films of near equiatomic Ni/Ti composition to produce partially crystallized highly sensitive R-phase spots surrounded by amorphous regions. Scanning electron microscopy having secondary and back-scattered electrons, field emission scanning electron microscopy, optical microscopy and X-ray diffraction patterns were used to characterize the laser treated spots. Effect of nanosecond pulse lasering on microstructure, morphology, thermal diffusion and inclusion formation was investigated. Increasing beam intensity and laser pulse-number promoted amorphous to R-phase transition. Lowering duration of the... 

    Fabrication of porous NiTi alloy via powder metallurgy and its mechanical characterization by shear punch method

    , Article Russian Journal of Non-Ferrous Metals ; Volume 53, Issue 2 , April , 2012 , Pages 169-175 ; 10678212 (ISSN) Parvizi, S ; Hasannaeimi, V ; Saebnoori, E ; Shahrabi, T ; Sadrnezhaad, S. K ; Sharif University of Technology
    Abstract
    In this study, Porous NiTi shape memory alloy has been produced by mechanical alloying of the elemental Ni and Ti powders. The compacting process was done at two temperatures (warm and cold press) and then sintering at 980 and 1050°C was performed on the specimens. Microstructure and mechanical prop-erties of the samples were investigated by optical microscopy, scanning electron microscopy (SEM) and X-ray diffraction. Moreover, shear punch test (SPT) employed to investigate the effect of compaction pressure and sintering temperature on the mechanical properties of the fabricated samples. It was revealed that warm compaction/sintering resulted in 15% yield stress improvement and 20% ultimate... 

    Biocompatibility and corrosion behavior of the shape memory NiTi alloy in the physiological environments simulated with body fluids for medical applications

    , Article Materials Science and Engineering C ; Volume 30, Issue 8 , 2010 , Pages 1112-1117 ; 09284931 (ISSN) Khalil Allafi, J ; Amin Ahmadi, B ; Zare, M ; Sharif University of Technology
    2010
    Abstract
    Due to unique properties of NiTi shape memory alloys such as high corrosion resistance, biocompatibility, super elasticity and shape memory behavior, NiTi shape memory alloys are suitable materials for medical applications. Although TiO2 passive layer in these alloys can prevent releasing of nickel to the environment, high nickel content and stability of passive layer in these alloys are very debatable subjects. In this study a NiTi shape memory alloy with nominal composition of 50.7 atom% Ni was investigated by corrosion tests. Electrochemical tests were performed in two physiological environments of Ringer solution and NaCl 0.9% solution. Results indicate that the breakdown potential of... 

    Effect of post weld heat treatment on mechanical and corrosion behaviors of niTi and stainless steel laser-welded wires

    , Article Journal of Materials Engineering and Performance ; 2016 , Pages 1-8 ; 10599495 (ISSN) Mirshekari, G. R ; Saatchi, A ; Kermanpur, A ; Sadrnezhaad, S. K ; Sharif University of Technology
    Springer New York LLC  2016
    Abstract
    Effects of post weld heat treatment (PWHT) on mechanical properties and corrosion behavior of NiTi shape memory wire, laser welded to the 304 stainless steel wire were investigated. The results showed that PWHT at 200 °C increased corrosion resistance and tensile strength of the joint up to ~1.8 times that of the as-weld joint, with no heat treatment. On the contrary, precipitation of neoteric intermetallic compounds like Fe2Ti, Cr2Ti, FeNi, Ni3Ti, and Ti2Ni in the welded region deteriorated these properties, when PWHT was conducted at 400 °C. Due to the vital effects of the PWHT performed after the laser welding, careful control of the PWHT temperature was found to be a prerequisite for... 

    On the effect of detwinning-induced plasticity in compressive cyclic loading of NiTi shape memory alloys

    , Article Mechanics of Materials ; Volume 148 , 2020 Ebrahimi, P ; Arghavani, J ; Naghdabadi, R ; McGarry, J. P ; Sharif University of Technology
    Elsevier B.V  2020
    Abstract
    A new inelastic mechanism, detwinning-induced plasticity (DIP), is proposed to model the response of NiTi SMAs to cyclic loading, based on thermodynamic considerations. DIP is incorporated into a constitutive framework for NiTi SMA. The constitutive framework also includes well-established inelastic mechanisms of phase transformation, transformation-induced plasticity, residual martensite, and detwinning. The model is constructed at the single crystal scale using the framework of thermodynamics and a crystal plasticity formulation. An explicit scale-transition rule is adopted for the simulation of polycrystalline materials, allowing direct comparison of the model predictions with published... 

    Effect of nano Al2O3 addition on mechanical properties and wear behavior of NiTi intermetallic

    , Article Materials and Design ; Volume 51 , October , 2013 , Pages 375-382 ; 02613069 (ISSN) Farvizi, M ; Ebadzadeh, T ; Vaezi, M. R ; Kim, H. S ; Simchi, A ; Sharif University of Technology
    Elsevier Ltd  2013
    Abstract
    It has been found that the high wear resistance of NiTi alloy is mainly attributed to its pseudoelasticity which is only effective within a small temperature range. It is believed that pseudoelasticity becomes ineffective by applying high-load wear condition which yields plastic deformation and temperature increment during wear test. Therefore, the enhanced wear resistance can be obtained from the improvement of mechanical property of the alloy without much reduction of pseudoelasticity. In this study, a low weight percentage of hard Al2O3 nanoparticles were added to NiTi atomized powders. The resultant powder mixture was homogenized by ball milling and sintered in a vacuum furnace in order... 

    Mechanism of reaction of molten NiTi with EBM graphite crucible

    , Article Materials Science and Technology ; Volume 25, Issue 6 , 2009 , Pages 699-706 ; 02670836 (ISSN) Sadrnezhaad, Kh ; Ahmadi, E ; Malekzadeh, M ; Sharif University of Technology
    2009
    Abstract
    Ultra clean NiTi shape memory alloy was produced by electron beam melting of Ni rich vacuum inductionally melted butts together with pure Ti chunks in both condensed and electrographite crucibles. A hollow cathode discharge gun was used for heating up to 1623, 1653 and 1693 K and holding the charge materials under vacuum for 300, 600, 900 and 1200 s. Effects of temperature, time and compactness of the crucible on formation/disappearance of the hard compounds like Ni3Ti, Ti4Ni2O, Ti4Ni 2C, Ti3Ni2OC and TiC were determined by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray analysis. A combination of the experimental results with the kinetic rate equations indicated...