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1
Penrod, Luke Edward. "Fabrication and characterization of porous shape memory alloys." Texas A&M University, 2003. http://hdl.handle.net/1969.1/145.
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This work details an investigation into the production of porous shape memory alloys (SMAs) via hot isostatic press (HIP) from prealloyed powders. HIPing is one of three main methods for producing porous SMAs, the other two are conventional sintering and selfpropagating hightemperature synthesis (SHS). Conventional sintering is characterized by its long processing time at near atmospheric pressure and samples made this way are limited in porosity range. The SHS method consists of preloading a chamber with elemental powders and then initiating an explosion at one end, which then propagates through the material in a very short time. HIPing provides a compromise between the two methods, requiring approximately 5 hours per cycle while operating in a very controlled environment. The HIPing method gives fine control of both temperature and pressure during the run which allows for the production of samples with varying porosity as well as for finetuning of the process for other characteristics. By starting with prealloyed powder, this study seeks to avoid the drawbacks while retaining the benefits of HIPing with elemental powders.
In an extension of previous work with elemental powders, this study will apply the HIP method to a compact of prealloyed powders. It is hoped that the use of these powders will limit the formation of alternate phases as well as reducing oxidation formed during preparation. In addition, the nearspherical shape of the powders will encourage an even pore distribution. Processing techniques will be presented as well as a detailed investigation of the thermal and mechanical properties of the resulting material.
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Aydogmus, Tarik. "Processing And Characterization Of Porous Titanium Nickel Shape Memory Alloys." Phd thesis, METU, 2010. http://etd.lib.metu.edu.tr/upload/12612232/index.pdf.
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Porous TiNi alloys (Ti-50.4 at. %Ni and Ti-50.6 at. %Ni) with porosities in the range 21%-81% were prepared successfully applying a new powder metallurgy fabrication route in which magnesium was used as space holder resulting in either single austenite phase or a mixture of austenite and martensite phases dictated by the composition of the starting prealloyed powders but entirely free from secondary brittle intermetallics, oxides, nitrides and carbonitrides. Magnesium vapor do not only prevents secondary phase formation and contamination but also provides higher temperature sintering opportunity preventing liquid phase formation at the eutectic temperature, 1118 °C resulting from Ni enrichment due to oxidation. By two step sintering processing (holding the sample at 1100 °
C for 30 minutes and subsequently sintering at temperatures higher than the eutectic temperature, 1118 °
C) magnesium may allow sintering probably up to the melting point of TiNi. The processed alloys exhibited interconnected (partially or completely depending on porosity content) open macro-pores spherical in shape and irregular micro-pores in the cell walls resulting from incomplete sintering. It has been found that porosity content of the foams have no influence on the phase transformation temperatures while deformation and oxidation are severely influential. Porous TiNi alloys displayed excellent superelasticity and shape memory behavior. Space holder technique seems to be a promising method for production of porous TiNi alloys. Desired porosity level, pore shape and accordingly mechanical properties were found to be easily adjustable.
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Chan, Wing Nin. "Comparison of the wearing of porous and dense NiTi shape memory alloy." access abstract and table of contents access full-text, 2006. http://libweb.cityu.edu.hk/cgi-bin/ezdb/dissert.pl?msc-ap-b21458406a.pdf.
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Thesis (M.Sc.)--City University of Hong Kong, 2006."Master of Science in Materials Engineering & Nanotechnology dissertation." Title from title screen (viewed on Nov. 23, 2006) Includes bibliographical references.
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Zhao, Ying. "Design of energy absorbing materials and composite structures based on porous shape memory alloys (SE) /." Thesis, Connect to this title online; UW restricted, 2006. http://hdl.handle.net/1773/7148.
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Popov, Petar Angelov. "Constitutive modelling of shape memory alloys and upscaling of deformable porous media." Texas A&M University, 2003. http://hdl.handle.net/1969.1/2273.
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Shape Memory Alloys (SMAs) are metal alloys which are capable of changing
their crystallographic structure as a result of externally applied mechanical or thermal
loading. This work is a systematic effort to develop a robust, thermodynamics based,
3-D constitutive model for SMAs with special features, dictated by new experimental
observations. The new rate independent model accounts in a unified manner for the
stress/thermally induced austenite to oriented martensite phase transformation, the
thermally induced austenite to self-accommodated martensite phase transformation
as well as the reorientation of self-accommodated martensite under applied stress. The
model is implemented numerically in 3-D with the help of return-mapping algorithms.
Numerical examples, demonstrating the capabilities of the model are also presented.
Further, the stationary Fluid-Structure Interaction (FSI) problem is formulated
in terms of incompressible Newtonian fluid and a deformable solid. A numerical
method is presented for its solution and a numerical implementation is developed.
It is used to verify an existing asymptotic solution to the FSI problem in a simple
channel geometry. The SMA model is also used in conjunction with the fluid-structure
solver to simulate the behavior of SMA based filtering and flow regulating devices.
The work also includes a numerical study of wave propagation in SMA rods.
An SMA body subjected to external dynamic loading will experience large inelastic
deformations that will propagate through the body as phase transformation and/or
detwinning shock waves. The wave propagation problem in a cylindrical SMA is
studied numerically by an adaptive Finite Element Method. The energy dissipation
capabilities of SMA rods are estimated based on the numerical simulations. Comparisons
with experimental data are also performed.
6
Kwan, Wai Ming. "Wear resistance of porous titanium-nickel shape memory alloy fabricated by reactive sintering with HIPping." access abstract and table of contents access full-text, 2005. http://libweb.cityu.edu.hk/cgi-bin/ezdb/dissert.pl?msc-ap-b21174155a.pdf.
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Thesis (M.Sc.)--City University of Hong Kong, 2005.At head of title: City University of Hong Kong, Department of Physics and Materials Science, Master of Science in materials engineering & nanotechnology dissertation. Title from title screen (viewed on Aug. 31, 2006) Includes bibliographical references.
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Chan, Benny See Tsun. "Corrosion behavior of porous NiTi shape memory alloy prepared by capsule free hot isolated pressing processing." access abstract and table of contents access full-text, 2005. http://libweb.cityu.edu.hk/cgi-bin/ezdb/dissert.pl?msc-ap-b21174003a.pdf.
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Thesis (M.Sc.)--City University of Hong Kong, 2005.At head of title: City University of Hong Kong, Department of Physics and Materials Science, Master of Science in materials engineering & nanotechnology dissertation. Title from title screen (viewed on Aug. 31, 2006) Includes bibliographical references.
8
Saedi, Soheil. "Shape Memory Behavior of Dense and Porous NiTi Alloys Fabricated by Selective Laser Melting." UKnowledge, 2017. http://uknowledge.uky.edu/me_etds/90.
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Selective Laser Melting (SLM) of Additive Manufacturing is an attractive fabrication method that employs CAD data to selectively melt the metal powder layer by layer via a laser beam and produce a 3D part. This method not only opens a new window in overcoming traditional NiTi fabrication problems but also for producing porous or complex shaped structures. The combination of SLM fabrication advantages with the unique properties of NiTi alloys, such as shape memory effect, superelasticity, high ductility, work output, corrosion, biocompatibility, etc. makes SLM NiTi alloys extremely promising for numerous applications.
The SLM process parameters such as laser power, scanning speed, spacing, and strategy used during the fabrication are determinant factors in composition, microstructural features and functional properties of the SLM NiTi alloy. Therefore, a comprehensive and systematic study has been conducted over Ni50.8 Ti49.2 (at%) alloy to understand the influence of each parameter individually. It was found that a sharp [001] texture is formed as a result of SLM fabrication which leads to improvements in the superelastic response of the alloy. It was perceived that transformation temperatures, microstructure, hardness, the intensity of formed texture and the correlated thermo-mechanical response are changed substantially with alteration of each parameter. The provided knowledge will allow choosing optimized parameters for tailoring the functional features of SLM fabricated NiTi alloys. Without going through any heat treatments, 5.77% superelasticity with more than 95% recovery ratio was obtained in as-fabricated condition only with the selection of right process parameters.
Additionally, thermal treatments can be utilized to form precipitates in Ni-rich SLM NiTi alloys fabricated by low energy density. Precipitation could significantly alter the matrix composition, transformation temperatures and strain, critical stress for transformation, and shape memory response of the alloy. Therefore, a systematic aging study has been performed to reveal the effects of aging time and temperature. It was found that although SLM fabricated samples show lower strength than the initial ingot, heat treatments can be employed to make significant improvements in shape memory response of SLM NiTi. Up to 5.5% superelastic response and perfect shape memory effect at stress levels up to 500 MPa was observed in solutionized Ni-rich SLM NiTi after 18h aging at 350ºC. For practical application, transformation temperatures were even adjusted without solution annealing and superelastic response of 5.5% was achieved at room temperature for 600C-1.5hr aged Ni-rich SLM NiTi.
The effect of porosity on strength and cyclic response of porous SLM Ni50.1 Ti49.9 (at%) were investigated for potential bone implant applications. It is shown that mechanical properties of samples such as elastic modulus, yield strength, and ductility of samples are highly porosity level and pore structure dependent. It is shown that it is feasible to decrease Young’s modulus of the SLM NiTi up to 86% by adding porosity to reduce the mismatch with that of a bone and still retain the shape memory response of SLM fabricated NiTi. The shape memory effect, as well as superelastic response of porous SLM Ni50.8Ti49.2,were also investigated at body temperature. 32 and 45% porous samples with similar behaviors, recovered 3.5% of 4% deformation at first cycle. The stabilized superelastic response was obtained after clicking experiments.
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Caputo, Matthew P. "4-Dimensional Printing and Characterization of Net-Shaped Porous Parts Made from Magnetic Ni-Mn-Ga Shape Memory Alloy Powders." Youngstown State University / OhioLINK, 2018. http://rave.ohiolink.edu/etdc/view?acc_num=ysu1525436335401265.
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Myers, Eric J. "Finite Element Modeling (FEM) of Porous Additively Manufactured Ferromagnetic Shape Memory Alloy Using Scanning Electron Micrograph (SEM) Based Geometries." Youngstown State University / OhioLINK, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=ysu149399154152881.
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Taheri, Andani Mohsen. "Modeling, Simulation, Additive Manufacturing, and Experimental Evaluation of Solid and Porous NiTi." University of Toledo / OhioLINK, 2015. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1438888243.
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Underhill, Daniel Martin Lennard. "Ferromagnetic shape memory alloys." Thesis, University of Cambridge, 2013. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.607746.
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Kelly, Brian L. "Beam shape control using shape memory alloys." Thesis, Monterey, Calif. : Springfield, Va. : Naval Postgraduate School ; Available from National Technical Information Service, 1998. http://handle.dtic.mil/100.2/ADA358806.
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Thesis (M.S. in Astronautical Engineering) Naval Postgraduate School, December 1998."December 1998." Thesis advisor(s): Brij N. Agrawal, Gangbing Song. Includes bibliographical references (p. 55). Also available online.
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Dai, Liyang. "Elasticity in ferromagnetic shape memory alloys." College Park, Md. : University of Maryland, 2004. http://hdl.handle.net/1903/2047.
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Thesis (Ph.D.) -- University of Maryland, College Park, 2004.Thesis research directed by: Material Science and Engineering. Title from t.p. of PDF. Includes bibliographical references. Published by UMI Dissertation Services, Ann Arbor, Mich. Also available in paper.
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Seaton, Alexander B. "Thermomechanical deformation of shape memory alloys." Thesis, Loughborough University, 2006. https://dspace.lboro.ac.uk/2134/20317.
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NiTi is a shape memory alloy and can undergo crystallographically reversible martensitic transformation under applied loads resulting in recoverable of strains of the order of 5 %. The single crystal properties of shape memory alloys have been studied extensively in the past and a good understanding of the mechanical properties of the material in this form has been acquired. However, when used in practical applications shape memory alloys are used in their polycrystalline form. In a polycrystalline form the deformation behaviour may be quite different to that of a single crystal due to the constraints of surrounding grains and anisotropy of material properties. In the case of shape memory alloys these are anisotropic elastic and transformation properties. The main focus of the work in this thesis is the deformation behaviour of commercial rod samples of NiTi while under thermomechanical loads. The grain-orientation-specific internal strain development and phase faction evolution within particular grain orientations is evaluated during deformation by the in-situ neutron diffraction technique. The experimental results presented include stress-induced martensitic transformation, cooling through the martensitic transformation under a fixed stress, the generation of recovery stresses while heated under constraint, and studies of the detwinning of the B 19' martensite phase under compressive and tensile loading. In addition, the effect of ageing on mechanical properties of NiTi is investigated via the method. Changes in the load partitioning behaviour is noted for NiTi cooled under a fixed tensile stress of 200 MPa which compare well with modelling predictions in the literature. Large changes in the mechanical properties of NiTi as a results of ageing are ascribed to the presence of the R-phase due to the formation of precipitates during ageing. Evidence of detwinning of B 19' martensite in both tension and compression is found, in contrast to other work in the literature.
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Vieira, Luís Manuel Alberty. "Laser welding of shape memory alloys." Master's thesis, Faculdade de Ciências e Tecnologia, 2010. http://hdl.handle.net/10362/4760.
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Dissertação apresentada na Faculdade de Ciências e Tecnologia da Universidade Nova de Lisboa para obtenção do grau de Mestre em Engenharia MecânicaA necessidade de desenvolver técnicas avançadas de união para ligas com memória de forma tem-se revelado um assunto da maior importância, uma vez que as suas propriedades funcionais,nomeadamente o efeito de memória de forma e a superelasticidade, se revestem de enorme valor para aplicações actuais ou emergentes. De entre as ligas com memória de forma, o NiTi é a mais aplicada em campos tecnológicos tão diversos como a indústria biomédica, aerospacial e automóvel,o que se deve às suas características, como sejam: as elevadas biocompatibilidade e resistência à corrosão. Por estas razões, tem sido investigadas técnicas de ligação para estas ligas. No entanto, a sua ligação a outros materiais constitui um desafio cada vez maior permitindo explorar novos domínios de aplicação. O principal objectivo deste estudo é compreender o efeito da soldadura laser em aspectos estruturais, mecânicos e funcionais, tanto em ligações similares envolvendo NiTi, como dissimilares. Foram produzidas juntas similares topo a topo utilizando um laser de Nd:YAG em modo contínuo e estudados os efeitos da direcção de laminagem na configuração de junta e dos parâmetros do processo nas caraterísticas das juntas. A soldadura dissimilar de NiTi com Ti-6Al-4V foi realizada com um laser de fibras operando em modo contínuo. Adicionalmente, soldaram-se arames de NiTi com aço inoxidável austenítico utilizando uma fonte laser de Nd:YAG operando em modo pulsado. Foram projectados e produzidos sistemas de fixação e de protecção gasosa específicos para estas aplicações. Foram desenvolvidos e/ou adaptados métodos de ensaio para a avaliação da macro e microestructura, do comportamento mecânico cíclico e da capacidade de memória de forma. Utilizaram-se técnicas de análise como a Calorimetria Diferencial de Varrimento (DSC), a Microscopia Electrónica de Varrimento (SEM), EDS para identificação de espécies químicas e microdureza para avaliar as juntas soldadas. Foram produzidas juntas soldadas sem defeitos de soldadura utilizando parâmetros de processo optimizados, as quais apresentaram elevada tensão de rotura (acima de 400 MPa), patamares superelásticos até níveis de deformação próximos de 8%, comportamento cíclico superior ao material base e fractura dúctil. Foi observada baixa tensão de rotura nas juntas dissimilares sobrepostas com aço inoxidável AISI 316LN, devido à fractura prematura pela zona afectada pelo calor, no lado do NiTi. Nas juntas topo a topo de NiTi com Ti-6Al-4V a zona revela uma estrutura de solidificação rápida do tipo dendrítica na qual se propagaram fissuras com origem em defeitos de soldadura, tais como falta de penetração.
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Kockar, Benat. "Shape memory behavior of ultrafine grained NiTi and TiNiPd shape memory alloys." Thesis, [College Station, Tex. : Texas A&M University, 2007. http://hdl.handle.net/1969.1/ETD-TAMU-2543.
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Brewer, Andrew Lee. "Shape memory response of ni2mnga and nimncoin magnetic shape memory alloys under compression." [College Station, Tex. : Texas A&M University, 2007. http://hdl.handle.net/1969.1/ETD-TAMU-1341.
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Toker, Guher P. "CHARACTERIZATION OF THE SHAPE MEMORY BEHAVIOR OF HIGH STRENGTH NiTiHfPd SHAPE MEMORY ALLOYS." UKnowledge, 2018. https://uknowledge.uky.edu/me_etds/114.
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NiTiHf alloys have emerged as potential materials for applications requiring high transformation temperatures (> 100 °C) with high strength and work output. Although they have high transformation temperatures, their low damping capacity, brittleness and poor superelastic responses (of Ti-rich NiTiHf) impedes their wider usage in many industrial applications. In this study, the quaternary alloying element of Pd has been added to NiTiHf alloys to improve and tailor their shape memory behavior,. NiTiHfPd alloys were systematically examined regarding the composition and heat treatments effects.
Effects of substituting Hf with Ti on the shape memory behavior of NiTHfPd alloys were investigated. There compositions were selected as Ni40.3Ti34Hf20Pd5 Ni40.3Ti39.7Hf15Pd5 and Ni40.3Ti44.7Hf10Pd5 (at.%). Their transformation temperatures, microstructure and shape memory properties were revealed and compared with conventional shape memory alloys. It was revealed that their transformation temperatures increases but transformation strain decreases with the increment of Hf content.
Additionally, superelastic responses of Ni45.3Ti29.7Hf20Pd5 andNi45.3Ti39.7Hf10Pd5 alloys were investigated. Transformation temperatures of polycrystalline Ni45.3Ti29.7Hf20Pd5are highly dependent on aging temperatures and they can be altered widely from room temperature to 250 oC.
Finally, the damping capacity of the Ni45.3Ti39.7Hf10Pd5 polycrystal and [111]-oriented Ni45.3Ti29.7Hf20Pd5 single crystal were investigated. The damping capacities were found to be 16-25 J.cm-3, and 10-23 J.cm-3 for the Ni45.3Ti39.7Hf10Pd5 and [111]-oriented Ni45.3Ti29.7Hf20Pd5 alloys, respectively.
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Liang, Chen. "The constitutive modeling of shape memory alloys." Diss., This resource online, 1990. http://scholar.lib.vt.edu/theses/available/etd-08232007-113153/.
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Page, David Gordon. "Electrodeposition of thin film shape memory alloys." Thesis, University of Newcastle Upon Tyne, 2001. http://hdl.handle.net/10443/470.
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There is considerable potential for the use of thin film shape memory alloys in the field of microtechnology due to their high power to volume ratio. The main obstacles for fabrication arise mainly due to the narrow regime over which shape memory behaviour is observed and the paucity of process techniques. Shape memory transition in brass only occurs in the alloy composition range 38.5 - 41.6 wt %% zinc. This study used a pyrophosphate electrolyte containing Cu2P2O7, Zn2P2O7 salts and an excess of K4P207 and KNO3, for brass deposition as a replacement for cyanide electrolytes because it is non-toxic and noncorrosive. A rotating disc electrode was employed to systematically examine polarisation data and a rotating cylinder electrode was employed to produce thin brass films and deduce the current efficiencies of copper, zinc and brass deposition with respect to deposition potential. Thin films were plated between 5- 301im, they all displayed a smooth, uniform homogenous deposit with no precipitates or oxide inclusions. The current efficiencies were found to be < 45% for copper, < 15% for zinc and between 10 - 30% for brass. The microstructural characterisation of the Cu-Zn thin alloys was undertaken by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray fluorescence (EDAX), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC). XRD showed all the electrodeposited Cu-Zn alloys to have same phase composition as those predicted by the equilibrium phase diagram for Cu-Zn. This confirmed the existence of the parent p-phase within the shape memory composition range, which undergoes the martensitic transformation. TEM showed these foils to be composed of a matrix of a, p and martensite nano sized grains (< 40nm) co-existing with a sparse distribution of larger grains (200-300nm). The larger grains were always martensite in nature, recognisable by their twinning planes. Differential scanning calorimetry analysis shows evidence of a martensitic transformation change for the thin brass films.
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Kelly, Alex Bhattacharya Kaushik Murray RIchard M. "A constitutive relation for shape-memory alloys /." Diss., Pasadena, Calif. : California Institute of Technology, 2009. http://resolver.caltech.edu/CaltechETD:etd-09292008-204618.
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Hartwell, Ashley (Ashley Jessica). "Shape Memory Alloys for small scale actuation." Thesis, Massachusetts Institute of Technology, 2018. http://hdl.handle.net/1721.1/118714.
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Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2018.Cataloged from PDF version of thesis.
Includes bibliographical references (pages 51-52).
Shape Memory Alloys (SMAs), materials that can undergo a fully recoverable strain change due to a thermal cycle, and which can be produced in a form that is superelastic are only utilized limitedly. In this thesis, I investigated the relationship between the material properties of shape memory alloy micro-wires and their mechanical performance. This study was conducted with two main types of SMAs, the first a commercially available NiTi wire, and the second an emerging Cu-based alternative. This comparison allows an understanding of the current state of the art for small scale actuation with SMA wires, and to evaluate the Cu-based alternative SMA, which has a reduced cost and improved thermal properties. This thesis evaluates Cu-based SMAs as substitutes for NiTi in terms of activation strain of wire during a shape memory cycle, power consumption during actuation, heating and cooling times during actuation, and cost. Furthermore this thesis includes studies on the processing of Cu-based alternatives to enhance shape memory properties of interests, such as transformation temperature and fatigue, and suggests future work to improve Cu-based SMA wires..
by Ashley Hartwell.
S.M.
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Guo, Weimin M. Eng Massachusetts Institute of Technology. "Orthopaedic applications of ferromagnetic shape memory alloys." Thesis, Massachusetts Institute of Technology, 2008. http://hdl.handle.net/1721.1/45957.
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Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2008.Includes bibliographical references (leaves 36-40).
Ferromagnetic shape memory alloys (FSMAs) are a new class of magnetic field-actuated active materials with no current commercial applications. By applying a magnetic field of around 0.4 T, they can exert a stress of approximately 1.5 MPa, exhibiting a strain of up to 6%. This thesis evaluates their technical and commercial feasibility in orthopaedic applications. Remote actuation is a key advantage FSMAs have over current implant materials. Also, the human body temperature is constant, providing a stable environment for FSMAs to operate. A number of potential orthopaedic applications are proposed and evaluated. Out of these, the most prominent application is the spinal traction device. It is a temporary implantable device, intended to perform internal spinal traction. A design has been proposed, with suggestions of suitable materials for its various components and appropriate device dimensions. Preliminary market and cost analyses have been conducted. This orthopaedic technology is currently in its infant stage. To commercialize this device, more trials are needed.
by Weimin Guo.
M.Eng.
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Zhang, Yahui. "Low cycle fatigue of shape memory alloys." Thesis, Université Paris-Saclay (ComUE), 2018. http://www.theses.fr/2018SACLY004/document.
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Dans cette thèse, nous proposons une analyse globale multi-échelles de la fatigue à faible nombre de cycles des matériaux à mémoire de forme (MMF). Dans un premier temps, une large campagne d’essais a été menée pour différents chargements thermomécaniques comprenant des tests de fatigue sous contrainte et déformation imposée et pour différentes fréquences de chargement. A partir des résultats des essais, un critère de fatigue, basé sur l’énergie de déformation, a été développé ; on montre que l’énergie de déformation est un paramètre pertinent pour prédire la fatigue des MMF en tenant compte du couplage thermomécanique et du type de chargement : contrainte ou déformation imposée. Ensuite, en prenant appui sur la répartition de l’énergie de l’hystérésis en dissipation et énergie stockée, on avance une interprétation physique du mécanisme de la fatigue des MMF. Dans la troisième partie, on propose une modélisation multi-échelles de l’initiation des fissures de fatigue dans les MMF à partir de la notion de plasticité de transformation (PlTr). Dans ce cadre, on montre que la fatigue de MMF est contrôlée par la (PlTr) et que la température maximale lors de la transformation de phase est le paramètre à retenir pour prédire la rupture par fatigue des MMF. Le modèle permet également de prédire le lieu d’initiation des premières fissures de fatigue. Enfin, un procédé – fondé sur l’«éducation» des MMF – permettant d’améliorer la résistance à la fatigue est proposéThe thesis proposes a multi-scale comprehensive analysis of low cycle fatigue of shape memory alloys (SMAs). First, low cycle fatigue of SMAs is experimentally investigated; comprehensive tensile-tensile fatigue tests under both stress and strain controlled loadings at different frequencies are carried out and results are discussed. Second, a new strain energy-based fatigue criterion is developed; it is shown that the use of total strain energy is a relevant parameter to predict fatigue lifetime of SMAs for different thermomechanical conditions and under different types (strain-control or stress-control) loadings. A physical interpretation of the mechanism related to the low-cycle fatigue of SMAs is then provided based on the conversion of hysteresis work into dissipation and stored energy. Third, fatigue crack initiation during cyclic stress-induced phase transformation is modeled based on transformation induced plasticity (TRIP); it is shown that the maximum temperature during the cyclic loading is a relevant indicator of the fatigue of SMA. Furthermore, the effect of the macroscopic mechanical load on the the fatigue lifetime is addressed as well as the spatial location of crack initiation. Finally, a mechanical training process that allows enhancing resistance to low cycle fatigue of SMAs is proposed
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Young, Avery W. "A Study on NiTiSn Low-Temperature Shape Memory Alloys and the Processing of NiTiHf High-Temperature Shape Memory Alloys." Thesis, University of North Texas, 2018. https://digital.library.unt.edu/ark:/67531/metadc1157642/.
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Shape memory alloys (SMAs) operating as solid-state actuators pose economic and environmental benefits to the aerospace industry due to their lightweight, compact design, which provides potential for reducing fuel emissions and overall operating cost in aeronautical equipment. Despite wide applicability, the implementation of SMA technology into aerospace-related actuator applications is hindered by harsh environmental conditions, which necessitate extremely high or low transformation temperatures. The versatility of the NiTi-based SMA system shows potential for meeting these demanding material constraints, since transformation temperatures in NiTi can be significantly raised or lowered with ternary alloying elements and/or Ni:Ti ratio adjustments. In this thesis, the expansive transformation capabilities of the NiTi-based SMA system are demonstrated with a low and high-temperature NiTi-based SMA; each encompassing different stages of the SMA development process. First, exploratory work on the NiTiSn SMA system is presented. The viability of NiTiSn alloys as low-temperature SMAs (LTSMAs) was investigated over the course of five alloy heats. The site preference of Sn in near-equiatomic NiTi was examined along with the effects of solution annealing, Ni:Ti ratio adjustments, and precipitation strengthening on the thermomechanical properties of NiTiSn LTSMAs. Second, the thermomechanical processability of NiTiHf high-temperature SMA (HTSMA) wires is presented. The evolution of various microstructural features (grain size reduction, oxide growth, and nano-precipitation) were observed at incremental stages of the hot rolling process and linked to the thermal and mechanical responses of respective HTSMA rods/wires. This work was carried out in an effort to optimize the rolling/drawing process for NiTiHf HTSMAs.
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Friend, C. M. "Factors affecting reversible shape-memory." Thesis, University of Surrey, 1985. http://epubs.surrey.ac.uk/847449/.
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In the last twenty years Reversible Shape-Memory (RSM) alloys have become the source of considerable technological interest as a result of their ability to generate spontaneous and reversible changes of shape on thermal cycling. This has led to the development of a range of reversible shape-memory devices for thermostatic sensing applications. In these devices the alloy is subjected to several thousand shape-memory cycles and the stability of the reversible shape-memory is therefore an important alloy property. Data on the effect of shape-memory cycling on the long-term stability of the reversible shape-memory, however, is extremely limited. The present work, conducted to fill this gap, has shown that there is an inherent instability in the reversible shape-memory, with changes in the operating temperatures and cumulative reductions in the maximum shape-strain output of actuators on long-term thermal cycling under conditions simulating real devices. Extensive investigation has shown that these instabilities result from a number of sources, ageing of the shape-memory martensites and most importantly from morphological disruptions in the "trained" martensites caused by two-stage stress-induced transformation and due to the build-up of transformation-induced dislocation debris. This shape-strain degradation has also been successfully modelled by means of a simple two-stage stress-induced martensitic transformation model.
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Fung, Cheung Kwan. "Thermal mechanical behaviour of NiTi shape memory alloy." access abstract and table of contents access full-text, 2004. http://libweb.cityu.edu.hk/cgi-bin/ezdb/dissert.pl?msc-ap-b21174076a.pdf.
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Thesis (M.Sc.)--City University of Hong Kong, 2004.At head of title: City University of Hong Kong, Department of Physics and Materials Science, Master of Science in materials engineering & nanotechnology dissertation. Title from title screen (viewed on Aug. 31, 2006) Includes bibliographical references.
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Penar, Bradley W. "Recentering Beam-Column Connections Using Shape Memory Alloys." Thesis, Georgia Institute of Technology, 2005. http://hdl.handle.net/1853/7185.
Full textAbstract:
Shape memory alloys are a class of alloys that display the unique ability to
undergo large plastic deformations and return to their original shape either
through the application of heat (shape memory effect) or by relieving the
stress causing the deformation (superelastic effect). This research takes
advantage of the unique characteristics of shape memory alloys in order to
provide a moment resisting connection with recentering capabilities.
In this study, superelastic Nitinol, a nickel-titanium form of shape memory
alloy that exhibits a flag-shaped stress versus strain curve, is used as the
moment transfer elements within a partially restrained steel beam-column
connection. Experimental testing consists of a one-half scale interior
connection where the loading is applied at the column tip. A pseudo-static
cyclic loading history is used which is intended to simulate earthquake
loadings. The energy dissipation characteristics, moment-rotation
characteristics, and deformation capacity of the connection are quantified.
Results are then compared to tests where A36 steel tendons are used as the
moment transfer elements. The superelastic Nitinol tendon connection showed
superior performance to the A36 steel tendon connection, including the ability
to recenter without residual deformation.
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Tarhan, Elif. "Ageing Characteristics Of Copper Based Shape Memory Alloys." Phd thesis, METU, 2004. http://etd.lib.metu.edu.tr/upload/3/593541/index.pdf.
Full textAbstract:
Martensite-to-Beta transformation temperatures of CuAlNiMn and CuAlNi shape memory alloys has been determined by differential scanning calorimetry (DSC). In CuAlNiMn alloys, each new betatizing treatment has resulted in randomly varying transformation temperatures on the same specimen and an anomalously diffuse and serrated Martensite-to-Beta transformation peaks in the first cycle. Therefore, as quenched alloy samples were thermally cycled for three times in DSC prior to ageing to obtain thermally stable and reproducible transformation temperatures and to eliminate the anomalous effect of betatizing on the transformation temperatures.
CuAlNiMn alloys were aged in martensitic condition at temperatures in the range 80
C to 150&
#61616
C for 24 hours to 312 hours ageing periods. Both A_s and A_f temperatures have increased with ageing temperature and time while M_s and M_f temperatures have not changed during martensite ageing. Transformation temperatures of CuAlNi alloys, on the other hand, have not changed during martensite ageing. In this respect, CuAlNiMn alloys were found to be more prone to martensite stabilization than the CuAlNi alloys. Through Transmission Electron Microscope investigation in the Cu-12.6wt%Al-5.9wt%Ni-1.8wt%Mn alloy aged at 150&
#61616
C for 312 hours has revealed no sign of precipitate formation and it has been concluded that the &
#65533
precipitates pinning martensite boundaries&
#65533
mechanism could not be responsible of martensite stabilization. Beta phase ageing of CuAlNiMn alloys at temperatures 200&
#61616
C, 230&
#61616
C, 250&
#61616
C and 270&
#61616
C, have drastically shortened the periods for stabilization to the extent that &
#946
-to-M transformation completely ceases. With regard to the Manganese content, highest Manganese bearing alloy was the one stabilized first and the lowest manganese containing one was the longest lasting alloy during beta phase ageing. Beta stabilization was not observed in any of the four CuAlNi alloys at the end of 96 hours ageing at 200&
#61616
C while beta stabilization was realized after 26, 38 and 11 hours ageing at the same temperature in the three Mn containing alloys studied. In conclusion, on the basis of ageing studies at 200&
#61616
C, with regard to beta stabilization, CuAlNi alloys were found to be more resistant to high temperature ageing than CuAlNiMn alloys. Equilibrium &
#947
_2 and &
#945
phases were observed with coupled-grown lamellar morphologies in Cu-13.6%Al-3.0%Ni alloy aged above 400&
#61616
C.
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Mirzaeifar, Reza. "A multiscale study of NiTi shape memory alloys." Diss., Georgia Institute of Technology, 2013. http://hdl.handle.net/1853/49071.
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Shape memory alloys (SMAs) are widely used in a broad variety of applications in multiscale devices ranging from nano-actuators used in nano-electrical-mechanical systems (NEMS) to large energy absorbing elements in civil engineering applications. This research introduces a multiscale analysis for SMAs, particularly Nickel-Titanium alloys (NiTi). SMAs are studied in a variety of length scales ranging from macroscale to nanoscale. In macroscale, a phenomenological constitutive framework is adopted and developed by adding the effect of phase transformation latent heat. Analytical closed-form solutions are obtained for modeling the coupled thermomechanical behavior of various large polycrystalline SMA devices subjected to different loadings, including uniaxial loads, torsion, and bending. Thermomechanical responses of several SMA devices are analyzed using the introduced solutions and the results are validated by performing various experiments on some large SMA elements. In order to study some important properties of polycrystalline SMAs that the macroscopic phenomenological frameworks cannot capture, including the texture and intergranular effects in polycrystalline SMAs, a micromechanical framework with a realistic modeling of the grains based on Voronoi tessellations is used. The local form of the first law of thermodynamics is used and the energy balance relations for the polycrystalline SMAs are obtained. Generalized coupled thermomechanical governing equations considering the phase transformation latent heat are derived for polycrystalline SMAs. A three-dimensional finite element framework is used and different polycrystalline samples are modeled. By considering appropriate distributions of crystallographic orientations in the grains obtained from experimental texture measurements of NiTi samples the effects of texture and the tension-compression asymmetry on the thermomechanical response of polycrystalline SMAs are studied. The interaction between the stress state (tensile or compressive), number of grains, and the texture on the thermomechanical response of polycrystalline SMAs is also studied. For studying some aspects of the thermomechanical properties of SMAs that cannot be studied neither by the phenomenological constitutive models nor by the micromechanical models, molecular dynamics simulations are used to explore the martensitic phase transformation in NiTi alloys at the atomistic level. The martensite reorientation, austenite to martensite phase transformation, and twinning mechanisms in NiTi nanostructures are analyzed and the effect of various parameters including the temperature and size on the phase transformation at the atomistic level is studied. Results of this research provide insight into studying pseudoelasticity and shape memory response of NiTi alloys at different length scales and are useful for better understanding the solid-to-solid phase transformation at the atomistic level, and the effects of this transformation on the microstructure of polycrystal SMAs and the macroscopic response of these alloys.
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Delemont, Michael A. "Seismic retrofit of bridges using shape memory alloys." Thesis, Georgia Institute of Technology, 2001. http://hdl.handle.net/1853/32787.
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TIYYAGURA, MADHAVI. "TRANSMISSION ELECTRON MICROSCOPY STUDIES IN SHAPE MEMORY ALLOYS." Master's thesis, University of Central Florida, 2005. http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/3913.
Full textAbstract:
In NiTi, a reversible thermoelastic martensitic transformation can be induced by temperature or stress between a cubic (B2) austenite phase and a monoclinic (B19') martensite phase. Ni-rich binary compositions are cubic at room temperature (requiring stress or cooling to transform to the monoclinic phase), while Ti-rich binary compositions are monoclinic at room temperature (requiring heating to transform to the cubic phase). The stress induced transformation results in the superelastic effect, while the thermally induced transformation is associated with strain recovery that results in the shape memory effect. Ternary elemental additions such as Fe can additionally introduce an intermediate rhombohedral (R) phase between the cubic and monoclinic phase transformation. This work was initiated with the broad objective of connecting the macroscopic behavior in shape memory alloys with microstructural observations from transmission electron microscopy (TEM). Specifically, the goals were to examine (i) the effect of mechanical cycling and plastic deformation in superelastic NiTi; (ii) the effect of thermal cycling during loading in shape memory NiTi; (iii) the distribution of twins in martensitic NiTi-TiC composites; and (iv) the R-phase in NiTiFe. Both in situ and ex situ lift out focused ion beam (FIB) and electropolishing techniques were employed to fabricate shape memory alloy samples for TEM characterization. The Ni rich NiTi samples were fully austenitic in the undeformed state. The introduction of plastic deformation (8% and 14% in the samples investigated) resulted in the stabilization of martensite in the unloaded state. An interlaying morphology of the austenite and martensite was observed and the martensite needles tended to orient themselves in preferred orientations. The aforementioned observations were more noticeable in mechanically cycled samples. The observed dislocations in mechanically cycled samples appear to be shielded from the external applied stress via mismatch accommodation since they are not associated with unrecoverable strain after a load-unload cycle. On application of stress, the austenite transforms to martensite and is expected to accommodate the stress and strain mismatch through preferential transformation, variant selection, reorientation and coalescence. The stabilized martensite (i.e., martensite that exists in the unloaded state) is expected to accommodate the mismatch through variant reorientation and coalescence. On thermally cycling a martensitic NiTi sample under load through the phase transformation, significant variant coalescence, variant reorientation and preferred variant selection was observed. This was attributed to the internal stresses generated as a result of the thermal cycling. A martensitic NiTi-TiC composite was also characterized and the interface between the matrix and the inclusion was free of twins while significant twins were observed at a distance away from the matrix-inclusion interface. Incorporating a cold stage, diffraction patterns from NiTiFe samples were obtained at temperatures as low as -160ºC. Overall, this work provided insight in to deformation phenomena in shape memory materials that have implications for engineering applications (e.g., cyclic performance of actuators, engineering life of superelastic components, stiffer shape memory composites and low-hysteresis R-phase based actuators). This work was supported in part by an NSF CAREER award (DMR 0239512).M.S.M.E.
Department of Mechanical, Materials and Aerospace Engineering;
Engineering and Computer Science
Materials Science and Engineering
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PAIVA, ALBERTO. "MODELING OF THERMOMECHANICAL BEHAVIOR OF SHAPE MEMORY ALLOYS." PONTIFÍCIA UNIVERSIDADE CATÓLICA DO RIO DE JANEIRO, 2004. http://www.maxwell.vrac.puc-rio.br/Busca_etds.php?strSecao=resultado&nrSeq=4942@1.
Full textAbstract:
CONSELHO NACIONAL DE DESENVOLVIMENTO CIENTÍFICO E TECNOLÓGICOO estudo de materiais inteligentes tem instigado várias aplicações nas mais diversas áreas do conhecimento (da área médica à industria aeroespacial). Os materiais mais utilizados em estruturas inteligentes são as ligas com memória de forma, as cerâmicas piezoelétricas, os materiais magneto-estrictivos e os fluidos eletro- reológicos. Nas últimas décadas, as ligas com memória de forma vêm recebendo atenção especial, sendo utilizadas principalmente como sensores ou atuadores. Existe uma gama de fenômenos associados a estas ligas que podem ser explorados. Visando uma análise mais precisa do comportamento destes materiais, tem se tornado cada vez maior o interesse no desenvolvimento de modelos matemáticos capazes de descrevê-los de maneira adequada, permitindo explorar todo o seu potencial. O objetivo deste trabalho é propor um modelo constitutivo unidimensional que considera quatro variantes de microconstituintes (austenita, martensita induzida por temperatura, martensita induzida por tensão trativa e martensita induzida por tensão compressiva) e diferentes propriedades para cada fase. O efeito das deformações induzidas por temperatura é incluído na formulação. O modelo contempla ainda o efeito das deformações plásticas e o acoplamento entre os fenômenos de plasticidade e transformação de fase. Além disso, são introduzidas modificações na formulação que permitem o alargamento do laço de histerese da curva tensão-deformação, fornecendo resultados mais coerentes com dados experimentais. Por fim, incorpora-se a assimetria no comportamento tração-compressão. A validação do modelo é obtida comparando os resultados numéricos obtidos através do modelo com resultados experimentais encontrados na literatura para ensaios de tração a diferentes temperaturas e para a assimetria no comportamento tração- compressão.
The study of intelligent materials has instigated many applications within the various knowledge areas (from medical field to aerospace industry). The most used materials in intelligent structures are the shape memory alloys (SMA), the piezoelectric ceramics, the magnetostrictive materials and the electrorheological fluids. In the last decades, SMAs have received special attention, being mainly used as sensors or actuators. There is a number of phenomena related to these alloys that can be explored. Aiming a more precise analysis of SMA behavior, the interest on the development of mathematical models capable of describing these phenomena properly has grown, allowing to explore all their potential. The aim of this work is to propose a unidimensional constitutive model which considers four microconstituent variants (austenite, martensite induced by temperature, martensite induced by tensile loading and martensite induced by compressive loading) and different material properties for each phase. The effect of thermal strains is included in the formulation. The model considers the effect of plastic strains and the plastic-phase transformation coupling. Besides, some changes are introduced in the formulation in order to enlarge the stress-strain hysteresis loop, resulting in better agreements with experimental data. Eventually, the tensioncompression asymmetry is incorporated. The model validation is obtained through the comparison between the numerical results given by the model and experimental results found in the literature for tensile tests at different temperatures and for tension- compression asymmetry.
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Koumatos, Konstantinos. "The formation of microstructure in shape-memory alloys." Thesis, University of Oxford, 2012. http://ora.ox.ac.uk/objects/uuid:1089932b-d36e-4414-b128-6f7bcfe9cdf3.
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The application of techniques from nonlinear analysis to materials science has seen great developments in the recent years and it has really been a driving force for substantial mathematical research in the area of partial differential equations and the multi-dimensional calculus of variations. This thesis has been motivated by two recent and remarkable experimental observations of H. Seiner in shape-memory alloys which we attempt to interpret mathematically. Much of the work is original and has given rise to deep problems in the calculus of variations. Firstly, we study the formation of non-classical austenite-martensite interfaces. Ball & Carstensen (1997, 1999) theoretically investigated the possibility of the occurrence of such interfaces and studied the cubic-to-tetragonal case extensively. In this thesis, we present an analysis of non-classical austenite-martensite interfaces recently observed by Seiner et al.~in a single crystal of a CuAlNi shape-memory alloy, undergoing a cubic-to-orthorhombic transition. We show that these can be described by the general nonlinear elasticity model and we make some predictions regarding the admissible volume fractions of the martensitic variants involved, as well as the habit plane normals. Interestingly, in the above experimental observations, the interface between the austenite and the martensitic configuration is never exactly planar, but rather slightly curved, resulting from the pattern of martensite not being exactly homogeneous. However, it is not clear how one can reconstruct the inhomogeneous configuration as a stress-free microstructure and, instead, a theoretical approach is followed. In this approach, a general method is provided for the construction of a compatible curved austenite-martensite interface and, by exploiting the structure of quasiconvex hulls, the existence of curved interfaces is shown in two and three dimensions. As far as the author is aware of, this is the first construction of such a curved austenite-martensite interface. Secondly, we study the nucleation of austenite in a single crystal of a CuAlNi shape-memory alloy consisting of a single variant of stabilized 2H martensite. The nucleation process is induced by localized heating and it is observed that, regardless of where the localized heating is applied, the nucleation points are always located at one of the corners of the sample - a rectangular parallelepiped in the austenite. Using a simplified nonlinear elasticity model, we propose an explanation for the location of the nucleation points by showing that the martensite is a local minimizer of the energy with respect to localized variations in the interior, on faces and edges of the sample, but not at some corners, where a localized microstructure can lower the energy. The result for the interior, faces and edges is established by showing that the free-energy function satisfies a set of quasiconvexity conditions at the stabilized variant throughout the specimen, provided this is suitably cut. The proofs of quasiconvexity are based on a rigidity argument and are specific to the change of symmetry in the phase transformation. To the best of the author's knowledge, quasiconvexity conditions at edges and corners have not been considered before.
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Chen, Xue. "Magneto-mechanical behaviors of ferromagnetic shape memory alloys." Palaiseau, Ecole polytechnique, 2013. http://pastel.archives-ouvertes.fr/docs/00/84/86/30/PDF/Thesis_xue-Chen_ENSTA.pdf.
Full textAbstract:
Ferromagnetic Shape Memory Alloys (FSMA) are promising candidates for sensors and actuators for their high-frequency response and large reversible strain. The aim of this dissertation is the analysis of the magneto-mechanical behaviors of FSMA. In this aim, we study, both experimentally and theoretically, the martensite reorientation in FSMA. Firstly, a 2D/3D magneto-mechanical energy analysis is proposed and incorporated into phase diagrams for a graphic study of path-dependent martensite reorientation in FSMA under 3D loadings. Criteria and material requirements for obtaining reversible strain in cyclic loadings are derived. The energy analysis predicts that FSMA in 2D/3D configurations (multi-axial stresses) has much more advantages than in 1D configuration, e. G. , higher output stress and more application flexibility. Secondly, to validate the predictions of the energy analysis, 2D experiments are performed on FSMA and results reveal that the intrinsic dissipation and the transformation strain due to martensite reorientation are constant in all tested 2D stress states. Moreover, preliminary results validate that the output stress of FSMA in 2D configuration (magnetic field with biaxial stresses) is larger than in 1D configuration, and the output stress can be increased by increasing the auxiliary stress. Finally, to predict the magneto-mechanical behaviors of FSMA in general multi-axial loadings, a 3D constitutive model is developed within the framework of thermodynamics of irreversible processes. All the martensite variants are considered and the temperature effect is also taken into account. Model simulations agree well with all the existing 1D/2D experiments. The model is further incorporated into finite element analysis for studying the non-linear bending behaviors of FSMA beams. The sample-geometry effect and the material anisotropic effect are systematically investigated.
37
Dabbaghi, Hediyeh. "Oxidation Analysis of Additive Manufacturing Shape Memory Alloys." University of Toledo / OhioLINK, 2020. http://rave.ohiolink.edu/etdc/view?acc_num=toledo1596450323778946.
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Azeem, Mohammed Abdul. "Diffraction investigations of high temperature shape memory alloys." Thesis, Imperial College London, 2013. http://hdl.handle.net/10044/1/11138.
Full textAbstract:
Shape memory alloys are intermetallic materials with a unique ability to revert to a predefined physical shape by virtue of diffusionless transformations. Recent interest by aerospace and automotive industries to exploit the functionalities of these materials in future energy efficient designs has renewed scientific research in this field. However, the current understanding of transformation hysteresis is inhibited by experimental difficulties associated with viewing the transformations and therefore most of our knowledge is confined to symptomatic bulk properties such as those accessible from calorimetry and dilatometry. In the current study, in situ synchrotron X-ray diffraction (SXRD) was used to accurately document the adaptability of unit cells of participating phases during transformation in a series of high temperature shape memory alloys (HTSMAs). Selected alloys based on NiTi, ZrCu and NiMnGa systems were prepared in vacuum arc melter, homogenized and rolled to grain size adequate for SXRD experiments. The resulting diffraction patterns were Rietveld refined and the evolution of unit cell parameters of participating phases were recorded as a function of temperature. It was observed that the lattices of participating phases undergo a significant overall dilation during transformation. The lower symmetry martensite unit cell was observed to undergo unprecedented anisotropic strains, reaching as high as 1.2% in certain alloys. The high temperature higher symmetry austenite was observed to complement the changes in martensite lattice during heating and vice versa was observed during cooling. These changes were mostly observed in final stages of transformation. Surprisingly, a negative coefficient of thermal expansion was observed in the b lattice parameter of the monoclinic martensite in NiTi based alloys. The implications of such strains on the current phenomenological martensitic transformation models that takes into consideration lattice parameters away from the transformation regime for prediction of orientation between participating phases has been discussed.
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Chatziathanasiou, Dimitrios. "Cyclic multiaxial behavior modeling of Shape Memory Alloys." Thesis, Paris, ENSAM, 2016. http://www.theses.fr/2016ENAM0015/document.
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De nouvelles approches phénoménologiques sur la modélisation du comportement des AMFs sont nécessaires pour tenir en compte leur réponse complexe sous chargement multiaxial. L’effet de l’anisotropie induit une dépendance de leur comportement inélastique de la direction du chargement pour des cas superélastiques. La réorientation martensitique affecte drastiquement la réponse du matériau sous chargement non-proportionnel. La charge répétitive modifie aussi certaines propriétés du matériau. L’objectif de cette étude est de proposer un nouveau modèle constitutif thermodynamique robuste pour les AMFs, focalisé surtout sur des compositions NiTi équiatomiques pour capter la transformation martensitique anisotrope et la réorientation des variantes martensitiques. Une nouvelle approche mathématique est introduite pour permettre la prise en compte de l’anisotropie de contraintes et l’évolution des déformations inélastiques lors de la transformation directe, causée par les conditions de mise en forme de structures en AMFs. Cette méthode est évaluée en employant des courbes contraintes-déformations résultant de chargements proportionnels simulés par un modèle micromécanique. Un modèle phénoménologique considérant surtout la réorientation martensitique et mettant en évidence le fort couplage thermomécanique est développé. Il est implémenté dans une plate-forme numérique en C++, SMART+, et évalué en exécutant des simulations des expériences non-proportionnelles existantes. Des structures complexes sont également simulées en employant la Méthode des Élements Finis. La dernière partie de ce travail concerne l’étude expérimentale des effets du chargement cyclique sur l’évolution des déformations résiduelles et le seuil de transformation des alliages NiTi sous sollicitation uniaxiale et biaxialeNew phenomenological approaches in modeling the behavior of SMAs are needed to account for their complex response under multiaxial loading. The effect of anisotropy induces a dependence of their inelastic behavior to the direction of the loading for superelastic cases. Martensitic reorientation affects drastically material response under non-proportional loading. Repeated loading also alters certain material properties. The goal of this study is to propose a new robust thermodynamic constitutive model for SMAs with focus on equiatomic NiTi compositions to capture anisotropic martensitic transformation and reorientation of martensitic variants, always taking in mind the strong thermomechanical coupling. A new mathematical approach is introduced to account for the anisotropy of stresses and the evolution of inelastic strains during forward transformation caused by the forming conditions of SMA structures. This method is evaluated by utilizing stress-strain curves resulting from proportional loading simulated with a micromechanical model. A phenomenological thermodynamic model considering especially martensitic reorientation and exhibiting the strong thermomechanical coupling is developed. It is implemented on a numerical platform in C++, SMART, and evaluated by simulating existing non-proportional experiments. Complex structures are also simulated using Finite Element Analysis. The last part of this work concerns the experimental study of the effects of cyclic loading to the evolution of residual strain and transformation threshold of NiTi under uniaxial and biaxial testing
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Muite, Benson K. "Analysis, modelling and simulation of shape memory alloys." Thesis, University of Oxford, 2009. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.543534.
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Saghaian, Sayed M. "SHAPE MEMORY BEHAVIOR OF SINGLE CRYSTAL AND POLYCRYSTALLINE Ni-RICH NiTiHf HIGH TEMPERATURE SHAPE MEMORY ALLOYS." UKnowledge, 2015. http://uknowledge.uky.edu/me_etds/65.
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NiTiHf shape memory alloys have been receiving considerable attention for high temperature and high strength applications since they could have transformation temperatures above 100 °C, shape memory effect under high stress (above 500 MPa) and superelasticity at high temperatures. Moreover, their shape memory properties can be tailored by microstructural engineering. However, NiTiHf alloys have some drawbacks such as low ductility and high work hardening in stress induced martensite transformation region. In order to overcome these limitations, studies have been focused on microstructural engineering by aging, alloying and processing.
Shape memory properties and microstructure of four Ni-rich NiTiHf alloys (Ni50.3Ti29.7Hf20, Ni50.7Ti29.3Hf20, Ni51.2Ti28.8Hf20, and Ni52Ti28Hf20 (at. %)) were systematically characterized in the furnace cooled condition. H-phase precipitates were formed during furnace cooling in compositions with greater than 50.3Ni and the driving force for nucleation increased with Ni content. Alloy strength increased while recoverable strain decreased with increasing Ni content due to changes in precipitate characteristics.
The effects of the heat treatments on the transformation characteristics and microstructure of the Ni-rich NiTiHf shape memory alloys have been investigated. Transformation temperatures are found to be highly annealing temperature dependent. Generation of nanosize precipitates (~20 nm in size) after three hours aging at 450 °C and 550 °C improved the strength of the material, resulting in a near perfect dimensional stability under high stress levels (> 1500 MPa) with a work output of 20–30 J cm– 3. Superelastic behavior with 4% recoverable strain was demonstrated at low and high temperatures where stress could reach to a maximum value of more than 2 GPa after three hours aging at 450 and 550 °C for alloys with Ni great than 50.3 at. %.
Shape memory properties of polycrystalline Ni50.3Ti29.7Hf20 alloys were studied via thermal cycling under stress and isothermal stress cycling experiments in tension. Recoverable strain of ~5% was observed for the as-extruded samples while it was decreased to ~4% after aging due to the formation of precipitates. The aged alloys demonstrated near perfect shape memory effect under high tensile stress level of 700 MPa and perfect superelasticity at high temperatures up to 230 °C. Finally, the tensioncompression asymmetry observed in NiTiHf where recoverable tensile strain was higher than compressive strain.
The shape memory properties of solutionized and aged Ni-rich Ni50.3Ti29.7Hf20 single crystals were investigated along the [001], [011], and [111] orientations in compression. [001]-oriented single crystals showed high dimensional stability under stress levels as high as 1500 MPa in both the solutionized and aged conditions, but with transformation strains of less than 2%. Perfect superelasticity with recoverable strain of more than 4% was observed for solutionized and 550 °C-3h aged single crystals along the [011] and [111] orientations, and general superelastic behavior was observed over a wide temperature range. The calculated transformation strains were higher than the experimentally observed strains since the calculated strains could not capture the formation of martensite plates with (001) compound twins.
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Kiefer, Bjoern. "A phenomenological constitutive model for magnetic shape memory alloys." Texas A&M University, 2006. http://hdl.handle.net/1969.1/4712.
Full textAbstract:
A thermodynamics-based constitutive model is derived which predicts the nonlinear
strain and magnetization response that magnetic shape memory alloys (MSMAs) exhibit
when subjected to mechanical and magnetic loads. The model development
is conducted on the basis of an extended thermo-magneto-mechanical framework.
A novel free energy function for MSMAs is proposed, from which the constitutive
equations are derived in a thermodynamically-consistent manner. The nonlinear and
hysteretic nature of the macroscopic material behavior is captured through the evolution
of internal state variables which are motivated by the crystallographic and
magnetic microstructures of MSMAs. Model predictions are presented for different
relevant loading cases and analyzed in detail. Finally, magnetostatic boundary
value problems for MSMAs are considered and numerically solved using the finite
element method. For these computations the developed constitutive model provides
the nonlinear magnetic properties of the MSMA. The knowledge of the magnetic field
distribution in the computational domain as a function of the applied field, which
results from this magnetostatic analysis, is useful for the proper interpretation of
experimental results as well as the design of experiments and applications.
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Wasylyszyn, Jonathan Allen. "Active control of underwater propulsor using shape memory alloys." Thesis, Texas A&M University, 2005. http://hdl.handle.net/1969.1/4926.
Full textAbstract:
The development of a leading edge propeller blade reconfiguration system using Shape Memory Allow (SMA) muscles is presented. This work describes the design and testing of a leading edge flap, which is used to alter the local camber of a propeller blade. The leading edge flap is deflected by SMA wires housed in the blade and maintained in a fixed position with a shaft locking and releasing mechanism. A locking and releasing mechanism is utilized so that constant actuation of the SMAs is not required to maintain leading edge deflection. The profile at 70% span of the propeller blade was used to create a two-dimensional blade for leading edge flap design implementation and load testing. Deflection of up to five degrees was obtained with the final design of the leading edge flap and locking and releasing mechanism. The SMA muscles used to deflect the leading edge were actuated electronically through resistive heating and were controlled by a proportional/integral gain control algorithm with closed-loop feedback from a linear displacement sensor within the blade. With the final design of the leading edge flap and locking and releasing mechanism, a preliminary design for a three-dimensional propeller was created.
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Subramaniam, Ameendraraj. "Fatigue behavior of copper zinc aluminum shape memory alloys." Thesis, National Library of Canada = Bibliothèque nationale du Canada, 1998. http://www.collectionscanada.ca/obj/s4/f2/dsk2/tape17/PQDD_0013/MQ32256.pdf.
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Rajagopalan, Sudhir. "INSTRUMENTED NANOINDENTATION STUDIES OF DEFORMATION IN SHAPE MEMORY ALLOYS." Doctoral diss., University of Central Florida, 2005. http://digital.library.ucf.edu/cdm/ref/collection/ETD/id/3283.
Full textAbstract:
Near equi-atomic nickel titanium (NiTi) shape memory alloys (SMAs) are a class of materials characterized by their unique deformation behavior. In these alloys, deformation mechanisms such as mechanical twinning and stress induced phase transformation between a high symmetry phase (austenite) and a low symmetry phase (martensite) additionally occur and influence mechanical behavior and thus their functionality. Consequently, applications of SMAs usually call for precise phase transformation temperatures, which depend on the thermomechanical history and the composition of the alloy. Instrumented indentation, inherently a mechanical characterization technique for small sampling volumes, offers a cost effective means of empirically testing SMAs in the form of centimeter scaled buttons prior to large-scale production. Additionally, it is an effective probe for intricate SMA geometries (e.g., in medical stents, valves etc.), not immediately amenable to conventional mechanical testing. The objective of this work was to study the deformation behavior of NiTi SMAs using instrumented indentation. This involved devising compliance calibration techniques to account for instrument deformation and designing spherical diamond indenters. Substantial quantitative information related to the deformation behavior of the shape memory and superelastic NiTi was obtained for the first time, as opposed to existing qualitative indentation studies. For the case of shape memory NiTi, the elastic modulus of the B19' martensite prior to twinning was determined using spherical indentation to be about 101 GPa, which was comparable to the value from neutron diffraction and was substantially higher than typical values reported from extensometry (68 GPa in this case). Twinning at low stresses was observed from neutron diffraction measurements and was attributed to reducing the elastic modulus estimated by extensometry. The onset of predominantly elastic deformation of the twinned martensite was identified from the nanoindentation response and the elastic modulus of the twinned martensite was estimated to be about 17 GPa. Finite element modeling was used to validate the measurements. For the case of the superelastic NiTi, the elastic modulus of the parent austenite was estimated to be about 62 GPa. The onset of large-scale stress induced martensite transformation and its subsequent elastic deformation were identified from the nanoindentation response. The effect of cycling on the mechanical behavior of the NiTi specimen was studied by repeatedly indenting at the same location. An increase in the elastic modulus value for the austenite and a decrease in the associated hysteresis and residual depth after the initial few cycles followed by stabilization were observed. As for the case of shape memory NiTi, finite element modeling was used to validate the measurements. This work has initiated a methodology for the quantitative evaluation of shape memory and superelastic NiTi alloys with instrumented spherical indentation. The aforementioned results have immediate implications for optimizing thermomechanical processing parameters in prototype button melts and for the mechanical characterization of intricate SMA geometries (e.g., in medical stents, valves etc.) This work was made possible by grants from NASA (NAG3-2751) and NSF (CAREER DMR-0239512) to UCF.Ph.D.
Department of Mechanical, Materials and Aerospace Engineering;
Engineering and Computer Science
Materials Science and Engineering
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Green, S. M. "The surface performance of Ni-Ti shape memory alloys." Thesis, University of Nottingham, 1995. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.294709.
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Scarsbrook, G. A. "Martensite stabilisation in Cu-Zn-Al shape memory alloys." Thesis, University of Cambridge, 1985. http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.373697.
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Kottenstette, Nicholas E. (Nicholas Eugene). "Designing mechanisms with shape memory alloys and permanent magnets." Thesis, Massachusetts Institute of Technology, 1997. http://hdl.handle.net/1721.1/43923.
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Vedantam, Srikanth 1972. "Constitutive modeling of Cu-Al-Ni shape memory alloys." Thesis, Massachusetts Institute of Technology, 2000. http://hdl.handle.net/1721.1/34342.
Full textAbstract:
Thesis (Sc.D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2000.Includes bibliographical references (leaves 103-110).
Certain alloys can exist in multiple phases, which, in the context of solids, essentially mean multiple crystallographic structures. For example, at certain compositions, a Cu-Al-Ni alloy can exist as a cubic lattice (austenite), an orthorhombic lattice ([beta]'1-martensite) or a monoclinic lattice (([beta]'1-martensite). The material changes from one phase to another under various conditions of thermal and/or mechanical loading. Under certain loads, multiple phases can coexist and when this happens, a sharp interface separates any two phases. As the stress or temperature changes, the interface propagates through the material and particles transform from one phase to the other as they cross the moving phase boundary. (A martensitic phase can exist in the form of many "variants", and an interface between co-existing variants is a twin boundary.) The constitutive modeling of such materials is made difficult by the inherent anisotropic nature of such materials and by the non monotonicity of the stress-strain curves. We develop a systematic method by which we can calculate the free-energy of such a material based on its symmetry. The velocity with which interfaces propagates controls the rate of phase transformations (i.e. the "kinetics"). It is well known that classical balance laws are insufficient for a complete description of the behavior of materials undergoing phase transformations. The classical continuum theory describes the bulk regions (regions away from the interfaces) in a satisfactory manner but leaves a gap in the information concerning the interface. This lacuna has been filled by either including nucleation and kinetic criteria that are consistent with the second law of thermodynamics, or by regularizing the continuum theory in some consistent manner. The above treatments seek to provide information on the boundaries between the phases. However, they suffer from the drawback that even though they are meant to be continuum scale descriptions of microscale phenomena which take place on the transformation front they do not model the physics of the transformations. A more natural way of obtaining the relevant information would be to directly study the transformation process at a microscale and then perform an appropriate homogenization so that the resulting law is applicable at a continuum scale. Such an approach would facilitate a deeper understanding of the transformation process as well as enable the continuum theory to reflect the micromechanical processes that govern the transformation. We develop a lattice model of twin and phase boundaries that accounts for microstructural effects. The model incorporates the effect of ledges in the interface. A quasi continuum model is obtained by approximating the resulting difference-differential equation of motion of the ledge, but retaining leading discreteness effects. The quasicontinuum model now models the interface at a continuum scale but incorporates lattice effects. The kinetic relation obtained from such a model explains the experimentally observed difference in the stress required for moving boundaries between different variants of martensite. The kinetic relation obtained for phase boundaries has the feature that the hysteresis loops do not decrease in size to zero for vanishing loading rates.
by Srikanth Vedantam.
Sc.D.
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Chen, Ta-Tung. "Electrochemical micromachining of microdevices from NiTi shape memory alloys." Thesis, Cranfield University, 1999. http://dspace.lib.cranfield.ac.uk/handle/1826/10697.
Full textAbstract:
This thesis aimed to develop a reproducible process for batch-fabricating microdevices
required for MEMS and medical applications, such as micro actuators and stents, from
heat-sensitive NiTi shape memory materials. Electrochemical micromachining was
chosen to carry out this work. This is a non-traditional machining process involving
photoresist processing and electrolytic etching which has received much attention
recently for the processing of thin films. The electrolyte used was a non-aqueous solution
of 5% sulphuric acid in methanol.
The optimum parameters for the photoresist processing were obtained by evaluation of
the thickness and exposure time of the KTFR photoresist coating. A quantitative
investigation of the electrolytic etching of NiTi was carried out to study the influence of
applied voltage, etch time and line width of the test pattern on the etching behaviour, e.g.
etch rate, undercut, depth of etch and etch factor. The anodic polarisation behaviour of
NiTi in 5% sulphuric acid in methanol was investigated under a potentiostatic control
system to establish the optimum etching parameters.
The materials used for the fabrication of micro actuators (required by Forschungszentrum
Karlsruhe, Germany to make a prototype microvalve) were NiTi alloy thin film materials
(sputtered or cold-rolled) with thicknesses ranging from 5 to 46J...lm displaying a one-way
or two-way shape m:emory effect. A variety of optimised designs of micro actuator were
successfully etched electrolytically at 8V. The etch rate was found to depend directly on
the anodic current density. The addition of a third alloying element such as Pd or eu
reduced the anodic current density and maintained a similar etch rate. However it
resulted in the breaking of the films during etching due to the reduction in the ductility of
the material.
The materials for the micro fabrication of stents were 100J...lm thick NiTi sheets. The
problem of non-uniform metal dissolution was observed. However, by adding a
sacrificial etch band as a current 'robber', periodic rotation of the anode and properly
adjusting the electrochemical and geometric parameters, the stents were etched
successfully with improved yield and dimensional accuracy.