Academic literature on the topic 'Cu-AI-NI Shape Memory Alloys'

Thesis (Sc.D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2000.<br>Includes bibliographical references (leaves 103-110).<br>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.<br>by Srikanth Vedantam.<br>Sc.D.

Les alliages Cu-Al-Ni sont les seuls à posséder une température de transformation allant jusque 200°C. Ceci leur confère un avantage par rapport aux alliages Cu-Al-Zn ou Ti-Ni dont les températures de transformation ne dépassent pas 100°C.<p><p>Néanmoins, un chauffage temporaire au dessus de 200°C peut provoquer une perte de l’effet mémoire des alliages Cu-Al-Ni.<p><p>Nous avons étudié trois alliages aves des teneurs en nickel comprises entre 3 % et 5 %.<p><p>L’objectif de notre étude est double :<p><p>• Étudier la cinétique des transformations structurales au cours d’un vieillissement en phase β dans le domaine de températures 200°C-350°C ;<p><p>• Quantifier la perte de l’effet mémoire au cours du vieillissement afin de déterminer les possibilités d’utilisation de ces alliages au dessus de 200°C.<p><p>La caractérisation structurale a été effectuée par microscope optique, diffraction des rayons X, microscopie électronique à balayage et microscopie électronique en transmission. <p><p>Les caractéristiques de la transformation martensitique ont été déterminées par analyse thermomécanique (TMA), par calorimétrie différentielle à balayage (DSC) et par des mesures de résistivité électrique. <p><p>La perte de l’effet mémoire simple sens a été quantifiée à partir des courbes de transformations obtenues par analyse thermomécanique(TMA) sur des échantillons comprimés.<p><p>Les résultats principaux sont :<p><p>&61636; Au dessus de 300°C, la précipitation de la phase d’équilibre у&8322; se produit au cours du vieillissement. Elle entraîne une augmentation de la température Mѕ.<p><p>Nous avons montré que cette augmentation de Ms peut être reliée à la fraction transformée par une loi de puissance.<p><p>&61636; Il n’y a pas de relation directe, en revanche, entre la perte de l’effet mémoire et la fraction transformée. Cela indique que le nombre et la taille des précipités ont une influence sur la perte de l’effet mémoire.<p><p>&61636; Pour un vieillissement de 256 minutes à 275°C, la perte de l’effet mémoire est inférieure à 15%. Par contre, au dessus de 300°C, la perte de l’effet mémoire est très rapide.<p><p>Nous pouvons donc considérer que 275°C est une température limite à ne pas dépasser pour ces alliages.<p><p><p><p><p><p><p>ABSTRACT<p><p>The shape memory alloys Cu-Al-Ni are the only ones to have a transformation temperature of up to 200°C. This gives them an advantage compared to shape memory alloys Cu-Zn-Al or Ti-Ni whose transformation temperatures do not exceed 100 ° C.<p><p>However, a temporary heating above 200 ° C can cause a loss of memory effect alloys Cu-Al-Ni.<p><p>We studied three alloys with nickel content between 3% and 5%.<p><p>The aim of our study is twofold:<p><p>• Studying the kinetics of structural changes during aging in β phase in the temperature range 200 °C-350 °C.<p><p>• Quantifying the loss of memory effect with aging in order to determine the potential use of these alloys above 200°C.<p><p>The structural characterization was carried out by optical microscope, XR-ray diffraction, scanning electron microscopy and transmission electron microscopy.<p><p>The characteristics of the martensitic transformation were determined by thermomechanical analysis (TMA), differential scanning calorimetry (DSC) and by measuring the electrical resistivity.<p><p>The loss of one way shape memory was quantified from the curves obtained by thermomechanical analysis (TMA) on compressed samples.<p><p>The main results are:<p><p>&61636; Above 300 ° C, the precipitation of equilibrium phase γ2 occurs during aging. It causes an increase in temperature Mѕ.<p><p>We showed that this increase of Ms may be related to the fraction transformed by a power law.<p><p>&61636; There is no direct relationship between the loss of memory effect and the fraction transformed. This indicates that the number and size of the precipitates have an influence on the loss of memory effect.<p><p>&<br>Doctorat en Sciences de l'ingénieur<br>info:eu-repo/semantics/nonPublished

Made available in DSpace on 2015-05-08T14:59:55Z (GMT). No. of bitstreams: 1 arquivototal.pdf: 8849589 bytes, checksum: 337394414de1a3c9ab9c071e9c18f59b (MD5) Previous issue date: 2014-02-04<br>Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - CAPES<br>The shape memory alloy, more specifically constituted of CuAlBe, have played an important role in the manufacture of electromechanical components, especially when worked requests from thermal and/or applied stresses. At first, this present research work, were obtained, by casting the gas, samples of alloys Cu-11.8%Al-0.60%Be; Cu-11.8%Al-0.55%Be; Cu-11.8%Al-0.55%Be-0.50%Nb-0.27%Ni (wt%). Sequentially, quenched to half of them, with the intention of doing them acquire the shape memory effect. In a third moment, said samples passed assays DSCs, aiming raise the temperature curves of phase transformation (austenite: Ai/Af and martensite: Mi/Mf). The main study, performed during machining by turning dry, using a piezoelectric dynamometer, were defined the cutting forces of samples tempered and not tempered. In compliance with the track methodology, were applied five replicates, with four levels of cutting speed for each of the samples tested. The signals from the dynamometer were processed in LabVIEW and further processed in by a program done on MatLAB. Secondary assays were also performed soon after machining by turning dry. In the first, by means of a K-type thermocouple, temperatures of machining were measured along the stride; on the second, by assay of circularity, it was verified if there was or no retention of the martensite in the samples of shape memory alloys. It was found that the percentage of beryllium and nickel present in the alloys investigated, as well as the introduction of niobium as a grain refiner were decisive for achieving the results<br>As ligas com memória de forma, mais especificamente as constituídas de CuAlBe, têm desempenhado um papel importante na fabricação de componentes eletromecânicos, sobretudo quando trabalhados a partir de solicitações térmicas e/ou tensões aplicadas. A princípio, neste presente trabalho de pesquisa, foram obtidas, por fundição a gás, amostras das ligas Cu-11,8%Al-0,60%Be; Cu-11,8%Al-0,55%Be; Cu-11,8%Al-0,55%Be-0,50%Nb-0,27%Ni (% em peso). Sequencialmente, temperou-se a metade delas, com o intuito de fazê-las adquirir o efeito memória de forma. Num terceiro momento, as referidas amostras passaram por ensaios de DSCs, visando levantar as curvas de temperatura de transformação de fases (da austenita: Ai/Af e da martensita: Mi/Mf). No ensaio principal, realizado durante a usinagem por torneamento a seco, por meio de um dinamômetro piezoelétrico, definiram-se as forças de corte das amostras temperadas e não temperadas. Dando cumprimento à trilha metodológica, foram aplicadas cinco réplicas, com quatro níveis de velocidade de corte para cada uma das amostras ensaiadas. Os sinais oriundos do dinamômetro foram processados no LabVIEW e posteriormente tratados por uma programação feita em MatLAB. Ensaios secundários também foram realizados logo após a usinagem por torneamento a seco. No primeiro, por meio de um termopar tipo K, foram medidas as temperaturas de usinagem ao longo da passada; no segundo, por ensaio de circularidade, foi verificado se ocorreu ou não a retenção da martensita nas amostras das ligas com memória de forma. Constatou-se que a percentagem de berílio e de níquel presente nas ligas investigadas, bem como a introdução do nióbio como refinador de grãos foram decisivos para o alcance dos resultados obtidos

Submitted by Kilvya Braga (kilvyabraga@hotmail.com) on 2018-04-23T13:59:14Z No. of bitstreams: 1 SUELENE SILVA ARAÚJO - DISSERTAÇÃO (PPGEM) 2015.pdf: 3709690 bytes, checksum: 1d6f1a6f3675dd7c45fb2cd75c6c92bf (MD5)<br>Made available in DSpace on 2018-04-23T13:59:14Z (GMT). No. of bitstreams: 1 SUELENE SILVA ARAÚJO - DISSERTAÇÃO (PPGEM) 2015.pdf: 3709690 bytes, checksum: 1d6f1a6f3675dd7c45fb2cd75c6c92bf (MD5) Previous issue date: 2015-08-07<br>CNPq<br>Neste trabalho foi avaliado o comportamento dinâmico e a fadiga termomecânica de fios de uma liga com memória de forma (LMF) CuAlNi submetidos a ensaios cíclicos utilizando um analisador dinâmico mecânico (DMA). Assim, foram realizadas análises para determinar a capacidade de amortecimento e o módulo de elasticidade dos fios e adicionalmente a fadiga estrutural sob controle de deformação em modo de flexão simples alternada, a partir da variação da amplitude de deslocamento, para três temperaturas distintas. O comportamento dinâmico revelou uma considerável capacidade de amortecimento dos fios, principalmente durante a transformação de fase martensitica, apresentando um expressivo pico de absorção de energia. Além disso, o módulo de elasticidade (rigidez) apresentado pelos fios revelou a existência de um importante aumento durante a transformação, diferenciando as duas distintas fases. Notou-se também a partir dos resultados dos ensaios de fadiga que as características funcionais dos fios dependem de forma direta da amplitude de deformação imposta, influenciando nos valores de tensão e no tempo de vida dos fios, revelando que a fadiga pode ser considerada de baixo ciclo.<br>This study evaluated the dynamic behavior and the thermomechanical fatigue of CuAlNi SMA wires subjected to cyclic tests using a DMA device. Therefore, analyzes were performed to determine the damping capacity and stiffness of SMA wires, in addition to structural fatigue under strain control from the amplitude variation at three different temperatures. The evolution of the wire’s behavior has shown a considerable damping capacity, mainly during the martensitic phase transformation, presenting a significant peak of energy absorption. Moreover, the storage modulus (stiffness), revealed a significant increase during transformation, differentiating the distinct phases. It has been also noted, from the results of fatigue tests, that the functional behavior of the SMA wires depend directly on the amplitude of the imposed strain, influencing on the stress values as well as on the wire’s lifetime, revealing that fatigue can be considered of low cycle type.

Submitted by Johnny Rodrigues (johnnyrodrigues@ufcg.edu.br) on 2018-04-04T20:26:12Z No. of bitstreams: 1 ALANA PEREIRA RAMOS - DISSERTAÇÃO PPG-CEMat 2014..pdf: 2592489 bytes, checksum: 5e82ab454a76e1050981da9019ce8fec (MD5)<br>Made available in DSpace on 2018-04-04T20:26:12Z (GMT). No. of bitstreams: 1 ALANA PEREIRA RAMOS - DISSERTAÇÃO PPG-CEMat 2014..pdf: 2592489 bytes, checksum: 5e82ab454a76e1050981da9019ce8fec (MD5) Previous issue date: 2017-08-07<br>As ligas ternárias de Ti-Cu-Ni com memória de forma são conhecidas por apresentarem transformação de fase característica e propriedades semelhantes às ligas binárias Ti–Ni. Estudos realizados com ligas ricas em cobre mostraram que a adição de cobre nas ligas de Ti-Ni reduz a histerese de resposta do efeito de memória de forma e aumenta a TFA (tendência de formação de fase amorfa) ainda pouco estudada com altas porcentagens de cobre. Diante disso, este trabalho teve como objetivo avaliar a influência da velocidade de resfriamento nas temperaturas de transformação e na tendência de amorfização em fitas de Ti-CuNi resfriadas rapidamente. Para tanto, foram produzidas duas fitas Ti01 (Ti 43,5 Cu 37,8 Ni 18,7) e Ti02 (Ti 58,4 Cu 25,6 Ni 16,0) pelo processo melt spinning, variando-se a velocidade linear da roda em 21 m/s e 63 m/s. As amostras foram caracterizadas utilizando-se técnicas DSC, DR-X, RET e MO. Após essa caracterização pode-se afirmar que a técnica de melt spinning permite a produção de fitas muito finas, da ordem de micrômetros, em apenas uma etapa de processamento, assim como também foi possível a produção de fitas amorfas, do sistema Ti-Cu-Ni, sem nenhuma fase cristalina como observado na fita Ti01 e Ti 02 obtidas com velocidade linear de 63m/s. O tratamento térmico foi suficiente para remover todos os defeitos produzidos pelo processo de solidificação rápida e produzir um rápido crescimento de grão, favorecendo o aumento das temperaturas de transformação martensíticas e austeníticas.<br>Ternary alloys with shape memory Ti-Cu-Ni are known to submit a characteristic phase transformation and properties similar to the and Ti-Ni binary alloy . Studies with rich-copper alloys showed that the addition of copper in alloys Ti-Ni reduces the hysteresis response of the shape memory effect and increases the TFA (tendency to form amorphous phase) still little studied with high percentages of copper. Thus, this study aimed to evaluate the influence of cooling rate on the transformation temperatures and on the tendency of Cu-Ni-Ti ribbons rapidly solidified. Therefore, two ribbons TI01 (Ti 43.5 Cu 37.8 Ni 18.7) and Ti02 (Ti 58.4 Cu 25.6 Ni 16.0) were produced by melt spinning process, varying the wheel linear velocity 21 m/s and 63 m/s. The samples were characterized using DSC, X-DR, RET and MO and techniques. Melt spinning technique allows the production of very thin ribbons of the order of microns, in one processing step, as it was also possible to produce amorphous ribbons, the system Ti-Cu-Ni, without crystalline phase as observed in TI01 and 02 Ti02 with linear velocity of 63m/s. The heat treatment was sufficient to remove all defects produced by rapid solidification process and produce a rapid grain growth, favoring the increase of temperatures of martensitic and austenitic transformation.

Submitted by Johnny Rodrigues (johnnyrodrigues@ufcg.edu.br) on 2018-04-06T20:54:52Z No. of bitstreams: 1 GEORGE CARLOS DOS SANTOS ANSELMO - TESE PPG-CEMat 2014..pdf: 19735821 bytes, checksum: c92e61b342c27548f43bd01d30640a96 (MD5)<br>Made available in DSpace on 2018-04-06T20:54:52Z (GMT). No. of bitstreams: 1 GEORGE CARLOS DOS SANTOS ANSELMO - TESE PPG-CEMat 2014..pdf: 19735821 bytes, checksum: c92e61b342c27548f43bd01d30640a96 (MD5) Previous issue date: 2014-08-29<br>Ligas com efeito de memória de forma possuem grande potencial para aplicações nos setores da robótica, automotivo, aeronáutico, medicina e na produção de atuadores miniaturizados. O objetivo desse trabalho foi investigar e desenvolver materiais com efeito de memória de forma (Shape Memory Effect - SME) das ligas Ti-Ni-Cu na forma de fitas micrométricas produzidos por meio de Melt Spinning. A metodologia utilizada para produção das ligas Ti-Ni-Cu foi via fusão a plasma (Plasma Skull Push-Pull), e para fabricação de fitas utilizou-se a técnica de solidificação rápida por injeção de metal líquido em volante de cobre nas velocidades de 38 e 50 m/s, logos após as ligas e fitas Ti-Ni-Cu foram caracterizadas por: DSC, SMRT, DRX, MEV. Inicialmente barras prismáticas da liga Ti-Ni50-x-Cux (x=3,4,5,6,7%at.Cu) foram produzidas via fusão a plasma. Por meio solidificação rápida obteve-se fitas com espessuras de 30 a 45 µm com a variação da velocidade do volante de cobre de 38 e 50 m/s no Melt Spinning. Ensaios de DRX revelam à presença da fase B19’ nas ligas brutas de fusão a temperatura ambiente. As ligas apresentaram transformações de fase em único estágio B2↔B19`. As temperaturas de transformação As das ligas Ti-Ni-Cu decrescem com o incremento de Cu. Concluise que as temperaturas de transformação martensíticas (Ms) de fitas Ti-Ni-Cu decrescem com a diminuição do tamanho de grão, e os valores de histerese e entalpia decrescem quando altas taxas de super-resfriamento são alcançadas no Melt Spinnig.<br>Alloys with shape memory effect have immense potential for applications in robotics, automotive and aeronautics industry, medicine and in the production of miniaturized actuators. The aim of this study was to investigate, develop and manufacture materials with shape memory effect of Ti-Cu-Ni alloys in the form of micrometer tapes produced by Melt Spinning. The methodology used for the production of Ti-Cu-Ni alloys by fusion plasma (Plasma Skull Push-Pull), and manufacturing tapes used the technique of rapid solidification injection of liquid metal into the copper wheel speeds 38 and 50 m/s. alloy and Ti-Ni-Cu ribbons were characterization by: DSC, SMRT, XRD, SEM. Initially prismatic bars of Ti-Ni50-x-Cux (x = 3,4,5,6,7 at.Cu%) had been produced by plasma fusion. With the rapid solidification is obtained tapes with thicknesses between 30 to 45 µm with the variation of the speed of the wheel covers 38 and 50 m / s the melt spinning. XRD tests reveal the presence of the B19' in gross phase alloy melting temperature. The alloys showed phase transformations in single stage B2↔B19`. The transformation temperatures of the alloy Ti-Cu-Ni decrease with the increase of Cu. We conclude that the temperatures of martensitic transformation (Ms) of Ti-Ni-Cu ribbons decreases with decreasing grain size, and hysteresis values and enthalpy decreases when high rates of super-cooling are achieved in Melt Spinnig.

Shape memory alloys witch exhibit magnetically induced phase transformations at room temperatures are the most interesting for magnetic cooling applications. Heusler type alloys (both stoichiometric and non-stoichiometric) are among the most studied systems, as they permit the change of the transition temperatures due to compositional variations. The first Heusler alloys that have been studied in depth are Ni- Mn-Ga alloys. However, to overcome the high cost of Gallium and low martensitic transformation temperature, the search for Ga-free alloys has been recently endeavoured, principally, by introducing In, Sn or Sb. The purpose of this work is the production and characterization of non-stoichiometric Heusler alloys based on a Ni-Mn-Sn composition, with copper or Palladium doping. The effect of doping element will be determined, on the crystalline structure, the phase transition temperatures, the thermodynamic and magnetocaloric parameters. On the other hand, a heat treatment will be exerted on Ni-Mn-Sn-Cu alloys then characterize them in the same way. Shape memory alloys will be produced in the first step, by arc melting technique, to produce the bulk and then by melt spinning, to obtain shape memory ribbons. Produced samples are characterized by scanning electron microscopy (SEM) technique, to check the morphological structure of the alloys, microanalysis technique (EDX) to have the final composition experimentally, X-ray diffraction technique (XRD) to extract information from the crystal structure, differential scanning calorimetry (DSC) to study the thermal and thermodynamic variations induced by structural phase transformations and finally magnetometry techniques (Physical property measurement system, PPMS: VSM, resistivity, hysteresis cycles) to characterize the phase transformations behaviour and magnetocaloric effect under applied external magnetic fields. Among the results obtained in the present work it can be concluded that both Cu-doped and Pd-doped alloys have similar morphology. The reversible austenite-martensite transformation was detected in all ribbons. Cu or Pd doping tends to increase the structural transition temperatures. These changes in the magnetic and martensitic transformation temperatures are confirmed report directly to the ratio (e/a) of the alloy. Moreover, it’s emphasized the fact that Pd doped alloys show a good magnetocaloric effect (the maximum variation of ΔS is approximately 4.5 J / (Kg.K) for an applied magnetic field of 50 kOe and the refrigeration capacity is 28 J / kg). This dependence must allow the selection of the appropriate composition for the production of alloys with transformation into a desired temperature range. On the other hand, measurements of DRX, DSC and magnometry carried out on Ni-Mn-Sn-Cu alloys after application of thermal cycles of heating-cooling of 100 times, allowed us to certify the great stability of the alloys and the shape memory effect (minor change in phase transformation temperatures and curie temperatures)<br>Els aliatges tipus Heusler (tant estequiomètrics com no estequiomètrics) es troben entre els sistemes més estudiats, ja que permeten el canvi de les temperatures de transició a causa de les variacions de la composició. Els primers aliatges de Heusler que s’han estudiat en profunditat són els aliatges de Ni-Mn-Ga. No obstant això, per superar l’elevat cost del gal i la baixa temperatura de transformació martensítica, recentment s’ha intentat buscar aliatges sense Ga, principalment, introduint In, Sn o Sb. L’objectiu d’aquest treball és la producció i caracterització d’aliatges de Heusler no estequiomètrics a partir d’una composició de Ni-Mn-Sn, amb dopatge de cobalt o pal·ladi. L'efecte de l'element dopant es determinarà, sobre l'estructura cristal·lina, les temperatures de transició de fase, els paràmetres termodinàmics i magnetocalòrics. D’altra banda, s’exercirà un tractament tèrmic sobre els aliatges de Ni-Mn-Sn-Cu per després caracteritzar-los de la mateixa manera. Els aliatges de memòria de forma es produiran en el primer pas, mitjançant la tècnica de la fusió de l’arc, per produir l’aliatge massiu i després per la fusió per obtenir cintes de memòria de forma. Les mostres produïdes es caracteritzen per la tècnica de microscòpia electrònica d’escombratge (SEM), per comprovar l’estructura morfològica dels aliatges, la tècnica de microanàlisi (EDX) per tenir la composició final de manera experimental, la tècnica de difracció de raigs X (XRD) per extreure informació de l’estructura cristal·lina, calorimetria d’escaneig diferencial (DSC) per estudiar les variacions tèrmiques i termodinàmiques induïdes per transformacions de fase estructurals i finalment tècniques de magnetometria (PPMS: VSM, resistivitat, cicles d’histèresi) per caracteritzar el comportament de les transformacions de fase i l’efecte magnetocalòric sota camps magnètics externs aplicats. Entre els resultats obtinguts en el present treball es pot concloure que tant els aliatges dopats amb Cu com els aliatges dopats amb Pd tenen una morfologia similar. La transformació reversible d'austenita-martensita es va detectar a totes les cintes. El dopatge Cu o Pd tendeix a augmentar les temperatures de transició estructural. Aquests canvis en les temperatures de transformació magnètica i martensítica es confirmen directament a la relació (e / a) de l'aliatge. A més, s’ha destacat el fet que els aliatges dopats amb Pd mostren un bon efecte magnetocalòric. Aquesta dependència ha de permetre seleccionar la composició adequada per a la producció d'aliatges amb transformació en un rang de temperatura desitjat. D’altra banda, les mesures de DRX, DSC i magnometria realitzades sobre aliatges Ni-Mn-Sn-Cu després de l’aplicació de cicles tèrmics de calefacció-refrigeració de 100 vegades, ens van permetre certificar la gran estabilitat dels aliatges i la efecte memòria de forma<br>Programa de Doctorat en Tecnologia

Protective and functional coatings have been undergoing development for decades and further improvement to their mechanical and tribological properties are more and more challenging. Nowadays most research is aimed at improving the tribological behaviour of hard and functional coatings through the optimisation of their microstructure on the nanoscale. Over time significant breakthroughs have been achieved, however improving material performance is becoming harder. Combining different layers is another possible way of improving the tribological performance of functional coatings. The use of bonding layers can mitigate the differences in mechanical and thermal properties between coating and substrate and might change the behaviour of the coating system in a tribological scenario. As a consequence, the nature of this bonding interlayer plays an important role in the response of the coatings to the complex stress conditions taking place in a tribological system. Among the possible interlayer candidates a layer with the capability of accommodating large deformation, thus protecting the substrate from plastic deformation as well as improving the adhesion of the top layer to the substrate, could represent a suitable choice. One of the potential classes of materials satisfying the above requirements are the Ni-Ti based alloys which are known to exhibit superelastic properties also when sputter deposited. In this study we first focus our investigation on the characterisation of sputter-deposited Ni-Ti based thin films. In particular, the Ni-Ti system is doped by a third element, Cu through which mechanical and microstructural properties can be changed without detrimental effects on the typical functional properties of Ni-Ti alloys. The effects of Cu, in the range 0 – 20 at.%, and of the post-deposition heat treatments, with particular regard to annealing temperature, on mechanical and microstructural properties of sputter-deposited Ni-Ti(-Cu) thin films are investigated by nanoindentation, X-ray diffraction and transmission electron microscopy. Based on the objective of using Ni-Ti(-Cu) thin films as the interlayer in tribological coatings, some of the Ni-Ti(-Cu) films are selected and integrated in a bilayer design. Among the tribological coatings self-lubricant W-S-C coatings are known for their excellent non-Amonton frictional behaviour with friction decreasing with increasing contact pressure. The low friction of W-S-C coatings is associated with the formation of a WS2 tribolayer on the sliding surface. When the basal plane of the WS2 tribolayer is aligned with the sliding direction, the friction coefficient drops to very low values owing to the weak bonding between chalcogenide atomic planes. The formation of the low-shear surface layer is directly related to contact pressure; therefore, a W-S-C coating is an ideal functional layer with which to study the effect of Ni-Ti(-Cu) interlayers on sliding properties. W-S-C/Ni-Ti(-Cu) bilayer coatings are fabricated following a three-step process consisting of deposition and annealing of the Ni-Ti(-Cu) layers and subsequent deposition of the top functional layer. Mechanical and nano scratch behaviour of these bilayers is investigated in order to study the functional role of different Ni-Ti(-Cu) interlayers on the response of the bilayers to nanoindentation and nano-scratch tests. The tribological performance of W-S-C single layer and selected W-S-C/Ni-Ti(-Cu) bilayers are investigated by sliding tests in humid air under different test conditions in order to assess the potential beneficial effects of the interlayer on the tribological properties of W-S-C. Correlation between the tribological properties measured and microstructural changes induced by sliding is achieved by investigating the tested coatings by focused ion beam and transmission electron microscopy. Chemical changes on the sliding surfaces are also investigated by Raman spectroscopy in order to highlight possible differences in the tribolayer formation under different test conditions and for different Ni-Ti(-Cu) interlayers. The study is also aimed at understanding how the stress-induced martensitic transformation is activated in the interlayers during sliding.

Thesis: S.B., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2015.<br>Cataloged from PDF version of thesis.<br>Includes bibliographical references (pages 37-38).<br>Shape memory alloys exhibit superelasticity and the shape memory effect by undergoing a diffusionless phase transformation between the austenite and martensite phases. Nickel-titanium alloys are currently the most common material used. However, due to their expensive cost, alternatives such as Cu-based alloys have been investigated. Cu-based alloys have exhibited the shape memory effect and have achieved 6-8% strain recovery. This work investigates Cu-Al-Ni- Mn shape memory alloys in the form of microwires with the potential application in smart textiles. Wire microstructure and composition, transition temperatures, and strain recovery were analyzed after the wires were subjected to varying annealing times and temperatures. These data were used to determine the ideal conditions to achieve the most shape memory and superelasticity.<br>by Keerti Shukla.<br>S.B.

Made available in DSpace on 2015-05-08T14:59:52Z (GMT). No. of bitstreams: 1 parte1.pdf: 2223950 bytes, checksum: 585473348fd57251708f0ba93240c53b (MD5) Previous issue date: 2008-10-28<br>Coordenação de Aperfeiçoamento de Pessoal de Nível Superior<br>The presented work consists of a study for observation of the martensite kept in the transformation of martensite phase - austenite in a alloy of Ni - Al - V, with memory shape effect, into, a room temperature. We defined that the alloy with composition the 3,5% (in weigh) of nickel, presents better results when happens successive loading and downloading cancel though, 3,0% of deformation into a room temperature, back to austenite phase after retreat the load. Above 3,0% of deformation became evident the appearance of retrain martensite. For her identification it was necessary the development of on equipment to traction, as well as bodies of proofs dimensioned for that. The analyses were done through X rays diffraction of optical microscopy and rehearsal of traction.<br>O trabalho apresentado consta de um estudo para observação da martensita retida na transformação de fase de martensita austenita em uma liga de Ni Al V, com efeito de memória de forma, a temperatura ambiente. Definimos que a liga com composição 3,5% (em peso) de níquel, apresenta melhor resultado quando submetida a carregamento e descarregamento sucessivos; chegando a 3,0% de deformação a temperatura ambiente, retornando ao estado austenítico depois de retirada a carga. Acima de 3,0% de deformação, evidencia-se o surgimento de martensita retida. Para sua identificação, foi necessária a confecção de um equipamento para tracionar, como também corpos de prova dimensionados para tanto. As observações foram realizadas através de análises de raios X, microscopia ótica e ensaios de tração.