Thesis ( MTech(Mechanical Engineering))--Cape Peninsula University of Technology, 2011 / This study deals with the development of aero morphing reinforced composite materials embedded with NiTi alloys. It is shown that the composite materials can be manufactured using resin infusion process to produce better mechanical properties such as tensile strength and material stiffness. These composite materials are modelled experimentally using temperature and time parameters. The object of the modelling is to determine the effect of process temperature on the smart material alloy (SMA). As a result, a composite structural designer would now possess an added dimension in optimising material design. In addition, the study is conducted to analyse the structural behaviour of composite materials when embedded and when not embedded with NiTi alloys. The analysis is constrained to the evaluation of material tensile strength and stiffness upon performance of composite structures. A macro mechanical approach is employed to perform the analysis in specimens with different fibre orientation [0°, 45° and 90°]. The estimation of tensile strength and stiffness parameters is based on the characteristics obtained from a macro mechanical approach. The orientation which posses the best material properties is selected to embed NiTi alloys. The experimental results of unembedded specimens are validated with the application of micromechanics equations and an Ansys software finite element modelling tool. There is fair agreement between the finite element simulation of macro mechanical test of the specimens and the measured experimental results. Although the macro mechanical approach is found to be successful, it is imperative to characterise the material interface strength of embedded specimens using a pull out test. The pullout test showed to some great extent the properties of reinforced composite embedded with NiTi alloys.

The synthesis of in-situ bulk metallic glass composites (BMGCs) with crystals that undergo a martensitic transformation under loading is possibly the most effective method to improve the plasticity of metallic glasses at room temperature. These martensitic or shape memory BMGCs are typically fabricated via solidification of glass-forming melts, which requires the meticulous selection of the chemical composition and the proper choice of the processing parameters (particularly the cooling rate) in order to ensure that the glassy matrix coexists with the desired amount of austenitic phase having suitable morphology and characteristics. Unfortunately, a relatively limited number of alloy systems, where austenite and glassy matrix coexist over a wide range of compositions, is available. Additionally, the necessity for rapid heat extraction and the corresponding high cooling rates essential for glass formation by melt solidification set an inherent limit to the achievable dimensions of BMGs and BMGCs specimens. The aim of this thesis is to study the effectiveness of powder metallurgy as an alternative to solidification for the synthesis of shape memory BMGCs. Ni50.6Ti49.4 and Zr48Cu36Al8Ag8 metallic glass powders were selected as the constituents of the composites because they have been extensively investigated and represent well the characteristic behavior of metallic glass and shape memory phases. BMGCs with different volume fractions of NiTi phase were fabricated using pressure-assisted sintering via hot pressing and their microstructure, mechanical properties and deformation mechanism were investigated. Particular focus was placed upon identifying the individual contributions of the martensitic transformation and shear band formation to plasticity as well as their mutual interaction at different length scales using a multidisciplinary approach involving experiments and simulations. BMG composites were synthesized by hot pressing of powder mixtures consisting of Zr48Cu36Al8Ag8 metallic glass and different amounts of Ni50.6Ti49.4 particles (10, 20, 40 and 60 vol.%) using the optimized consolidation parameters (temperature-time-pressure) determined for the monolithic BMG. All composites are characterized by a relatively uniform particle distribution and good interface bonding without any sign of reaction between the metallic glass and NiTi. The NiTi particles are progressively less isolated with increasing volume fraction of NiTi up to 40 % and, for the BMGC with 60 vol.% NiTi, the glassy particles are no longer connected and the NiTi phase becomes the continuous matrix. This is not a trivial achievement as the change of matrix while maintaining the structure of the constituent phases would not be easily obtained by solidification of melts with such different compositions. The size of the samples (10 mm diameter and 9 - 11 mm height) is larger than the characteristic BMGCs synthesized by casting and can, in principle, be scaled up to larger dimensions, demonstrating the effectiveness of this approach for overcoming the size limitation inherent to glass formation via solidification. In contrast to the monolithic BMG, which does not show any sign of plasticity, the BMGCs exhibit macroscopic plastic deformation that progressively increases with increasing NiTi content along with distinct strain-hardening. The BMG composites have similar fracture strength, which is comparable with the monolithic BMG, and exhibit a distinct double yield behavior, similar to shape memory BMGCs fabricated by casting. The deformed BMGCs exhibit a high density of shear bands, again in agreement with what observed for similar BMGCs fabricated by casting. These findings not only demonstrate that BMGCs with tunable microstructures and thus with optimized deformability can be synthesized by pressure-assisted sintering but, thanks to the phase stability of the components across such a wide range of compositions, also offer an excellent platform to examine fundamental aspects in the field of martensitic BMGCs. The confining stress exerted by the surrounding glassy matrix was quantified at the macroscale via a hybrid Voigt-Reuss mixture, which considers intermediate weighted combinations of stiff and compliant behaviors. In this way, the macroscopic stress required to initiate the martensitic transformation from B2 to B19´ can be described with rather good accuracy. The confining effect was further investigated by in-situ high-energy X-ray diffraction to have access to the strain tensor of the B2 phase as a function of loading. The results indicate that the confining stress along the direction perpendicular to the loading axis is particularly strong because the expansion of the B2 phase is constrained by the elastic matrix. A mechanism responsible for shear band formation in shape memory BMGCs is proposed. The stress field generated by the martensitic transformation in the contiguous glass would activate the adjacent shear transformation zone (STZ, the elementary units of plasticity in BMGs). The stress field induced by the activated STZ in the surrounding material then triggers the activation of the following STZs along the path of a potential shear band, in an autocatalytic process resembling the domino effect. The shear band formed in this way propagates through the glassy phase and, when impinging a B2 particle, the associated stress field would locally trigger the martensitic transformation, starting again the process. Molecular dynamics (MD) simulations of a martensitic BMGC show that the structural perturbation generated by the martensitic transformation is indeed transmitted to the adjacent glassy matrix and, in turn, to the developing shear band, in agreement with the proposed mechanism. The individual contribution of the glassy phase to the residual strain after each loading-unloading cycle was quantified assuming that the NiTi phase behaves in the same manner across the different specimens. The glass contribution was then correlated to the shear band density to obtain the plastic strain resulting from shear banding for a given amount of NiTi phase, a quantity that could be effectively used in the design of plastically-deformable BMGCs with shape memory particles. The martensitic transformation in the composites becomes progressively more irreversible with increasing strain. A large contribution to the martensite stabilization may come from the residual stress induced by the shear bands, in accordance with the finite element method (FEM) simulations, showing that residual stresses in the composites suppress the reverse transformation after unloading. These finding corroborates the hypothesis that the residual elastic stress field generated by the shear bands may be fundamental for stabilizing the martensitic phase by restraining the atoms at the glass-crystal interface from rearranging back to form austenite. This process can be reversed by proper heat treatment. The findings presented in this thesis offer the opportunity to synthesize shape memory BMG composites with enhanced plasticity and strain-hardening capability along with larger dimensions than those typically achieved by solidification. The powder metallurgy approach provides the necessary versatility in materials design and resulting properties of the composites via the control over the fundamental microstructural features, such as volume fraction, size, morphology and distribution of the second phase. Additionally, materials processing in the solid state gives a virtually infinite choice among the possible composite components, a degree of freedom not usually given when processing via solidification.:Abstract iii Kurzfassung vii Motivation and objectives xi 1 Theoretical background and state-of-the-art 1 1.1 Bulk metallic glasses (BMGs) 1 1.1.1 Formation of metallic glasses 2 1.1.2 Mechanical properties of BMGs 5 1.1.3 Shear bands in metallic glasses 8 1.2 Bulk metallic glass matrix composites 19 1.2.1 Fabrication of BMG composites 20 1.2.2 In-situ BMG composites 27 1.2.3 Ex-situ BMG composites 43 2 Experiments and simulations 57 2.1 Sample preparation 57 2.1.1 Starting materials 57 2.1.2 Powder mixing 59 2.1.3 Powder consolidation 60 2.2 Materials characterization 61 2.2.1 Composition analysis 61 2.2.2 Laboratory X-ray diffraction 61 2.2.3 High-energy X-ray diffraction and strain analysis 62 2.2.4 Viscosity measurements 63 2.2.5 Differential scanning calorimetry 64 2.2.6 Density measurements 64 2.2.7 X-ray computed tomography 65 2.2.8 Optical microscopy and scanning electron microscopy 65 2.2.9 Transmission electron microscopy 66 2.2.10 Elastic constants measurements 66 2.2.11 Mechanical tests 67 2.3 Molecular dynamic simulations 67 2.4 Finite element simulations 68 3 Pressure-assisted sintering of single-phase Zr48Cu36Al8Ag8 metallic glass and Ni50.6Ti49.4 powders 73 3.1 Synthesis and properties of single-phase Zr48Cu36Al8Ag8 bulk metallic glass 73 3.2 Synthesis and properties of single-phase Ni50.6Ti49.4 shape memory alloy 80 4 Pressure-assisted sintering of BMG composites with shape memory crystals: Microstructure and mechanical properties 87 4.1 Microstructure of BMG composites 87 4.2 Effect of NiTi volume fraction on mechanical properties 90 4.3 Effect of confinement of the glassy phase on the martensitic transformation 95 5 Deformation mechanism of shape memory BMG composites 101 5.1 Martensitic transformation and shear band formation 101 5.2 Mechanism of shear band formation in shape memory BMG composites 107 6 Reversibility of the martensitic transformation in shape memory BMG composites 113 6.1 Martensite stabilization in NiTi alloy and BMG composites 113 6.2 Simulation of the martensite stabilization effect in BMG composites 119 6.3 Effect of heat treatment on the martensitic reverse transformation 121 7 Summary and outlook 125 References 131 Acknowledgements 155 Publications 157 Erklärung 159

NiTi alloys with nearly equiatomic chemical composition are the most studied and the most practically utilized materials from the group of the shape memory alloys (SMA). The NiTi alloy has a thermoelastic martensitic transfor- mation (MT), which its two most important properties are based on: the shape memory and the superelasticity. Thin films of NiTi alloys with thicknesses from hundreds of nm to units of µm have a wide use mainly in microelectromechanical systems (MEMS). The doctoral thesis deals with a fabrication of thin NiTi films using a deposition on silicon substrates with two techniques - the magnetron sputtering and the pulsed-laser deposition (PLD), an investigation of their mi- crostructure using x-ray diffraction, a heat treatment of amorphous NiTi films and with a verification of MT using resistometry. Furthermore, thermomechan- ical properties of a system NiTi-polyimide (NiTi-PI), namely a dependence of a curvature radius of a NiTi-PI bilayer on temperature, are studied. 1

Shape memory alloys (SMAs) can recover large strains (e.g., up to 8%) by undergoing a temperature-induced phase transformation. This strain recovery can occur against large forces, resulting in their use as actuators. The SMA elements in such actuators integrate both sensory and actuation functions. This is possible because SMAs can inherently sense a change in temperature and actuate by undergoing a shape change, associated with the temperature-induced phase transformation. The objective of this work is to develop an SMA based cryogenic thermal conduction switch for operation between dewars of liquid methane and liquid oxygen in a common bulk head arrangement for NASA. The design of the thermal conduction switch is based on a biased, two-way SMA actuator and utilizes a commercially available NiTi alloy as the SMA element to demonstrate the feasibility of this concept. This work describes the design from concept to implementation, addressing methodologies and issues encountered, including: a finite element based thermal analysis, various thermo-mechanical processes carried out on the NiTi SMA elements, and fabrication and testing of a prototype switch. Furthermore, recommendations for improvements and extension to NASA's requirements are presented. Such a switch has potential application in variable thermal sinks to other cryogenic tanks for liquefaction, densification, and zero boil-off systems for advanced spaceport applications. The SMA thermal conduction switch offers the following advantages over the currently used gas gap and liquid gap thermal switches in the cryogenic range: (i) integrates both sensor and actuator elements thereby reducing the overall complexity, (ii) exhibits superior thermal isolation in the open state, and (iii) possesses high heat transfer ratios between the open and closed states. This work was supported by a grant from NASA Kennedy Space Center (NAG10-323) with William U. Notardonato as Technical Officer.

NiTi based shape memory alloys (SMA) cover most of the commercially produced shape memory devices and components. The reversible martensitic transformation between the phases B2 (austenite) and B19′ (martensite) is responsible for the shape memory effect in these alloys. The amount of strain which can be regained after a permanent deformation through thermal activation, known as the recoverable strain, is a strong function of crystallographic texture and microstructure. Texture influences the activation of a specific martensite variant during stress induced martensitic (SIM) transformation and also the re-orientation of twinned variants during further deformation. The variant selection decides the amount of recoverable strain. Since the NiTi based shape memory alloys inevitably undergo thermo-mechanical processing in the course of component design, the consequent evolution of texture and microstructure regulate the shape memory behavior. The present thesis is aimed to address this issue in some NiTi alloys that are technologically important for different applications, namely a binary Ni-rich NiTi alloy, a copper containing NiTi alloy and a hafnium containing NiTi alloy. The Ni rich NiTi alloy displays pseudoelastic behavior that can be used for couplings, the NiTiCu alloy provides a controlled thermal hysteresis suitable for actuator applications and the NiTiHf alloy can be used for high temperature applications. The first Chapter of the thesis provides a detailed overview of the existing knowledge related to evolution of microstructure and texture during processing, the transformation texture and its role on the shape memory behavior in NiTi alloys. The second chapter includes the experimental procedure followed to generate different textures, namely unidirectional and cross rolling with and without a subsequent annealing and also the details of the techniques used to characterize the structure, microstructure, texture and mechanical properties. The evolution of texture during thermo-mechanical processing of a Ni rich NiTi alloy and its impact on shape memory behavior is addressed in Chapter 3. The two modes of rolling employed at higher temperature led to the formation of different textures. The texture of unidirectionally rolled samples was characterized by a strong <111>||ND fiber, while a strong Goss {100}<110> component along with <111>||ND fiber was observed in the texture of the cross rolled samples. Annealing of the unidirectionally rolled samples generated a strong <100>||ND fiber, and a weak <111>||ND fiber was observed for the cross rolled samples. Microtexture analyses indicated that dynamically recrystallized grains had significantly different texture compared to the statically annealed material. One of the salient features of this study is the analysis of different twin boundaries with coincident site lattice (CSL) relations that has been observed in the hot rolled material. The origin of these twins has been attributed to deformation. The evolution of twin boundaries with CSL relation has strong influence on texture formation. A few of the important texture components have been found to have CSL relation amongst them. The origin of different texture components were found using intra-grain misorientation parameters. In-situ transformation studies in a scanning electron microscope have confirmed the formation of different types of twins at very low amount of strain in the Ni rich NiTi alloy. A Schmid factor based criterion was used to identify the activation of a particular variant. Trace analysis of the surface relief due to SIM transformation was utilized to confirm the theoretically predicted variant. Schmid criterion has been found to be valid in all the cases. Modulus variation with temperature and strain was studied using dynamical mechanical analysis. Microstructural changes during thermal and thermo-mechanical cycling revealed higher orientation gradient along grain boundaries compared to grain interior. The compatibility condition at the grain boundaries were attributed to higher misorientation development. Misorientation development during cycling loading process is also found to be a strong function of texture. Processing condition and texture has a strong influence on the recoverable strain. Particularly, the strength of <111>||ND fiber is influential in deciding the recoverable strain. Study of microstructure and texture evolution in the TiNiCu SMA and subsequent study on its impact on recoverable strain is presented in Chapter 4. Convincing evidences for the mechanisms operating during different dynamic restoration processes have been presented through microstructural investigation. Texture analysis of the austenite phase showed the formation of <111>||ND fiber. Despite the weakening of texture at larger strain, strength of certain deformation texture components like S {123}<634> and Cu {112}<111> increased, which suggested that texture evolution in TiNiCu alloy deviates from the texture of binary NiTi at large strains. Transformation texture analysis was carried out through electron back scattered diffraction technique, using an in-situ heating stage. The analysis of the results showed predominant activation of <011> type II as well as {11 1 } type I twins. A comparison of martensite and austenite pole figures indicated strong variant selection during phase transformation. Like the binary NiTi alloy, cross rolling of TiNiCu alloy also showed ample changes in the texture of martensite phase through the formation of different texture components. Annealing of both unidirectionally and cross rolled samples led to the weakening of texture. The change in volume fraction of Ti2NiCu precipitates, resulting from different processing conditions, influenced the transformation temperature. In this case also, texture and large intra-grain misorientation governed the recoverable strain. Chapter 5 is dedicated to the study of high temperature NiTiHf alloy. X-ray diffraction and differential scanning calorimetric studies confirmed a two step martensitic transformation, a B19` monoclinic and rhombohedral R-phase martensite in the studied alloy (Ni49.4Ti38.6Hf12). Microstructural investigations showed the formation of dendritic (Ti,Hf)2Ni precipitates along the grain boundary. Evolution of R-phase martensite was always observed along with (Ti,Hf)2Ni precipitates, irrespective of the processing condition. Dissolution of (Ti,Hf)2Ni precipitates by solution treatment suppressed the R phase formation. Strong texture of R-phase martensite confirmed variant selection during martensitic transformation. On the contrary, texture of B19` martensite was always weak, suggesting no preference for variant selection. Rolled material with a relatively strong texture exhibited higher recoverable strain compared to annealed material. Finally, all the significant outcomes of the present investigation are summarized in Chapter 6. Based on the conclusions, suggestions for future work have been mentioned.

Dans cette étude, nous avons élaboréun revêtementde TiO₂ par sol-gel sur des alliages super-élastiques de NiTi. L’idée générale était de développer couche mince de TiO₂ protectrice et flexible. Le film mince est formé en immergeant les échantillons de NiTi, préalablement chimiquement gravés, dans une solution réactive à 7,5 mm/s, puis en effectuant plusieurs traitements thermiques :un premier traitement thermique à 100 °C pendant 45 minutes dans une atmosphère humide, un deuxième traitement à 110 °C pendant 2 heures dans une atmosphère sèche et enfin un dernier traitement à 500 °C pendant 10 minutes. Les couches minces de TiO₂ ont été caractérisé par de la flexion trois points, par MEB, par MET, par AFM, par GIXRD, par XPS et par de la cartographie Raman. Les résultats de l’étude ont montré un film nanocomposite, avec ~100nmde TiO2 amorphe formé à l’interface externe de la couche et d'un mélange de grains cristallisés de ~10 nm, d'anatase et de rutile à l'interface interne métal/oxyde.Cette hétéro-structure est capable de soutenir 6,4% de déformation sans l’apparition de défauts plastiques majeurs (cloques, fissures...). Une faible concentration de Ni a été observé au niveau de la surface externe des couches minces de TiO₂, ce qui se traduit par une augmentation de la biocompatibilité du matériau. La technique sol-gel a été utilisée pour revêtir des instruments endodontiques de RaCe. Ce deuxième système a été testé à la fois en fatigue pour estimer sa durée de vie, et à la résistance à la corrosion en NaClO, et à des températures correspondant aux transformations de phase. Les résultats ont montré une augmentation statistiquement significative de la durabilité en fatigue, en particulier après les essais de corrosion. L'efficacité de la « coupe », mesurée par une procédure originale, a été similaire aux instruments revêtus et non revêtus. Le traitement thermique n'a pas été suffisant efficient pour modifier de façon significative les températures de transformation de phase : le comportement mécanique d'origine de l'instrument a été maintenu. / In this work, a dip-coating sol-gel deposition route was developed to coat superelastic NiTi alloy with a flexible TiO₂ protective layer. The film was formed by emerging the samples at 7.5 mm/s and thermally treating at 100ºC in a humid atmosphere for 45 min, 110ºC in a dry atmosphere for 2 hours and at 500ºC for 10 minutes.The film was first deposited over chemically etched substrates and characterized by SEM, TEM, AFM, GIXRD, XPS, Raman cartographyand three-point bending tests. Results showed that a ~100 nm nanocomposite film constituted of amorphous TiO₂ on the upper half and a mixture of ~10 nm anatase and rutile grains on the oxide/metal interfacewas formed. This film was capable of sustaining up to 6.4% strain without cracking or peeling. A high decrease in the concentration of Ni at the surface was measured, indicating an that an increase in the biocompatibilityof the material was achieved. This route was used to coat RaCe endodontic instruments, which were tested regarding fatigue life, cutting efficiency and corrosion resistance in NaClO. Results showed a statistically significant improvement in fatigue life for the coated instruments, mainly after corrosion tests. Cutting efficiency measured by an original developed technique was similar for coated and uncoated samples. / Neste trabalho foi desenvolvida uma rota de deposição sol-gel por imersão para revestirligas de NiTi superelásticas com uma camada protetora e flexível de TiO2. O filmeformado pela emersão de amostras a 7,5 mm/s seguida de tratamentos térmicos a 100ºCpor 45 minutos em atmosfera úmida, 110ºC por 2 horas em atmosfera seca e 500ºC por10 minutos. O filme foi depositado sobre substratos decapados quimicamente ecaracterizados por MEV, MET, AFM, GIXRD, XPS, cartografia Raman e dobramentode três pontos. Resultados mostraram que um filme nanocompósito com ~100 nmconstituído de TiO2 amorfo na metade superior e uma mistura de grãos de 10 a 50 nmde anatase e rutila na interface metal/óxido foi formado. Este filme é capaz de sustentar6,4% de deformação sem trincar ou descamar. Uma grande redução na concentração deNi na superfície foi detectada, indicando um aumento na biocompatibilidade domaterial. A rota foi usada para revestir instrumentos endodônticos de NiTi modeloRaCe 25/0.06 que foram testados em relação à vida em fadiga, eficiência de corte,resistência à corrosão em NaClO. Detectou-se um aumento estatisticamentesignificativo na vida em fadiga, especialmente após os ensaios de corrosão. A eficiênciade corte, medida por um procedimento original desenvolvido, foi similar parainstrumentos revestidos e não revestidos. O tratamento térmico não foi suficiente paraalterar significativamente as temperaturas de transformação de fases, mantendo ocomportamento mecânico original do instrumento.

Cíle této práce jsou: (i) vyhodnocení vlivu Y2O3 kelímku na kontaminaci indukčně tavené NiTi slitiny obohacené niklem, (ii) optimalizace podmínek, při kterých jsou tavby uskutečněny a (iii) získání nových DSC a 3D AP dat o vlivu vodíkové atmosféry na martenzitickou transformaci a na strukturu NiTi slitin obohacené niklem s tvarovou pamětí. Byly provedeny následující experimenty: – Pět taveb bylo navrženo a provedeno tak, aby byla snížena maximální tavící teplota. – Pět přetaveb bylo uskutečněno při teplotě 1500 C s dobou výdrže 2, 10 a 20 minut a při teplotách 1450 C a 1550 C s 20 minutovou dobou výdrže. Experiment byl navržen tak, aby byl vyšetřen vliv tavící teploty a doby výdrže na obsah kyslíku pocházejícího z kelímku Y2O3 v tavenině. – Tepelné zpracování NiTi slitiny obohacené niklem v režimu I (žíhání) a v režimu II (kombinace žíhání se stárnutím) pod atmosférou vodíku, směsi vodíku s héliem a pod referenční atmosférou čistého hélia. Získané výsledky jsou: – Navržené tavící postupy vedou ke snížení maximální tavící teploty a to z 1800 C na 1400 C. Přesto toto velké snížení maximální tavící teploty nevedlo k významnému poklesu obsahu kyslíku. – Během přetavby uskutečněné při teplotě 1500 C s dobou výdrže 2 minuty se obsah kyslíku navýšil o trojnásobek jeho počáteční hodnoty a příliš se nelišil od obsahu kyslíku naměřeného ve slitině, která byla přetavena při stejné teplotě s dobou výdrže 10 minut. K nárustu o čtyřnásobek počáteční hodnoty obsahu kyslíku došlo u přetavby vedené na teplotě 1450 C po dobu 20 minut a hodnota obsahu kyslíku se příliš nelišila od hodnoty naměřené ve slitině přetavené při teplotě 1550 C se stejnou dobou výdrže. – S rostoucím parciálním tlakem vodíku dochází k potlačení jednokrokové martenzitické transformace. Významný pokles výšky DSC píku nastává při parciálním tlaku 100 mbar. 3D AP analýza odhalila, že nedochází k žádné lokální změně koncentrace a nebo pozic niklových a titanových atomů ve vzorku, který byl žíhán v režimu I ve vodíku. Bylo objeveno, že vodík tvoří stabilní intersticiální tuhý roztok v NiTi B2 mřížce, kde vytváří systém nanodomén s obsahem vodíku vyšší než 10 at%.