Dissertations / Theses on the topic 'High strength shape memory alloy'

To date the commercial use of shape memory alloys (SMAs) has been mostly limited to binary NiTi alloys with transformation temperatures approximately in the -100 to 100 &"186;C range. In an ongoing effort to develop high-temperature shape memory alloys (HTSMAs), ternary and quaternary additions are being made to binary NiTi to form NiTi-X (e.g., X: Pd, Pt, Au and Hf) alloys. Stability and repeatability can be further increased at these higher temperatures by limiting the stress, but the tradeoff is reduced work output and stroke. However, HTSMAs operating at decreased stresses can still be used effectively in actuator applications that require large strokes when used in the form of springs. The overall objective of this work is to facilitate the development of HTSMAs for use as high-force actuators in active/adaptive aerospace structures. A modular test setup was assembled with the objective of acquiring stroke, stress, temperature and moment data in real time during joule heating and forced convective cooling of Ni19.5Ti50.5Pd25Pt5 HTSMA springs. The spring actuators were evaluated under both monotonic axial loading and thermomechanical cycling. The role of rotational constraints (i.e., by restricting rotation or allowing for free rotation at the ends of the springs) on stroke performance was also assessed. Recognizing that evolution in the material microstructure results in changes in geometry and vice versa in HTSMA springs, the objective of the present study also included assessing the contributions from the material microstructural evolution, by eliminating contributions from changes in geometry, to overall HTSMA spring performance. The finite element method (FEM) was used to support the analytical analyses and provided further insight into the behavior and heterogeneous stress states that exist in these spring actuators. Furthermore, with the goal of improving dimensional stability there is a need to better understand the microstructural evolution in HTSMAs that contributes to irrecoverable strains. Towards this goal, available Ni29.5Ti50.5Pd20 neutron diffraction data (from a comparable HTMSA alloy without the solid solution strengthening offered by the Pt addition) were analyzed. The data was obtained from in situ neutron diffraction experiments performed on Ni29.5Ti50.5Pd20 during compressive loading while heating/cooling, using the Spectrometer for Materials Research at Temperature and Stress (SMARTS) at Los Alamos National Laboratory. Specifically, in this work emphasis was placed on neutron diffraction data analysis via Rietveld refinement and capturing the texture evolution through inverse pole figures. Such analyses provided quantitative information on the evolution of lattice strain, phase volume fraction (including retained martensite that exists above the austenite finish temperature) and texture (martensite variant reorientation and detwinning) under temperature and stress. Financial support for this work from NASA's Fundamental Aeronautics Program Supersonics Project (NNX08AB51A), Subsonic Fixed Wing Program (NNX11AI57A) and the Florida Center for Advanced Aero-Propulsion (FCAAP) is gratefully acknowledged. It benefited additionally from the use of the Lujan Neutron Scattering Center at Los Alamos National Laboratory, which is funded by the Office of Basic Energy Sciences (Department of Energy) and is operated by Los Alamos National Security LLC under DOE Contract DE-AC52-06NA25396.<br>ID: 030646204; System requirements: World Wide Web browser and PDF reader.; Mode of access: World Wide Web.; Thesis (M.S.A.E.)--University of Central Florida, 2011.; Includes bibliographical references (p. 102-106).<br>M.S.A.E.<br>Masters<br>Mechanical and Aerospace Engineering<br>Engineering and Computer Science<br>Aerospace Engineering; Space System Design and Engineering Track

Cette thèse présente une approche globale d’optimisation vis-à-vis de la fatigue des matériaux et structures en alliages à mémoire de forme (AMF). Cette approche s’articule en trois étapes : i) Le développement d’une loi de comportement capable de prédire la réponse thermomécanique à l’état stabilisé d’une structure en AMF sous chargement cyclique multiaxial non proportionnel. On prend notamment en compte la dépendance de la déformation résiduelle par rapport à la température. Par ailleurs, la méthode LATIN à grand incrément de temps a été généralisée pour les AMF dans le cadre du modèle ZM. Ceci permet de résoudre les problèmes de convergence numérique rencontrés lorsque le processus de transformation de phase se produit avec une pente du plateau de transformation faible. ii) Le développement d’un critère de fatigue à grand nombre de cycles pour les AMF. Ce critère s’inscrit dans le cadre de la théorie d’adaptation à l’instar du critère de Dang Van pour les métaux élasto-plastiques. Le critère proposé permet de calculer en chaque point de la structure en AMF un facteur de fatigue indiquant son degré de dangerosité. iii) Le développement d’une approche d’optimisation structurale qui peut être utilisée pour améliorer la durée de vie en fatigue prédite par le critère proposé dans la deuxième partie. Des exemples numériques sont traités pour valider chaque étape. L‘approche globale a par ailleurs été testée et validée pour l’optimisation structurale d’un stent<br>This thesis presents a comprehensive and effi cient structural optimization approach for shape memory alloys (SMAs) with respect to fatigue. The approach consists of three steps: First, the development of a suitable constitutive model capable of predicting, with good accuracy, the stabilized thermomechanical stress state of a SMA structure subjected to multiaxial nonproportional cyclic loading. The dependence of the saturated residual strain on temperature and loading rate is discussed. In order to overcome numerical convergence problems in situations where the phase transformation process presents little or no positivehardening, the large time increment method (LATIN) is utilized in combination with the ZM (Zaki-Moumni) model to simulate SMA structures instead of conventional incremental methods. Second, a shakedown-based fatigue criterion analogous to the Dang Van model for elastoplastic metals is derived for SMAs to predict whether a SMA structure subjected to high-cycle loading would undergo fatigue. The proposed criterion computes a fatigue factor at each material point, indicating its degree of safeness with respect to high-cycle fatigue. Third, a structural optimization approach, which can be used to improve the fatigue lifetime estimated using the proposed fatigue criterion is presented. The prospects of this work include the validation of the optimization approach with experimental data

A simulator is developed to model and design high strain shape memory alloy (SMA) tension actuators. The simulator may be used to predict characteristics of a given actuator, or to design its geometry under specifications such as force, speed, stroke and size. The accuracy of the model is verified experimentally in reference to an existing NiTi shape memory alloy prototype actuator. Having developed some confidence in the model, the performance of the proposed actuation mechanism is compared to other existing technologies. In particular, the force-displacement and speed characteristics of a micro-solenoid electro-magnetic actuator and a muscle-size pneumatic actuator are compared to those of the SMA actuators with same dimensions.<br>A new concept of designing shape memory alloy bending actuator is presented in the end of the thesis. Part of the modelling work is accomplished in this research by developing a software simulator which is capable of predicting the geometric transformation of the actuator during bending. As a result, the dynamic strain of each SMA fiber in the actuator can be computed given a bending axis and angle. Graphical display of the bending transformation is implemented using in house software package. Further investigation of modelling and control of the SMA bending actuator is left as future work.

NiTi-based shape memory alloys (SMAs) offer a good combination of high-strength, ductility, corrosion resistance, and biocompatibility that has served them well and attracted the attention of many researchers and industries. The alloys unique thermo-mechanical ability to recover their initial shape after relatively large deformations by heating or upon unloading due to a characteristic reversible phase transformation makes them useful as damping devices, solid state actuators, couplings, etc. However, there is a need to increase the temperature of the characteristic phase transformation above 150 °C, especially in the aerospace industry where high temperatures are often seen. Prior researchers have shown that adding ternary elements (Pt, Pd, Au, Hf and Zr) to NiTi can increase transformation temperatures but most of these additions are extremely expensive, creating a need to produce cost-effective high temperature shape memory alloys (HTSMAs). Thus, the main objective of this research is to examine the relatively unstudied NiTiZr system for the ability to produce a cost effective and formable HTSMA. Transformation temperatures, precipitation paths, processability, and high-temperature oxidation are examined, specifically using high energy X-ray Diffraction (XRD) measurements, in NiTi-20 at.% Zr. This is followed by an in situ XRD study of the phase growth kinetics of the favorable H-phase nano precipitates, formed in NiTiHf and NiTiZr HTSMAs, based on prior thermo-mechanical processing in a commercial NiTi-15 at.% Hf HTSMA to examine the final processing methods and aging characteristics. Through this research, knowledge of the precipitation paths in NiTiZr and NiTiHf HTSMAs is extended and methods for characterization of phases and strains using high energy XRD are elucidated for future work in the field.

This thesis presents methods for fabricating MicroElectroMechanical System (MEMS) actuators and high-flow gas microvalves using wafer-level integration of Shape Memory Alloys (SMAs) in the form of wires and sheets. The work output per volume of SMA actuators exceeds that of other microactuation mechanisms, such as electrostatic, magnetic and piezoelectric actuation, by more than an order of magnitude, making SMA actuators highly promising for applications requiring high forces and large displacements. The use of SMAs in MEMS has so far been limited, partially due to a lack of cost efficient and reliable wafer-level integration approaches. This thesis presents new methods for wafer-level integration of nickel-titanium SMA sheets and wires. For SMA sheets, a technique for the integration of patterned SMA sheets to silicon wafers using gold-silicon eutectic bonding is demonstrated. A method for selective release of gold-silicon eutectically bonded microstructures by localized electrochemical etching, is also presented. For SMA wires, alignment and placement of NiTi wires is demonstrated forboth a manual approach, using specially built wire frame tools, and a semiautomatic approach, using a commercially available wire bonder. Methods for fixing wires to wafers using either polymers, nickel electroplating or mechanical silicon clamps are also shown. Nickel electroplating offers the most promising permanent fixing technique, since both a strong mechanical and good electrical connection to the wire is achieved during the same process step. Resistively heated microactuators are also fabricated by integrating prestrained SMA wires onto silicon cantilevers. These microactuators exhibit displacements that are among the highest yet reported. The actuators also feature a relatively low power consumption and high reliability during longterm cycling. New designs for gas microvalves are presented and valves using both SMA sheets and SMA wires for actuation are fabricated. The SMA-sheet microvalve exhibits a pneumatic performance per footprint area, three times higher than that of previous microvalves. The SMA-wire-actuated microvalve also allows control of high gas flows and in addition, offers benefits of lowvoltage actuation and low overall power consumption.<br>QC 20120514

Thesis (M.S.)--University of Akron, Dept. of Mechanical Engineering, 2007.<br>"December, 2007." Title from electronic thesis title page (viewed 02/22/2008) Advisor, D. Dane Quinn; Co-Advisor, Graham Kelly; Department Chair, Celal Batur; Dean of the College, George K. Haritos; Dean of the Graduate School, George R. Newkome. Includes bibliographical references.

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.

Shape memory alloys (SMAs) exhibit the ability to absorb large dynamic loads and, therefore, are excellent candidates for structural components where impact loading is expected. Compared to the large amount of research on the shape memory effect and/or pseudoelasticity of polycrystalline SMAs under quasi-static loading conditions, studies on dynamic loading are limited. Experimental research shows an apparent difference between the quasi-static and high strain rate deformation of SMAs. Research reveals that the martensitic phase transformation is strain rate sensitive. The mechanism for the martensitic phase transformation in SMAs during high strain rate deformation is still unclear. Many of the existing high strain rate models assume that the latent heat generated during deformation contributes to the change in the stress-strain behavior during dynamic loading, which is insufficient to explain the large stress observed during phase transformation under high strain rate deformation. Meanwhile, the relationship between the phase front velocity and strain rate has been studied. In this dissertation, a new resistance to phase transformation during high strain rate deformation is discussed and the relationship between the driving force for phase transformation and phase front velocity is established. With consideration of the newly defined resistance to phase transformation, a new model for phase transformation of SMAs during high strain rate deformation is presented and validated based on experimental results from an austenitic NiTi SMA. Stress, strain, and martensitic volume fraction distribution during high strain rate deformation are simulated using finite element analysis software ABAQUS/standard. For the first time, this dissertation presents a theoretical study of the microscopic band structure during high strain rate compressive deformation. The microscopic transformation band is generated by the phase front and leads to minor fluctuations in sample deformation. The strain rate effect on phase transformation is studied using the model. Both the starting stress for transformation and the slope of the stress-strain curve during phase transformation increase with increasing strain rate.

Les alliages à mémoire de forme ferromagnétique (FSMAs) possèdent la capacité d’accommoder une large déformation réversible à haute fréquence à l’aide d'une réorientation de la martensite induite par un champ magnétique. Cependant, cette réorientation à haute fréquence induit un frottement au niveau des interfaces entre les variantes de martensite provoquant une dissipation et par suite une élévation significative de la température dans le matériau, ce qui pose des problèmes d'instabilité nuisant à la performance du comportement dynamique des FSMAs. En particulier, l'amplitude de la déformation induite par le champ magnétique est réduite de façon significative lorsque l’augmentation de la température est suffisante pour déclencher la transformation de phase martensite-austénite. Un tel effet thermique sur les réponses dynamiques de FSMA n'a pas encore été étudié dans la littérature où la plupart des expériences dynamiques existantes ont été réalisées sur une courte période de temps (quelques secondes) afin d’éviter la variation de la température.Le but de cette thèse est l’analyse et la modélisation de ce phénomène. Pour ce faire, des analyses expérimentales et théoriques multi-échelles des performances des FSMAs soumis à un champ magnétique de longue durée et à haute fréquence sont réalisées.Tout d’abord, des expériences systématiques d'actionnement magnétique de longue durée (&gt; 100 secondes) sur une éprouvette en monocristal Ni-Mn-Ga sont effectuées à différents niveaux de la fréquence du champ magnétique, de la contrainte de compression initiale et du flux d'air ambiant (échange de chaleur) afin d’étudier leur influence sur la réponse des FSMAs. Par ailleurs, un modèle unidimensionnel de transfert de chaleur a été est développé permettant d’interpréter les nouveaux phénomènes liés aux effets thermiques mises en lumière expérimentalement. Ainsi, les conditions nécessaires à l’obtention d’une réponse dynamique stable ont été déduites. De plus, afin de comprendre la dépendance de la déformation nominale induite par le champ magnétique par rapport aux échanges thermiques à partir d'une analyse microscopique, la distribution/évolution de la déformation locale ainsi que la transformation/réorientation associée parmi les différentes phases/variantes au cours de l'actionnement à haute fréquence sous divers conditions d'échange de chaleur sont analysées via des observations in-situ à l’aide la corrélation d'images numériques (DIC). Un nouveau mécanisme est ainsi révélé : le mouvement des interphases induit par la variation de température (transformation de phase) et le mouvement des variantes de martensite induit par le champ magnétique (réorientation de martensite) peuvent être activés simultanément, sous l'actionnement magnéto-thermique-mécanique (i.e, le champ magnétique à haute fréquence, la force de ressort mécanique et le flux d'air ambiant) dans la mesure où le matériau peut auto-organiser les fractions volumiques des différentes phases/variantes afin de satisfaire toutes les conditions aux limites thermo-magnéto-mécaniques. En outre, la morphologie des bandes de déformations et des différentes phases/variantes auto-organisées est révélée et expliquée à l’échelle microscopique à l’aide des conditions de compatibilité géométrique<br>Ferromagnetic Shape Memory Alloys (FSMAs) have ability to provide large high-frequency reversible strain via magnetic field-induced martensite reorientation. But, the high-frequency frictional twin boundary motion of the martensite reorientation can induce a rapid accumulation of dissipation heat and cause a significant temperature rise in the material, which poses instability problems about the dynamic performance of FSMA. Particularly, the output strain amplitude would be reduced significantly when the temperature increases to be high enough to trigger the Martensite-Austenite phase transformation. However, such thermal effect on the dynamic responses of FSMA has not yet been investigated in literature where most existing dynamic experiments were performed only for a short-time period (a few seconds) to avoid the temperature variation. In this thesis, multi-scale experimental and theoretical analyses of the long-time performance of FSMA under high-frequency magnetic actuation are performed. Systematic experiments of the long-time magnetic actuation (&gt; 100 seconds) on a Ni-Mn-Ga single crystal bar are conducted at various levels of magnetic field frequency, initial compressive stress and ambient airflow (ambient heat-exchange efficiency) to investigate their influences on the stable state of the high-frequency FSMA-actuator. A one-dimensional heat-transfer model is developed and the new experimental phenomena of the thermal effects are well understood. Based on the experimental results and theoretical analysis, critical conditions to achieve the large and stable output strain amplitude in the high-frequency actuation are derived. Moreover, to understand the heat-exchange dependence of the output nominal-strain from a microscopic view, the local strain distribution/evolution and the associated transformation/reorientation among the different phases/variants during the high-frequency actuation under various heat-exchange efficiencies are demonstrated via the in-situ Digital Image Correlation observations. A novel mechanism is revealed: the temperature-driven phase boundary motion (phase transformation) and the magnetic field-driven twin boundary motion (martensite reorientation) can be activated at the same time under the magneto-thermal-mechanical actuation (i.e., the high-frequency magnetic field, the mechanical spring force and the varying ambient airflow) as the material can self-organize its volume fractions of the different phases/variants to satisfy all the thermo-magneto-mechanical boundary conditions. Further, the self-organized morphology/pattern composed of various variants and phases during cyclic deformation (with the moving habit plane and twin boundaries) can be explained by microstructure compatibility analyses

Since NiTi shape memory alloy (SMA) was discovered in the early 1960s, great progress has been made in understanding the properties and mechanisms of NiTi SMA and in developing associated products. For several decades, most of the scientific research and industrial interests on NiTi SMA has focused on its superelastic applications in the biomedical field and shape memory based “smart” devices, which involves the low strain rate (around 0.001 s^-1) response of NiTi SMA. Due to either stress-induced martensite phase transformation or stress induced martensite variant reorientation under the applied load, NiTi SMA has exhibited a high damping capacity in both austenitic and martensitic phase. Recently, there has been an increasing interest in exploitation of the high damping capacity of NiTi SMA to develop high strain rate related applications such as seismic damping elements and energy absorbing devices. However, a systematic study on the influence of strain, strain rate and temperature on the mechanical properties, phase transformation, microstructure and crystal structure is still limited, which leads to the difficulties in the design of products being subjected to high strain rate loading conditions. The four main objectives of the current research are: (1) achieve the single loading and the control of strain, constant strain rate and temperature in high strain rate compression tests of NiTi SMA specimens using Kolsky (split Hopkinson) compression bar; (2) explore the high strain rate compressive responses of NiTi SMA specimens as a function of strain (1.4%, 1.8%, 3.0%, 4.8%, and 9.6%), strain rate (400, 800 and 1200 s^-1), and temperature (room temperature (294 K) and 373 K); (3) characterize and compare the microstructure, phase transformation and crystal structure of NiTi SMAs before and after high strain rate compression; and (4) correlate high strain rate deformation with the changes of microstructure, phase transformation characteristics and crystal structure. Based on the results from this study, it was found that: (1) the compressive stress strain curves of martensitic NiTi SMAs under quasi-static loading conditions are different from those under high strain rate loading conditions, where higher strain hardening was observed; (2) the critical stress and stress plateau of martensitic NiTi SMAs are sensitive to the strain rate and temperature, especially at 373K, which results from the interplay between strain hardening and thermal softening; (3) the microstructure of martensitic NiTi SMA has changed with increasing strain rate at room temperature (294 K), resulting in the reduction in the area of ordered martensite region, while that area increases after deformation at elevated temperature (373K); (4) the phase transformation characteristic temperatures are more sensitive to deformation strain than strain rate; (5) the preferred crystal plane of martensitic NiTi SMA has changed from (11 ̅1)M before compression to (111)M after compression at room temperature (294 K), while the preferred plane remains exactly the same for martensitic NiTi SMA before and after compression at 373 K. Lastly, dynamic recovery and recrystallization are also observed after deformation of martensitic NiTi SMA at 373K.

The overarching goal of this research was to generate a menu of shape memory alloys (SMAs) actuator materials capable of meeting the demands of aerospace applications. Material requirements were recognized to meet the demand for high temperature SMAs with actuating temperatures above 85 °C and provide material options capable of performing over 100K actuation cycles. The first study is a preliminary characterization for the down selection of Ni-rich NiTiHf15 compositions chosen for a more in-depth examination of the nano-precipitation and evolution of the H-phase. To make this selection, the effect of Ni content in Ni-rich NiTiHf high temperature shape memory alloys (HTSMAs) on processability, microstructure, and hardness was analyzed for three compositions (Ni50.1TiHf15, Ni50.3TiHf15, Ni50.5TiHf15). Each composition was characterized under three conditions: homogenized, 25%, and 50% thickness reduction through hot-rolling. The second study emphasized the processing and aging response of an industrially produced, hot-extruded Ni50.3Ti29.7Hf20 (at%) HTSMA. The samples were sectioned into two halves with half remaining as-extruded and the other half hot-rolled to a 25% reduction in thickness. A portion of both conditions underwent conventional aging for 3 hours at various temperatures ranging from 450-750 °C, and the other portion was pre-aged for 12 hours at 300 °C followed by conventional aging treatments. After processing, the samples were characterized by differential scanning calorimetry (DSC), Vickers hardness (HV) testing, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and synchrotron radiation X-ray diffraction (SR-XRD). The relationship between the introduction of texturing, pre-aging, and aging on Ni-rich and high Hf-content compositions was investigated.

Le dépôt d'un revêtement sur la surface d'un alliage à mémoire de forme (AMF) est un problème rendu complexe par les déformations mécaniques importantes, réversibles et répétées subies par ces matériaux en cours d'usage (effet mémoire double sens, superélasticité). L'électropolymérisation sous polarisation cathodique a conduit à la construction de films de polyacrylonitrile (PAN) de 100 à 200 m d'épaisseur et de polyméthacrylonitrile (PDAN) de 15 micromètres d'épaisseur chimiquement greffés sur la surface d'un AMF à base de cuivre (Cu-Zn-Al). L'utilisation conjuguée de techniques électrochimiques (voltampérométrie, spectroscopie d'impédance électrochimique) et de techniques d'analyse de surface (XPS, UPS, Auger, IRRAS) a permis de décrire les mécanismes de greffage et de croissance des films. Lors d'essais de la tenue mécanique de films déposés sur la surface de l'AMF, une nette amélioration a été observée lorsque le film est construit par électropolymérisation, par rapport à un film simplement déposé ou trempé. Soumis à des tests de corrosion accélérée, les films de PMAN électropolymérisés présentent des propriétés protectrices permettant d'envisager leur utilisation pour le revêtement des AMF cuivreux en milieu corrosif

碩士<br>國立臺灣大學<br>材料科學與工程學研究所<br>107<br>In this study, the effect of heat treatment on transformation behaviors of Ni-rich Ti48.5Ni49.5Fe2 shape memory alloy (SMA) and on mechanical properties of equiatomic HfNbTiZr high entropy alloy (HEA) are systematically studied. Ti48.5Ni49.5Fe2 SMA exhibits strain glass transition in as solution-treated state, and a B2↔R transformation induced by Ti3Ni4 ppts when the specimens aged at 300 ºC/ 350 ºC. When the specimens aged at 400 ºC/500 ºC, a two-stage R-phase transformation, which was followed by a R↔B19’ transformation occurs, in which the R-phase transformation occurs at higher temperature is also induced by Ti3Ni4 ppts whereas that at lower temperature is related to the matrix away from ppts. If the ageing temperature is raised to 550 ºC/ 600 ºC, a B2↔R transformation occurs for long ageing time due to the lower Ni and higher Fe contents in the matrix. As for HfNbTiZr HEA, a single BCC-phase solid solution is identified in as cold-rolled state and in recrystallized state at above 900 ºC, and its hardness increases as the recrystallization temperature increases When 80% cold-rolled HfNbTiZr HEA aged at 350 ºC/450 ºC, the hardness increment of ~48% can be attained, while the hardening effect on 550 ºC aged specimens is not so pronounced due to the recovery effect. Though multiple phases are observed after aged at 350 ºC~550 ºC, the HEA’s age hardening is mainly contributed from HCP ppts.

Shape memory alloys (SMAs) exhibit unique combination of structural and functional properties and hence have a variety of current and potential applications. The mechanical behaviour of SMAs, in particular the influence of processing on the microstructure, which in turn influences the performance of the alloy, mechanical properties at the nano-scale, and under cyclic loading conditions, are of great current interest. In this thesis, specific issues within each of these broad areas are examined with a view to suggest further optimize/characterize SMAs. They are the following: (a) For thermo-mechanical secondary processing of SMAs, can we identify the optimum combination of temperature- strain rate window that yields a desirable microstructure? (b) How can indentation be used to obtain information about functional properties of shape memory alloys so as to complement traditional methods? (c) How can the information obtained from indentation be utilized for the identification of the alloy composition that yields a high temperature SMA through the combinatorial diffusion couple approach? Towards achieving the first objective, we study the hot deformation behavior of a cast NiTi alloy with a view of controlling the final microstructure. The “processing maps” approach is used to identify the optimum combination of temperature and strain rate for the thermomechanical processing of a SMA system commonly used in actuators applications (NiTiCu). Uniaxial compressions experiments are conducted in the temperature range of 800- 1050 °C and at strain rate range of 10-3 and 102 s-1. 2-D power dissipation efficiency and instability maps are generated and various deformation mechanisms, which operate in different temperature–strain rate regimes, are identified with the aid of these maps. Complementary microstructural analysis of specimens (post deformation) is performed with the help of electron backscattered diffraction (EBSD) analysis to arrive at a processing route which produces stress free grains. A safe window suitable for industrial processing of this alloy which leads to grain refinement and strain-free grains (as calculated by various methods of misorientation analysis representation) is suggested. Regions of the instability (characterized by the same analysis) result in strained microstructure, which in turn can affect the performance of the SMA in a detrimental manner. Next, to extract useful information from indentation responses, microindentation experiments at a range of temperatures (as the shape memory transformation is in progress) are conducted underneath the Vickers indenter. SME was observed to cause a change in the calculated recovery ratios at temperatures above As. Spherical indentation of austenite and martensite show different characteristics in elastic and elasto- plastic regimes but are similar in the plastic regime. NanoECR experiments are also conducted under a spheroconical indenter at room temperature, where the resistance measured is observed to increase during the unloading of room temperature austenite SMA. This is a signature of the reverse transformation back to austenite during the withdrawal of the indenter. Lastly, recovery ratios are monitored in the case of a NiTiPd diffusion couple before and after heat treatment at different temperature intervals using non- contact optical profilometry. The recovery ratio approach is successfully used to determine the useful temperature and %Pd range for a potential NiTiPd high temperature SMA. The method makes high throughput identification of high temperature shape memory alloys possible due to promising alloy compositions being identified at an early stage.

Kupferbasierte Legierungen mit Formgedächtniseffekt (z.B. Cu-Al-Ni-Mn) sind vergleichsweise kostengünstige Vertreter im Bereich der Hochtemperatur-Formgedächtnislegierungen mit vielversprechenden Umwandlungseigenschaften. Üblicherweise werden diese über konventionelle schmelzmetallurgische Prozesse hergestellt und dann einer thermomechanischen Behandlung unterzogen. Für die vorliegende Arbeit wurden die Formgedächtnislegierungen Cu-11.85Al-3.2Ni-3Mn und Cu-11,35Al-3,2Ni-3Mn-0,5Zr (m-%) unter Nutzung des selektiven Laserschmelzens (Selective Laser Melting – SLM) verarbeitet und Bauteile, nach einer Optimierung der Prozessparameter, mit einer hohen relativen Dichte (ca. 99%) hergestellt. Anschließend wurde der Einfluss des Energieeintrags, eines zusätzlichen Umschmelzschrittes (Mehrfachbelichtung) und einer Substratheizung auf das Gefüge, das Umwandlungsverhalten, die mechanischen Eigenschaften und die Rückverformung (Zweiweg-Effekt, Pseudoelastizität) untersucht. Zum Vergleich wurden weitere Probenkörper mittels Rascherstarrung der Schmelze hergestellt. Besonders die Korngröße und die thermische Stabilisierung der unterschiedlichen Phasen wirken sich unmittelbar auf die Umwandlungstemperaturen sowie das Rückverformungsverhalten aus. Durch die Nutzung des selektiven Laserschmelzens ergeben sich neue Möglichkeiten bei der Herstellung von endkonturnahen sowie geometrisch komplexen Bauteilen mit Formgedächtniseffekt. Zudem können die Gefüge, und damit die Umwandlungseigenschaften des Materials, bereits während der Herstellung eingestellt werden.