Shape memory alloys are a class of alloys that display the unique ability to undergo large plastic deformations and return to their original shape either through the application of heat (shape memory effect) or by relieving the stress causing the deformation (superelastic effect). This research takes advantage of the unique characteristics of shape memory alloys in order to provide a moment resisting connection with recentering capabilities. In this study, superelastic Nitinol, a nickel-titanium form of shape memory alloy that exhibits a flag-shaped stress versus strain curve, is used as the moment transfer elements within a partially restrained steel beam-column connection. Experimental testing consists of a one-half scale interior connection where the loading is applied at the column tip. A pseudo-static cyclic loading history is used which is intended to simulate earthquake loadings. The energy dissipation characteristics, moment-rotation characteristics, and deformation capacity of the connection are quantified. Results are then compared to tests where A36 steel tendons are used as the moment transfer elements. The superelastic Nitinol tendon connection showed superior performance to the A36 steel tendon connection, including the ability to recenter without residual deformation.

Hybrid Titanium Composite Laminates (HTCL) have vast potential for future commercial aircraft development. In order for this potential to be properly utilized the HTCLs material properties must first be well understood and obtained through experimentation. Crack initiation and crack growth characteristics of HTCLs are dependent on the heat treatment of the embedded constituent titanium foil. While high strength titanium foils may delay crack initiation, there may be an adverse effect of unsuitable crack growth rates in the HTCLs. Literature has indicated that when properly designed, cracks in HTCLs can arrest due to fiber bridging mechanisms and other crack closure mechanisms. Traditional surface inspection techniques employed on facesheet laminate evaluations will not be able to properly monitor the internal crack growth and damage progression for the internal plies. The main objective of the this joint Georgia Tech/Boeing research project was to determine and compare crack initiation and crack growth characteristics of different heat-treated -Ti 15-3 titanium foil embedded in HTCLs. Georgia Tech utilized a unique capability of x-raying the internal foils of the HTCL specimen in a servo-hydraulic test frame while under load. The titanium foil in this study represented four different heat treatments that result in four increasing levels of strength and decreasing levels of elongation. Specifically, open-hole HTCL coupons were tested at four stress load levels under constant amplitude fatigue cycles to determine a-N curves for the HTCL layups evaluated. The layup evaluated was [45/0/-45/0/Ti/0/-45/0/45]. Crack growth rates were determined once the initiated crack was detected via radiographic exposure. Radiographic delamination analysis and thermoelastic stress analysis techniques were employed to determine additional damage mechanisms in the laminate. Analytical and finite element methods were utilized to determine ply stresses. Additionally, titanium foil properties were determined via dog-bone coupons for each of the four heat treatment conditions.

M. Tech. Metallurgical Engineering. / Studies the corrosion behavior of sintered Ni-Ti shape memory alloys containing Cu and Nb additions.Objectives are: 1. Investigate structural and phased interactions occurring during the sintering of the allloys. 2. Study the effects of niobium and copper addition on the general corrosion behavior of the sintered nickel-titanium alloys in sodium chloride and sulphuric acid. 3. Study the effects of copper and niobium addition on localized corrosion behavior of the alloys in chloride environments.

M. Tech. Metallurgical Engineering. / StudIes the tribocorrosion behaviour of copper-nickel-titanium shape memory composite reinforced by zirconia,synthesized through powder metallurgy process. The research aims to achieve the following objectives: 1. Study the tribocorrosion mechanisms of the composites in NaCl solution (typical human body fluid). 2. Investigate the tribocorrosion mechanisms of the composites in other environments typical of some engineering applications.The proposed study on incorporating zirconia into the matrix NiTiCu through powder metallurgical process and investigations of the phenomenon of joint wear-corrosion synergism occurring in sodium chloride considered typical of human body system and sulphuric acid environment typical of wide range engineering applications is therefore very novel. It is therefore aimed that information on the tribocorrosion behaviour of NiTiCu as well as with zirconia incorporation will form basis for typical compositional formulation approaches for improved bio-tribocorrosion improvement in biomedical applications and actuators used in other engineering applications.

Shape memory alloys (SMAs) are widely used in a broad variety of applications in multiscale devices ranging from nano-actuators used in nano-electrical-mechanical systems (NEMS) to large energy absorbing elements in civil engineering applications. This research introduces a multiscale analysis for SMAs, particularly Nickel-Titanium alloys (NiTi). SMAs are studied in a variety of length scales ranging from macroscale to nanoscale. In macroscale, a phenomenological constitutive framework is adopted and developed by adding the effect of phase transformation latent heat. Analytical closed-form solutions are obtained for modeling the coupled thermomechanical behavior of various large polycrystalline SMA devices subjected to different loadings, including uniaxial loads, torsion, and bending. Thermomechanical responses of several SMA devices are analyzed using the introduced solutions and the results are validated by performing various experiments on some large SMA elements. In order to study some important properties of polycrystalline SMAs that the macroscopic phenomenological frameworks cannot capture, including the texture and intergranular effects in polycrystalline SMAs, a micromechanical framework with a realistic modeling of the grains based on Voronoi tessellations is used. The local form of the first law of thermodynamics is used and the energy balance relations for the polycrystalline SMAs are obtained. Generalized coupled thermomechanical governing equations considering the phase transformation latent heat are derived for polycrystalline SMAs. A three-dimensional finite element framework is used and different polycrystalline samples are modeled. By considering appropriate distributions of crystallographic orientations in the grains obtained from experimental texture measurements of NiTi samples the effects of texture and the tension-compression asymmetry on the thermomechanical response of polycrystalline SMAs are studied. The interaction between the stress state (tensile or compressive), number of grains, and the texture on the thermomechanical response of polycrystalline SMAs is also studied. For studying some aspects of the thermomechanical properties of SMAs that cannot be studied neither by the phenomenological constitutive models nor by the micromechanical models, molecular dynamics simulations are used to explore the martensitic phase transformation in NiTi alloys at the atomistic level. The martensite reorientation, austenite to martensite phase transformation, and twinning mechanisms in NiTi nanostructures are analyzed and the effect of various parameters including the temperature and size on the phase transformation at the atomistic level is studied. Results of this research provide insight into studying pseudoelasticity and shape memory response of NiTi alloys at different length scales and are useful for better understanding the solid-to-solid phase transformation at the atomistic level, and the effects of this transformation on the microstructure of polycrystal SMAs and the macroscopic response of these alloys.

In this thesis, I present studies in the field of terahertz (THz) spectroscopy. These studies are divided into three areas: Development of a narrowband THz source, the study of carrier transport in metal thin films, and the exploration of coherent dynamics of quasi-particles in semiconductor nanostructures with both broadband and narrowband THz sources. The narrowband THz source makes use of type II difference frequency generation (DFG) in a nonlinear crystal to generate THz waves. By using two linearly chirped, orthogonally polarized optical pulses to drive the DFG, we were able to produce a tunable source of strong, narrowband THz radiation. The broadband source makes use of optical rectification of an ultra-short optical pulse in a nonlinear crystal to generate a single-cycle THz pulse. Linear spectroscopic measurements were taken on NiTi-alloy thin films of various thicknesses and titanium concentrations with broadband THz pulses as well as THz power transmission measurements. By applying a combination of the Drude model and Fresnel thin-film coefficients, we were able to extract the DC resistivity of the NiTi-alloy thin films. Using the narrowband source of THz radiation, we explored the exciton dynamics of semiconductor quantum wells. These dynamics were made sense of by observing time-resolved transmission measurements and comparing them to theoretical calculations. By tuning the THz photon energy near exciton transition energies, we were able to observe extreme nonlinear optical transients including the onset of Rabi oscillations. Furthermore, we applied the broadband THz waves to quantum wells embedded in a microcavity, and time-resolved reflectivity measurements were taken. Many interesting nonlinear optical transients were observed, including interference effects between the modulated polariton states in the sample. / Graduation date: 2012

Thesis (MScEng (Industrial Engineering))--University of Stellenbosch, 2009. / The titanium alloys have found wide application in the aerospace, biomedical and automotive industries. Soaring fuel prices and environmental concerns are the fundamental drivers that intensify the demand situation for titanium. From a machining viewpoint, one of the challenges companies face, is achieving high material removal rates while maintaining the form and function of the part. The ultimate aim for a machining business remains to make parts quickly. Conventional cutting speeds range from 30 to 100 m/min in the machining of Ti-6Al-4V. Milling this alloy faster however is challenging. Although titanium is becoming a material of choice, many of the same qualities that enhance titanium‟s appeal for most applications also contribute to its being one of the most difficult materials to machine. The author explored the potential for Polycrystalline diamond (PCD) inserts in high speed milling of Ti-6Al-4V, by trying to understand the fundamental causes of tool failure. The objective was to achieve an order of magnitude increase in tool life, while machining at high speed, simply by reducing some of the failure mechanisms through different cutting strategies. Tool wear is described as a thermo-mechanical high-cycle fatigue phenomenon. The capability of a higher material removal per tool life is achieved in the case of PCD inserts compared to Tungsten carbide (WC). The average surface roughness produced was relatively low. The collected chips were also analyzed. The work demonstrated progress over the performance reported in current literature. The work confirms that there is a region where a sufficiently high temperature in the cutting zone may contribute to extended tool life, provided that the tool material can withstand these extreme conditions.

Thesis (MScEng)--Stellenbosch University, 2013. / ENGLISH ABSTRACT: The aerospace- and automotive industries have an urgency to save space and reduce weight, as well as a need to increase fuel efficiency and reduce emissions. This has led to the use of lightweight structural materials, such as Ti6A14V alloy, which is the most widely used titanium alloy in the aerospace industry. This alloy has an exceptional strength-to-density ratio. The work also covers studies on tool steel 40CrMnMo7 that is used in applications in the tooling-, aerospace and automotive industry. In the quest for improved performance new alternative methods of efficiently machining these materials are investigated. One of the important criteria during machining of these materials is their machinability. This study discusses current research in high performance machining strategies and techniques for advanced materials such as Ti6Al4V and 40CrMnMo7. The properties that make these materials advantageous for the use in the aerospace- and automotive industry also make them difficult to cut. The widespread application of Ti6Al4V in the aerospace industry has encouraged investigations into cooling strategies or -techniques to maintain and improve tool life. Ti6Al4V has a low thermal conductivity causing the heat generated during machining to accumulate on the cutting edge of the tool. During various experiments the application of external compressed air blow cooling (dry cutting), flood cooling, high pressure through spindle cooling (HPTSC) and modifications thereof were investigated. The research project also evaluated the performance of a coating (TiAlN) and various coating treatments. The objectives of the HPTSC modifications were to improve the coolant stream impingement on the tool surface, effectively compressing the thermal barrier, and to reduce the chip-tool contact area. This would lead to a decrease in tool heating and wear. The modified techniques failed to increase tool life but showed signs of increased heat removal capability under the given conditions. It was observed that air blow cooling (dry cutting) delivered the best results when considering cutting materials, coating, coating treatment and cooling strategies or –techniques throughout the experiments conducted. / AFRIKAANSE OPSOMMING: Die Ruimte-en motor-industrie het 'n dringendheid om ruimte te bespaar en gewig te verminder, sowel as 'n behoefte om brandstofdoeltreffendheid te verbeter en emissies te verminder. Dit het gelei tot die gebruik van liggewig strukturele materiale, soos Ti6A14V Allooi , wat die mees gebruikte titanium allooi in die Ruimte is. Hierdie allooi het 'n uitsonderlike krag-tot-digtheid-verhouding. Die studie dek ook gereedskapstaal 40CrMnMo7 wat in die gereedskap, Ruimte-en motor-industrie aangewend word. In die soeke na verbeterde prestasie word nuwe alternatiewe metodes om effektief bewerking van hierdie materiaal ondersoek. Een van die belangrikste kriteria tydens bewerking van hierdie materiaal is die bewerkbaar daarvan. Hierdie studie bespreek die huidige navorsing in hoë prestasie bewerking strategieë en tegnieke vir gevorderde materiale, soos Ti6Al4V en 40CrMnMo7. Die eienskappe wat hierdie materiaal voordelig maak vir die gebruik in die lug-en Ruimte-en motor-industrie, maak dit terselfdetyd moeilik om te sny. Die wydverspreide toepassing van Ti6Al4V in die lug-en Ruimte industrie moedig ondersoeke aan na koelstrategieë of -tegnieke om die instrumentlewe te handhaaf en te verbeter. Ti6Al4V het lae termiese geleidingsvermoë wat veroorsaak dat die hitte, wat gegenereer word tydens bewerking, versamel op die voorpunt van die instrument. Tydens verskillende eksperimente was die toepassing van eksterne saamgeperste lugblaas-verkoeling (droë sny), vloed verkoeling, hoë-druk-deur-die-spil-afkoeling (HPTSC) en aanpassings daarvan geondersoek. Die navorsingsprojek het ook die prestasie van 'n bedekkingslaag (TiAlN) en verskeie bedekkingslaagbehandelings geëvalueer. Die doelwit van die HPTSC aanpassing was om die koelmiddelstroom beklemming op die instrument oppervlak te verbeter, en effektiewelik die termiese versperring saam te pers, asook die skerf-teenoorinstrument kontak te verminder. Dit sou lei tot 'n afname in die instrumentverwarming en -slytasie. Die gewysigde tegnieke het daarin misluk om die instrumentlewe te verhoog, maar het tekens getoon van 'n toename in hitte verwydering vermoë onder die gegewe omstandighede. Dit is dus waargeneem dat lugblaasverkoeling (droë sny) die beste resultate gelewer het in die oorweging van sny materiale, bedekkingslaag, bedekkingslaagbehandelings en verkoeling strategieë of -tegnieke wat regdeur die eksperimente uitgevoer was.

The aim of this thesis is to provide microstructural and mechanical characterisation of α-β titanium alloys exposed to a range of thermo-mechanical conditions, in particular under-going high rate deformation at elevated temperatures, representative of the Linear Friction Welding (LFW) manufacturing process. Three α-β titanium alloys provided by Rolls-Royce are studied: Ti-64 blade, disc and Ti-6246 disc. Ti-64 and Ti-6246 show complex deformation behaviour with strain, strain rate and temperature, especially near the transus temperature, where the low temperature α phase is transformed into the high temperature β phase. The microstructure and mechanical properties evolve in an interconnected fashion, and understanding this mutual influence is necessary to better predict the behaviour of these alloys. Characterisation of the mechanical properties was performed through uniaxial compression tests at strain rates from 0.001 to 3000 s^-1, using an Instron screw-driven machine at quasi-static rates, a servo-hydraulic machine at medium rates and a Split-Hopkinson Pressure Bar and a drop-weight tower at high strain rates. The tests were performed over a range of temperatures from room temperature to 1300 °C. The main focus was on high strain rate and high temperature tests, with the development of a gravity driven direct impact Hopkinson bar, referred as a drop-weight system, which is intended to evaluate the mechanical response of metals to high strain rate loading at temperatures up to c. 1300 °C. The design and principles of operation of the system are presented, along with calibration and validation data. Preliminary tests were performed on stock Ti-64, heated at two rates: 1 and 20 °C s^-1. The evolution of the mechanical properties was analysed, focussing on the strain rate, temperature and phases dependencies. Characterisation of the microstructure was realised by performing interrupted compression tests, first at room temperature, three plastic strains, 4%, 10% and 20%, and two different strain rates, 0.001 and 2000 s^-1; then at 4% plastic strain, a strain rate of 2000 s^-1 and three elevated temperatures, 700, 900 and 1100 °C. A better understanding of the microstructure evolution with strain, strain rates and temperature, including the macrotexture and microtexture of the specimens, was obtained using Electron Backscatter Diffraction (EBSD) to characterise the texture of the undeformed and deformed materials. The better understanding of the flow stress and microstructural evolution of both Ti-64 and its individual α and β phases with various strain rates and temperatures is intended to be used in the development of more accurate models representing the behaviour of these alloys. Predicting the microstructure evolution and then the mechanical properties of a material is essential to optimise the final mechanical properties of the alloys when welded by manufacturing processes such as the LFW process.

En raison d’une combinaison de propriétés physiques, chimiques et mécaniques remarquables, les alliages de titane et de titane sont devenus des candidats prometteurs dans le domaine de l'industrie chimique, de l’aéronautique, de l'aérospatiale et des matériaux biomédicaux. Durant les procédures de fabrication ainsi qu’en service, les composants sont exposés à des environnements tels que la surface de ces composant seront exposées à l’hydrogène. Par conséquent, la compréhension de l’interaction de ces matériaux avec l'hydrogène lors de divers procédés de d’élaboration et de mise en forme est importante afin que leurs propriétés et leurs performances puissent être contrôlées et prédites de manière fiable. Dans le but d'améliorer les propriétés et les performances du titane et de réduire le coût de fabrication des produits en titane, le présent travail se concentre principalement sur les effets de l'hydrogène sur le laminage à froid et le phénomène de recristallisation de deux titanes caractérisés par une structure cristalline différente, c'est-à-dire une structure hexagonale et cubique cubique pour respectivement le titane de pureté commerciale Ti50A et l'alliage de titane β-métastable β-21S. Étant donné que la microstructure du titane et de ses alliages est le facteur contrôlant leurs propriétés et leurs performances, l'évolution microstructurale en présence d'hydrogène introduit par deux méthodes distinctes a été analysée par une combinaison de techniques expérimentales incluant DRX, SEM-EBSD et TEM. L'introduction de l'hydrogène dans le Ti50A par une méthode électrolytique a conduit à la précipitation de deux types d'hydrures (δ-TiHx, ε-TiH2) dans la matrice de phase , et il a été constaté que la fraction volumique de ces hydrures augmentait avec la durée du temps de chargement électrolytique. En raison du mode de formation par précipitation à partir des joints de grains, cinq relations d'orientation (OR) entre la -phase et l'hydrure δ-TiHx ont été déterminées par analyses des projections stéréographiques, et parmi celles-ci trois nouvelles relations d'orientation ont pu être mise en évidence. En outre, la corrélation entre la texture de laminage et la précipitation à l'hydrure a pu être établie. On a constaté que l'existence de la texture de de laminage facilitait la précipitation d'hydrure δ suivant l'orientation d’orientation de type OR2. Les analyses de rayons X révélaient un élargissement des pics de diffraction de la phase , ce qui indiquait une augmentation de la densité de dislocation, ces dislocations étant nécessaires pour tenir compte de l'inadéquation du réseau entre les hydrures et la matrice. Sur la surface d’échantillons déformés en compression, l'observation des traces de glissement et de macle de tension de type TT1 {10 2} < 011> dans les -grains contenant des hydrures a suggéré que les hydrures avaient une certaine capacité à supporter une déformation de cisaillement, en fonction des relations d'orientation entre la matrice et les hydrures mais aussi de leur épaisseur. Bien qu'aucune corrélation directe entre la nucléation des macles et la présence des hydrures n’ait été établie dans cette étude, l'effet des hydrures sur le développement des macles a été constaté. En étudiant l’effet de l'hydrogène sur le comportement au laminage à froid dans Ti50A, il a été possible de montrer que la formation de macle de type TT1 peut être facilitée par l'augmentation du rapport c/a de la maille hexagonale résultant de l’addition de l'hydrogène et de l'existence de contraintes locales générées par la précipitation des hydrures. Un raffinement de la microstructure a également été observé dans le Ti50A hydrogéné, ce qui suggère que la présence d'hydrures a tendance à générer de nouveaux grains de fortes désorientations (HAB). En outre, de nombreuses dislocations géométriquement nécessaires (GND) permettant de tenir compte de l'incompatibilité de contrainte entre l'hydrure et [...] / Due to an attractive combination of physical, chemical and mechanical properties, titanium and titanium alloys have become promising candidates in the field of chemical industry, aerospace and biomedical materials. During manufacturing procedures and applications, components are exposed to environments that can act as sources of hydrogen. Therefore, understanding their interaction with hydrogen upon various mechanical/thermal processing is important so that their properties and performance can be controlled and reliably predicted. With the aim of enhancing the properties and performance of titanium and reducing the cost of manufacturing products, the present work is primarily focusing on the effect of hydrogen on the cold rolling behavior and the subsequent annealing of titanium and titanium alloy characterized by different crystalline structure, i.e. hexagonal and body cubic centered (bcc) structure for the commercial pure titanium Ti50A and metastable β-titanium alloy β-21S, respectively. Since the microstructure of titanium and its alloys is the governing factor that determines their properties and performance, the microstructural evolution in the presence of hydrogen upon various procedures was analyzed by combination of XRD, SEM-EBSD and TEM. The introduction of hydrogen in Ti50A by electrolytic method induced the precipitation of two types of hydrides (δ-TiHx, ε-TiH2) in the  phase matrix, and it was found that the volume fraction of these hydrides increased as the charging time increased. Five orientation relationships (ORs), three of them being new orientation relationships ever reported, between the -phase and the hydride δ-TiHx were determined. Moreover, the correlation between the rolling texture and the hydride precipitation was also established. It was found that the existence of the rolling texture facilitated the precipitation of δ-hydride following the OR2-type orientation relationship. X-ray analyses revealed a broadening of the diffraction peaks corresponding to the  phase, indicating a increase of the dislocation density, these dislocations being necessary to accommodate the lattice misfit between hydrides and the matrix. Under compression loading, the observation of slip traces and tension twin {10 2}< 011> TT1 in the -grains containing hydrides, suggested that the hydrides had a certain ability to accommodate the imposed shear strain, depending on the orientation relationships between the matrix and the hydrides as well as on their thickness. Although no correlation between the nucleation of twinning and the hydride could be established in this study, the hydrides seemed to play an important role on the development of twinning deformation. The effects of hydrogen on the cold rolling behavior in Ti50A showed that, the formation of TT1tension twins can be facilitated due to the increase of the c/a ratio owing to the hydrogen addition and the existence of local stresses generated by the precipitation of hydrides. The refinement of the microstructure was also observed in the hydrogenated Ti50A-H suggesting that the presence of hydrides can enhance the generation of high angle boundaries (HABs). In addition, the formation of numerous geometrically necessary dislocations (GND) allowing the accommodation of the strain incompatibility between the hydride and matrix could be worked out by SEM-EBSD, which also confirmed analyses of the X-ray traces. In the case of β-21S alloy, with bcc structure that can accommodate a larger concentration of interstitial atoms, hydrogen was introduced by gas method. The effect of hydrogen on the microstructure was found to be closely related to the hydrogen concentration. In the range of hydrogen/metal ratio 0.052 < H/M < 0.300, the microstructure consisting of the single β-phase showed that the dissolved hydrogen atoms expanded the bcc β-lattice and suppressed the decomposition of the β phase upon cooling [...]