Universiẗat, Diss., 2003--München.

Prior work in this system has indicated that, under favourable conditions, the formation of a martensitic transformation product on cooling may be preceded by a second-order cubic-rhombohedral transformation. In the present investigation, thin-film electron microscopy and diffraction have been employed to study the pre-transformation phenomena in the temperature range 20-300 °C. The results are interpreted in terms of localized thermal diffuse scattering, resulting from the transformation, in terms of lattice dynamics, is suggested. dark field micrographs of the rhombohedral phase suggest a moderate degree of co-operation between adjacent domains. Additional data is provided in the form of volumetric and x-ray intensity measurements. / Thesis / Master of Engineering (ME)

Survey work has resolved certain reported anomalies in this system. In particular, TiNi was found to undergo two displacive phase transformations at temperatures near room temperature. Below 50°C the pseudo body-centered cubic compound undergoes a gradual distortion in an unusual second order phase transformation producing the "transition phase”, the existence of which is terminated by a typical martensitic burst transfor­mation at 20°C which produces the "martensitic phase". The crystal structures of the parent phase and transformation products, and the trans­formation characteristics have been studied with X-ray and electron diffrac­tion, electron microscopy, electrical resistance and density measurements. / Thesis / Master of Science (MS)

TiNi films were prepared by co-sputtering TiNi target and a separate Ti target. Crystalline structure and phase transformation behaviors of TiNi films were investigated. Results showed that TiNi films had fine grain size of about 500 nm and fully martensitic structure at room temperature. X-ray photoelectron spectroscopy (XPS) results indicated that there is adherent and natural TiO₂ film, which is beneficial to its corrosion resistance and biocompatibillity. Results from differential scanning calorimeter (DSC), in-situ X-ray diffraction (XRD) and curvature measurement revealed clearly martensitic transformation upon heating and cooling. The TiNi films were further deposited on micromachined silicon cantilever and membrane structures in order to form micro-gripper or microvalve with large deformation due to shape-memory effect. / Singapore-MIT Alliance (SMA)

In this paper, some critical issues and problems in the development of TiNi thin films were discussed, including preparation and characterization considerations, residual stress and adhesion, frequency improvement, fatigue and stability, as well as functionally graded or composite thin film design. Different types of MEMS applications were reviewed and the prospects for future advances in fabrication process and device development were discussed. / Singapore-MIT Alliance (SMA)

TiNi shape memory alloy is characterized by shape memory and superelastic effects which occur due to reversible martensite transformation. It has been recently found that TiNi alloy has superior dent and wear resistance compared to other conventional materials. The stress-induced martensite transformation exhibited by this alloy contributes to its dent and wear resistance. Much work is required to establish the fundamental principals governing the superelastic behavior of TiNi under wear and indentation conditions. Understanding the superelastic behavior helps to employ superelastic TiNi in applications where high impact loading is expected as in gears and bearings. In this study the superelastic behavior of shape memory alloys under reciprocating sliding wear and indentation loading conditions was investigated. The deformation behavior of superelastic Ti-Ni alloys was studied and compared to AISI 304 stainless steel. Dominant wear and deformation mechanisms were identified.

TiNi shape memory alloys (SMAs) have been extensively used in various applications. The great interest in TiNi alloys is due to its unique shape memory and superelasticity effects, along with its superior wear and dent resistance. Shape memory and superelastic effects are due to a reversible martensitic transformation that can be induced either thermally or mechanically. In this study, indentation tests at different temperatures, loads and strain rates have been performed on superelastic TiNi alloy. Deformation characteristics of superelastic TiNi under indentation have been compared to AISI 304 steel as a conventional material. Also, in-situ optical microscopy tests with interrupted heating have been employed in order to gain an insight into the coupled deformation and reversible martensitic transformation behaviour of TiNi SMAs under tensile loads. An understanding of the impacts of strain rate and temperature on the deformation behaviour of TiNi SMAs under localized compressive loads has been proposed.

No description available.

Cette thèse vise à améliorer les propriétés d'alliage à mémoire de forme et de stockage d'hydrogène du TiNi par des substitutions chimiques sur le sous-réseau de Ni. L'effet de la substitution de Ni par Pd, Cu et Co sur les propriétés structurales, la transformation martensitique et les propriétés d'hydrogénation de TiNi, a été étudiée par des techniques structurales (diffraction des Rayons-X et des neutrons sur poudre), calorimétriques, par réaction solide-gaz et par mesure électrochimique. Des calculs de type DFT ont été effectués pour mettre en évidence les modifications de la structure électronique sur les propriétés d'hydrogénation. Les trois substitutions, TiNi1-zMz (M = Pd, Cu et Co; z ≤ 0,5), conduisent à la formation de composés pseudo-binaires. La substitution par Pd et par Cu augmentent le volume de la maille de TiNi, alors que celle par Co possède un effet inverse. Les températures de transformation martensitique suivent la même tendance que les changements de volume. Ils augmentent fortement pour M = Pd et légèrement pour M = Cu, tandis qu'elles diminuent pour M = Co. Les propriétés d'hydrogénation sont très sensibles à la nature chimique des substitutions. La capacité diminue fortement avec la substitution par Pd, modérément avec le Cu et reste stable pour le M = Co. Contrairement à l'effet attendu par des considérations géométrique, la substitution par Pd et par Cu diminuent la stabilité des hydrures. Les calculs DFT montrent qu'un effet électronique, et non géométrique, régit la stabilité des hydrures pour M = Pd. La substitution par le cobalt induit une formation de plusieurs hydrures par étapes successives, observée par mesure d'isothermes pression-composition en multiple plateaux. Pour les applications, la substitution de Ni par Cu avec une faible teneur (z = 0,2) augmente la capacité de décharge électrochimique de TiNi de 150 à 300 mAh/g en raison de la déstabilisation de l'hydrure. Cela ouvre de nouvelles perspectives pour l'utilisation d'alliages TiNi comme électrodes de batteries Ni-MH. En revanche, la substitution par Pd ou Cu (avec z = 0,5) est très efficace pour réduire la réactivité de TiNi avec l'hydrogène, et est donc intéressante pour des applications à mémoire de forme sous un environnement réducteur / The PhD thesis aims to improve shape memory and hydrogen storage properties of TiNi by chemical substitutions in the Ni sub-lattice. The effect of Pd, Cu and Co substitutions on crystal structure, martensitic transformation and hydrogenation properties of TiNi has been studied by structural (X-ray and neutron powder diffraction), calorimetric, solid-gas and electrochemical means. Ab initio DFT calculations were done to highlight electronic effects on hydrogenation properties. The three substitutions, TiNi1-zMz (M = Pd, Cu and Co; z ≤ 0.5), lead to the formation of pseudobinary compounds. Substitutions by Pd and Cu increase the unit-cell volume of TiNi, whereas the reverse effect occurs for Co. Martensitic transformation temperatures correlate with volume variations. They increase strongly for M = Pd and slightly for M = Cu, whereas M = Co decreases it. Hydrogenation properties are very sensitive to chemical elements substitution. The capacity decreases strongly for M = Pd, moderately for M = Cu and remains stable for M = Co. Contrary to expected effect by geometric model, both Pd and Cu substitutions decrease the stability of hydrides. DFT calculations show that electronic rather than geometric effects govern hydride stability for M = Pd. Co substitution induces step-wise formation of hydrides with a multi-plateau behaviour in pressure-composition isotherms. As concerns applications, low amount of Cu substitution (z =0.2) increases the electrochemical discharge capacity of TiNi from 150 to 300 mAh/g due to hydride destabilization. This opens new perspectives for using TiNi-based alloys in Ni-MH batteries. In contrast, Pd and high amount of Cu substitution (z = 0.5) are effective to decrease TiNi reactivity towards hydrogen, and therefore attractive for shape memory applications under reductive environment

In the scope of this study, TiNi foams with porosities in the range of 39-64 vol% were processed from prealloyed powders by Mg space holder technique. Porous TiNi alloys displayed homogeneously distributed spherical pores with interconnections, which is suitable for bone ingrowth. Porous Ti-50.8 at%Ni alloys were processed by sintering at 1200 &deg / C for 2 h to analyze the microstructure as well as mechanical behavior. SEM, TEM and XRD studies were conducted for the characterization of microstructure and phase analyses in addition to the mechanical characterization performed by monotonic and superelasticity compression tests as well as compressive fatigue tests. It was observed that stress required to trigger martensitic transformation was decreased via increasing porosity. The monotonic compression test results also indicated that altering the porosity content of TiNi foams leads to different monotonic compression behaviors. It was observed that the foams display more bulk deformation like behavior as a composite structure composed of TiNi and macropores when the porosity content was low. As the porosity content has increased, the struts became more effective and deformation proceeds by the collapse of favorable struts. On the other hand, cyclic superelasticity tests results indicated that maximum achieved and recovered strain values at the end of fifth cycle increase while the fraction of strain recovered at the end of fifth cycle decreases with decreasing porosity content. Furthermore, the fatigue lives of the processed foams were observed to vary within a band which has a width decreasing with decreasing &sigma / max / &sigma / y yielding an endurance limit ranging in between 26-89 MPa or 0.5-0.6 &sigma / y. Fractography studies on the failed foams after fatigue testing revealed that the failure occurs by the coalescence of micro-cracks initiated from pore walls leading to macro-cracks aligned at 45o with respect to the loading axis. In addition to the mentioned characterization studies, the effects of sintering temperature and time on TiNi foams with 58 vol% porosity as well as heat treatment on the microstructure and the mechanical behavior of TiNi foams with 49 vol% porosity were analyzed with SEM and compression tests. Aging of TiNi foams with 49 vol% porosity at 450 &deg / C for 1.5 h has shown that the presence of Ti3Ni4 precipitates improve the superelastic response.