Thesis (M.Sc.Eng.)-University of KwaZulu-Natal, Durban, 2009. / Full text also available online. Scroll down for electronic link.

Thesis (M.Sc.)--City University of Hong Kong, 2004. / At head of title: City University of Hong Kong, Department of Physics and Materials Science, Master of Science in materials engineering & nanotechnology dissertation. Title from title screen (viewed on Aug. 31, 2006) Includes bibliographical references.

Thesis (M.S.)--Worcester Polytechnic Institute. / Keywords: thermomechanical, SMAs, NiTiNOL, ACES, OEH. Includes bibliographical references.

Thesis (Ph. D.)--Civil and Environmental Engineering, Georgia Institute of Technology, 2006. / Gall, Kenneth, Committee Member ; Leon, Roberto, Committee Member ; Kurtis, Kimberly, Committee Member ; Jacobs, Laurence, Committee Member ; DesRoches, Reginald, Committee Chair.

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.

Le titane, comme ses alliages, présente des caractéristiques remarquables qui peuvent être modulées du fait des nombreuses microstructures qu’il est possible d’obtenir. Grâce à cette grande variété, le titane et ses alliages possèdent un grand nombre de propriétés. Parmi les plus intéressantes, on peut citer leur résistance à la corrosion, leur biocompatibilité, mais aussi leurs excellentes propriétés mécaniques (résistance, ductilité, ténacité, fluage…). Pour toutes ces raisons, l’attrait pour les alliages de titane n’a cessé de croître dans de nombreux secteurs. En effet ils sont maintenant largement utilisés dans les industries aéronautique et chimique, mais aussi l’architecture, le naval, l’industrie automobile, le sport ou encore la médecine. La nanoindentation est utilisée couramment de nos jours pour déterminer les propriétés mécaniques locales des matériaux. Elle permet notamment de caractériser des alliages métalliques possédant une microstructure polycrystalline. La taille de l’indenteur en nanoindentation étant faible (de quelques micromètres à quelques dizaines de micromètres), cette technique est idéale pour caractériser les propriétés mécaniques de surface des différents grains d’un matériau. Elle permet notamment de mesurer simultanément la dureté et le module d’élasticité. Si les essais de nanoindentation sont associés à un banc motorisé X-Y, une matrice étendue d’indents peut être réalisée avec un pas de quelques micromètres. Grâce à cette technique et dans le cadre de ce travail de thèse, nous avons réalisé dans un premier temps des cartographies de dureté et de module d’élasticité (HIPF et EIPF). Dans un second temps, nous avons évalué des propriétés non-conventionnelles d’alliages de titane, telles que l’effet mémoire de forme et la superélasticité. Dans la première partie de l’étude, la nanoindentation a été corrélée à l’EBSD (diffraction des électrons rétro-diffusés) afin d’identifier la relation entre l’orientation cristallographique d’un grain et ses propriétés mécaniques. L’étude a été menée sur les alliages de composition Ti-30Nb et Ti-27Nb (%at) de structure cubique centrée (phase ), et sur le titane de pureté commerciale T40, de structure hexagonale compacte (phase ). Dans la seconde partie de l’étude, la nanoindentation a été utilisée pour mesurer l’effet mémoire de forme (SM) et la superélasticité (SE) de différents alliages de titane à travers une large gamme de profondeur d’indentation. La mesure de ces propriétés non-conventionnelles a été réalisée à partir de l’étude des courbes charge-déplacement obtenues pour chaque essai d’indentation. L’amplitude de l’effet SE et SM a été caractérisée par des ratios de hauteur et de travail déterminés par l’étude des courbes de nanoindentation ainsi que des profils AFM réalisés au microscope à force atomique. / Titanium and titanium alloys presents remarkable characteristics which can be modulated due to the many different microstructures that is possible to obtain. Thanks to this huge variety, titanium and its alloys can exhibit many properties. Among the most interesting, there may be mentioned their corrosion resistance, biocompatibility, but also their excellent mechanical properties (strength, ductility, toughness, creep…). For all these reasons, interest for of titanium alloys has been growing in many areas. Indeed they are now widely used in the aerospace and chemical industries, but also in architecture, naval, automotive, sports or medicine. Nanoindentation is commonly used nowadays to determine local mechanical properties of materials. For example, this technique allows the characterization of metallic alloys having a polycrystalline microstructure. The size of the indenter in nanoindentation being small (few microns to few tens microns), and consequently this technique is ideal for characterizing the surface mechanical properties of different grains of a material. It allows simultaneous measurement of the hardness and the elastic modulus. If nanoindentation tests are associated with a XY motorized test bed, a wide array of indents can be achieved with a step of few micrometers. Thanks to this technique and as part of this thesis, we have realized at first hardness and elastic modulus mapping (HIPF and EIPF). In a second time, we have evaluated unconventional properties of titanium alloys, such as shape memory effect and superelasticity. In the first part of the study, nanoindentation was correlated with EBSD (Electron backscattered diffraction) to identify the relationship between the crystallographic orientation of a grain and its mechanical properties. The study was conducted on the Ti-30Nb and Ti-27Nb (at.%) alloy compositions having a bodycentered cubic structure ( phase), and the commercially pure titanium (CP-Ti) having a hexagonal close packed structure ( phase). In the second part of the study, nanoindentation was used to measure the shape memory effect (SM) and the superelasticity (SE) of various titanium alloys through a range of indentation depth. The measurement of these unconventional properties was performed from the study of load-displacement curves for each indentation test. The magnitude of the SE and SM effect was characterized by depth and work ratios determined from the study of nanoindentation curves and AFM profiles.

NiTi (also known as Nitinol) is an almost equiatomic alloy of nickel and titanium and has many applications in various industries, such as biomedical, automotive, and aerospace. NiTi shape memory alloys undergo martensitic phase transformations under both thermal and mechanical loading and exhibit unique properties, such as superelasticity (SE) and shape memory effects (SME). Modeling the fatigue behavior of this alloy is very challenging due to the unique mechanical response of the material. Moreover, there are very limited studies on the fatigue behavior of this alloy under more realistic loading conditions, such as variable amplitude loading and multiaxial loading. In this study, strain-controlled cyclic experiments have been conducted in different conditions to study the variable amplitude fatigue behavior of superelastic NiTi. Nonzero mean strain/stress behavior of superelastic NiTi is investigated, and it is demonstrated that the classical fatigue models for mean strain/stress correction do not appropriately model the nonzero mean strain/stress fatigue behavior of superelastic NiTi. It is shown that, despite common metals (e.g., steel, aluminum, and titanium alloys), mean strain also affects the fatigue behavior of superelastic NiTi, as the resulting mean stress does not fully relax under cyclic load. Two energy-based fatigue models have been proposed based on the results in this study and provide acceptable correlation with experimental observations. The models proposed in this research, account for the effects of mean strain/stress and variations in cyclic deformation. The variations in the cyclic deformation can be due to several factors, such as slight changes in chemical composition, heat treatment processes, texture, etc. The predicted fatigue lives using the proposed fatigue model fall within scatter bands of 1.5 times the experimental life for constant amplitude loading. Analyses also show that the proposed total fatigue toughness parameter, ΣWt, together with the Rainflow cycle counting technique can accurately predict the fatigue life under more realistic loading condition, such as two-step (i.e. high-low and low-high) and variable amplitude load-paths.

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