The current study focuses on phase stability and evolution in the titanium-zinc titanium-copper and titanium-antimony systems. The study utilized the Laser Engineering Net Shaping (LENS™) processing technique to deposit compositionally graded samples of three binary system in order to allow the assessment of phase stability and evolution as a function of composition and temperature the material is subjected to. Through LENS™ processing it was possible to create graded samples from Ti-xSb (up to 13wt%) and Ti-xCu (up to 16wt%). The LENS™ deposited gradient were solutionized, and step quenched to specific aging temperature, and the resulting microstructures and phase were characterized utilizing XRD, EDS, SEM, FIB and TEM. The Ti-Zn system proved incapable of being LENS™ deposited due to the low vaporization temperature of Zn; however, a novel processing approach was developed to drip liquid Zn onto Ti powder at temperatures above β transus temperature of Ti (882 ◦C) and below the vaporization temperature of Zn (907 ◦C). The product of this processing technique was characterized in a similar way as the graded LENS™ depositions. From measurements performed on Ti-Sb it seems that Sb could be a potential α stabilizer in Ti due to the presence of a mostly homogeneous α grains throughout the gradient; however, from XRD it can be understood that a titanium antimonide phase is present. From results obtained from the Ti-Zn samples, it can be surmised that the eutectoid reaction seems to be active, i.e. The eutectoid reaction is kinetically fast, as concluded by the presence of pearlitic structures. Finally, for the Ti-Cu system this work has been attempted to prove or disprove the existence of the Ti3Cu through the use of XRD and TEM SAD patterns. From XRD spectra collected there are peaks belonging to the Ti3Cu orthorhombic phase along with Ti2Cu and α-Ti phase. In addition to the Ti-Cu system displayed structures associated with divorced eutectoid decomposition mechanism, and at low undercooling seems to be prone to forming solid state dendrites.

NiTiCu is a shape memory alloy that regains its original shape after large amount of shape changing deformation when heated above a critical temperature called reverse martensitic trans-formation temperature( Af). When external load is applied on the sample in twinned martensite phase at low temperature, it deforms by detwinning, accommodating large amount of strains. When it is heated above Af, the shape recovers by transformation of the martensite to austenite phase. However, the amount of shape recovery degrades over time due to internal factors such as precipitates, residual strains and thermal history as well as external factors such as stresses. Severe localized stresses induced by contacts result in plastic deformation that affect the reverse martensitic transformation and hence the shape recovery. In this work, we study how varying levels of contact stresses induced in NiTiCu thin film affect its shape recovery. NiTiCu thin films of six different compositions are deposited on Si(100) wafer by co-sputtering from elemental targets. After deposition, the films are annealed at 500 C for 4 h to make them crystalline. The composition of the films varied linearly with applied power to the targets. Uniformity in composition over a 4 inch substrate area is achieved by substrate rotation. All the films show ne grain microstructure after annealing. The subsurface of the Ni-rich films is columnar. Ni-rich films have annealing cracks and the crack width increases with Ni composition in the films. The roughness of as-deposited films is found to be more for Ni-rich films compared to Ti-rich films. The roughness of the Ni-rich and Ti-rich films increased after annealing. From the X-ray diffraction studies, it was observed that the films are nanocrystalline. Indentation is carried out using a Berkovich diamond indenter with spherical apex, at nine different locations with loads ranging from 0.25 mN to 25 mN. A predefined array is chosen for indentation such that the larger indents act as a guide to precisely locate minute indents generated at lower loads, with residual depth as small as 10 nm, for imaging in high-resolution microscopes like Scanning Electron Microscope as well as in Atomic Force Microscope . In Ti60 (a Ti-rich) lm, the residual indents generated at loads greater than 10 mN show radial cracks originating at corners. Average crack length increases with the maximum load used for generating the indent. Sequential sectioning of Ti48 (a Ni-rich) lm using Focused Ion Beam microscope, revealed that the cracks originate at the lm-substrate interface and reach the surface. In Ti48 lm, residual indents do not show any indentation cracks. The indentation stresses are accommodated by breaking of the columnar structure and the voids between them. Delamination of the film from the substrate is observed on either sides of the indent in both the Ti60 and Ti48 films. The hardness of the films is high at low loads and decrease as the load increases. The deformation by indentation at lower loads is mainly due to detwinning as only the apex of the indenter, which is nearly spherical, is in contact with the sample and the resulting stresses are low. As the load increases, the deformation starts getting accommodated through dislocations along with detwinning as the stress beneath the indenter increases. Spherical cavity model extended to SMA shows that inner hemisphere near the tip contains dislocations where stresses are very high, surrounded by detwinned region with stresses that are relatively low. When the sample is heated above reverse martensitic transformation temperature to induce shape recovery in the indents, only the detwinned region recovers to the original shape. Recovery ratio, quantification of shape recovery, is calculated from the depth of the indents before and after heating. Recovery ratio in Ti60 films is found to be large at low loads and decreases with increase in load. The decrease in shape recovery in Ti60 is attributed to the increase in the amount of plastic deformation at the expense of detwinning. Three-dimensional mapping of the surfaces shows that the recovery ratio is high at the apex of the indent at the maximum depth and reduces towards the edges of the indent. There is no evident recovery in Ti48 films. The shape recovery of SMAs can be achieved by Joule heating. When electric current is passed through the material, it heats up by Joule heating because of the intrinsic resistivity. The resistivity and hence the resistance would get effected by the dislocation based plastic deformation induced by the contact. This might result in shape recovery through resistive heating. Towards understanding this, the effect of contact stresses on electrical contact resistance is studied. Experimental setup is designed, developed and calibrated for studying the variation of electrical contact resistance of the NiTiCu thin films as a function of load. Electrical contact resistance is found to decrease with increase in applied load. Contact stresses in sub-micron NiTiCu thin films are simulated by carrying out nanoindentation at different loads. The recovery ratio is high when the stresses induced by the contact is less, at lower loads. The shape recovery ratio is reduced when the induced contact stresses in-creases. There is no shape recovery at the sharp edges of the indentation where contact stresses are very high. Hence, by carefully designing the features to reduce the stress concentrations, the performance of the device can be improved.

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.