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.

A shape memory alloy (SMA) is an intermetallic compound able to recover, in a continuous and reversible way, a predetermined shape during a thermal cycle while generating mechanical work. In this thesis, its use in developing an actuator for a machining process is investigated. The actuator is to drive the tool cross feed into an aluminium workpiece in a finishing lathe operation. The actuator structure was designed with an output shaft to transfer the movement and force of the SMA wire outside the device. The actuator was fabricated and the experimental setup was assembled which also included a power supply control circuit, displacement sensor, temperature sensor and current sensor for feedback, and data collection and monitoring within software. PID control was implemented within the software that regulated the power supplied to the SMA, thereby providing the position control. This study covers the mechatronics system design and development of the actuator, the experiments carried out to determine performance and the results. Open loop tests were conducted to determine the maximum stroke, the effect of cooling and response to radial forces. These tests revealed the expected non-linearity of the SMA. The actuator achieved the rated maximum stroke of 3-4 percent. The forced cooling test showed a general improvement of approximately 65 percent with fans. The radial force tests showed the value of the maximum stroke remained unaffected by force. The results from the closed loop tests responses with a tuned PID controller produced a stable system for various displacement setpoints. The actuator had a feed rate of 0.25 mm/s and an accuracy of 0.0153mm, which was within the acceptable accuracy for turning operations. The system was deemed accurate for a conventional lathe machine cross feed.

Actuator applications of shape memory alloys have typically been limited by their phase transformation temperatures to around 100 degrees C. However, recently with a focus on aerospace and turbomachinery applications there have been successful efforts to increase the phase transformation temperatures. Several of these alloy development efforts have involved ternary and quaternary elemental additions (e.g., Pt, Pd, etc.) to binary NiTi alloys. Experimentally assessing the effects of varying composition and thermo-mechanical processing parameters can be cost intensive, especially when expensive, high-purity elemental additions are involved. Thus, in order to save on development costs there is value in establishing a methodology that facilitates the fabrication, processing and testing of smaller specimens, rather than larger specimens from commercial billets. With the objective of establishing such a methodology, this work compares thermo-mechanical test results from bulk dog-bone tensile Ni29.5Ti50.5Pd20 samples (7.62 mm diameter) with that of thin wires (100 μm-150 µm diameter) extracted from comparable, untested bulk samples by wire electrical-discharge machining (EDM). The wires were subsequently electropolished to different cross-sections, characterized with Scanning Electron Microscopy, Transmission Electron Microscopy and Energy Dispersive X-Ray Spectroscopy to verify the removal of the heat affected zone following EDM and subjected to Laser Scanning Confocal Microscopy to accurately determine their cross-sections before thermo-mechanical testing. Stress-strain and load-bias experiments were then performed on these wires using a dynamic mechanical analyzer and compared with results established in iv previous studies for comparable bulk tensile specimens. On comparing the results from a bulk tensile sample with that of the micron-scale wires, the overall thermomechanical trends were accurately captured by the micron-scale wires for both the constrained recovery and monotonic tensile tests. Specifically, there was good agreement between the stress-strain response in both the martensitic and austenitic phases, the transformation strains at lower stresses in constrained recovery, and the transformation temperatures at higher stresses in constrained recovery. This work thus validated that carefully prepared micron-diameter samples can be used to obtain representative bulk thermo-mechanical properties, and is useful for fabricating and optimizing composition and thermomechanical processing parameters in prototype button melts prior to commercial production. This work additionally assesses potential applications of high temperature shape memory alloy actuator seals in turbomachinery. A concept for a shape memory alloy turbine labyrinth seal is also presented. Funding support from NASA’s Fundamental Aeronautics Program, Supersonics Project (NNX08AB51A) and Siemens Energy is acknowledged.

No description available.

The @lab is a group dedicated to the research and development of electronic textiles for architectural applications; this thesis presents the structural analyses performed by the author to improve the @lab’s projects. Also included are three investigations performed by the author that pertain to smart material applications in responsive architecture and textiles. The first investigation evaluated the feasibility of using piezoelectric materials to harvest power from human foot traffic; overall, it was determined to not be feasible. The second investigation experimentally tested how six parameters of shape memory alloy spring actuators affect their reaction times and stroke; all six parameters affected the reaction times and/or stroke. The third investigation experimentally tested how three parameters of superelastic SMA springs influence their stiffness and resonant frequencies; overall, it was found that traditional spring mechanics can be used to predict their behavior providing the internal stress does not reach the upper plateau stress.

published_or_final_version / abstract / Orthopaedics and Traumatology / Master / Master of Philosophy

Approved for public release; distribution is unlimited / Solder joints provide both electrical and mechanical interconnections between a silicon chip and the packaging substrate in an electronic application. The thermomechanical cycling (TMC) in the solder due to the mismatch of the coefficient of thermal expansion (CTE) between the silicon chip and the substrate causes numerous reliability challenges. This situation is aggravated by the ongoing transition to lead-free solders worldwide, and the trend towards larger, hotter-running chips. Therefore, improved solder joints, with higher resistance to creep and low-cycle fatigue, are necessary for future generations of microelectronics. This study reports in the development a process to fabricate solder joints with a fine distribution of shape memory alloys (SMA) NiTi particulates. The microstructure and interface zone of the as-reflowed solder-SMA composite has been characterized. / Lieutenant, United States Navy

NiTiHf alloys have emerged as potential materials for applications requiring high transformation temperatures (> 100 °C) with high strength and work output. Although they have high transformation temperatures, their low damping capacity, brittleness and poor superelastic responses (of Ti-rich NiTiHf) impedes their wider usage in many industrial applications. In this study, the quaternary alloying element of Pd has been added to NiTiHf alloys to improve and tailor their shape memory behavior,. NiTiHfPd alloys were systematically examined regarding the composition and heat treatments effects. Effects of substituting Hf with Ti on the shape memory behavior of NiTHfPd alloys were investigated. There compositions were selected as Ni40.3Ti34Hf20Pd5 Ni40.3Ti39.7Hf15Pd5 and Ni40.3Ti44.7Hf10Pd5 (at.%). Their transformation temperatures, microstructure and shape memory properties were revealed and compared with conventional shape memory alloys. It was revealed that their transformation temperatures increases but transformation strain decreases with the increment of Hf content. Additionally, superelastic responses of Ni45.3Ti29.7Hf20Pd5 andNi45.3Ti39.7Hf10Pd5 alloys were investigated. Transformation temperatures of polycrystalline Ni45.3Ti29.7Hf20Pd5are highly dependent on aging temperatures and they can be altered widely from room temperature to 250 oC. Finally, the damping capacity of the Ni45.3Ti39.7Hf10Pd5 polycrystal and [111]-oriented Ni45.3Ti29.7Hf20Pd5 single crystal were investigated. The damping capacities were found to be 16-25 J.cm-3, and 10-23 J.cm-3 for the Ni45.3Ti39.7Hf10Pd5 and [111]-oriented Ni45.3Ti29.7Hf20Pd5 alloys, respectively.

This thesis details an investigation into the properties and applications of shape memory alloy (SMA) composites. SMA-composites are a new material which have the possibility of having a large impact on what the structures as we know today, are constructed with. SMA-composites are adaptive materials which can be used to control the shape and frequencies of vibration of a structure. In order to determine the effectiveness of such a material, research into the functional properties of SMAs and SMA-composites was conducted. As an initial step, the transformation behaviour of constrained SMAs was investigated in order to obtain a better understanding into the recovery stress generation of these wires when embedded into a composite material. It is known that the transformation is based on two types of martensite within the alloy; self accommodating and preferentially oriented martensite. The amounts of each type and how they vary with differing pre-strain were determined through DSC measurements and an explanation for why preferentially oriented martensite is not observed during DSC testing was made. The next step was to investigate the effectiveness of embedding SMA wires into composites and the thermomechanical properties of the SMA wires and the SMA-composites were determined. This was completed using a specially designed tensile testing machine which was capable of having the whole specimen immersed into an oil bath and heated and cooled repeatedly. The stress-strain, strain-temperature, stress-temperature, resistance-strain and cyclic properties of various wires were obtained, giving a better understanding of the behaviour of SMA wires under different test conditions. NiTiCu SMA wires were embedded into kevlar composite materials and the recovery stress generation (stress-temperature), stress-strain, and strain-temperature behaviour was determined. If SMA-composites are to be used as new materials for structural applications, verification that the embedment of pre-strained SMA wires into the material doesn't adversely affect the impact behaviour needs to be carried out. SMA-composite specimens with varying volume fractions of superelastic SMA wires, pre-strain and position through the thickness were made up for impact damage characterisation. These specimens were impacted at three different energy levels. The results showed that by embedding SMA wires into composite materials there is a reasonably low damage accumulation after impact. There is also no adverse impact effect on the structure compared with structures without wires as well as structures with other types of wires such as steel and martensitic SMA wires. The SMA-composites showed good damping and energy absorption capabilities. A novel application of SMA-composites is their use as a SMA patch in order to repair damage in existing cracked metallic structures. An analytical study and finite element modelling showing the closure stresses obtainable for use as patches was made.

Shape memory alloys (SMAs) have recently attracted much attention as a smart material that can be used in passive protection systems such as energy dissipating devices and base isolation systems. For the purpose of investigating the potential use of SMAs in seismic engineering applications a soft computing approach, namely a neurofuzzy technique is used to model dynamic behavior of CuAlBe shape memory alloy wires. Experimental data are collected from two test programs that have been performed at the University of Chile. First, in order to evaluate the effect of temperature changes on the behavior of superelastic SMA wires, a large number of cyclic, sinusoidal, tensile tests are conducted at various temperatures. Second, to assess dynamic effects of the material, a series of laboratory experiments are conducted on a scale model of a three story model of a building that is stiffened with SMA wires and given excitation by a shake table. Two fuzzy inference systems (FISes) that can predict hysteretic behavior of CuAlBe wire have been created using these experimental data. Both fuzzy models employ a total of three input variables (strain, strain-rate, and temperature or prestress) and one output variable (predicted stress). Values of the initially assigned membership functions for each input are adjusted using a neural-fuzzy procedure to accurately predict the correct stress level in the wires. Results of the trained FISes are validated using test results from experimental records that had not been previously used in the training procedure. Finally, numerical simulations are conducted to illustrate practical use of these wires in a civil engineering application. In particular, dynamic analysis of a single story frame and a three story benchmark building that are equipped with SMA damping elements are conducted. Then, an isolated bridge that utilizes a linear rubber bearing together with SMA elements is analyzed. Next, in order to show recentering ability of SMAs, nonlinear time history analysis of a chevron like braced frame is implemented. The results reveal the applicability for structural vibration control of CuAlBe wire whose highly nonlinear behavior is modeled by a simple, accurate, and computational efficient FIS.