The successful use of Nickel-Titanium (Nitinol) in biomedical applications requires an accurate control of its unique mechanical properties. The purpose of this study is to analyze the effects of a wide range of heat treatments on the mechanical behavior of hot-rolled and cold-drawn Nitinol. Results comprise an understanding of the effect of heat treatment temperature and time variation on final material response which is imperative for optimization of material properties. Thirty-three heat treatment variations are tested by combining three durations, 10 minutes, 90 minutes, and 8 hours, with eleven different heat treatment temperatures between 200°C and 440°C. Following heat treatment, the Nitinol samples undergo tensile testing with upper plateau strength, lower plateau strength, ultimate tensile strength, strain to failure, and residual elongation compared for all test groups. Heat treatment "power" is used to describe the efficacy of different combinations of heat treatment temperature and duration. When using hot-rolled Nitinol, results show a low heat treatment power does not create significant precipitation hardening or a significant decrease in martensite transformation stress, resulting in a high upper plateau strength, high residual strain values, and evidence of plastic deformation upon unloading. Moderate power treatments lead to sufficient hardening of the material and a decrease in martensite transformation stress resulting in a pseudoelastic response. Increasing to a high treatment power further decreases the transformation stress and increases the martensite transformation temperature leading to a shape-memory response in hot rolled Nitinol. When using cold-drawn Nitinol, low and moderate heat treatment power levels result in the material exhibiting a pseudoelastic response. Increasing heat treatment power shows the same effects on martensite transformation stress and temperature as seen with the hot-rolled material resulting in a material response transition from pseudoelastic to shape memory.

published_or_final_version / Dentistry / Doctoral / Doctor of Philosophy

M. Tech. Metallurgical Engineering / Titanium alloys exhibit poor tribological characteristic which include abrasion resistance, metal to metal wear resistance and solid particle erosion and cavitation due to low surface hardness and high coefficient of friction. These poor properties have limited the application of titanium alloys as engineering tribological components, tools and parts that operate in severe wear and friction conditions. Laser processing defects such as pores, cracks and segregation pose a huge threat to the quality and the microstructure of the deposited layer. Defects caused by the parameters lead to severe wear and corrosion occurrence, hence, precise control of the parameters are crucial and it depends on the properties of the material used. It is postulated that Nickel Titanium (NiTi) is a promising candidate as reinforcement matrix material for wear resistant alloy. The wear resistance would be further enhanced if NiTi is in-situ incorporated onto titanium matrix by laser cladding to yield hard intermetallic phases. The main goal of the study is to develop corrosive-wear resistant thin surface coatings on Ti-6Al-4V alloy for automotive and aerospace applications by depositing Titanium and Nickel elemental powders to form in-situ NiTi intermetallic using laser cladding technique.

Thesis (M.S.)--University of Alabama at Birmingham, 2007. / Additional advisors: Peter G. Anderson, Brigitta C. Brott, Jack E. Lemons. Description based on contents viewed Feb. 4, 2008; title from title screen. Includes bibliographical references (p. 69-70).

Thesis (MTech (Mechanical Engineering))--Cape Peninsula University of Technology, 2010. / Hybrid polymeric composites (HPC) are widely used for the design of aerospace, automobile and civil engineering structures. One of the major challenges posed by these materials and structures is their brittle nature. When subjected to impact and dynamic loads, the polymeric composite structures undergo micro cracking. The cracks coalesce, propagate and can lead to catastrophic failure of the material and structures. In this thesis, an intelligent hybrid polymeric composite (IHPC) beam with healing ability was developed and tested. The IHPC beam developed consisted of a 3% prestrained 1mm diameter Ni-Ti shape memory alloy (SMA) wire actuator embedded in the polymeric host matrix. The function of the embedded Ni-Ti shape memory alloy was to enhance intelligence and healing ability to the IHPC beam. Upon electric current resistance heating, the Ni-Ti SMA actuator responds by contracting as a result of detwinned martensite → austenite phase transformation. Contraction of the SMA in the IHPC beam was utilized to stiffen and enhance healing by retarding crack growth and recovery of the strain induced in the loaded IHPC beam. This can result to increase of the flexural stiffness EI (defined as the product of the Young’s Modulus E of the material and the moment of inertia I of the geometry of the beam) and mode I fracture stress intensity factor KIC of the IHPC beam. One (1) mm diameter Ni-Ti SMA wire was used in the experimental work in this thesis. The wire was cut into 35 pieces, 200 mm long each. Ni-Ti SMA wires were heated in the furnace to a temperature of 250ºC for ten (10) hours then were left to cool in the ambient air. The heat treatment was aimed to release any residual stress and to stabilize the austenite start (AS) and austenite finish (Af) transformation temperatures of the Ni-Ti SMA. After heat treatment, the Ni-Ti SMA wires were prestrained by 3% (based on a gauge length of 150mm) on a tensile testing machine. Prestraining of the Ni-Ti SMA wires was aimed to induce detwinned martensite volume fraction in them hence increasing the transformation strain and recovery force of the Ni-Ti SMA actuator. Intelligent hybrid polymeric composite (IHPC) beams and polymeric virgin (PV) beams, all of dimensions 150mmx25mmx10mm were manufactured by casting 60D polyurethane thermosetting epoxy resin in a silicon mould. transformation strain and recovery force of the Ni-Ti SMA actuator.

Orientador: Jorge Otubo / Dissertação (mestrado profissional) - Universidade Estadual de Campinas, Faculdade de Engenharia Mecânica / Made available in DSpace on 2018-08-16T22:36:53Z (GMT). No. of bitstreams: 1 Minetto_RobertoTadeu_M.pdf: 5159616 bytes, checksum: a09b62580c76b0c4d2322045c6ba6a3d (MD5) Previous issue date: 2010 / Resumo: Idealmente, as juntas rigidamente fixadas por parafusos deveriam manter as forças de aperto aplicadas em cada parafuso até o final da vida útil e com um mínimo de variação entre os parafusos de um mesmo conjunto. No entanto, ensaios mecânicos comprovam que a dispersão das forças de aperto, nos processos atuais de fixação do cabeçote de cilindros, é muito grande e que a perda de pré-carga pode chegar a 30%. Ou seja, se aplica 100 kN e ter-se-á efetivamente 70 kN de força resultante após algumas horas de utilização. Esta perda é devido à acomodação (relaxamento) dos picos de usinagem da rosca e da face de assentamento da cabeça do parafuso, das deformações plásticas dos componentes envolvidos, do "endireitamento" parafuso, da velocidade do fuso de aperto, etc. Como o sistema de aperto convencional não usa arruela entre o parafuso e o cabeçote, a proposta deste projeto foi estudar o uso da liga Ti-50,67at%Ni com efeito de memória de forma (superelástico) para a utilização na confecção desta arruela. Uma vez montada no conjunto, a arruela estará sujeita a uma carga de compressão, e, devido ao efeito da memória de forma, o componente tenderá a voltar à forma original minimizando ou mesmo eliminando a perda de carga sofrida pelos parafusos usados na montagem / Abstract: Gaskets rigidly attached by bolts should keep the applied forces to each bolt until the end of its life and presenting a small force variation among the bolts of the same set. However, mechanical tests confirms that the bolt forces dispersion, in the current cylinder head fixation process, is large and the pre load loss reach 30 per cent. It means that if we apply 100 kN, we are going to have 70 kN of resultant bolt forces after few hours of running tests. This loss is caused by machining peaks accommodation in the thread contact and bolt head seating, plastic deformation (thermal and mechanical) of the involved components, bolts beating and others important aspects of bolt torque strategy. As the current system does not use flat washers between the bolt head and cylinder head bosses this project purposes to study the use of Ti-50,67at%Ni alloy, with shape memory effect (superelasticity) , for fabricating this washer. When assembled in the set, the washer will suffer a compressive load and because of the shape memory effect, the component will try to return to the original form, minimizing or even eliminating the pre load loss by bolts used to set / Mestrado / Materiais e Processos de Fabricação / Mestre em Engenharia Automobilistica

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.

Shape memory and superelastic applications of NiTi based alloys have typically been limited to near room temperature or to bulk length scales. The objective of this work is two-fold: first, to investigate shape memory behavior at low temperatures in the context of the R-phase transformation in NiTiFe alloys by recourse to arc-melting, differential scanning calorimetry (DSC), transmission electron microscopy (TEM) and mechanical testing at low temperatures; and second, to investigate superelasticity and two-way shape memory behavior at reduced length scales in the context of NiTi by recourse to micro-compression, micro-indentation and TEM studies. Selected compositions of ternary NiTiFe shape memory alloys were arc-melted and thermomechanically processed to investigate the influence of composition and processing parameters on the formation of the R-phase. The methodology used for the processing and characterization of the alloys was established and included microprobe analysis, DSC, TEM and mechanical testing. No phase transformation was observed in alloys with Fe content in excess of 4 at.%. Thermomechanical treatments facilitated the formation of the R-phase in Ni-rich alloys. The range of the transformation between the R-phase and austenite, and the hysteresis associated with it were influenced by the distribution and size of metastable Ni4Ti3 precipitates. The investigation of the microstructural, thermal and mechanical properties of the R-phase transformation in NiTiFe alloys revealed a complex dependence of these properties on processing parameters. The present work also highlighted the hitherto unexplored competition between the two inelastic deformation modes operating in the R-phase (detwinning and stress-induced transformation) and established the preference of one mode over the other in stress-temperature space. iv The complete micromechanical response of superelastic NiTi was examined by performing careful micro-compression experiments on single crystal pillars of known orientations using a nanoindenter tip. Specifically, the orientation dependence of the elastic deformation of austenite, the onset of its transformation to martensite, the gradient and the hysteresis in the stress-strain response during transformation, the elastic modulus of the stress-induced martensite and the onset of plasticity of the stress-induced martensite were analyzed in separate experiments. A majority of the results were explained by recourse to a quantitative determination of strains associated with austenite grains transforming to martensite variants or twinning in martensite. Microstructural studies were also performed on a micro-indentation trained NiTi shape memory alloy specimen to understand the mechanisms governing the two-way shape memory effect. In situ TEM studies at temperature on specimens obtained at different depths below the indent showed the presence of retained martensite along with the R-phase. Previously, while such twoway shape memory behavior has typically been associated with large dislocation densities, this work provides evidence of the role of retained martensite and the R-phase in cases with reduced dislocation densities. Funding support for this work from NSF (CAREER DMR-0239512), NASA (NAG3-2751) and SRI is acknowledged.

A heightened interest in intelligent material systems has occurred in recent years due to their remarkable adaptive abilities. Intelligent materials systems, which contain sensors and actuators coupled by means of active control, frequently utilize composite materials as the skeletal structure. In order for composite materials to be utilized in intelligent material systems to their utmost capability, many material properties, including the interfacial shear strength between the embedded sensor or actuator and the matrix must be thoroughly understood.. Investigations were performed in order to examine the effects of different variables on the interfacial characteristics between a nitinol fiber and a composite matrix. First, rough, clean fiber surfaces were found to provide the best adhesion to the matrix due to the mechanical interaction of the matrix with the rough surface finish. Second, it was determined that the interfacial shear strength is not dependent upon embedded fiber length. Third, a very small diameter fiber will break before pulling out of the matrix, but overall, large fibers have a greater interfacial strength. Fourth, it was found that the initial prestrain on the fiber during processing had no effect on the interfacial shear strength of the fiber to the matrix. Fifth, it was determined that fatigue does not degrade the shear strength of any of the different initial pres trains. Finally, it was found that a coating that does not adhere well to the fiber neither macroscopically degrades nor enhances interfacial strength. / Master of Science

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.