Deformation Behavior of Heat-Treated Ni-Rich NiTi Shape Memory Alloy
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While much is known about the shape memory and supereleasticity of equiatomic alloy of Ni and Ti (NiTiNoL), the deformation behavior of Ni-rich NiTi has attracted relatively some attention. The increase in nickel content, however, also hardens the alloy, which can make the alloy difficult to process. A slightly Ni-rich composition of NiTi can be, on the other hand, advantageous in applications where a higher stiffness of NiTi coupled with its shape memory and superelasticity is required e.g. in orthodontic wires, cardiovascular stent or pressure valves. In this study, we investigate the influence of heat treatment on the deformation behavior of superelastic nickel–titanium for biomedical applications. For this, NiTi alloy composed of 56 wt.% (51 at.%) nickel has been investigated after heat treatment within the thermal window of between 400 and 800°C. Heat treatment significantly influenced both the plasticity and the transformation behavior of Ni-rich NiTi. A detailed examination of the microstructural evolution, calorimetric response and tensile test response with respect to the superelasticity allowed us to establish protocols for obtaining nearly ideal superelastic properties in Ni-rich NiTi shape memory alloys. Our findings can enable use of these alloys in e.g. medical devices that require higher stiffness and a larger surface area.Keywords:
Nickel titanium
Pseudoelasticity
Diffusionless transformation
Titanium alloy
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Nickel titanium
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This chapter contains sections titled: Introduction Experimental setup and test matrix Fatigue tests results Conclusions Acknowledgments
Nickel titanium
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The present study investigates the superelasticity properties of spark plasma sintered (SPS) nickel titanium shape memory alloy (NiTi SMA) with the influence of sintering temperature and particle size. The nanoindentation is conducted on the surface of the NiTi SMA at various loads such as 100, 300 and 500[Formula: see text]mN. The nanoindentation technique determines the quantitative results of elasto-plastic properties such as depth recovery in the form of superelasticity, stiffness, hardness and work recovery ratio from load–depth ([Formula: see text]–[Formula: see text]) data during loading and unloading of the indenter. Experimental findings show that the depth and work recovery ratio increases with the decrease of indentation load and particle size. In contrast, increasing the sintering temperature exhibited a better depth and work recovery due to the removal of pores which could enhance the reverse transformation. The contact stiffness is influenced by [Formula: see text] which leads to attain a maximum stiffness at the highest load (500[Formula: see text]mN) and particle size (45[Formula: see text][Formula: see text]m) along with the lowest sintering temperature (700 ∘ C). NiTi alloy exhibited a maximum hardness of 9.46[Formula: see text]GPa when subjected to indent at the lowest load and particle size sintered at 800 ∘ C. The present study reveals a better superelastic behavior in NiTi SMA by reducing the particle size and indentation load associated with the enhancement of sintering temperature.
Pseudoelasticity
Nickel titanium
Spark Plasma Sintering
Indentation
Particle (ecology)
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NiTi-Zr high temperature alloys possess relatively poor shape memory properties and ductility in comparison with NiTi-Hf and NiTi-Pd alloys. During martensite transformation of the newly developed NiTi-Zr high temperature shape memory alloys (SMAs) the temperature increases along with Zr content when the Zr content is more than 10 at%. As the Zr content increases, the fully reversible strain of the alloys decreases. However, complete strain recovery behavior is exhibited by all the alloys studied in this paper, even those with a Zr content of 20 at%. Stability of the NiTi-Zr alloys during thermal cycling was also tested and results indicate that the NiTi-Zr alloys have poor stability against thermal cycling. The reasons for the deterioration of the shape memory effect and stability have yet to be determined.
Nickel titanium
Temperature cycling
Diffusionless transformation
Ductility (Earth science)
Thermal Stability
Strain (injury)
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Nickel titanium
Temperature cycling
Constant (computer programming)
Strain (injury)
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Pseudoelasticity
Nickel titanium
Diffusionless transformation
Texture (cosmology)
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Pseudoelasticity
Nickel titanium
Texture (cosmology)
Ductility (Earth science)
Microscale chemistry
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Nickel titanium (NiTi) alloy is a unique alloy that exhibits special behavior that recovers fully its shape after being deformed to beyond elastic region. However, this alloy is sensitive to any changes of its composition and introduction of inclusion in its matrix. Heat treatment of NiTi shape memory alloy to above 600 °C leads to the formation of the titanium oxide (TiO2) layer. Titanium oxide is a ceramic material that does not exhibit shape memory behaviors and possess different mechanical properties than that of NiTi alloy, thus disturbs the shape memory behavior of the alloy. In this work, the effect of formation of TiO2 surface oxide layer towards the thermal phase transformation and stress-induced deformation behaviors of the NiTi alloy were studied. The NiTi wire with composition of Ti-50.6 at% Ni was subjected to thermal oxidation at 600 °C to 900 °C for 30 and 60 minutes. The formation of the surface oxide layers was characterized by using the Scanning Electron Microscope (SEM). The effect of surface oxide layers with different thickness towards the thermal phase transformation behavior was studied by using the Differential Scanning Calorimeter (DSC). The effect of surface oxidation towards the stress-induced deformation behavior was studied through the tensile deformation test. The stress-induced deformation behavior and the shape memory recovery of the NiTi wire under tensile deformation were found to be affected marginally by the formation of thick TiO2 layer.
Nickel titanium
Titanium alloy
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The thermal cycling tests and high temperature aging tests were performed to characterize the stability of NiTi-Pd and NiTi-Hf high temperature shape memory alloys. These alloys have better stability than NiTi during thermal cycling. In addition, it also found that the NiTi-Pd and NiTi-Hf alloy have a very good stability in high temperature aging.
Nickel titanium
Temperature cycling
Thermal Stability
Cycling
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Superelasticity and mechanical damping are important functional properties of nickel–titanium (NiTi) shape memory alloys (SMAs). Owing to the generation and accumulation of dislocation during loading, the recovery strain of commercial NiTi SMAs is usually smaller than 10%, which limits their ability to dissipate energy. In this paper, the superelasticity and mechanical damping of a nanocrystalline NiTi SMA was studied. The results show that the nanocrystalline NiTi SMA alloy possessed a large recovery strain of about 14%, greater than that of the commercial NiTi SMAs, and a high level of absorbed energy, or toughness, of 111 MJ m−3, which is higher than the highest value (about 81 MJ m−3) of all SMAs reported so far. The transmission electron microscopy (TEM) studies suggest that few full dislocations were generated in the nanocrystalline NiTi alloy during loading. Instead, the dominant deformation modes after stress induced martensitic transformation were elastic deformation and detwinning. The detwinning process decreased the twin boundary energy, which stabilised the martensitic phase.
Pseudoelasticity
Nickel titanium
Nanocrystalline material
Diffusionless transformation
R-Phase
Damping capacity
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