TiNi超弾性合金細線の強度に及ぼす環境の影響

Ti-Ni shape memory alloy (SMA) thin wires have been applied in dental corrective devices and the shape memory treatment was done by electric current heating in a dental clinic. However, there are only a few studies done on the shape memory characteristics of the electric current heat treatment. This paper explores the effects of electric current heat treatment on shape memory behavior. The thin Ti-Ni wires with diameters of 0.3mm were made by melting ingot followed by drawing the wires. These wires were electric current heat treated at 8V 0.7A and 16V 1.7A in the range of 300s, 900s and 1.8ks respectively. The transformation temperatures were measured by a Differential Scanning Calorimetly (DSC) during cooling and heating. Mechanical properties were measured by a tensile machine.

Since NiTi shape memory alloy (SMA) was discovered in the early 1960s, great progress has been made in understanding the properties and mechanisms of NiTi SMA and in developing associated products. For several decades, most of the scientific research and industrial interests on NiTi SMA has focused on its superelastic applications in the biomedical field and shape memory based “smart” devices, which involves the low strain rate (around 0.001 s^-1) response of NiTi SMA. Due to either stress-induced martensite phase transformation or stress induced martensite variant reorientation under the applied load, NiTi SMA has exhibited a high damping capacity in both austenitic and martensitic phase. Recently, there has been an increasing interest in exploitation of the high damping capacity of NiTi SMA to develop high strain rate related applications such as seismic damping elements and energy absorbing devices. However, a systematic study on the influence of strain, strain rate and temperature on the mechanical properties, phase transformation, microstructure and crystal structure is still limited, which leads to the difficulties in the design of products being subjected to high strain rate loading conditions. The four main objectives of the current research are: (1) achieve the single loading and the control of strain, constant strain rate and temperature in high strain rate compression tests of NiTi SMA specimens using Kolsky (split Hopkinson) compression bar; (2) explore the high strain rate compressive responses of NiTi SMA specimens as a function of strain (1.4%, 1.8%, 3.0%, 4.8%, and 9.6%), strain rate (400, 800 and 1200 s^-1), and temperature (room temperature (294 K) and 373 K); (3) characterize and compare the microstructure, phase transformation and crystal structure of NiTi SMAs before and after high strain rate compression; and (4) correlate high strain rate deformation with the changes of microstructure, phase transformation characteristics and crystal structure. Based on the results from this study, it was found that: (1) the compressive stress strain curves of martensitic NiTi SMAs under quasi-static loading conditions are different from those under high strain rate loading conditions, where higher strain hardening was observed; (2) the critical stress and stress plateau of martensitic NiTi SMAs are sensitive to the strain rate and temperature, especially at 373K, which results from the interplay between strain hardening and thermal softening; (3) the microstructure of martensitic NiTi SMA has changed with increasing strain rate at room temperature (294 K), resulting in the reduction in the area of ordered martensite region, while that area increases after deformation at elevated temperature (373K); (4) the phase transformation characteristic temperatures are more sensitive to deformation strain than strain rate; (5) the preferred crystal plane of martensitic NiTi SMA has changed from (11 ̅1)M before compression to (111)M after compression at room temperature (294 K), while the preferred plane remains exactly the same for martensitic NiTi SMA before and after compression at 373 K. Lastly, dynamic recovery and recrystallization are also observed after deformation of martensitic NiTi SMA at 373K.

The phase transformation temperature of nickel titanium (NiTi) shape memory alloy (SMA), which is commonly used in biomaterial fields, is strongly influenced by aging heat treatments. In this study, we apply a new aging heat treatment under loading conditions to near-equiatomic NiTi SMA wires to investigate its effect on phase transformation temperatures. We determine changes in the phase transformation temperatures via differential scanning calorimetry measurements. We analyze transformation temperatures, hysteresis properties, and enthalpy changes and discuss significant results with the help of micrographs. Moreover, we evaluate our results using two-way analysis of variance, axiomatic design methods, and customary analysis in order to make reliable inferences. We observe a significant difference between first and second heating-cooling cycle results for NiTi SMA samples on the basis of austenitic transformation temperatures. Consequently, we are able to theorize correlations between design parameters and functional requirements.

Crevice Corrosion Behavior of CP Ti, Ti-6Al-4V Alloy and Ti-Ni Shape Memory Alloy in Artificial Body Fluids

ABSTRACTShape memory alloys (SMA) are metallic attractive engineering materials due to their capacity to store pre-defined shapes through a thermally induced phase transition from a solid state. This paper aims to evaluate the influence of solubilization thermal treatments on a NiTi shape memory alloy originally fabricated by vacuum induction melting and then reprocessed by plasma melting followed by injection molding (Plasma Skull Push Pull process) into different metal molds (steel, aluminum, brass and copper) in order to compare the thermal properties regarding to its raw state. The thermal treatments of solubilization were carried out at 850°C in different times (2n function, n = 0, 1, 2 and 3, in hours). The influence of solubilizing treatments in the NiTi shape memory alloy was analyzed using the following characterization techniques: Differential Scanning Calorimetry (DSC) and Electrical Resistance as a function of Temperature (ERT). The results demonstrate that the solubilization heat treatments applied on the reprocessed NiTi shape memory alloy through the plasma skull push pull process, provides important changes in the phase transformation of the material. Therefore, it was demonstrated that it is necessary to solubilize the material after melting or remelting the NiTi shape memory alloy via this process to obtain mini-actuators products with homogeneous properties.

Restoration phenomena of neutron-irradiated TiNi shape memory alloys

Enhancing biocompatibility of NiTi shape memory alloys by simple NH3 treatments

The possibility of uniting two or more different materials to obtain structures capable to feel and adapt to environmental alterations and operational conditions, has been leading to the development of active composites with functional properties that makes possible the control of shape, vibration, rigidity and/or structural integrity monitoring. These characteristics are very well accepted in modern technological applications. In this context, active composites were manufactured using pre-impregnated (Pre-Pregs) of carbon fiber reinforced polymer (CFRP) and NiTi shape memory alloy (SMA) thin wires in different conditions. The functional capacity of the obtained CFRP-NiTi smart composites was verified through detection of buckling and thermal contraction/expansion effects by electrical heating of the NiTi wires inside the CFRP matrix. Although the NiTi wires represent a minimum volumetric fraction in the CFRP-NiTi produced systems, its influence was evidenced.

Effect of manganese content on the hydrogen embrittlement of twinning-induced plasticity (TWIP) steels under hydrogen charging and hydrogen environment

The electro-discharge machining (EDM) characteristics of TiNi shape memory alloys (SMAs) have been investigated in this study. Experimental results show that the material removal rate of TiNi SMAs in the EDM process significantly relates to the electro-discharge energy mode, involving the pulse current IP and pulse duration τP. It also has a reverse relationship to the product of the melting temperature and thermal conductivity of TiNi SMAs. In addition, a longer pulse duration τP and a lower pulse current IP should be selected to have a precise EDM machining of TiNi SMAs. Many electro-discharge craters and re-cast materials are observed on the EDM surface of TiNi SMAs. The re-cast layer consists of the oxides TiO2, TiNiO3 and the deposition particles of the consumed Cu electrode and dissolved dielectric medium. The thickness of the re-cast layer initially increases, reaches a critical value, and then decreases with increasing pulse duration τP. The specimen's hardness near the outer surface can reach 750 Hv for EDM TiNi SMAs. This feature originates from the hardening effect of the re-cast layer. The EDM TiNi SMAs still exhibit a nearly perfect shape recovery at a normal bending strain, but a slightly reduced shape recovery at a higher bending strain due to the depression of the re-cast layer. All the Ti49Ni51, Ti50Ni50 and Ti50Ni40Cu10 SMAs exhibit similar EDM characteristics although they have different crystal structures and mechanical properties at room temperature.

Acoustoplastic effect in the shape memory alloy Ni–Ti–Re at ultrasonic frequency

Thermodynamic ripening induced multi-modal precipitation strengthened NiTi shape memory alloys by directed energy deposition

Ni-Ti, a shape memory alloy (SMA), can recover from deformation to its initial shape when heated. With using this effect, Ni-Ti SMA is applied for several industries such as a medical industry, an aerospace, electrical application on a part of micro structure. The Ni-Ti alloy used for SMA is composed of approximately 56% nickel and 44% titanium. With this composition, Ni-Ti alloy cannot be machined efficiently using traditional machining tools and methods such as the lathe, milling, and drilling because it shares the poor heat dispersion characteristics of titanium. Thus, Ti-Ni SMA should be machined using non-traditional machining methods. Electrochemical micro machining (EMM) is one form of non-traditional machining that can be applied to Ni-Ti SMA. As an anodic dissolution process, EMM allows the machining of complex shapes in Ti-Ni SMA without the generation of heat and without causing tool wear during the machining process. In this study, through the experiment that making the micro-groove, the characteristic of short pulse electrochemical machining (SPECM) process is accomplished to Ni-Ti SMA. And an evaluation of various machining factors for Ni-Ti SMA is also performed, through the substitution of different types of power source and machining time, and through simulation of the material removal rate (MRR). The experimental and simulation results are analyzed and compared.

Biocompatibility of TiO2 and TiO2/heparin coatings on NiTi alloy

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